Prevention and control of
membrane fouling:
practical implications and
examining recent innovations
By assignment from: DSTI
Performed by:
dr.ir. A.C.M. Franken
Membraan Applicatie Centrum Twente b.v.
Date:
June 2009
Summary
The project “prevention and control of membrane fouling: practical implications and examining recent
innovations” is carried out as a techno-project within DSTI. The original objectives of this project are: to
present a brief outline of the state-of-the-art on research into membrane fouling and to evaluate some
novel techniques for the reduction and prevention of membrane fouling.
In a first instance the phenomenon of flux decline is described based on a resistances model. Based on
this model the different types of membrane fouling are presented. A state-of-the-art of cleaning procedures for the different types of fouling is given.
Reduced fouling strategies involving membranes are discussed in chapter 3. The modification of polymer
surfaces using functionalised coatings is described at the hand of the coatings applied by Poly-An, a
company specialised in chemical modification of polymer surfaces. Another approach, developed at
Wageningen University, uses physical attachment of complex coacervate core micelles (C3Ms) on membranes to improve the antifouling properties of the membrane using a coating of polymeric brush layers
that are formed by adsorption.
For feed streams with higher concentration and/or increased viscosity, the modification of the membrane
surface is not enough to prevent flux decline and membrane fouling. Chapter 4 discusses briefly the wellknown approach of cross-flow and back-flush/back-shock. Specific, relatively new, developments to
improve mechanical support of the filtration process are presented, being vibration enhanced membrane
separation (V-SEP) and (preventive) cleaning of RO-membranes using air (AiRO).
Two case studies are described in chapter 5 and 6, being (i) clarification of inuline juice (Sensus) and (ii)
nanofiltration of a lactic acid residual stream. Both cases could not be solved using modification of
membrane properties. In the case of the inuline juice, major improvements could be achieved using
improved methods of membrane cleaning, and furthermore improving the method of conservation and
maintenance. For the lactic residual stream tests were performed using a V-SEP pilot system. The results
show that the fluxes were improved with reduced energy costs.
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Table of contents
Summary ...................................................................................................................................................... 2
Table of contents .......................................................................................................................................... 3
1.
Introduction ........................................................................................................................................ 5
2.
What is membrane fouling? ............................................................................................................... 6
2.1. Flux decline .................................................................................................................................... 6
2.1.1.
Resistances during filtration process ...................................................................................... 6
2.1.2.
Concentration polarisation ...................................................................................................... 7
2.1.3.
Gel layer formation.................................................................................................................. 8
2.1.4.
Characterization methods: SDI and MFI ................................................................................ 8
2.2. Types of membrane fouling ........................................................................................................... 9
2.2.1.
Colloidal fouling....................................................................................................................... 9
2.2.2.
Organic fouling...................................................................................................................... 10
2.2.3.
Scaling .................................................................................................................................. 10
2.2.4.
Biofouling .............................................................................................................................. 10
2.3. Membrane Cleaning..................................................................................................................... 12
2.3.1.
Four variables ....................................................................................................................... 12
2.3.2.
Cleaning agent...................................................................................................................... 13
2.3.3.
Mechanical action ................................................................................................................. 13
2.3.4.
Mechanical action ................................................................................................................. 14
2.3.5.
Temperature and time .......................................................................................................... 15
2.4. Cleaning strategies ...................................................................................................................... 15
2.4.1.
How to identify the type of membrane fouling? .................................................................... 15
2.4.2.
Foulant type and phenomena ............................................................................................... 16
2.4.3.
Cleaning schedule ................................................................................................................ 17
2.4.4.
Membrane autopsy ............................................................................................................... 17
3.
Reduced-fouling strategies .............................................................................................................. 19
3.1. Poly-An - modification of polymer surfaces.................................................................................. 19
3.1.1.
Functionalized surfaces for anti-fouling applications ............................................................ 19
3.1.2.
Affinity materials for life science ........................................................................................... 19
3.1.3.
Molecular Surface Engineering............................................................................................. 20
3.2. LU Wageningen - C3M coating .................................................................................................... 21
3.2.1.
Complex coacervate core micelle (C3M).............................................................................. 21
3.2.2.
Coating of RO membranes ................................................................................................... 22
3.2.3.
Evaluation of the physically attached coating....................................................................... 22
3.3. In-line coagulation ........................................................................................................................ 22
4.
Fouling control by mechanical means ............................................................................................. 24
4.1. Critical flux.................................................................................................................................... 24
4.2. Cross-flow .................................................................................................................................... 26
4.3. Rotation enhanced membrane separation................................................................................... 27
4.3.1.
Cross-rotation filtration.......................................................................................................... 27
4.3.2.
Rotating membranes ............................................................................................................ 27
4.4. Vibration enhanced membrane separation .................................................................................. 28
4.4.1.
Principle of vibration enhanced membrane filtration............................................................. 28
4.4.2.
V-SEP ................................................................................................................................... 29
4.5. Back-flush / backwash / (dynamic) back-pulsing ......................................................................... 31
4.6. Cleaning with air........................................................................................................................... 33
4.6.1.
Application of air in membrane filtration ............................................................................... 33
4.6.2.
AiRO - air/water for the control of particulate fouling............................................................ 33
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5.
Case study: Royal Cosun / Sensus ................................................................................................. 35
5.1. Background information ............................................................................................................... 35
5.1.1.
Royal Cosun ......................................................................................................................... 35
5.1.2.
Sensus .................................................................................................................................. 35
5.1.3.
Inulin ..................................................................................................................................... 35
5.2. Separation process at Sensus ..................................................................................................... 36
5.2.1.
Inulin extraction..................................................................................................................... 36
5.2.2.
Clarification process.............................................................................................................. 36
5.3. Problem definition / observations................................................................................................. 37
5.4. Recommendations ....................................................................................................................... 38
6.
Case study: Purac ........................................................................................................................... 39
6.1. Background information ............................................................................................................... 39
6.1.1.
Purac and lactic acid............................................................................................................. 39
6.1.2.
Lactic acid residual stream ................................................................................................... 39
6.1.3.
Problem (re)definition ........................................................................................................... 39
6.2. V-SEP pilot system ...................................................................................................................... 40
6.3. Results and discussion ................................................................................................................ 41
6.3.1.
Flux as function of time......................................................................................................... 41
6.3.2.
Flux as function of pressure.................................................................................................. 42
6.3.3.
Flux as function of concentration .......................................................................................... 42
6.3.4.
Flux as function of vibration .................................................................................................. 42
6.3.5.
Effect of irreversible fouling - cleaning.................................................................................. 43
6.4. Conclusions of V-SEP tests ......................................................................................................... 44
7.
Concluding remarks......................................................................................................................... 45
8.
Literature.......................................................................................................................................... 46
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1. Introduction
My first introduction into the difficulties of (defining) the topic “membrane fouling” occurred nearly twenty
years ago at the University of New South Wales in Sydney. At that time I was working as a post-doc
“down-under” and we were in a meeting discussing the topic of one of the starting Ph.D. students concerning biofouling in reverse osmosis. After some time, one of the elderly Ph.D. students said: “Bet you
for a pint, that in a year from now you still won’t have defined the topic of your Ph.D. study”. Needless to
say that the elderly student won the bet with ease.
This anecdote tells the difficulty of what is exactly understood by “membrane fouling”. To the (nonexperienced) user of membrane systems in fact any decrease in flux is a hindrance and is likely to be
termed fouling. To the biologist it is a process of bacterial growth, to the desalination specialist it is
scaling, to the chemical engineer it is a nasty control problem and to the company director it is a nuisance
that costs money.
On “Wikipedia” membrane fouling is defined as the process in which solute or particles deposit onto the
membrane surface or into membrane pores such that membrane performance is deteriorated. It presents
major obstacle for the wide spread use of this technology. Membrane fouling can cause severe flux
decline and affect the quality of the water produced. Severe membrane fouling may require intense
chemical cleaning or membrane replacement. As a result, operating costs of a treatment plant is therefore
increased. There are various types of foulants namely colloidal (clays, flocs), biological (bacteria, fungi),
organic (oils, polyelectrolytes, humics) and scaling (mineral precipitates) [1]. Using the definition “the
process in which solute or particles deposit onto the membrane surface or into membrane pores such that
membrane performance is deteriorated” is a broad and adequate definition.
This study is not a theoretical essay, but uses simplified expressions to illustrate the effect of a deterioration of the membrane performance. Furthermore, so hands-on experience will be presented, among
others case-study that were performed for Purac and Cosun.
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2. What is membrane fouling?
In this chapter an attempt will be made to define the concepts of “membrane fouling”. First the “basics”,
such as flux decline and resistances during a filtration process will be discussed. Than the terms “fouling”
and “scaling” will be discussed in more details and a special section will be dedicated to “biofouling”.
Based on these description methods of membrane cleaning and fouling control will be discussed.
2.1. Flux decline
In the most simple description “flux decline” is the decrease of the permeation through a membrane as a
function of time. This decline is caused by several phenomena, taking place during the filtration process.
This flux decline, related to the pure water flux, can be small for relatively clean feeds in ultrafiltration
(UF), nanofiltration (NF) or reverse osmosis (RO), but can also be more than 90% especially in more
open filtration processes like microfiltration (MF). In general, flux decline is caused by a decreasing
driving force and/or an increased resistance.
2.1.1. Resistances during filtration process
The volumetric membrane flux is described by the following formula:
driving force (e.g. ∆P, ∆C, ∆T)
Flux =
viscosity * total resistance
The resistances that can occur during a filtration process are schematically given in Figure 1. Resistance
Rm (the resistance of the membrane itself) is an intrinsic resistance that is always present during filtration.
This value can be calculated from the pure water flux.
Figure 1: Possible resistances against solvent transport [2]
Resistances that are a result of the filtration process are: Rp (blocking of pores by the solute), Ra (adsorption of the solute onto the walls of the pores of the membrane), Rg (formation of a gel layer on top of the
membrane) and Rcp (concentration polarisation).
The resistance Rp as a result of blocking of pores depends on particle size and membrane structure only.
This phenomenon often occurs when using symmetric membranes and is sometimes called “depth-filtration”. In processes were dense membrane (top)layers are used (such as RO) pore blocking does not
occur.
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Adsorption of the solute onto the walls of the pores or on top of the membrane (resulting in resistance Ra)
depends on the interaction between the membrane and the solute. Adsorption of proteins on the membrane is one of the most well-known types of fouling as a result of adsorption, but in fact also “biofouling”
can be placed in this category. The major difference between protein adsorption and biofouling is the
time-scale. Whereas the effect of protein adsorption is measured in hours, the effect of biofouling is
measured in days or weeks.
The effect of concentration polarisation (resulting in Rcp) and gel layer resistance (resulting in Rg) is
discussed in more detail in the next sections.
For pressure driven processes, the volumetric membrane flux Jv can thus be presented as:
∆P - ∆π
Jv =
η * (Rm + Rp + Rcp + Rg)
In the above equation ∆P is the applied pressure difference over the membrane, ∆π is the osmotic pressure difference over the membrane, η is the viscosity of the feed solution and Rx are the different
membrane resistances as described in this section.
The osmotic pressure effect differs with the type of membrane process. For instance, in RO all components in water are retained to a certain degree. Taking the example of seawater desalination using a
reverse osmosis membrane with retention for NaCl of more than 99%, it means that the osmotic pressure
difference across the membrane as a result of the salt in the feed is around 26 bars. This means that the
applied pressure has to exceed 26 bars in order to produce permeate. On the other hand, ultrafiltration of
seawater does not have the problem of osmotic pressure as these membranes do not have retention for
NaCl (thus there is no osmotic pressure difference over the membrane).
Although the above equation (in theory) offers the opportunity to calculate the exact amount of each of
these resistances, in practice this is nearly impossible. This report/study takes a practical approach and
will only use the equation to establish an understanding of the phenomena involved in a declining flux as
a result of increasing resistances.
2.1.2. Concentration polarisation
Concentration polarisation is a phenomenon causing a flux decline that is due to solute being retained by
the membrane and the solvent passing the membrane. As such every membrane process has to deal
with an increasing concentration of retained solutes at the feed/concentrate side.
It has to be realised that concentration polarisation and osmotic pressure are two different phenomena. In
general, osmotic pressure is calculated based on the properties of the feed solution (or better: the difference in properties of the (bulk) feed and (bulk) permeate solution), whereas concentration polarisation is
a phenomenon taking place near the membrane surface.
Due to the filtration process, the concentration of the retained solutes near the membrane surface rises.
This leads to the following equation:
Cm/Cb =
exp (Jv/km)
R + (1-R).exp (Jv/km)
In the above equation Cm is the concentration of the retained solute at the membrane surface and Cb is
the bulk concentration of the solute. Jv is the volumetric flux through the membrane, km is the mass
transfer coefficient for the solute and R is the retention (in fraction) of the solute [3].
As can be seen from this equation the extent of concentration polarisation depends on anumber of
factors, being:
a. Filtration flux Jv.
If the filtration flux is higher, then solutes are dragged more in the direction of the pores. This can
result in pore blocking and/or a cake layer on top of the membrane. The optimum way to proceed
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would be to have a flux that is “low” enough to make sure that no deposition takes place on top of the
membrane. This concept has been described in literature as “critical flux” [4]. The level of the critical
flux depends, among others, on cross-flow velocity, membrane type, type of solute and bulk concentration of the solute. In practice this, however, leads to very low fluxes and a huge membrane surface.
b. Mass transfer coefficient near the membrane surface km.
The mass transfer coefficient depends on the cross-flow velocity (the most common method to control
km) an on the type of solute. The cross-flow velocity is one of the most important parameters to
control concentration polarisation. By moving the fluid, the mass transfer coefficient near the membrane surface can be increased. As a result of this mixing action the concentration of solutes at the
membrane surface will be lowered, thus decreasing the effect of concentration polarisation. Methods
of increasing the mass transfer coefficient are axial cross-flow (most commonly used), transversal
cross-flow (e.g. TNO’s DAM module) or by moving the membrane itself (e.g. V-SEP).
