Clay & organic fouling efficiently removed
M. W.Armstrong*, S. Gallego*, S. P. Chesters*
* Genesys International Ltd, 3A Aston Way, Middlewich, Cheshire CW10 0HS, UK
(e-mail : [email protected] ; [email protected])
Abstract
Long term membrane autopsy feedback from the Genesys Laboratory showed that RO system failure is
mainly caused by fouling rather than scaling. Main foulants are : aluminosilicates (clays), organic matter and
biofilm. Of all the foulants, clays are the most difficult to clean due to their specific structural characteristics :
plasticity, presence of different structural cations, relative impermeability to water.
When subject to clay fouling, membranes experience a rapid flux decline, requiring a feed pressure increase
to maintain product flow. This only causes further compaction of the clay deposit, which becomes
increasingly impermeable to water and thus harder and harder to clean with traditional chemicals. Cleaning
frequency increases, enhancing the risk of membrane damage. In order to avert irreversible fouling, clay
deposits must therefore be removed early and effectively.
This paper explores the nature of clay, the mechanisms behind membrane fouling and the subsequent
development of Genesol 703, a new type of cleaning product for RO/NF/UF membranes. Comparative lab
measurement and site trials showed that Genesol 703 is a technically and economically viable cleaning
chemical for the removal of organic deposits in general and clay deposits in particular from membranes.
Keywords. RO membranes, cleaning, fouling, aluminosilicates, organics, Genesol 703
INTRODUCTION
Membrane autopsies performed at the Genesys laboratories in Madrid over a 5 year period showed that the
majority of RO membrane failure was caused by fouling rather than scaling. The most common foulants
found in RO plant lead elements were : aluminosilicates (or clay minerals), organic matter and biofilm. (see
figure 1). Clays are naturally occurring compounds present in most waters, which are only partially removed
by RO pre-treatment. Their specific structural properties make them much more difficult to clean than other
types of foulant. This paper details the processes behind clay fouling of membranes and the subsequent
development of a new cleaning product. Results from laboratory and field testing are presented, with an
explanation of the unique mechanisms behind its efficiency.
3%
27%
35%
8%
5%
Aluminium
Calcium Phospate
22%
Aluminosilicates
No deposits detected
Biofilm
Organic Matter
Figure 1. The main types of foulant identified on membrane elements from the first position during autopsy
(2001-2007). Source: GMP laboratories statistics
CLAY CHARACTERISTICS
Clays are formed by the long-term weathering of silicate based rocks by carbonic acid and other solvents.
They end up in surface waters, constituting a potential problem for all reverse osmosis feed waters. Their
structure is composed of parallel sheets of silicate tetrahedron rings interlinked by oxygen atoms. The sheets
are weakly bonded by layers of cations. Water and other neutral molecules can get trapped between the
sheets, giving clays their soft compressible nature and their ability to absorb & lose water. Swaddle [1] refers
to them as aluminosilicates when some of the Si4+ ions in the silicates structure are replaced by Al 3+ ions. In
order to balance the charge, other cations such as iron, magnesium or alkali metals are integrated in the
structure. These minerals are extremely resistant to chemical ation, high pH or temperature, which explains
the resistance of clays to conventional cleaning products. Clay minerals are divided into 3 major groups,
each with a specific structure, properties and adverse effects on membrane performance :
- Kaolinite : silicate sheets bonded to aluminium oxide & hydroxide layers (most common type)
- Illite : contains a wide range of cations (Al, Mg, Fe & K)
- Montmorillonite/Smectite : contains Ca, Na, Al, Mg and is notable for its ability to take up and lose water
Clays have 2 major properties which make them hard to clean from the membrane surface :
- Water impermeability : the specific structure of clays prevents water & dissolved cations from entering
reaching the internal layers
- Plasticity : clays can deform under pressure, blocking the pores at the membrane surface in the process
CLAY FOULING MECHANISMS
Small colloidal matter (<2µm) and fine suspended particles are not retained by conventional pretreatment
(including utltrafiltration) and can therefore cause membrane fouling [2]. First position elements in a RO
system are affected first, but the problem can spread to the whole train if untreated. Fouling mechanisms on
the membrane surface are complex and poorly understood. Physical adsorption, electrostatic charge and
bridge-formation have all been reported [3]. They involve water velocity, water turbulence along the
membrane surface and cake formation by particle accumulation. Cake formation is influenced by many
parameters which affect the porosity of the deposit and hence the membrane flux. Compression also has an
effect on the porosity. The formation of water resistant clay deposits leads to membrane flux reduction, salt
passage increase and ∆P increase, with frequent - yet partially inefficient - cleaning being required to
maintain system operation. Abrasion of the membrane surface by clay minerals has also been observed as
their crystalline structure is compressed against the membrane surface while the operating pressure
increases.
GENESOL 703
Foulant cleaning can be have 3 different effects [5] :
- deposit removal
- morphology changes (swelling / compaction)
- surface chemistry changes (hydrophobicity, charge modification…)
The choice of an inappropriate cleaner can seriously impair the membrane performance, the membrane
lifetime and the O&M costs [6]. Genesol 703 was specifically developed to overcome the limitations of
existing cleaners against clay deposits and provide effective cleaning. Genesol 703 is a combination of high
pH phosphate cleaners, chelants, surfactants and other active compounds. It is designed to have a detergent
& surfactant effect on colloidal foulants as well as creating high ionic strength at the membrane surface. First,
it reduces the deposit surface tension, enabling the surfactant to overcome the cake impermeability. The
cleaning solution can penetrate the clay inter-layers, thus the deposit becomes more porous and therefore
more permeable, allowing more chemical to enter. In order to overcome pore blockages, Genesol 703
salinity also causes a forward osmosis effect, enabling clay deposits to be lifted from the surface membrane
and probably minimising membrane abrasion. The processes involved require further research to be fully
understood. The Genesol 703 also reduces the biofouling potential through the lysis of microorganisms.
CONCLUSIONS
Laboratory and field tests proved the Genesol 703 product effectiveness. The lab tests were performed on 3
actual RO elements, after deposit characterization by scanning electron microscopy. Cleaning experiments
were performed on a flat sheet test rig, enabling precise measurement of membrane flux and rejection under
normalized conditions before & after cleaning. The data obtained demonstrates the Genesol 703 superiority
at removing clay deposits and improve flux compared to other chemical blends tested. However, salt
rejection measurement proved inconclusive for all cleaners (decline), probably because of membrane
abrasion. Genesol 703 also proved particularly efficient at cleaning severely fouled membranes at a low
concentration (1%). Testing in thirty operating plants in different countries confirmed these results through
noticeable membrane flux improvement and stable or improved salt rejection.
REFERENCES
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Limited (1990)
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over multiple operational cycles. Journal of Membrane Science 216 (2003) 1-2.
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