CHEM-E3140 – Bioprocess technology II
Dialysis and reverse osmosis
Emilie Baroux, 583051
Anik Nath, 547233
Sirkku Whitney, 598198
Report returned: Dec 5, 2016
Abstract
This literature review’s aim is to feature an introduction to both dialysis and reverse osmosis
and explains what makes these technologies different from other filtration techniques. A short history of
filtration is included to reveal how and why these methods were developed in the first place. The main
mechanisms and principles behind their functioning are studied. A closer look at the membrane materials is
taken to discover the limitations and the possible uses to these systems as well as the cost and scale of
filtering operations. Also some of the most common current uses for each are presented as case studies.
Background
Filtration is often the most efficient and practical process of separating large volumes of liquid
from dissolved solids or other liquids. It incorporates a wide range of scales - from a few millilitres of a single
sample to continuous process of hundreds of cubic meters of drinking water. There are also benefits to being
able to automate the process with control values that can monitored and adjusted (pressure, flow rate etc.).
As always with any process, a single-stage process such as filtration is preferred to other, multi-stage
operations that might be more difficult to control and troubleshoot due to their complexity. Reverse osmosis
and dialysis are both special cases of ultrafiltration based on the particle exclusion size.
Osmosis is an over 250-year old find and is commonly found in nature as a way of cell walls
permitting the passage of water through it via diffusion from less concentrated liquid to one with higher
concentration. It was only in the 1950s that membranes which would allow the phenomenon to be reversed
were starting to appear when desalination of seawater was researched. While making seawater potable is
still the main use for reverse osmosis and the fundamentals are the same as 60 years ago, a tremendous
amount of development has happened with the membranes allowing the lab experiment to be scaled-up to
produce drinking water to a large part of residents of the Arabian Peninsula. As world freshwater supplies
are privatized, polluted and used up the reverse osmosis is likely to become an even more widely used
technology than it is today.
Dialysis is a separation technique that facilitates the removal of small, unwanted compounds
from macromolecules in solution by selective and passive diffusion through a semipermeable membrane [1].
The world’s first dialysis machine may have been made out of sausage casings, orange juice cans and a clothes
washing machine but a lot of development has happened since 1940s [2]. In biotechnologies, this technique
is a commonly used method for desalting and buffer exchange of proteins despite the slow speed and large
volumes of buffers often required, and is typically performed overnight, especially for large samples. Another
well-known use of dialysis is the medical use to remove urea from urine in artificial kidney dialysis devices
[3].
Mechanism
Dialysis
Dialysis works by diffusion, a process that results from the thermal, random movement of
molecules in solution [4]. Sample molecules that are larger than the membrane-pores are retained on the
sample side of the membrane, but small molecules and buffer salts pass freely through the membrane,
reducing the concentration of those molecules in the sample [4]. At the end, diffusion of small molecules
leads to the net movement from areas of higher to lower concentration, until an equilibrium is reached: [3]
C =C
1
2
(1)
Figure 1: Schematic description of a dialysis dispositive in laboratory [5].
The efficiency of dialysis largely depends on the difference between the volumes of the inside
(sample, V ) and outside (buffer, V ) liquid spaces. This is why we generally seek to use as large volume (V ) of
the dialysate as possible. However, the efficiency of dialysis can be further increased by performing multistep dialysis by exchanging the outer solution after the equilibrium has been reached [5]. In this case, the
attainable dilution of the inside solution will be (n : number of steps) :
1
2
2
[V /(V +V )]
1
1
2
n
(2)
For example, when dialysing 1 mL of sample against 200 mL of dialysis buffer, the
concentration of unwanted dialysable substances will be decreased 200-fold when equilibrium is attained.
Following two additional buffer changes of 200 mL each, the contaminant level in the sample will be reduced
by a factor of 8 x 10 (200 x 200 x 200) [4].
6
There are many factors influencing the rate of dialysis. Firstly, because heat affects the
movement of molecules, increasing temperature speeds diffusion. In selecting the most appropriate
temperature, it is important to take into account the thermal stability of the molecule of interest. The rate
of diffusion is also directly proportional to the concentration of a molecule: as the concentration of a
molecule increases, so does the probability that one of those molecules will contact the dialysis membrane
and then diffuse across to the other side. However, as a molecule's molecular weight increases, the rate of
movement in solution decreases along with the chance of diffusion through the membrane. Finally, the rate
of dialysis is also directly proportional to the surface area of the membrane and inversely proportional to its
thickness, and stirring the buffer during the dialysis process also increases the diffusion rate [4].