The type of solute is also important. Proteins will act differently than e.g. salts and will therefore result
in different km’s.
c. Retention of the solute R.
If the retention of a solute is higher then the effect of concentration polarisation is more pronounced.
d. Concentration of solute Cb.
A higher bulk concentration of the solute leads to a higher concentration of solute at the membrane
surface.
2.1.3. Gel layer formation
As long as the solutes remain in solution, the phenomena occurring at the membrane surface can be
described as concentration polarisation. In membrane filtration, however, the solutions to be treated are
very often not ideal solutions and often problems occur with deposition on top of the membrane.
The "classic way" of the description of gel layer formation comes from filtration of macromolecules (in
specific proteins). At a certain concentration of solutes, the mixture of solutes and solvent does not
behave as a solution any more. The simplified mechanism of what happens at the membrane surface is:
Cm increases viscosity η increases km decreases extra increase in Cm etc. Once a gel layer is
formed, this layer cannot be removed simply by reducing the bulk concentration again. Often special
cleaning has to be used.
There is thin line between operating in the area of concentration polarisation and gel layer formation
especially in the field of micro- and ultrafiltration. Often a slight disturbance in the process might already
be the cause that an irreversible (gel) layer is formed on top of the membrane or, even worse, blocks
(part) of the feed channel completely.
Another characteristic of (gel) layer formation is the time-scale. In general these phenomena occur within
a short time-scale (usually within several hours).
In this simplified description of flux decline and membrane resistances, all phenomena in which a layer is
formed on top of the membrane are described as a gel layer (thus resulting in a resistance Rg). According
to the theory of membrane fouling one should give different names to the different phenomena, but for the
description of “flux decline as a result of a layer formed on top of the membrane” this does not differ
significantly. In fact there are many types of membrane fouling that can occur. The most important types
are colloidal fouling, organic fouling, scaling, biofouling and all type of combinations. These phenomena
are discussed in section 2.2.
2.1.4. Characterization methods: SDI and MFI
To conclude this section on flux decline, some methods for determining the “fouling potential” of a feed
solution are presented. The first method is the “SDI-method” [5] in which water is passed through a 0.45
µm membrane filter at a constant applied gauge pressure of 207 kPa (30 psig), and the rate of plugging of
the filter is measured. In this method the SDI is calculated from the filtration times. Although this method
is not the most accurate way for determining the fouling potential of the feed water, it is often used by
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membrane manufacturers for stating the warranty limits for their RO-membranes. In Figure 2 a typical SDI
curve and the calculation methodology is given in the left hand side.
Figure 2: Determination of the Silt Density Index (SDI) and the Modified Fouling Index (MFI)
from experimental data [6].
Already back in 1980, Schippers et al suggested a new method for determining the fouling potential of
feed waters for reverse osmosis, called the Modified Fouling Index (in Dutch: Membrane Fouling Index)
[7]. Although this method gives a better insight into the fouling potential of the feed water and can also be
used for water with a higher fouling potential, the MFI-method never replaced the SDI-method. For a
complete description of the methods, see the references.
2.2. Types of membrane fouling
The main difference between the types of fouling (colloidal fouling, organic fouling, scaling and biofouling)
is the nature of the particles that cause the fouling. The difference between types of fouling is made
because each type of foulant has an effect on membrane performance and also has its own type of
counter measures (feed pre-treatment (before) and cleaning (afterwards)).
In addition, fouling can be divided into reversible and irreversible fouling based on the attachment
strength of particles to the membrane surface. Reversible fouling can be removed by means of strong
shear force or backwashing. Formation of a strong matrix of fouling layer with the solute during continuous filtration process will result in reversible fouling being transformed into irreversible fouling layer.
Irreversible fouling is normally caused by strong attachment of particles, which is impossible to be
removed by physical cleaning method [8].
2.2.1. Colloidal fouling
Colloidal particles are ubiquitous in natural waters. Colloids cover a wide size range, from a few nanometers to a few micrometers. Examples of aquatic colloids are clay minerals, colloidal silica, aluminium,
iron and manganese oxides, organic colloids and suspended matter, and calcium carbonate precipitates.
In the pH range of natural waters, most colloids carry a negative surface charge. The surface charge of
aquatic colloids reflects their surface properties and the chemical composition of natural waters.
During membrane fouling, colloids accumulate on the membrane surface or within the membrane pores
and adversely affect both the quantity (permeate flux) and quality (solute concentration) of the product
water.
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When analysing colloidal fouling of pressure-driven membranes, it is important to distinguish between two
cases. For reverse osmosis, nanofiltration and perhaps “tight” ultrafiltration membranes, colloidal fouling
is caused by the accumulation of particles on the membrane surface in a so-called cake layer. This cake
layer provides an additional hydraulic resistance to water flow through the membrane and, thus, reduces
the product water flux. For microfiltration membranes, pore plugging by colloidal particles can be an
important fouling mechanism, in addition to particle accumulation on the membrane surface. The extent of
pore plugging and cake layer formation depends on the relative size of the particles compared to the
membrane pore size [9].
2.2.2. Organic fouling
The term organic fouling is applied for those substances that are dissolved in the feed solution and that
tend to stick to the surface of the membrane. Foulants like oil, macromolecules, proteins, anti-foaming
agents are all contributing to an organic gel layer on top of the membrane or in pores. The initial built-up
of a layer is caused by adsorption.
The main difference between colloidal fouling and organic fouling is that the aforementioned are particles
and the latter are dissolved. One has to bear in mind that both types of foulant can lead to the same type
of gel layer and often a “mixed” layer is formed.
2.2.3. Scaling
Scaling or precipitation fouling involves crystallization of solid salts, oxides and hydroxides from solutions.
Through changes in temperature, or water removal (as in reverse osmosis), the concentration of salts
may exceed the saturation, leading to a precipitation of salt crystals. Precipitation fouling is not only a
problem in reverse osmosis, but is also a very common problem in boilers and heat exchangers operating
with hard water and often results in limescale.
As an example, the equilibrium between the readily soluble calcium bicarbonate - always prevailing in
natural water - and the poorly soluble calcium carbonate, the following chemical equation may be written:
Ca(HCO3)2 (aq) CaCO3 + CO2 + H2O
The calcium carbonate that has formed through this reaction precipitates. Due to the temperature
dependence of the reaction, and increasing volatility of CO2 with increasing temperature, the scaling is
higher at increased temperatures. In general, the dependence of the salt solubility on temperature or
increased concentration will often be the driving force for precipitation fouling [10].
The major scaling ions include calcium, magnesium, bicarbonate, sulphate, silica, iron and barium. The
following inorganic scales may be present in fouled RO membranes: calciumcarbonate (CaCO3), calcium
sulphate (CaSO4) and barium sulphate (BaSO4). Calcium carbonate is the most likely inorganic scale to
be deposited. Use of an effective antiscalant will inhibit scale formation [11].
One of the methods to prevent scaling is to lower the pH of the solution. However, this is not always
possible. Therefore, during the past two decades, new generations of antiscalants have emerged commercially, in which the active ingredients are mostly proprietary mixtures of various molecular weight polycarboxylates and polyacrylates. Optimal molecular weights have been reported in the range of 1,000 3,500 Dalton. Other polyelectrolytes including polyphosphonates and polyphosphates have also been
applied successfully with certain types of feed waters.
2.2.4. Biofouling
Biofouling is a special class of organic fouling and is the result of complex interactions between the
membrane material, dissolved substances, fluid flow parameters and microorganisms.
A biofilm is defined as a structured community of microorganisms encapsulated within a self-developed
polymeric matrix and adherent to a living or inert surface. Biofilms are also often characterized by surface
attachment, structural heterogeneity, genetic diversity, complex community interactions, and an extracellular matrix of polymeric substances [12].
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Single-celled organisms generally exhibit two distinct modes of behavior. The first is the familiar free floating form in which single cells float or swim independently in a liquid medium. The second is an attached
state in which cells are closely packed and firmly attached to each other and usually form a solid surface
(biofilm).
Formation of a biofilm begins with the attachment of free-floating microorganisms to a surface. Figure 3
shows the different stage of development of a biofilm. The first colonists adhere to the surface initially
through weak, reversible Van der Waals forces (stage 1: initial attachment). If the colonists are not immediately separated from the surface, they can anchor themselves more permanently using cell adhesion
structures such as pili (stage 2, irreversible attachment) [12].
Figure 3: Five stages of biofilm development (see text for explanation).
The first colonists facilitate the arrival of other cells by providing more diverse adhesion sites and beginning to build the matrix that holds the biofilm together. Some species are not able to attach to a surface
on their own but are often able to anchor themselves to the matrix or directly to earlier colonists (stage 3,
maturation I). Once colonization has begun, the biofilm grows through a combination of cell division and
recruitment (stage 4, maturation II). The final stage of biofilm formation is known as development (stage
5), and is the stage in which the biofilm is established and may only change in shape and size. This
development of biofilm allows for the cells to become more antibiotic resistant [12].
The biofilm is held together and protected by a matrix of excreted polymeric compounds called EPS. EPS
is an abbreviation for either extracellular polymeric substance or exopolysaccharide. This matrix protects
the cells within it and facilitates communication among them through biochemical signals. Some biofilms
have been found to contain water channels that help distribute nutrients and signalling molecules. This
matrix is strong enough that under certain conditions, biofilms can become fossilized [12].
As it is practically impossible to keep an industrial system completely sterile, micro-organisms will always
be present on surfaces. Investigations into the beginning of the fouling process have revealed that the
micro-organisms settle on the membrane already in the first hours of operation. Biofilm growth turns into
biofouling only when a certain “threshold of interference” is exceeded [13]. With this definition, Flemming
(a world-renowned expert in this field) states that “biofouling” as the point where the presence of microorganisms becomes a nuisance. Furthermore, by stating that biofouling can be caused by a change in
nutrient concentration, shear forces, temperature or “other factors”, Flemming admits that there is no
direct answer to solving the problem of biofouling.
Controlling biofouling is a major challenge in membrane filtration installations. Curative or preventive
measures are not always effective. Flocculants provide a suitable habitat for microbial growth, whereas
conditioning agents are potential sources of microorganisms and of nutrients for the biofilm. Another
source of microbial contamination is the piping, storage tanks and treatment systems prior to RO, such as
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ion exchangers, active carbon filters and degasifiers. Biofilm are able to grow in very low nutrient habitats
with total organic carbon levels as low as 5 to 100 µg/L [3].
Biofouling is a comparatively slow process and its effect is seen in a gradual decline in the water flux rate,
a gradual increase in transmembrane and differential pressure and a gradual decrease in mineral rejection. The flux decline associated with biofilm formation oftern occurs in two phases: a rapid decline over a
period of two weeks and then an asymptotic approach to an equilibrium value usually around 60-80% of
the initial permeation rate.
The importance of biofouling can be illustrated by the amount of research that is performed on this topic.
All major universities and institutes in The Netherlands, involved in membrane technology research, are
conducting research into biofouling. The most encouraging aspect about these projects is the cooperation
between these institutes.
Figure 4: Integrated approach of research topics at Wetsus
Especially Wetsus should be mentioned as they have formulated a specific research theme in which the
following projects are being conducted [14]:
• Prevention of biofouling from biological perspective (fundamental study of the development of biofilms
by studying the microbial population as well as the spatial distribution of the various aerobes and
anaerobes using various molecular techniques).
• Influence of operational parameters on biofouling of membranes (when a “threshold of interference” is
exceeded, biofilms are developed into biofouling [13]; by adjusting membrane modules and the
operational parameters (e.g. adjusting configuration of spacers, developing an effective cleaning
procedure, offer a more favourable surface for biofilm development in front of the membrane
elements (“biologische hangplek“) biofouling could be prevented).
• Influence of environmental and process conditions on biofouling development (determine key parameters that influence membrane fouling using a biofilm monitor; the results will be used to develop
membrane filtration processes that are less susceptible to membrane fouling).
• Prevention of membrane biofouling by surface coating (develop surface coatings in order to prevent
biofilm formation; focus on antifouling properties of polymeric brush layers formed by adsorption of
complex coacervate core micelles).
In chapter 3 some “anti-fouling strategies” such as coatings and the use of flocculants are discussed. In
chapter 4 mechanical methods such as air-water cleaning and cross-flow will be discussed.
2.3. Membrane Cleaning
Whatever action is taken, membrane fouling cannot be completely excluded. At some point in time
membrane cleaning needs to take place. The type of cleaning depends on the type of fouling of the
membranes. In the next sections various aspect of membrane cleaning are discussed.
2.3.1. Four variables
Membrane cleaning is in fact comparable to doing laundry or dishes. The same principles apply when
cleaning a membrane (module) or when doing laundry. Comparable to laundry where different types of
clothing (cotton, wool, silk, etc.) need a different type of detergent, different types of membrane materials
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(cellulose acetate, polyamides, polypropylene, etc.) need different types of cleaning agents and cleaning
processes.
The four variables that are important in cleaning are:
a. Cleaning agent
b. Mechanical action
c. Temperature
d. Time
In the next sections these variables will be described shortly as to provide a small knowledge basis for the
reader. For detailed information please contact cleaning specialists.
2.3.2. Cleaning agent
The cleaning agent is the major component in the cleaning process. Some membrane manufacturers
supply their own cleaning agents to be used with their membranes, but very often membrane manufacturers refer to the cleaning process in terms of pH limitation and restrictions into the use of oxidative
agents.
Some major companies in the field of cleaning chemicals (such as washing powders and industrial (CIP)
cleaners) also supply chemicals for use in membrane systems. Some of these companies (including
specialized subsidiaries) are:
- GE Water: special cleaning products for membranes (GE acquired BetzDearborn).
- Henkel: supplies Ultrasil-products through subsidiary Ecolab.
- Johnson Diversey: specialized product line Divos.
- Nalco: specialized product line Permacare (formerly Houseman).
The product lines for membrane chemicals of the above companies comprise of products such as:
- Anti-scalants.
- Cleaning products.
- Biocides.
- Pre-treatment chemicals (such as flocculants and coagulants).
The cleaning agents are built-up of basic components. In general a first major division is made based on
the pH of the cleaning solution. The three main type of cleaners (acid, neutral, alkaline) consist of the
following components:
- Acid cleaners (consisting of acids, surfactants, defoamers and inhibitors).
- Neutral cleaners (consisting of surfactants, defoamers, builders and enzymes).
- Alkaline cleaners (consisting of alkalis, builders, sequestering agents, complexing agents, defoamers,
soil dispersing agents, surfactants, corrosion inhibitors and oxiders (boosters)).
Which type of cleaner has to be used depends on the type of fouling, the type of membrane (material)
and the type of process (e.g. food or non-food). In the Table 1 some general rules on the use of cleaning
chemicals in relation to the fouling type are presented. This table is adapted from the Handbook of
Industrial Membranes [3].
2.3.3. Mechanical action
Comparable to the way a washing machine works, mechanical action will help in the removal of foulants.
In general, the mechanical action in membrane systems is provided by using a high crossflow velocity in
combination with low transmembrane pressure.
Other methods of supplying mechanical action are:
- Back-flush and back-shock can also be used as supporting mechanical actions in the cleaning
process. A description of these methods is given in section 4.3.
- Vibration as supporting technology in the cleaning of membranes is very effective. This method can
only be applied when using a vibration enhanced membrane separation (see section 4.2).
- Sponge balls. This method can only be used in tubular membrane systems with internal diameter of
the membrane tube of more than 10 mm. In this method, sponge balls with diameters that are slightly
Prevention and control of membrane fouling
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-
larger than the membrane tube diameter are used. In this way the foulants are mechanically washed
from the surface of the membrane. This method of cleaning is commonly used for tubular condensors
and heat-exchangers, and adapted for use in tubular membranes [15].
Ultrasound is another form of mechanical action to aid the cleaning process. Ultrasound irradiation
increases the flux by breaking the concentration polarisation and cake layer at the membrane
surface. Damage due to ultrasound irradiation on the membrane surface has been discovered in
some research, whereas in other studies even a frequent use of ultrasound did not affect the
membranes. Ultrasonically enhanced membrane filtration has not yet been widely commercialised.
The main reasons that this method has not had its break-through yet are the development stagnation
of transducer technology for membrane filtration and the control of membrane erosion [16].
Table 1: General rules on the use of cleaning chemicals in relation to the type of fouling
Foulants
Description
Scale
Precipitate of sparingly soluble
salts (minerals) caused by the
concentration of salts in the
feed/brine solution during passage
across the membrane surface.
E.g.: CaCO3, CaSO4, BaSO4,
Sr.SO4, SiO2.
-
Agglomeration of suspended
matter on the membrane surface.
E.g.: SiO2, Fe(OH)3, Al(OH)3,
FeSiO4.
-
Colloidal
Clay / Silt
Effects on RO performance
-
-
Method of control / cleaning
Major loss of salt rejection
Moderate increase in differential
pressure
Slight loss of production
Effects generally occur in the final
stage of the membrane system
-
Rapid increase in differential
pressure.
Moderate loss of production.
Moderate loss of rejection.
Effects usually occur in the first
stage
-
-
-
Biological
Formation of bio-growth upon
membrane surface. E.g.: iron
reducing bacteria, sulphur
reducing bacteria, mycobacterium,
Pseudo-monas.
-
Major loss of production.
Moderate loss of salt rejection.
Possible moderate increase in
differential pressure.
Effects occur slowly, steadily.
-
Organic
Attachment of organic species to
the membrane surface. E.g.:humic
acid, oil, polyelectrolytes, grease.
-
Rapid and major loss of
production.
Stable or moderate increase in salt
rejection.
Stable or moderate increase in
differential pressure.
Lower recovery.
Adjust pH.
Use scale inhibitor.
Clean with citric acid or EDTA-based
solution.
Clean silicate-based foulants with
ammonium bifluoride-based solutions.
(Ultra)filtration as first step.
Charge stabilisation.
Higher Feed-Brine flows.
Clean with EDTA or sodiumtripoly
phospate at high pH.
Clean silicate-based foulants with
ammonium bifluoride-based solutions.
Lower recovery
Sodium bisulphite addition.
Chlorination with or without activated
carbon filtration.
Clean with EDTA-based solutions at
high pH.
Shock disinfection program with
formaldehyde, hydrogen peroxide,
peracetic acid.
Ultrafiltration before RO.
Filtration with active carbon.
Cleaning is rarely successful but
isopropanol or proprietary solutions
have been effective.
2.3.4. Mechanical action
Comparable to the way a washing machine works, mechanical action will help in the removal of foulants.
In general, the mechanical action in membrane systems is provided by using a high crossflow velocity in
combination with low transmembrane pressure.
Other methods of supplying mechanical action are:
- Back-flush and back-shock can also be used as supporting mechanical actions in the cleaning
process. A description of these methods is given in section 4.3.
- Vibration as supporting technology in the cleaning of membranes is very effective. This method can
only be applied when using a vibration enhanced membrane separation (see section 4.2).
- Sponge balls. This method can only be used in tubular membrane systems with internal diameter of
the membrane tube of more than 10 mm. In this method, sponge balls with diameters that are slightly
larger than the membrane tube diameter are used. In this way the foulants are mechanically washed
Prevention and control of membrane fouling
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-
from the surface of the membrane. This method of cleaning is commonly used for tubular condensors
and heat-exchangers, and adapted for use in tubular membranes [15].
Ultrasound is another form of mechanical action to aid the cleaning process. Ultrasound irradiation
increases the flux by breaking the concentration polarisation and cake layer at the membrane
surface. Damage due to ultrasound irradiation on the membrane surface has been discovered in
some research, whereas in other studies even a frequent use of ultrasound did not affect the
membranes. Ultrasonically enhanced membrane filtration has not yet been widely commercialised.
The main reasons that this method has not had its break-through yet are the development stagnation
of transducer technology for membrane filtration and the control of membrane erosion [16].
2.3.5. Temperature and time
Temperature and time are discussed in combination. An elevated temperature is an advantage for the
cleaning process as foulants are dissolved more readily when the temperature is raised (oil and grease
tend to dissolve in hot water and are removed more easily). Some type of cleaning agents (e.g. the
enzymatic cleaning agents) have an optimum temperature of 35-40ºC.
The use of temperature depends on the type of membrane (module). For instance, most RO modules
have a temperature restriction of around 45ºC (this is a module limitation as nearly all RO-membranes
can easily withstand temperatures in excess of 80ºC). For ceramic membranes, especially when used in
separation of oil-water emulsions, often elevated cleaning temperatures (around 80ºC) are recommended
and for most polymer membranes, the recommended cleaning temperature is chosen at 30 to 40ºC.
And as for time is concerned, the longer a cleaning procedure lasts, the more effective the cleaning will
be. But as "time is money" and cleaning time reduces the production time, the cleaning time should be
limited. Furthermore, one should realise that cleaning agents contribute to the wear and tear of the
membrane. Especially when specific agents are used in combination with certain membranes.
A specific example is the use of chlorine bleach for composite RO-membranes. The majority of composite
RO-membranes have a polyamide-toplayer, which are readily affected by chlorine. Chlorine tolerance is
generally specified by membrane manufacturers in terms of “ppm-hours”, which is the maximum number
of hours a membrane can be exposed to a certain concentration of chlorine before seriously affecting
membrane properties. For RO-membranes with a polyamide toplayer a “ppm-hour” number of 1000 is
given. In reference, ultrafiltration membranes made of Polyethersulfone (PES) Polyvinylpyrrolidone (PVP)
Blends are highly oxidant tolerant (>250000 ppm hours for chlorine, tolerant to permanganate and
ozone).
2.4. Cleaning strategies
The question always remains: how do I know which type of fouling I am dealing with and which type of
cleaning should be used? In this section the different steps in a cleaning strategy are explained.
2.4.1. How to identify the type of membrane fouling?
The first step in a cleaning strategy is to identify the type of membrane fouling. This is not always as easy
as it seems, but the following scheme can be useful for identifying the type of membrane fouling:
a. What is the type of process and which components are being retained?
Knowing the type of process and which components are being retained gives a first indication of the
type of fouling. For instance, the presence of high concentrations of Ca or Mg in RO or NF can point
towards scaling, whereas in MF or UF this type of fouling hardly occurs.
b. Analysis of the feed stream.
Based on the analysis of the feed stream (in combination with the type of process), a further
indication can be obtained on the type of fouling.
Prevention and control of membrane fouling
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c.
Which phenomena occur?
This question should be asked in terms of flux, retention, pressure drop between inlet and outlet, in
which part of the system do the effects occurs (first stage, last stage) and/or do the effects occur
rapidly or gradually. For instance, the column “effects on RO performance” in the table in section
2.3.2 (page 13) gives a description for phenomena that can occur during fouling of RO-membranes
(see also next section (2.4.2)).
d. Analysis of the deposit (if possible).
In some cases a clear deposit is formed in the membrane unit at one end of the module. Analysing
this deposit (TOC, element analysis) can give valuable information especially if the stream to be
treated is a complex stream and exists of more process streams. For instance, a deposit occurred in
a RO-installation for the recovery of several diluted acid streams. Based on the composition of all the
streams no deposit was expected. Upon analysing the deposit it turned out that large amounts of Sn
and P were found (a deposit of tin(IV)phosphate). In this case the problem could be solved by
decoupling the tin rinsing bath.
e. What is the history of the installation and the membranes?
Sometimes the key to the solution of a fouling problem lies in the history of the membrane installation,
e.g. membrane damage occurring due to failed cleaning attempts or filtration of - or cleaning with
chemicals that should not have been used on this specific membrane system. Furthermore, this
information also gives insight in what should not be done.
f.
Membrane autopsy.
If a membrane is irreversibly fouled and no it is not quite clear what has caused the problem an
autopsy of the membrane element might provide insight in the fouling process (see section 2.4.3).
2.4.2. Foulant type and phenomena
The relation between foulant type and membrane process parameters is clearer in reverse osmosis (and
also nanofiltration) than in other membrane processes. In reverse osmosis there is no built-up of cake
layers and other filtration phenomena. Therefore, any change in membrane process parameter is due to
fouling. The qualitative observations in the Table 1 have been quantified in Table 2 below [17].
Table 2: Foulant type in relation to filtration phenomena [17]
Foulant type
Salt passage
Pressure drop
Product flow
Other phenomena
Calcium and inorganic salts
10 - 25% increase.
10 - 40% increase.
< 10% decrease.
Effects occur mainly in
last stage
Metal oxides and hydroxides
> 2x rapid increase
> 2x rapid increase
20 - 40% decrease
Effects usually occur in
the first stage
Colloids
> 2x gradual increase
> 2x gradual increase > 50% gradual decrease
Organic matter
increase or decrease
small increase.
> 50% decrease
Biofouling
> 2x rapid decrease
> 2 x rapid increase
> 50% decrease
Effects usually occur in
the first stage
Identifying salt passage, pressure drop and product with the table above would result in the most likely
type of foulant. As such the cleaning strategy can be adjusted towards this type of foulant.
Although the above table and the identification steps from the previous section look straightforward, this
will not always give an answer to the question what type of fouling occurred. This can, among others, be
due to fact that the fouling layer is a combination of several foulant types (a combination of biofouling and
scale has been observed frequently).
In case of fouling of micro- and/or ultrafiltration the observations are not as clear as in reverse osmosis.
The reason is that only a small amount of particles are necessary to give a substantial flux reduction.
Prevention and control of membrane fouling
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Furthermore, this flux reduction can be attributed to several phenomena as was already shown in section
2.1.1, where the resistances in membrane filtration were described. More about this topic is discussed in
chapter 4 (fouling control by mechanical means).
2.4.3. Cleaning schedule
Based on the identification steps from section 2.4.1 and the observations as described in section 2.4.2 a
cleaning schedule can be formulated for cleaning fouled reverse osmosis (and nanofiltration) membranes.
If the major foulant is identified as inorganic salts or metal oxides or hydroxides, the first step should
always be an acid cleaner. If also organic fouling is present the second step should consist of an alkaline
cleaner. If necessary the steps can be repeated.
On the other hand, if organic fouling is the major type of fouling then an alkaline cleaner should be used.
An alkaline cleaner can be preceded by an enzymatic cleaner if persistent fouling is present.
Furthermore, in cases were the amount of fouling is substantial the cleaning solution should be changed
more frequently.
In biofouling cases, the best thing to do is to follow a three step protocol: (i) identification of the cause and
localisation of the problem, followed by (ii) sanitation (cleaning is more important than killing the microorganisms), and (iii) prevention [13]. It is needless to say that the first step is in fact the most important
step for solving the problem.
In general, cleaning of (heavy) biofouling should be focussed on “breaking” the biofilm. This can be done
using the following cleaning schedule:
1. alkaline cleaner combined with mechanical action preferably at elevated temperature. This step is
aimed at removing the biggest part of the fouling and also the loosely attached fouling. Mechanical
action is often applied using a higher cross-flow velocity, but also changing the velocity or the
direction of the velocity helps to remove part of the biofouling. The temperature can be a high as the
membranes or installation can handle (very often there is a limit of about 45ºC).
2. rinsing with water.
3. enzymatic cleaner at temperatures of 35-40ºC (soaking and/or gently pumping around). This step
aimed at breaking the film.
4. alkaline cleaner combined with mechanical action preferably at elevated temperature. This alkaline
cleaner can be directly added to the enzymatic cleaner of step 3.
5. rinsing with water.
6. optional: acidic cleaning to remove trace of inorganic scale and iron oxide.