Dialysis is performed with semipermeable membranes. The average or maximum pore sizes
of a dialysis membrane determines what size molecules can diffuse across it, which defines its molecular
weight cutoff (MWCO). The MWCO rating of a membrane refers to the smallest average molecular mass of
a standard molecule that will not effectively diffuse across the membrane. Typically, the smallest size globular
macromolecule that is retained by greater than 90 % upon extended dialysis (overnight) defines the nominal
MWCO. Thus, a dialysis membrane with a 10000 MWCO will generally retain proteins having a molecular
mass of at least 10 kDa. The MWCO should be chosen as high as possible in order to maximize the dialysis
rate [4].
The limited use of dialysis as a filtration method is not because of the cost of membranes.
Most common membranes are made of cellulose and are single-use due to the high hygiene standards of
their usual applications. The membranes have a tendency to foul and are generally too fragile to withstand
cleaning or high pressure. Because of these properties, it is likely that they are not a suitable filtration method
for upscaling [6].
Reverse osmosis
In order to understand the Reverse Osmosis (RO), it first requires to understand the concept
of most common natural phenomena known as osmosis. Osmosis is general and important process in the
nature, which can be easily defined by water passing from less concentrated solution towards the highly
concentrated solution through semipermeable barrier or membrane. The transfer carries on occurring until
the both solution gain equal concentration. A figure 2 below describes simple dynamics of the osmosis
process [7].
Figure 2 : Illustration of natural osmosis process [7].
Following the illustration in figure 2 it can be seen that, solution level rise in the more
concentrated solution. At equilibrium state of concentration of both sides, it develops a head which is called
osmotic pressure.
RO is completely alter process of osmosis. Osmosis occurs naturally without requiring external
energy, whereas RO requires to apply external energy/pressure using a high pressure pump on the side of
the highly concentrated solution. At osmotic pressure, the flow between two solutions will be ceased
whereas applying pressure or energy greater than osmotic pressure reverse the flow form highly
concentrated solution towards the less concentrated solution [8]. The amount of pressure required depends
on the salt concentration of the feed water. The higher the concentration of the feed water, the more
pressure is required to overcome the osmotic pressure. A simple schematic of reverse osmosis is given below.
Figure 3: Schematic of reverse osmosis process [7].
Reverse osmosis is membrane filtration technique. Within the membrane filtration, there are
many other technologies are also well known along Reverse Osmosis : Nanofiltration (NF) , Ultrafiltration
(UF) and Microfiltration (MF) [9]. Membrane filtration techniques are well employed in different fields
because of their efficiency, process maturity and cost effectiveness mainly. The picture below clarifies the
possible components that can be separated using these technologies.
Figure 4: comparison chart of different filtration various membrane filtration technologies [10].
As can be seen in figure 4, the RO is very efficient in terms of separating and concentrating,
where water only can transfer through the membrane while the other compounds are stopped. However,
reverse osmosis membranes are fragile in a way that requires prefiltration of the feed water to avoid clogging
or tearing the membranes.
As described above, RO is very effective techniques for treating water. According to the
literature, RO is capable of removing up to 99% of the dissolved salts (ions), particles, colloids, organics,
bacteria and pyrogens from the feed water. The membrane within the RO rejects contaminants based on
their size and charge. Molecules with molecular weight greater than 200 are likely to be rejected by the fully
functioning RO system. Besides, the greater the ionic charge of contaminants has higher chances of getting
stuck in membrane. However, the gases are not removed by the RO because gases such as CO is not highly
ionized while in the solution and have very low molecular weight.
2
Figure 5: Operation of RO water purification system [8].
In general, feed water is pumped into a Reverse Osmosis (RO) system which end up with two
types of liquid coming out of the RO system: the permeate and the reject stream. The reject stream, also
called concentrate or brine stream, contains the contaminants, as they are not allowed to pass through the
membrane. The water that makes it out through the membrane, with very little contaminants, is called
permeate or product water [8].
Most RO filters utilize a thin-film composite structure consisting of a relatively thick structural
support layer of polyester web, a thin micro-porous polysulfonic polymer interlayer giving the membrane
strength in high-pressure compression, and the extremely thin semi-permeable selective membrane that is
the functional heart of the filter. There are different types of membranes for RO process. The most dominant
RO membrane material is polymeric material due to its technological maturity and lower cost fabrication.