7. repeating steps 3 to 6 if necessary.
In general, one has to realise that there is no “standard” cleaning procedure. Especially if dealing with
“biofouling”, the cleaning strategy sometimes has to be altered because of adaptation of the microorganisms to a cleaning procedure.
2.4.4. Membrane autopsy
As mentioned in all sections before, identification and localisation of the fouling problem is the key factor
to the solution of the problem. Sometimes a membrane element is fouled in such a way that the element
cannot be cleaned again. In that case the fouled can still be of use by performing a (destructive) membrane autopsy on the element.
Figure 5 shows a photograph of a spiral wound reverse osmosis (SWRO) element of which the outer shell
is removed. It shows a massive amount of biofouling on the membrane surface.
In general a membrane autopsy can either be destructive or non-destructive, although only minimal tests
can be carried out on a non-destructive autopsy. The physical dissection of an element and analysis of
foulants is one of the most definitive troubleshooting tools for membrane systems.
Prevention and control of membrane fouling
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Figure 5: Biofouling of membrane- and spacer of an SWRO (left) and an electron microscope photograph
of the membrane surface (right).
Although procedures for membrane autopsies can differ slightly, in general it exists of the following steps
as shown in the following procedure as used by Avista Technologies [18]:
- External visual examination. A thorough examination is made of the exterior of the membrane
element. The inspection looks for damage or defects in the o-rings and brine seal. An examination of
the feed and concentrate ends and the outer fiberglass wrapping is also completed.
- Wet test of the element. In this test the membrane is placed in a single element pressure vessel and
operated under laboratory conditions. Both feed and pressure drops as well as salt rejection are
measured. Fluxes are measured and normalized. The data from this test is compared to the cell test
data to differentiate between the condition of the full membrane element vs the cut membrane
sample.
- Internal visual examination. The outer fiberglass wrapping is removed and the membrane is dissected
to examine glue lines and to note any colors and odors. If foulants are present on the membrane
surface, a sample is removed for further evaluation.
- Chemical identification of surface foulants using FTIR, EDX, SEM.
- Loss on Ignition (LOI) test. LOI-tests are designed to measure the amount of moisture or impurities
lost when the sample is ignited under the conditions specified in the individual monograph. In this
case an LOI test is used to obtain a relationship between the amount of organic foulant vs. inorganic
foulant.
- Cell and dye test. In this test a cut sample of the membrane is tested for flow and rejection. This data
is compared with the test results obtained during the full element wet test. A dye test is also performed to determine if the membrane has been exposed to oxidation or is physically damaged.
- Fujiwara Test. This is currently the most valid indicator of halogenated organics such as chlorine.
Especially if the membrane is a polyamide, this test is conducted to identify a possible source of
membrane failure.
- Report with recommendations.
In The Netherlands, an independent institute that can perform a membrane autopsy is “Waterlaboratorium
Noord” (WLN) [19]. Furthermore, membrane autopsies can be performed by suppliers of cleaning
chemicals (see section 2.3.2).
Prevention and control of membrane fouling
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3. Reduced-fouling strategies
One of the initial objectives of this project was to determine whether “non-fouling” coatings, such as
developed by Wageningen University in cooperation with Wetsus can be used in the cases at Sensus
and/or Purac. Although it turned out that the coatings could not be used in these specific cases, a short
review of coatings and other (chemical) strategies to reduce fouling will be presented.
The first statement of this chapter is its title (“reduced-fouling strategies”). In literature surface coatings
are often referred to as “non-fouling” or “anti-fouling”. Specifically the term “reduced-fouling” is chosen,
because it has to be realised that every coating will be susceptible to fouling at some time.
Furthermore, the use of coagulant in the feed and the use of “low fluxes” (critical flux concept) is
discussed in this section.
3.1. Poly-An - modification of polymer surfaces
Surface coatings are specifically used to counter the effect of biofouling (also referred to as biocorrosion,
membrane fouling or protein fouling). This unwelcome adsorption and bond of biomolecules on the surface of implants, membranes and synthetic containers in a liquid or biological milieu has a negative effect
on the performance. Particularly in the field of life science, in medical technology and in the food processing industry biofouling causes negative impacts and high consequential damage or costs: filtration membranes accrue (pore-blocking), vascular implants clog (thromboses) or implants hold the risk to be limited
in their function by infections or inflammations and thus have to be removed [20].
In this section some of the products of Poly-An, a company specialised in chemical modification of polymer surfaces, are discussed.
3.1.1. Functionalized surfaces for anti-fouling applications
PolyAn offers a modification of polymer surfaces in such a way that they will be biocompatible. According
to the field of application biofouling can be slashed, in individual cases even be prevented. Test series
with strongly adherent osteoblast cells have shown that microtiter-plates with MSE (Molecular Surface
Engineering) modified surfaces can reduce cell adhesion to a minimum. Growth and vitality of cells have
not been affected. The modified surfaces repel the cells, but do not act cytotoxically.
Figure 6: Schematic drawing of modification of polymer surface
3.1.2. Affinity materials for life science
The highly selective, specific and reversible interaction of a biomolecule with its complementary bond
partner facilitates and requires the use of affinity materials. The bond partner is immobilized on a matrix.
These materials recognize the individual chemical structure or the biological function of the biomolecule.
Thus separation and evidence of the biomolecule from a complex alloy can be achieved. Polymer
materials such as membranes (filter) can in particular be used in bioseparation and chromatography or as
adsorber, biomaterial or bioreactor.
MSE-technology can tailor varied materials in respect of anti-fouling, hydrophilicity, charge, immobilization
and affinity. By applying a very thin functional surface layer the materials are able to covalently immobilize
Prevention and control of membrane fouling
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molecules (bond partners, receptors, enzymes, dyes, molecular cues). This method is commonly used in
the traditional affinity technology.
An advantage of PolyAn’s technology consists in the fact that the surface layer is covalently bound to the
material. This also means that the dimensional stability and the physical characteristics of the source
material are maintained. For membranes this means that only the surface and the pore structure are
significantly changed [21].
Figure 7: Schematic drawing of modification of membrane surface
3.1.3. Molecular Surface Engineering
MSE-technology opens a new dimension in the molecular design of surfaces. Specific structures and
characteristic features of surfaces for special use can be adjusted on a chemical-molecular basis.
Figure 8: Schematic drawing of introducing specific functionality to the surface.
A wide range of combinations from morphology, source material (substrate) and functional groups is
possible. No matter if inorganic polymers like glass, natural polymers like cellulose, or artificial polymers
like polypropylene, if planar or porous – via spacer nearly any functionality can be covalently bound to the
substrates, without influencing their physical characteristics.
The surface characteristics are solely specified by the functional matrix. Thus the mechanical characteristics of the source material are combined with the biological-chemical characteristics of the surface. The
surface can contain both simple functional groups such as –COOH or –NH2 and complex peptide or DNA
molecules. The morphology of the functional matrix is alterable, and that is a core competence of PolyAn:
not only “brushes”, “tentaculars” or dendritic structures are possible, but also diagonally integrated layers,
so that the result is a covering density that can be navigated specifically [22].
Prevention and control of membrane fouling
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Figure 9: MSE “tool box” introducing specific functionality.
3.2. LU Wageningen - C3M coating
In March 2006, ms. Agata Brzozowska started a Ph.D. project at Wetsus in Leeuwarden. The project is
carried out in cooperation with Wageningen University (promotors are professor Willem Norde and
professor Martien Cohen Stuart; Dr. Arie Keizer (also WU) is co-promotor) [23].
3.2.1. Complex coacervate core micelle (C3M)
Complex coacervate core micelles (see figure 10 for a schematic representation) can be used to apply
specific layers on a surface in order to change its surface characteristeristics.
Figure 10: Schematic drawing of a complex coacervate core micelle (C3M).
The term"complex coacervation" refers to the interaction of two macromolecules of opposite charge. In
the literature coacervation addresses the separation of colloidal systems in liquid phases, wherein the
Prevention and control of membrane fouling
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phase more concentrated in hydrophilic colloid component is called the coacervate. The term"complex"
indicates that the driving force for separation of colloids is of electrostatic origin. Micelles that are formed
by the principle of complex coacervation are called complex coacervate core micelles (C3M) [24].
By mixing solutions containing polyanions and a polycations, where at least one of these carries a neutral
hydrophilic block, micelles are formed with a complex coacervate core.
In the “Laboratorium voor Fysische chemie en kolloïdkunde” at Wageningen University these types of
micelles, consisting of enzymes and synthetic block copolymers, are studied. The structure and stability
of these micelles, the activity of these enzymes in the micelles, characterisation techniques and other
functionalities are topic of investigation.
3.2.2. Coating of RO membranes
One of the topics of investigation is the use of complex coacervate core micelles (C3M’s) to improve
fouling characteristics of membranes. The main focus of the PhD-project of ms. Agata Brzozowska is the
study of the antifouling properties of polymeric brush layers formed by adsorption of complex coacervate
core micelles (C3M’s) on reverse osmosis (RO) membranes. The aim of this project is to develop cheap,
sustainable and removable polymer surface coatings preventing biofilm formation on membrane surfaces.
The antifouling properties of the coating are attributed to the polymeric brush layers that are formed by
adsorption of complex coacervate core micelles (C3Ms) on reverse osmosis (RO) membranes.
For the application of the coating a solution of 10-20 ppm of C3M is prepared. The solution is applied
directly onto the membranes in the modules. The coating is physically attached and can also be removed
if desired. Desorption of the coating occurs at increased salt concentrations (> 0.5 m NaCl) and/or at a
low pH (< 2). Furthermore, it has to be realised that the effect of the coating wears of in about 5 days. As
the concentrations are very low and the used polymers (block co-polymers of PEO) are food grade (at
least in a number of countries), this does not have to be a barrier on using the coating.
3.2.3. Evaluation of the physically attached coating
At this stage it is not possible to make a thorough evaluation of the physically attached coating using
complex coacervate core micelles (C3M’s). The major reason is that the development stage is still far
from practical implementation. Whether the fact that the coating is physically attached is an advantage or
a disadvantage remains to be tested. The practical limitations of this physically attached coating, being
the salt concentration should be less than 0.1 m NaCl (about 6 g/L) and the pH should be higher than 4,
means that one is at least limited in applications. The low salt concentration means that any desalination
process is not possible.
There might be applications in which the coating has reduced fouling properties, and even when fouling
occurs on the coating, it might be possible to remove coating and fouling in one action. This might be
compared to the peeling-off of the skin of natural organisms. However, it needs to be investigated
whether the process of removing the coating can be applied in commercial membrane modules and,
more important, whether a new can be applied on the membrane again. As the removal of the old coating
(+ fouling) in commercial modules (especially spiral wound modules) is not that easy, any remainder of
the old coating might interfere with the newly applied coating.
If using this mode of operation, it resembles the process of “precoat filtration” [25], although the time scale
would be considerably different. Another aspect is of course the cost of the coating in relation to the
frequency of the change of the coating.
In short, the applications will be restricted and many practical obstacles need to be overcome.
3.3. In-line coagulation
Instead of changing the properties of the membrane another strategy for reduced fouling is to change the
properties of the feed solution. Coagulation is known pre-treatment method for removing natural organic
Prevention and control of membrane fouling
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matter (NOM) from surface water. This can be done either by pre-coagulation or in-line coagulation. Precoagulation is a separate process followed by flotation and sedimentation. The supernatant is then used
as feed for the filtration process. The disadvantage of pre-coagulation is that a separate unit has to be
installed and this contributes considerably to the costs of the separation process. With heavily fouled feed
streams a pre-coagulation is preferred.
In-line coagulation is the application of a coagulant before membrane filtration without the pre-coagulation
and/or pre-filtration step. In-line coagulation helps the performance of the filtration process as a reduction
of the hydraulic resistance is observed. Other advantages of in-line coagulation are the fact that hydraulic
cleaning is more effective (mainly because the internal membrane surface is better protected from
foulants) and that the permeate quality is better due to enhanced NOM and turbidity removal.
Until recently, the optimal dosing concentration of a coagulant is determined by jar tests. In general, the
optimal dosing concentration determined in this way is too high. Furthermore, the coagulant concentration
determined in this way is not optimised, as changing conditions, such as seasonal influences, are not
taken into account.
Blankert et al. have developed a control configuration using a feed-back controller for dosing a polyalumina coagulant in a membrane system (see Figure 11) [26].
Figure 11: Schematic drawing of basic feedback configuration [26].
Pilot plant tests using Norit Xiga ultrafiltration membranes were performed using surface water from the
Twentekanaal. The tests were performed with fixed setting and at constant flux. Based on an increase or
decrease of the total resistance of the membrane (see also section 2.1.1) the coagulant dosing was
adjusted. As the total resistance of the membrane at constant flux is proportional to the transmembrane
pressure this will be the steering parameter in practice. It was found that the control system performs well;
adaptation to changing conditions is achieved adequately and sufficiently fast. The initial resistance of the
last filtration before the chemical cleaning phase can be controlled within an accuracy of approximately
3% (of the total resistance) or 9% (of the fouling resistance). Compared to the current dosing strategy, a
significant reduction of coagulant consumption of around 30% can be achieved [26].
In a later study, a multi-objective control system was developed and experimentally tested. By using a
multi-objective control system the short-term fouling as well as long-term fouling can be effectively
controlled. To control long-term resistance increase during sequential chemical cleaning cycles, it is
necessary that the cleaning efficiency be used as a control variable. This means that cleaning time,
chemical composition of the cleaner will be used as steering parameters [27].
The study resulted in a model that can be used to predict the fouling status of a membrane during
multiple chemical cleaning cycles. The proposed model is used to minimize the overall operating costs based on chemicals consumption, energy consumption and investment costs - over a fixed time horizon,
guaranteeing production of a specified volume of permeate, where the number of cycles, the net
production flux, the duration of a production phase and the duration of a subsequent cleaning phase are
calculated. It was found that at the chemical cleaning cycle level, optimization of the cleaning variables
does not strongly influence operational costs, however, optimization of the chemical cleaning cycle may
be useful as a means of effective fouling control [27].
Prevention and control of membrane fouling
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4. Fouling control by mechanical means
The coatings as described in the previous chapter have limited use if frequent chemical cleaning is
required (most coatings cannot withstand these vigorous conditions) and/or if more concentrated
solutions have to be treated. Especially in the latter case membrane surface properties hardly have any
influence on the membrane performance. In these cases other mechanical means have to be used.
In this chapter cross-flow and vibration enhanced membrane separation are described as means of
process enhancement. Back-flush / back-shock and cleaning with air are described as methods of
cleaning.
4.1. Critical flux
One of the topics that does not belong to the previous chapter of “reduced fouling strategies” or this
chapter, but should be discussed in relation to both topics, is “critical flux”. Field et al. introduced the
concept of “critical flux” to membrane technology [28, 29]. It defines a permeate flux below a critical value
where no irreversible fouling occurs. The critical flux is determined by several factors including hydrodynamic forces (e.g. crossflow velocity), transmembrane pressure, electrostatic interaction between feed
components and the membrane surface.
In a paper discussing several constant-flux filtration experiments for yeast cell suspensions, yeast cell
debris, and dodecane-water emulsions at various operating conditions in both flat-sheet and tubularmembrane systems, Field introduced this concept of critical flux. Whilst it had long been recognised that
low-pressure microfiltration is much more effective than high-pressure microfiltration, the emphasis in the
research of Field was on the possible existence of a critical flux and the desirability of starting filtration
operations at a low flux. The critical-flux hypothesis is that on start-up there exists a flux below which a
decline of flux with time does not occur [28].
In a later paper, the concept was further refined. The focus then shifted more from flux towards membrane resistance. A window of operation was defined where “the membrane resistance remains constant
as the flux is increased and thus there is a linear relationship between flux and transmembrane pressure”.
This also resulted in the definition of a “strong form of critical flux” and a “weak form of critical flux”. In the
first definition a “strong form” of critical flux exists if the flux of a suspension is identical to the flux of clean
water at the same transmembrane pressure. In the second definition a “weak form” of the critical flux
exists if the relationship between transmembrane pressure and flux is linear, but the slope of the line
differs from that for clean water [29].
In the design and operation of present day micro- and ultrafiltration equipment, there is a strong emphasis
on avoiding an increase membrane resistance. Nearly all membrane installation in water treatment are
designed on the basis “constant flux” and using a transmembrane pressure that is as low as possible.
This method of operation has also become the “standard” for Membrane Bio Reactors (MBR). Whatever
way the phenomenon or the process is described (critical flux, limiting flux, low pressure), it all comes
down to one thing (as was already described in section 2.1.1): to keep the membrane resistance as low
as possible.
The most common method for measuring the critical flux at certain conditions (feed, concentration, crossflow) is the constant-flux mode filtration run. The run starts at a low specified flux at which the transmembrane pressure (TMP) is measured. A step-by-step increase in flux is carried out and the corresponding
TMP is recorded. The flux at which a disproportional rise in TMP is recorded is termed the “critical flux”.
At different conditions, different values of the critical flux can be observed. For instance, upon filtration of
a colloidal silica suspension (0.5% HT50) at Reynolds numbers of 373 and 580, Field et al. measured
2
values of the critical flux of 50 and 80 l/m .hr respectively [29].
An improved flux-step method to determine the critical flux and the critical flux for irreversibility was
developed by the University of Twente and Wetsus. The critical flux for irreversibility is defined as the flux
Prevention and control of membrane fouling
page 24 of 24
where fouling cannot be removed by intermediate physical cleaning techniques like relaxation (stopping
permeation) and backwashing (reversing the permeate flow). In this case irreversible fouling has taken
place (compression of cake layer, pore blocking and/or gel concentration) [30].
The improved flux-step method applies successive fluxes up to a maximum flux and back in the same
way as the common flux-step method. The big difference is that after a given time (arbitrarily chosen to be
15 minutes) at that the flux is maintained at the high level (JH) the flux is lowered to a “relaxation level”
(flux JL). After 15 minutes at the JL-level the flux is raised again to the next high level. The TMP is
measured during the whole of the measuring cycle (see Figure 12).
The test run starts at JL and after 15 minutes the flux is raised to the first JH. Directly Pinitial is measured
and after 15 minutes (just before lowering the flux to the relaxation level JL again) Pend is measured. At the
first measurements Pinitial is the same as Pend (please note that this situation is not shown in Figure 12).
From a certain level of JH, the TMP starts to rise during the constant-flux measurement (see Figure 12).
Pend is now higher than Pinitial. The critical flux is now the highest JH at which Pinitial is still the same as Pend.
The critical flux gives information that the membrane resistance is increased. However, it does not give
any information on the type of fouling that might have occurred. To determine what type of fouling
(reversible or irreversible) has occurred the difference between two subsequent TMPs at the relaxation
flux JL (P2 - P1 in Figure 12) is taken.
Figure 12: Transmembrane pressure as a function of time and set level of the flux using the improved
flux-step method [30].
In measurement with PVDF ultrafiltration membranes and activated sludge as a feed, both the critical flux
and the critical flux for irreversibility were determined. The measurements showed that the critical flux
2
2
was about 55 l/m .hr and the critical flux for irreversibility was higher than 100 l/m .hr [30].
Taking in mind the figure with the membrane resistances (Figure 1) and taking a close look at the test run
using the improved flux-step method, one can roughly identify the following situation:
1. JL is the flux level at which no concentration polarisation and gel layer formation occurs. Due to the
relaxation even blocked pores can get unblocked. Thus the resistance of the membrane exists of
membrane resistance Rm and adsorption resistance Ra. This is roughly the same as the so-called
“pure water flux” measurements after a filtration experiment.
2. Below the critical flux also no concentration polarisation and gel layer formation occurs. In fact the
membrane resistance is the same as with JL.
3. Above the critical flux concentration polarisation starts to play a role, thus the total resistance is
increased and now consists of membrane resistance Rm, adsorption resistance Ra and concentration
polarisation resistance Rcp.
Prevention and control of membrane fouling
page 25 of 25
4. Although Rcp has to potential to grow into a cake or a gel, this is usually not happening as long as the
condition for irreversibility is not exceeded.
So what is the importance of the critical flux and the critical flux for irreversibility in relation to membrane
fouling for practical applications?
First of all, it shows that micro- and ultrafiltration installations should be run in a constant-flux mode. This
avoids a built-up of foulants due to concentration polarisation and subsequent gel layer formation in the
first instances of the membrane process. Most of the commercial equipment is using the constant-flux
mode, especially in water application and membrane bioreactors.
Secondly, the critical flux shows that a built-up of membrane resistance due to gel layer formation and
concentration polarisation (and in some cases pore-blocking) can be avoided. However, one should
realise that fouling due to adsorption on top of the membrane or into the pores also occurs when the
process is run below the critical flux. This means that (chemical) cleaning is inevitable.
4.2. Cross-flow
Cross-flow is one of the oldest methods to avoid membrane fouling. For detailed description of the crossflow principle the reader is referred to the many textbooks on membrane technology [4, 31, 32]. This principle of operation is applied to the difficult filtration of solutions and suspensions, where high shear and
good mass transport are necessary to avoid the build up of particles or macromolecules at the membrane
surface. The most common method to realise the high shear is by pumping the feed solution at high
speed in relation to a stagnant membrane (module).
One of the biggest drawbacks of cross-flow membrane filtration, especially when the high shear is
realised by pumping the feed around, is the energy consumption. The relation between flow going into the
membrane module (thus feed flow + recirculation flow) and permeate flow can be as high as 50. This
means that a large proportion of the energy is not used for filtration, but for moving the feed along the
membrane. In contrast, (semi) dead-end operations or reverse osmosis are operated in single pass,
meaning that all the pumping energy is effectively used for filtration in this case.
Several investigations have been conducted to reduce the energy consumption in the filtration process
while maintaining a high mass transfer coefficient and to lower the membrane resistance. Some of these
methods are:
- Module design. One of the developments to be mentioned is the Transverse Flow Membrane module
[33] or the DAM-module (Dwars Aangestroomde Membraan module) [34]. Both developments use
hollow fibre or capillary membranes with the selective layer on the outside. Due to the rather difficult
module design and construction in relation to the limited improvement in flux and/or energy savings,
both types of module have not been used on a large scale, although the latter type is still used by
TNO in research on membrane contactors [35].
- Other improvements to module design to stimulate the mass transfer involve constructions like the
use of flow diverters and curved and corrugated-plate membrane modules [36]. Flow diverters can be
used in any type of membrane, whereas the use of corrugated-plate is restricted to flat sheet
membranes. With the exception of flow diverters (at least the systems that can be applied relatively
easily), none of these designs have been applied on a large scale.
- Cross-rotation filtration in which a rotating shaft between the membrane plates is used to create a
high cross-flow velocity (see section 4.2.1).
- The other method using rotation in membrane system is by using rotating discs (see section 4.2.2).
- Vibration enhanced membrane separation is the last method to be treated in this section (see 4.3).
Of course, one has to realise that critical flux and flow-controlled permeation (see section 4.1) are still the
preferred method to reduce energy consumption. However, this method cannot be supplied in any
situation, but one has to realise that this concept can also be used in combination with the abovementioned methods.
Prevention and control of membrane fouling
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4.3. Rotation enhanced membrane separation
The principle of rotation enhanced membrane separation can be applied in two different ways. The first
option is to have a membrane stack and use a rotating shaft in between the membrane plates to create a
high cross-flow velocity. The second concept is to have the membranes rotating.
4.3.1. Cross-rotation filtration
The principle of cross-rotation filtration is shown in Figure 13. In a cross-rotation filtration system plates,
support layers, membranes and rotors are assembled in a vertical sandwich form. An outer frame and two
massive plates at the bottom and the top, along with the plate and rotor stack comprise a compact unit. A
rotating shaft in the middle of the plate stack moves the rotors, creating a velocity > 10 m/s over the membrane surface [37].
The filtrate is routed to the filtrate channel formed by the plate stack. The medium is distributed inside the
plate stack to each of the plates. Higher concentration of the medium takes place within the different
sections of the plate stack. Due to the fact that feed (pump) and flow over the membrane surface (rotors)
are independent of one another, high specific filtrate flows can be achieved at a low system pressure.
rotating shaft
membrane
drainage support
filter plate
concentrate
filtrate
feed
Figure 13: The principle of cross-rotation filtration (left) and view onto an open plate stack (right) [37].
CR-Filters are developed for use in "open" ultrafiltration and microfiltration applications. Compared to conventional membrane systems, this design allows higher specific filtrate flow rates to be achieved. Pressure drop within the CR-Filter is minimised because feed and flow over the membranes are independent
of one another. Concentration polarisation and cake layer formation on the membrane surface is suppressed by high cross flow conditions. This system is specifically used when a high concentration of the
feed is present or required. Typical applications are fermentation broths, pulp bleaching solutions, sludge
and polymer solutions [37].
4.3.2. Rotating membranes
As mentioned earlier the improvement of the mass transfer using rotating membranes is by rotation of the
membranes. Membrane systems, that use a stack of rotating membranes in different configurations, have
Prevention and control of membrane fouling
page 27 of 27
been developed in various configurations. In contrast to the system of cross-rotation filtration as described in the previous section in these case the membrane stack is rotating. Very often membrane
stacks using ceramic disks are used as they provide the necessary stiffness for the membrane stack. In
these systems the following module types are used:
1. Single Shaft Disk Filter System (see figure 14 left). In this system one stack of rotating membranes is
used. In most cases a stagnant flow diverter is used in between the rotating stack for an improved
shear at the membrane surface. Also the method of supplying the feed to the system influences the
shear forces at the membrane surface.
2. Double Shaft Disk Filter System (see figure 14 right). In this system two stacks of rotating membranes
are used. Preferably the two shaft rotate counter-current to create the maximum shear forces at the
membrane surface. This membrane overlapping can increase the permeate flux considerably (the
magnitude depends very strongly on the type of feed and the cross-flow conditions [38].
Figure 14: Single shaft disk separator (left) double shaft disk separator (right).
For an overview of dynamic shear-enhanced membrane filtration (a review of rotating disks, rotating
membranes and vibrating systems) the reader is referred to the paper by Jaffrin [38]. This paper reviews
various systems of dynamic filtration, also called shear-enhanced filtration, which consists in creating the
membrane shear rate necessary to maintain the filtration by a rotating disk, or by rotating or vibrating the
5 −1
membranes. This mode of operation permits to reach very high shear rates, of the order of (1-3) × 10 s
and to increase both permeate flux and membrane selectivity. Several types of industrial dynamic
filtration systems are available, but their share of the market is still small. This paper reviews the
operating principles and fluid dynamics basics of various types, cylindrical rotating membranes, disks or
blades rotating near a fixed membrane, rotating flat circular membranes, multi-shaft systems with
overlapping rotating ceramic membranes and vibrating systems with toroidal membrane oscillations
around an axis, or vibrating hollow fibers cartridges. It also reviews their main applications published in
the literature in microfiltration, ultrafiltration, nanofiltration and reverse osmosis with a comparison of
permeate fluxes with cross-flow filtration data when available. A comparison of performances between the
vibrating VSEP system and a rotating disk module in MF of yeast suspensions and in UF of skim milk is
also presented. The discussion is focused on a comparison of merits of various designs in the light of fluid
mechanics and energetic considerations [38].
4.4. Vibration enhanced membrane separation
This section discusses vibration enhanced membrane separation. This topic receives extra attention as
tests with a VSEP system have been carried out as part of the DSTI research project.