Other types include asymmetric RO membranes (cellulose acetate, aromatic polyamide, polypiperzineamide, polyoxadiazole etc.) and Thin Film Composite (TFC) membranes (polyfurane, polyether-polyfurane,
sulfonated polysulfine etc.). These different types of membranes have different efficiencies and
performances in terms of recovery, salt rejection, pressure tolerance, concentration and component types.
Some membrane materials are also sensitive to chlorine or organic solvents so the quality of the feed
determines the choice of membrane [11].
The performance of a RO system depends on several parameters given below as a list [6]:




Feed, permeate and concentrate pressure
Feed, permeate and concentrate conductivity (µS)
Feed, permeate and concentrate flow (gallons or liters per minute)
Temperature
The more salts are rejected in the reject stream, the more efficient the RO is. On the other
hand, the more there are salts in the permeate, the less it is efficient. If the salt passage is high, as well as if
the salt rejection is low, the system will require the membrane cleaning or replacement. [8]
The percentage of recovery is defined by the amount of permeate recovered. A high recovery
means that less water is collected as reject stream, and also that contaminants recovered in the reject stream
are more concentrated. However high recovery could cause problems due to scaling (high concentration of
contaminants on the surface of the membrane) and fouling (high pressure). By calculation recovery
percentage is described in below with an equation [8].
% Recovery = x 100
(3)
To improve the performance, multistep RO can be implied into the process. There is some
pretreatment using mechanical and chemical treatments can be considered in order to avoid the fouling,
scaling, membrane failure, frequent cleaning requirements. Sometimes multi media filters are used to
prevent the RO fouling [8].
However, RO is a proven and widely used technology to produce water that is suitable for
many industrial applications that require demineralized or deionized water. In addition, post treatment after
the RO system such as mixed bed deionization can increase the quality of the RO permeate and make it
suitable for the most demanding applications. Proper pretreatment and monitoring of an RO system is crucial
to preventing costly repairs and unscheduled maintenance. With the correct system design, maintenance
program, and experienced service support, your RO system should provide many years of high purity water.
[8]
Importance in filtration toolkit – case studies
Dialysis
Dialysis is primarily used to remove or exchange low molecular weight mineral solutes like
salts of solutions. Most of these applications occur on the laboratory scale, because on industrial scale the
removal of salts can be done more rapidly via diafiltration [12]. However, dialysis can be used for other
applications: in the medical domain, dialysis is used to remove waste and fluid from the bloodstream of
kidney-deficient patients [3].
Besides, it is being used in removing low molecular weight alcohols from fermentation broths,
thus reducing the alcohol content, without affecting the taste and appearance of the beverages, for the
production of low and non-alcohol beers [13]. Dialysis is also used in fermentation to remove growthinhibiting products, therefore permitting a better control of the concentrations and composition of the feed
material that enters any downstream processing scheme [12]. Dialysis can also be used in acid or alkali
recovery [14].
Reverse osmosis
RO is one of the most important ways of purifying water: for example, this technology is used
in mining industry to recover or remove the metal before releasing the effluent wastewater into the
environment. RO can be used as complete solution for treating solution or combination with other
technologies as treatment. For example, at El Aguilar city of Argentina, a company called Minera Aguilar has
employed reverse osmosis in combination with ultrafiltration for mining effluent treatment to remove heavy
metals like nickel, lead etc. [15].
In terms of bioprocess technology, this membrane filtration process is also attracting many
researchers in their research areas. A study conducted by the Stanford university which aimed for sustainable
water reuse, envisioned membrane bioreactors that will permit efficient and direct conversion of sewage
organics into useful fuels (methane and hydrogen) and removal of inorganic electrolytes even at cool
temperature. Energy from the fuels could be used to fill up demand of energy demands of water purification.
That will make water reuse sustainable. Also, it aimed for developing RO membranes that could remove
inorganic electrolytes with minimal energy loss due to chemical and biological fouling [16]. A schematic of
their concept is given below in figure 6.
Figure 6: Sustainable water reuse process scheme by the Criddle group, Environmental
Biotechnology at Stanford [16].
Therefore, the RO technology seemed useful with combination of other technology to achieve
sustainability on water use and fill up the demand of pure water globally.