4.4.1. Principle of vibration enhanced membrane filtration
The traditional method of reducing the effect of fouling in membrane systems is to operate with cross-flow
of the feed over the membrane. The economical limit to cross-flow velocity (mainly due to limits in module
Prevention and control of membrane fouling
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-1
design and energy costs) is given by a shear rate of typically 10,000 - 15,000 s . As such the membranes in cross-flow operation will still be subject to fouling, because the flow cannot remove solids and
particulate retained within the turbulent boundary layer.
An alternative method of creating increased shear rates at the membrane surface is to move the membrane itself. The principle of vibratory membrane filtration is already known for more than 20 years. Pall
introduced the Pallsep VMF Filter that uses an oscillating disc filter stack vibrating at approximately 50 Hz
-1
about a vertical axis. With such a system shear rates in the order of 100,000 to 150,000 s are generated
at the membrane surface (see figure 15 for a comparison of cross-flow and vibratory membrane filtration).
Figure 15: Boundary layer resistance in cross-flow (left) and V-SEP (right).
The shear developed at the membrane surface is independent of the feed flow rate. This allows
independent control of system pressure and shear rate. This operational feature makes a vibratory
system well adapted to handle high viscosity fluids. It also permits operation with high recoveries (high
permeate to feed ratio) such as 0.95 versus less than 0.1 for most cross-flow operations [39].
The main application of the Pallsep VMF system is in biotechnological applications (e.g. microfiltration of
fermentation broths).
4.4.2. V-SEP
An industrial version of the Pall system is developed by New Logic. They introduced the V-SEP (short for
“vibratory shear enhanced processing”) [39].
Figure 16: V-SEP resonating drive system [39].
Prevention and control of membrane fouling
page 29 of 29
Like the Pall system, V-SEP moves the membrane (leaf) elements in a vibratory motion tangential to the
face of the membrane. The feed slurry moves at a low velocity between the parallel membrane leaf
elements. The shear waves induced by vibration of the membranes repels solids and foulants from the
surface giving free access for liquid to the membrane pores.
A V-SEP system has only two moving parts: the torsion spring (on which the membrane module is
mounted) and the bearings. The vibration is induced using a motor with an eccentric weight that is
mounted on a metal plate (the seismic mass) supported by a rubber mount. The induced vibration
frequency (typically 50 to 60 Hz) is transferred to the membrane module using the torsion spring. The VSEP resonating drive system is given in figure 16.
The commercial module for the V-SEP consists of an array of parallel leaf membrane discs separated by
gaskets. Figure 17 gives a flow diagram of the process (left) and the cross section of a module showing
the individual membrane leafs.
Figure 17: V-SEP module with construction of filter pack [39].
The stack of discs is moved at high speed in a torsional oscillation with an amplitude of up to 1.5 inch at
-1
50 - 60 Hz thus creating a shear rate of around 150,000 s which is more than 10 times higher than the
maximum shear in cross-flow operation. Unlike crossflow filtration, nearly 99% of the total energy utilised
is converted to shear at the membrane surface.
As mentioned earlier, the typical energy input of a V-SEP system (industrial type module with around 130
2
m ) is around 15 kW for the vibration system. Depending on the type of process a feed pump is needed
with a capacity for single pass operation (this means the feed pump only has to supply the required
pressure and the total volume feed going into the module). As no cross-flow is needed the total energy
consumption for a V-SEP system is estimated at 10 to 20% of the energy costs for cross-flow operation.
In addition, due to the high shear in a V-SEP system, filtration rates of up to five times higher than in
cross-flow filtration can be obtained.
It has to be noted that the magnitude of the flux increase as compared to conventional crossflow strongly
depends on the type of membrane process and the type of application. For instance, in particle filtration
using micro- or ultrafiltration membranes, a flux of more than five times can be achieved. In this case the
increased shear not only lifts the particles but also allows the process to be carried out at increased
pressure without the adverse effect of cake-layer formation. The filtration efficiency of typical strong
fouling processes like broth filtration is increased by a factor of five to ten.
In addition to a better filtration efficiency another advantage of this process is that it has the ability to
concentrate to a much higher end solid content and that the slurry can become viscous without blocking
the modules.
As might be expected, the effect of relative flux increase is less pronounced with processes like nanofiltration or reverse osmosis. In these processes the stagnant boundary layer is less pronounced. However, the other advantages (high concentration and high viscosity possible) remain. If a V-SEP module is
Prevention and control of membrane fouling
page 30 of 30
compared to spiral wound elements, also there is also the advantage that feeds with high SDI (Silt
Density Index) can be used without the fear of plugging the feed spacer.
4.5. Back-flush / backwash / (dynamic) back-pulsing
Back-flushing, backwashing and back-pulsing are all methods of operation in which the transmembrane
pressure is periodically inverted by the use of a secondary pump, so that permeate flows back into the
feed, lifting the fouling layer from the surface of the membrane. The main difference in the methods is
mainly the time-frame in which the process operates.
Technically, the back-pulsing process is very similar to back-flushing or backwashing that is widely used
in micro- and ultrafiltration for commercial applications. However, the fundamental difference between a
back-pulse and a back-flush is the force and time used to lift accumulated deposits off the membrane.
Generally in back-flushing, flow reversal occurs for a few seconds once every several minutes, while
back-pulsing occurs at a higher frequency and the pulses applied for a very short time (<1 s) [40].
Back-pulsing has been demonstrated as a proven technique to reduce fouling and enhancing flux for
conventional microfiltration and ultrafiltration. Furthermore, it was observed that flux decline during the
filtration of BSA with microsieves was slowed down when the frequencies of permeate back-pulsing were
increased. The commonly used technique to generate the back-pulses is either based on a series solenoid valves that regulate the flow in the module and the permeate flow from a reservoir or a piston which
moves to and fro and causes flow reversal through the membrane.
Silicon nitride microsieves are microfiltration membranes characterised by uniform pore size and high
porosity. Furthermore, due to their extremely thin selective layer and the relative open support structure,
the clean water fluxes are usually very much higher (an order of magnitude of at least 100 times higher)
than the conventional microfiltration. This extremely high fluxes causes the microsieves to be extremely
susceptible to fouling and flux decline during filtration. As such, a new operating technique, i.e. high
frequency back-pulsing is developed by FluXXion B.V. (The Netherlands) to keep the microsieve surface
cleaned and to maintain the high fluxes [40].
Figure 18: Filtration set-up including Dynamic Cross Flow Pulse (DCP) unit [40].
In Figure 18 a filtration set-up is shown that uses a system for frequent back-pulsing called “Dynamic
Cross Flow Pulse (DCP) unit (see “red box” in figure 18). Backpulses are generated with the DCP unit
Prevention and control of membrane fouling
page 31 of 31
with frequencies of up to 50 Hz. It comprises a rotating shut-off valve driven by a stepper motor, which is
placed upstream of the microsieve. As it rotates, the flow in the feed channel is temporarily interrupted
and this causes the feed pressure to fluctuate. The permeate pressure is kept constant using a pressure
retainer. As illustrated in Figure 19, the shut-off valve is normally closed during filtration. During backpulsing, the shut-off valve is opened and this causes feed to flow out from the feed channel and the
pressure in the feed channel to decrease [40].
Figure 19: Schematic illustrations of the position of the shut-off valve during filtration (left) and
back-pulsing (right) [40].
The pressure changes in the feed and permeate channels are shown in Figure 20. The shut-off valve, i.e.
DCP unit, disrupts the feed flow intermittently. Filtration takes place when the feed pressure is higher than
the permeate flow. When the feed pressure drops below the permeate pressure, a reversal of the
permeate flow occurs and back-pulsing takes place. The back-pulse duration created by using the DCP
unit ranges typically between 5 to 200 ms and this amounts to 10% to 50% of the filtration time per
filtration cycle [40].
Figure 20: Pressure changes in the feed and permeate channels during the filtration cycle.
Back-pulse frequency 10 Hz. [40].
The simplicity of the DCP system is remarkable and high frequency backpulsing is to be considered a
promising technique to reduce fouling for both microsieves and conventional membranes. However, one
has to realize that the techniques can only be applied with rigid membranes and probably scaling-up of
the techniques will pose the necessary problems.
Prevention and control of membrane fouling
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4.6. Cleaning with air
The last topic in this section on fouling control by mechanical means is a development that was started
with the introduction of the Membrane Bio Reactors (MBR). As it turned out that the use of air for the
necessary aeration of the sludge had a positive effect on the reduction of membrane fouling,
investigations were started on the use of air specifically for cleaning purposes and reduction of membrane
fouling.
4.6.1. Application of air in membrane filtration
The application of air in membrane filtration was systematically investigated in the Ph.D. thesis of Jasper
Verberk [41]. The starting point of his Ph.D. study were the pilot experiments with injection of air in the
feed during a forward flush cleaning in a dead-end operated ultrafiltration installation. This combined
®
water and air flow cleaning is called AirFlush [42].
®
Verberk et al. studied the fundamentals of air-water flushing (AirFlush ) for membrane cleaning. The
research is focused on (i) the velocities of water and air for optimal cleaning and (ii) the distribution of
water and air over the cross sectional area of the module. Two different applications of water-air twophase flow in membranes were investigated, viz. (i) application of water-air two-phase flow in dead-end
operated ultrafiltration, (ii) application of water-air two-phase flow in capillary nanofiltration. From the
results of filtration experiments it was concluded that a water-air two-phase flow can be used
advantageously in cleaning and operation of membranes. In dead-end ultrafiltration the transport of
material from the membrane surface was improved by using a combined water and air flow. Also less
permeate is needed resulting in an improved overall recovery and a more concentrated retentate [41].
In cross-flow operated capillary nanofiltration the water-air two-phase flow was able to reduce the concentration polarization, resulting in a higher permeate flux and retention. The energy consumption of a twophase flow operated system was lower than a system with out air injection. In distribution experiments the
upscaling of the water-air two-phase flow from laboratory experiments to full-scale application was
investigated. It appeared that an equal distribution of water and air over tubular membranes is difficult,
while in capillary membranes the distribution of water and air is good. A practical solution to this
maldistribution in tubular membranes is to inject air intermittently [41].
4.6.2. AiRO - air/water for the control of particulate fouling
The experiments of Verberk already showed that a good distribution of air in capillary membranes was
achieved, whereas the distribution of air in tubular membranes was more difficult due to “channelling”.
The fact that a good distribution was obtained in “narrow channels” was a reason to start investigations
into air/water cleaning for control of (bio)fouling in spiral wound membrane elements [43].
In order to control biofouling and particulate fouling in spiral wound membrane elements, both daily
air/water cleaning (AWC) and daily copper sulphate dosing (CSD) were investigated and compared to a
reference without daily cleaning. A pilot study was carried out at KIWA for 110 days with three parallel
spiral wound membrane elements (AWC, CSD and the reference), which were fed by tap water enriched
with a biodegradable compound (100 µg acetate-C/L). The CSD element, which combined daily copper
sulphate dosing and sporadically air/water cleaning, performed best with an increase in pressure drop of
2
18% and a biomass concentration of 8000 pg ATP/cm within 110 days. This was followed by the AWC
2
element with a pressure increase of 37% and biomass concentration of 20,000 pg ATP/cm within 110
days. The reference element showed a pressure increase of 120% within 21 days. The presented
approach is considered very successful in controlling particulate fouling and biofouling, especially when
air/water cleaning is combined with copper sulphate dosing [43].
The advantage of air-water cleaning is that the dosage can be applied during the filtration process without
much trouble. The only thing that is needed is to lower the pressure, insert the air and once the air has
left the installation the pressure can be raised again. Compared to a cleaning chemical process this is
quite uncomplicated.
Prevention and control of membrane fouling
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The tests at KIWA have all been carried out using vertically mounted spiral wound elements. During these
tests no specific arrangements were necessary for introducing the air into the modules. A simple valve in
the feed line is enough to give a good distribution of the air in the spiral wound module.
Although it has to be confirmed by tests, it is expected that it is also be possible to get a good (or at least
reasonable) distribution of air in horizontally mounted spiral-wound modules. This would mean that the
system of air-water cleaning can also be applied on existing installations [44].
Prevention and control of membrane fouling
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5. Case study: Royal Cosun / Sensus
5.1. Background information
In this section background information is given on the company (Royal Cosun, Sensus) and the main
product (inulin) that will be subject of investigation.
5.1.1. Royal Cosun
Royal Cosun is an international developer, manufacturer and supplier of natural food ingredients. The
business comprises a number of activities:
• Single ingredients: sugar and fructose (Suiker Unie), inulin (Sensus), and alcohol (Nedalco).
• Customer-specific ingredients: Fine bakery (Unifine Döhler), Foodservice & Snacks (Unifine), and
Fruit & vegetable applications (SVZ).
• Aviko, producer of potato specialties.
As specialists in the taste, functionality, and healthiness of food ingredients every business group
develops concepts and applications. These are closely tailored to the individual wishes of customers in
Europe, North America, and Australasia. Via the Cosun Food Technology Centre Sensus has access to
areas of expertise such as dairy, beverages, bakery, fruit applications, snacks, and sauces.
Royal Cosun was established in 1899 when Dutch arable farmers set up their own co-operative. Their cooperative philosophy still applies today: co-operation throughout the food chain is Cosun's guiding
principle. The Cosun business groups increasingly work with their customers, suppliers, members and
each other to develop products that meet the needs of today's and tomorrow's world food markets [45].
5.1.2. Sensus
Netherlands-based Sensus is a well-established player in the inulin market, producing and marketing
Frutafit® and Frutalose® inulin/fructo- oligosaccharides (FOS) worldwide. Frutafit® and Frutalose® are
soluble, dietary fibres with added health benefits, making them ideal ingredients for functional foods.
Each year, Sensus contracts Dutch and Belgium arable farmers for the production and supply of cichory
roots. Cichory is a plant type that stores a substantial level of inulin in its roots which makes the
production of cichory very appealing.
After chicory has been sown in the spring (beginning of April) it quickly emerges and then grows relatively
slowly. First the root is shaped. During the first year of cultivation, the inulin is stored in the vacuoles of
the cells in the phloem and the xylum (woody tissue) in the root. The root is harvested from September
until December [45].
5.1.3. Inulin
Inulins are a group of naturally occurring polysaccharides (several simple sugars linked together) produced by many types of plants. They belong to a class of carbohydrates known as fructans. Inulin is used by
some plants as a means of storing energy and is typically found in roots or rhizomes. Most plants which
synthesize and store inulin do not store other materials such as starch [46].
Inulin is used increasingly in foods because it has unusual nutritional characteristics. It ranges from completely bland to subtly sweet and can be used to replace sugar, fat, and flour. This is particularly advantageous because inulin contains a third to a quarter of the food energy of sugar or other carbohydrates
and a sixth to a ninth of the food energy of fat. It also increases calcium absorption and possibly magnesium absorption, while promoting intestinal bacteria. Nutritionally, it is considered a form of soluble fiber,
and it is important to note that consuming large quantities (particularly for sensitive and/or unaccustomed
individuals) can lead to gas and bloating. Inulin has a minimal impact on blood sugar, making it generally
considered suitable for diabetics and potentially helpful in managing blood sugar-related illnesses [46].
Prevention and control of membrane fouling
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Figure 21: Structure formula of inulin.
Inulins are polymers mainly comprised of fructose units and typically have a terminal glucose; the molecular formula of inulin is: C6nH10n+2O5n+1. The fructose units in inulins are joined by a beta-(2-1) glycosidic
link. Plant inulins generally contain between 20 to several thousand fructose units. Smaller compounds
are called fructo-oligosaccharides, the simplest of these is 1-ketose, which has 2 fructose units and 1
glucose unit. Hydrolysis of inulins may yield fructo-oligosaccharides, which are oligomers with a degree of
polymerization (DP) of <= 10.
Plants that contain high concentrations of inulin include: elecampane (inula helenium), dandelion (taraxacum officinale), wild yam (dioscorea spp.), Jerusalem artichokes (helianthus tuberosus), chicory (cichorium intybus), jicama (pachyrhizus erosus), burdock (arctium lappa), onion (allium cepa), garlic (allium
sativum) and agave (agave spp.).
5.2. Separation process at Sensus
5.2.1. Inulin extraction
Inulin is extracted from the chicory by means of hot water. As a result of the extraction salts, proteins,
colouring agents and inulin are present in the solution. The proteins are removed by means of coagulation. The resulting clear juice is subsequently treated by ion exchange resulting in the removal of salt and
the colouring agents. Subsequent treatment with active carbon removes the components that give the
juice a bitter taste. As the previous processes do not remove all components for the full 100% an ultrafiltration process is used for clarification of the juice.
5.2.2. Clarification process
For the clarification process a membrane installation is used. This process removes all particles (including
micro-organisms) from the juice. The direct effect of the clarification process is checked by means of
visual inspection of turbidity. The installation is built-up of 6 units of which 4 are being used, 1 is being
cleaned and 1 is on standby. In this way a continuous process can be realised. The installation is
equipped with spiral wound membrane elements from different suppliers. The clarification process takes
place at a temperature of around 70ºC.
The membrane cleaning process takes place every 8 to 12 hours. The cleaning process involves a
chlorine wash cycle. The cleaning process takes place at a temperature of around 70ºC and pH of 10.
Prevention and control of membrane fouling
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The membranes are used during a campaign from September to midway December. In general the
membranes last 1,5 campaign. If problems occur with membranes often these problems occur during the
start-up of the installation at the beginning of the campaign.
5.3. Problem definition / observations
In the 2007 campaign problems occurred in several of the installations during the start-up of the installation. In this case, “problems” have to be defined as “there is turbidity in the permeate”. The turbidity is
checked for each membrane vessel individually based on visual inspection. Thus, if the permeate is not
clear, the membrane in the said vessel are changed. In a campaign company like Sensus, there is no
time to find out the causes of the problem, but action has to be taken straight away.
Based on the observation that the most likely time for problems is the start-up, the following analysis was
made of the situation, together with questions on how to solve the problem:
1. Turbidity of the permeate means that the system contains a “leak”. This can be anything from a membrane defect to a faulty seal or a badly positioned valve. The latter can be excluded, because after
the membrane change (which automatically means that the membrane seals (mostly O-rings) are
being replaced) the problem was solved.
2. The cleaning takes place every 8 to 12 hours during the campaign seasons using among others a
cleaning with sodium hypochlorite. It is known that the oxidative effect of this bleach has an adverse
effect on the membranes. Very often a maximum in “ppm-hours” is given for a certain type of
membranes. The polysulfone membranes used in this application in general have a “ppm-hour” of
500,000.
3. Given the cleaning time of 15 minutes and 2-3 times a day for about 100 days (the campaign time)
would result in 70 hours of hypochlorite cleaning. Given a concentration 1000 ppm, this would mean
that the ppm-hour number for 1,5 campaign would be about 100,000, which is below the 500,000
barrier. However, the elevated temperature of the cleaning (70ºC) might have an extra detrimental
effect.
4. The observation that very often membrane defects occur during the start-up of the installation led to
questions on the method of membrane conservation and maintenance. The procedure at Sensus was
as follows: after the last day of the campaign, the installation is cleaned several times following the
same cleaning procedure as during the process. As a last procedure the installation is rinsed and the
membranes are stored in a glycerine solution in the membrane installation from mid December till
September.
5. As most of the malfunctioning of the installation occurred during the start-up, it should not be excluded that part of the malfunctioning is due to O-ring failure. In fact, O-ring failure is one of the most
underestimated causes of problems with membrane installations. Although the O-rings used in the
installation can withstand the hypochlorite cleaning without any problem, it should be realised that the
oxidative nature of hypochlorite might result in accelerated aging. Combined with the standstill of the
installation during 8 months, a fatigue problem with the O-rings might occur during renewed start-up.
6. Based on information supplied by Eriks (a manufacturer of O-rings), it was found out that O-rings
made of EPDM have a good resistance against hypochlorite, but in combination with elevated temperature do tend to “age” quickly.
7. Not so much an observation, but the use of a coating, as proposed in the project, is excluded beforehand. The oxidative environment of hypochlorite cleaning (which is not only necessary but also
required for this food application) would remove the physical attached coating immediately.
Prevention and control of membrane fouling
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5.4. Recommendations
As can be seen from the observation in the previous section, much of the malfunctioning of the
installation does not occur from fouling of the installation, but are more the results of the method of
cleaning and the method of conservation and maintenance. In close cooperation with Ecolab, a supplier
of membrane cleaning agents, it is discussed how to improve the procedures in such a way that malfunctioning of the installation is reduced to a minimum, or that it does not occur during production.
The following procedure is suggested:
1. At the end of the campaign, the membranes are cleaned in the same way as before. Thus, a double
cleaning with hypochlorite.
2. In addition a cleaning using P3 Ultrasil 73 is carried out, which is an acid surfactant based cleaning
agent specifically to remove fat residues. Although this type of membrane cleaner would normally not
be used for this application, it can be used for preservation of membrane elements as well.
3. After one week the membranes are cleaned again using a fresh solution of P3 Ultrasil 73 and the
membranes are conserved again. This procedure is then repeated monthly while the membranes are
stored in the installation.
4. Before the start-up of the installation for the campaign season a check-up of the installation including
membranes takes place and the O-rings are replaced. Furthermore, the membranes are registered
on date of purchase and date they are placed in the installation. In this way, more information can be
obtained on the lifetime of the membrane modules.
Prevention and control of membrane fouling
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6. Case study: Purac
6.1. Background information
In this section background information is given on the company (Purac) and its main product (lactic acid).
Furthermore more information is given on the lactic acid residual stream, that will be subject of
investigation.
6.1.1. Purac and lactic acid
Purac is the world's largest and most experienced producer of natural lactic acid, derivatives, gluconates,
lactides and polylactides. PURAC is a subsidiary of CSM, a global player in Bakery Supplies & Food
Ingredients.
Lactic acid is a natural organic acid with a long history in the food, leather, wool-dyeing and cosmetics
industries. Long before it became commercially available, lactic acid was formed by natural fermentation
in products such as cheese, yoghurt, soy sauce, sourdough, meat products, pickled vegetables, beer and
wine. Today, lactic acid, its salts and esters are extensively used in food, industrial, cosmetic and
pharmaceutical industries [47].
Lactic acid was discovered in 1780 by Swedish chemist, Carl Wilhelm Scheele, who isolated the lactic
acid from sour milk as an impure brown syrup and gave it a name based on its origins: 'Mjölksyra'. The
French scientist Frémy produced lactic acid by fermentation and this gave rise to industrial production in
1881. Lactic acid is produced by the fermentation of sugar and water or by chemical process and is
commercially usually sold as a liquid.
PURAC's aim is to improve its customers' products using advanced biotechnology. Production is based
on renewable resources - an example of PURAC's concern for the environment. This applies to both
lactic acid and lactic-acid-derived products [47].
6.1.2. Lactic acid residual stream
In one of the production processes of PURAC a process stream containing lactic acid and some contaminants (typically polysaccharides, proteins and sugars) is obtained. This stream needs to be treated in
such a way that the polysaccharides and proteins are removed. A suitable process to be used is therefore
nanofiltration. Due to the nature of the contaminants crossflow conditions have to be used to avoid fouling
and to maintain an acceptable flux.
PURAC has some experience with this kind of equipment using a system with tubular nanofiltration
membranes operated in crossflow.
6.1.3. Problem (re)definition
In a first instance the use of non-fouling coatings, as developed by Wageningen University, was discussed. However, it appeared that the development of coatings was still in the experimental phase. Furthermore, the coatings are attached physically to the membrane surface. One of the limitations of the
coatings is the pH of the feed solution; once the pH gets below 4 the coating is detached from the membrane surface. As the pH of the lactic acid solution is around 2, the use of these non-fouling coatings is
therefore excluded beforehand.
One of the characteristics of the lactic acid solution is that it has a high concentration of several components and its viscosity seems rather high (based on visual observation; no analysis performed by
MACT). As such it is proposed to perform tests with a system that uses an increased mechanical support.
The system being used in the test is a V-SEP, in which the mechanical support is delivered by vibration.
The principles of vibration enhanced membrane separation are described in section 4.2. The experimental system is explained in the section 6.2.
Prevention and control of membrane fouling
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6.2. V-SEP pilot system
The tests with lactic acid residual stream are carried out with a vibrating membrane system obtained from
New Logic. The system used is a so-called “LP machine”. A schematic drawing of this system is given in
figure 22. This system is equipped with a membrane stack with nanofiltration membranes having a
MWCO (“molecular weight cut-off”) of around 250 Dalton (retention for NaCl is 30-40%). The total
2
membrane area of the module is 1.55 m [48].
Figure 22: Schematic drawing of V-SEP LP machine [48].
Prevention and control of membrane fouling
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The LP-system consists of a vibration drive system with a Baldor motor (1.5 kW, 1725 RPM). The
oscillation amplitude can be adjusted through frequency control and can be adjusted up to 1.25”; in the
2
tests amplitudes of ¾” and 1” were used. The filter pack (P mode) consists of 1.55 m and can be
operated with a maximum pressure of 600 psi (41 bar); the hold-up volume of the filter pack is 3 litres.
The feed pump is a Hydra-Cell (1.5 kW). The system is equipped with temperature probes, pressure
gauges and conductivity and pH sensors [48].
The method of operation is as follows: the feed is pumped from a feed container by the feed pump. By
means of a valve a pressure is set; the vibratory system can be started once the pressure exceeds 40 psi
(3 bar). After the vibratory system is started the desired pressure is set. During the tests the feed solution
is pumped around. This means that the feed solution is concentrated in the feed vessel. This mode is
used for two reasons: first the feed pump is over-dimensioned and therefore some circulation is required
and, secondly, this makes it easy to compare the vibratory measurement with the tests in which the
vibration is stopped (in the latter case only cross-flow is used).
6.3. Results and discussion
The results of the test are discussed in this section. During the tests pressure, temperature, flux,
conductivity, pH and concentration factor are measured as a function of time.
In the following sections the influence of different parameters is discussed.
6.3.1. Flux as function of time
The flux as a function of time is presented in figure 23. In this figure all the results are plotted as a
function of the time elapsed. In this figure the effect of pressure, concentration factor and vibration are all
presented. Please note that the temperature changes during the test. The flux values have not been
corrected for temperature.
17,00
500 psi
Flux [l/m2.hr]
15,00
13,00
400 psi
500 psi
11,00
9,00
400 psi
300 psi
7,00
vibration off
5,00
10:04
11:16
12:28
13:40
14:52
Time [hr]
Figure 23: Flux as a function of time.
If one looks at the first hour of operation, it can be observed that the flux at a given pressure (20 bar)
remains fairly constant. If a temperature correction is applied, actual a small decrease is observed. A
2
stable flux of around 10 l/m .hr is obtained at this pressure.
Prevention and control of membrane fouling
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6.3.2. Flux as function of pressure
2
From the stable flux obtained at 20 bar (10 l/m .hr), the pressure is increased to 27.5 bar. An immediate
2
increase to around 13 l/m .hr is observed. Raising the pressure further to a level of 34 bars, results in a
2
flux that is slightly higher than 15 l/m .hr. If these flux values would be expressed as permeability (in fact
flux divided by pressure), only a small decrease in permeability would occur.
From these measurements it is appears that a higher pressure is advantageous for a higher flux. This is
due to the high shear stress at the membrane surface, which means that a higher pressure does not
result in a higher concentration of solitudes at the surface. Practically spoken a pressure of 35 bar should
be considered as a maximum pressure in full-scale operation.
6.3.3. Flux as function of concentration
After about 1½ hour operation, a concentration run is started. The results of these tests are shown in
figure 23 as a function of time and in figure 24 where the flux as a function of the concentration factor is
given.