Figure 7: A large reverse osmosis water purification plant on the coast of Saudi-Arabia. This plant produces
the drinking water for an entire city by filtering salt out from seawater. [17]
Conclusion
The literature review revealed dialysis and reverse osmosis differing from other filtering
methods by featuring a specific membrane type instead of a pore size. Filtration is not done by molecule’s
size but its weight. Membranes are semipermeable instead of porous like a regular filter is and molecules of
smaller weight collide with and migrate through the membrane more often than heavy molecules. This
enables both extremes of small-particle filtration – the high-pressure, high throughput and what is close to
no-pressure, natural diffusion. Membranes can also feature other, molecule-specific properties. Most
common application for dialysis is blood dialysis done on medical patients with serious kidney problems.
With reverse osmosis it is water purification.
References
1. ‘Other methods for desalting and buffer exchange’, in Strategies for Protein Purification
Handbook, General Electric Healthcare, Uppsala 2010, pp. 36.
2. Blakeslee, S., Willem Kolff, Doctor Who Invented Kidney and Heart Machines, Dies at 97, New
York Times, 12 February 2009. Collected
from <http://www.nytimes.com/2009/02/13/health/13kolff.html?pagewanted=all>
3. ‘Dialysis’, in Bioprocess Engineering - Basic Concepts, 2nd edn, Shuler, M. L. and Kargi, F. (ed.),
Prentice Hall PTR, 2002, pp. 355-356.
4. ‘Dialysis methods for protein research’, thermofisher.com. Thermo Fisher Scientific Inc. 2016.
Collected from <https://www.thermofisher.com/fi/en/home/life-science/proteinbiology/protein-biology-learning-center/protein-biology-resource-library/pierce-proteinmethods/dialysis-methods-protein-research.html>
5. Hegyi, G., Kardos, J., Kovacs, M. et al., ‘Units, solutions, dialysis’ in Introduction to practical
biochemistry, Eötvös Loránd University, Hungary 2013. Web.
<http://elte.prompt.hu/sites/default/files/tananyagok/IntroductionToPracticalBiochemistry/c
h02s04.html>
6. ‘Tech notes - The dialysis process’, membrane-mfpi.com. Membrane Filtration Products Inc.
2009. Collected from <http://www.membrane-mfpi.com/home/tech-notes>
7. ‘Reverse Osmosis’, in The ABC’s of the filtration process and bioprocessing for the third
millennium, Ballew, W. H., Martinez, J.F., Markee, C. & Eddleman, T. R. (ed.), Compton 2002,
spectrum laboratories Inc, pp. 14-15. Collected from <www.spectrumlabs.com>
8. ‘What is reverse osmosis’, puretecwater.com. Puretech Industrial Water 2016. Collected from
<http://puretecwater.com/reverse-osmosis/what-is-reverse-osmosis>
9. ‘Introduction in membrane filtration’, in Membrane filtration – Reverse Osmosis, Nanofiltration,
UltraFiltration and MicroFiltration, GEA Process Engineering, Hudson 2012, pp. 4. Collected from
<http://www.gea.com/en/binaries/general-membrane-filtration_tcm11-17109.pdf>
10. ‘Reverse Osmosis vs. Nanofiltration and Other Filtration Technologies - Comparison of Reverse
Osmosis with Nanofiltration and Other Filtration Technologies’, aquaclearllc.com. Aqua Clear
Water Treatment Specialists 2015. Collected from <http://www.aquaclearllc.com/technicalinfo/reverse-osmosis-vs-nanofiltration-and-other-filtration-technologies/>
11. Lee, K. P., Arnot, T. C. and Mattia, D. A review of reverse osmosis membrane materials for
desalination-development to date and future potential, Journal of Membrane Science 370 (1- 2)
(2011) pp. 1-22. ISSN 0376-7388
12. Fane, A. G., Radovich, J. M., ‘Membrane Systems’ in Separation processes in biotechnologies,
Asenjo, J. A. (ed.) CRC Press, New York 1990 pp 249-250.
13. Moonen, H., Niefind, H. J. Alcohol reduction of beer by means of dialysis, Desalination 41 (1982)
327-335.
14. Luo, J., Wu, C., Xu, T., Wu, Y. Diffusion dialysis-concept, principle and applications, Journal of
Membrane Science 366 (2010) 1-16.
15. ‘Case study - Minera Aguilar’, rwlwater.com. RWL Water LLC 2016. Collected from,
<https://www.rwlwater.com/minera-aguilar-ultrafiltration-and-reverse-osmosis/>
16. The Cridle group, ‘Membrane Biotechnology’, stanford.edu. Collected from
<http://web.stanford.edu/group/evpilot/membrane.htm>
17. Doosan Heavy Industries and Construction: History
<http://www.doosanheavy.com/en/intro/history.do>