17,00
Flux [l/m2.hr]
15,00
13,00
500 psi
11,00
400
psi
9,00
7,00
5,00
1,00
1,50
2,00
2,50
3,00
CF [-]
Figure 24: Flux as a function of the concentration factor.
During these tests the pressure was kept at around 34 bars for most of the time; only in the last part of
the test the pressure was decreased to 27.5 bar. The temperature was not controlled and increased from
42ºC to 65ºC during the test run. The vibration amplitude was kept at 1”.
From figure 24 it can be seen that the flux decreased by about 25% when the concentration was
increased with a factor of 2.4. At this point the pressure was decreased from 34 bars to 27.5, which
resulted in an extra flux decrease of 15%. After this direct decrease the flux decrease is “stabilised” again.
In other words the steady decrease as a function of concentration seems to be stable.
6.3.4. Flux as function of vibration
The flux as a function of vibration is given in figure 25. The measurement is performed in the following
way: at the end of the concentration test (at 14:04 and after nearly 4 hours of operation), the collected
permeate is again added to the feed solution. In this way the concentration of the feed solution is about
the same as in the start of the measurement. After a short time the first flux measurement is performed.
2
As can be seen in figure 25 (but also in figure 23), the flux is around 11.6 l/m .hr. This value is lower than
the value measured at 11:30 (same pressure, but before the concentration run was started) that was 15.3
2
l/m .hr. Even though the 11:30 measurement was taken at a lower temperature, this value is about 25%
higher than the 14:11 measurement. This indicates that during the concentration run some material is
deposited on top of the membrane despite the vigorous vibration that was applied.
Prevention and control of membrane fouling
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From figure 25 it can be seen that once the vibration is stopped, the flux is decreased dramatically. From
2
2
a level of around 11 l/m .hr, the flux drops below 7 l/m .hr in about 5 minutes time. Once the vibration is
started again the flux immediately rises with 25%. This means that vibration is effective in reducing the
concentration polarization effect.
12,00
11,00
Flux [l/m2.hr]
10,00
9,00
8,00
7,00
6,00
vibration
off
5,00
4,00
14:02
14:16
14:31
14:45
15:00
15:14
15:28
Time [hr]
Figure 25: Flux as a function of time; influence of vibration.
However, if one takes a closer look at the results and extrapolates the flux tendencies, it can be seen that
the flux level, after the vibration is restarted, is not restored to the level as expected. Although this
conclusion has to be made with some reservation, it indicates that once material is deposited on the
membrane it cannot be removed easily using vibration.
2
From the extrapolations it would be reasonable to assume that a flux level of 9.5 - 10 l/m .hr would be a
stable level for vibratory processing and around 5.5 for the crossflow option. This means that the flux level
in vibratory operation is 70 - 80% higher than in comparable crossflow operation. Furthermore, it is
expected that vibratory operation is able to maintain a higher flux rate for a longer period (please note that
this is expected; there is no direct evidence from these experiments as this parameter is not extensively
investigated).
6.3.5. Effect of irreversible fouling - cleaning
From a number of observations it can be seen that an irreversible deposition of feed material on the
membrane occurs during the test. Despite vigorous vibration a certain flux decline as function of time
occurs. The observations are:
- When the first four flux measurements (taken at the start with a pressure of 20 bars) are corrected for
temperature increase a steady decrease of the corrected flux occurs. The same decrease is also
found for the permeability.
- The increase in pressure gives an increase in flux (also in temperature corrected flux), but a steady
decrease in permeability is observed.
- When the collected permeate is returned to the feed after the concentration run, the measured flux is
lower than expected; the temperature corrected flux or even the permeability is at a level which is
more 50% lower than their corresponding starting values.
- After the interruption of the vibration, the flux (c.q. permeability) does not return to the expected
values. At the end of the measurements the permeability is about 1/3 of the starting value.
Another important observation is made after the feed solution is pumped out of the membrane module
2
and is replaced by water. The pure water flux of the membrane module is as low as 11 l/m .hr (26ºC; 20
Prevention and control of membrane fouling
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2
bar) whereas the values before the test were more than 60 l/m .hr under comparable conditions.
Furthermore, the flux did not change during rinsing. This means that the fouling is attached firmly to the
membrane and cannot be removed easily despite vibration.
Once a cleaning agent was introduced to the feed vessel a strong increase in flux was observed almost
instantaneously. After replacing the cleaning solution with water, the original pure water flux was restored
almost totally. An extra cleaning cycle only resulted in a small extra increase in flux. This increase was
less than 10% and could be within the measurement uncertainties.
6.4. Conclusions of V-SEP tests
The observations lead to the following conclusions:
1. Vibratory enhanced filtration is a good method in avoiding flux decline in a nanofiltration process.
2. Pressure increase leads to a near proportional flux increase if vibration is used to minimize fouling.
3. During a concentration run, V-SEP can avoid a strong flux decrease despite a concentration factor of
nearly 3.
4. During filtration an irreversible deposition of feed components occurs on the membrane.
a. The deposition cannot be removed by shear alone.
b. The deposition can be easily removed using a cleaning solution.
c. The vibration enhances the cleaning process minimising the cleaning time.
5. No conclusion can be drawn with respect to the permeate quality as no analysis was performed on
feed and permeate samples by MACT.
Prevention and control of membrane fouling
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7. Concluding remarks
In this report recent innovations (with an emphasis on the practical short to medium term application)
have been discussed and two practical cases have been studied.
As far as the problem of fouling is concerned, the best thing is not to talk about fouling anymore, but to
tackle the problem as a case of “reduced flux under given process conditions”. This statement is not
“struisvogelpolitiek”, but a way of tackling a filtration problem taking all conditions into account. Comparable to, for instance, the fire triangle, a filtration problem has three variables as is indicated in Figure 26
below. It is worthwhile to take any of the three variables into consideration for modification.
mechanical support
membrane
feed solution
First there is the membrane. Next to the optimisation of pore size and pore size distribution, the hydrophilicity/hydrophobicity of the membrane surface can be changed. This can be done using chemical modifications (including grafting), but also by physical attachment of functional groups as demonstrated by the
University of Wageningen (see section 3.2). The latter development might turn out to be promising, but for
there are many problems to be solved before any commercial application can take place.
The second variable is modification of the feed solution. The system as developed by Blankert et al. is a
nice demonstration of how the use of coagulants can be optimised in ultrafiltration of surface water (see
section 3.3). Other applications include: (i) the use of anti-scalants in reverse osmosis; (ii) the use of
flocculants in manure and/or digestate treatment, and (iii) thermally induced coagulation of proteins in
broths.
Probably the most versatile method for improvement of the membrane productivity is to use mechanical
support. On purpose the term membrane productivity is used, as the productivity of the process is more
important than flux alone. In this paper the following methods have been discussed in chapter 4, all of
them helping to improve the goal of membrane productivity; some general, some in specific membrane
configurations.
But, however good the membrane process is designed and executed, one always has to take a certain
amount of fouling (among others by adsorption) into account. Cleaning is therefore inevitable at certain
intervals and should be an integral part of the process. As a last note on the “membrane process
triangle”, the dot in the middle of the triangle stands for knowledge and co-operation. Or as it has been
put by DSTI: “Together we can take bigger steps, have more impact, and share the risks”.
Prevention and control of membrane fouling
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8. Literature
1. http://en.wikipedia.org/wiki/Membrane_fouling (R.W. Baker; Membrane Technology and Applications;
John Wiley & Sons Ltd. (England), 2004).
2. G.B. van den Berg; Concentration Polarization in Ultrafiltration; Ph.D. thesis, University of Twente,
September 1988.
3. K. Scott; Handbook of Industrial Membranes; Elsevier Advanced Technology, Oxford (UK).
4. R.W. Field, D. Wu, J.A. Howell and B.B. Gupta; Critical flux concept for microfiltration fouling; Journal
of Membrane Science 100 (1995), pp. 259–272.
5. Standard Test Method for Silt Density Index (SDI) of Water; ASTM D 4189-07.
6. W.G.J. van der Meer; Membraanfiltratie; presteren onder druk; inaugurele rede Universiteit Twente,
27 maart 2008.
7. J.C. Schippers & J. Verdouw; The Modified Fouling Index, a method of determining the fouling characteristics of
water; Desalination 32 (1980) 137-148.
8. H. Choi, K. Zhang, D.D. Dionysiou, D.B. Oerther & G.A. Sorial; Effect of permeate flux and tangential
flow on membrane fouling for wastewater treatment; Journal Separation and Purification Technology
45 (2005) 68-78.
9. X. Zhu & M. Elimelech; Colloidal Fouling of Reverse Osmosis Membranes: Measurements and
Fouling Mechanisms; Environmental Science & Technology 31 (1997) 3654-3662.
10. http://en.wikipedia.org/wiki/Crystallization.
11. L. Dudley; Membrane Autopsies for Reversing Fouling in Reverse Osmosis; Membrane Technology
95 (1998) 9-12.
12. http://en.wikipedia.org/wiki/Biofilm.
13. H.-C. Flemming; Biofouling in water systems - cases, causes and countermeasures; Applied Microbiology and Biotechnology 59 (2002) 629-640.
14. http://www.wetsus.nl/ (biofouling van membranen).
15. http://www.spongeballs.com/index.asp.
16. H.M. Kyllönen, P. Pirkonen & M. Nyström; Membrane filtration enhanced by ultrasound: a review;
Desalination 181 (2005) 319-335.
17. L.Y. Dudley; Membrane Autopsies for Reversing Fouling in Reverse Osmosis; Membrane Technology
95 (1998) 9-12.
18. http://www.avistatech.co.uk/pdf/Membrane_Autopsy.pdf.
19. http://www.wln.nl/nieuws/membraanautopsie.html.
20. http://www.poly-an.de/index.php?mid=25&vid=&lang=en.
21. http://www.poly-an.de/index.php?docid=73&vid=&lang=en.
22. http://www.poly-an.de/index.php?mid=21&vid=&lang=en.
23. B. Hofs, A. Brzozowska, A. de Keizer, W. Norde and M.A. Cohen Stuart; Reduction of protein adsorption to a solid surface by a coating composed of polymeric micelles with a glass-like core; Journal of
Colloid and Interface Science 325 (2008) 309-315.
24. M.A. Cohen Stuart, S. van der Burgh, R.G. Fokkink & A. de Keizer; Complex coacervate core micelles as
surface modification or surface treatment; WO/2005/030282.
25. http://www.lenntech.com/precoatfilter_eng.htm.
26. B. Blankert, B.H.L. Betlem & B. Roffel; Development of a control system for in-line coagulation in an
ultrafiltration process; Journal of Membrane Science 301 (2007) 39-45.
27. E. Zondervan, B. Blankert, B.H.L. Betlem & B. Roffel; Development of a multi-objective coagulation
system for long-term fouling control in dead-end ultrafiltration; Journal of Membrane Science 325
(2008) 823-830.
28. R.W. Field, D. Wu, J.A. Howell & B.B. Gupta; Critical flux concept for microfiltration fouling; Journal of Membrane
Science 100 (1995) 259-272.
Prevention and control of membrane fouling
page 46 of 46
29. D. Wu, J.A. Howell & R.W. Field; Critical flux measurement for model colloids; Journal of Membrane Science 152
(1999) 89-98.
30. P. van der Marel, A. Zwijnenburg, A. Kemperman, M. Wessling, H. Temmink & W. van der Meer; An
improved flux-step method to determine the critical flux and the critical flux for irreversibility in a membrane reactor; Journal of Membrane Science 332 (2009) 24-29.
31. M. Mulder; Basic principles of membrane technology; Kluwer Academic Publishers, Dordrecht.
32. http://en.wikipedia.org/wiki/Cross-flow_filtration.
33. H. Futselaar & I.G. Racz; Counter-current flow membrane module for liquid separations; European
Patent 0,466,945 A1 (1990).
34. B.Ph. ter Meulen; Transfer device for the transfer of matter and/or heat from one medium flow to
another medium flow; European Patent 0,509,03 (1990).
35. P.H.M. Feron & A.E. Janssen; CO2 separation with polyolefin membrane contactors and dedicated
absorption liquids: performances and prospects; Separation and Purification Technology 27 (2002)
231-242.
36. M.J. van der Waal & I.G. Racz; Mass transfer in corrugated-plate membrane modules. II.
Ultrafiltration experiments; Journal of Membrane Science 40 (1989) 261-275.
37. http://www.membransysteme.de/pdfs/CR-Filter_Brochure.pdf.
38. M.Y. Jaffrin; Dynamic shear-enhanced membrane filtration: A review of rotating disks, rotating membranes and vibrating systems; Journal of Membrane Science 324 (2008) 7-25.
39. http://www.vsep.com/pdf/VSEP_Brochure.pdf.
40. C.N. Koh, T. Wintgens, T. Melin & F. Pronk; Microfiltration with silicon nitride microsieves and high
frequency backpulsing; Desalination 224 (2008) 88-97.
41. J. Verberk; Application of air in membrane filtration; Ph.D thesis TU Delft, 2005.
42. W. van der Meer; R. Termeulen, P. de Moel & H. van Dalfsen; Luchtspoeling bij ultrafiltratie; H2O
1999, nr. 4, p. 20-22.
43. E.R. Cornelissen, J.S. Vrouwenvelder, S.G.J. Heijman, X.D. Viallefont, D. Van Der Kooij & L.P.
Wessels; Periodic air/water cleaning for control of biofouling in spiral wound membrane elements;
Journal of Membrane Science 287 (2007) 94-101.
44. P. Wessels; personal communication, 2008.
45. http://content.cosun.nl/sensus/en/sensus.html.
46. http://en.wikipedia.org/wiki/Inulin.
47. http://www.purac.com/purac_com/2cca37a174132ec642c0f21e5118bc20.php.
48. http://www.vsep.com/products/lp_std.html.
Prevention and control of membrane fouling
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