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Restoring Clay Fouled Membranes
Ghanshyam Daimiwal, Wex Technologies Pvt. Ltd and Anand Avadhani, Head - Operation Wex Technologies Pvt. Ltd The most frequent foulants in the lead element of an RO plant are Aluminosilicates or Clays (35 per cent). The other types of foulants identified on membrane elements from the fi rst position during autopsy [2001-2007, GMP laboratories statistics] include Aluminium (3 per cent), Biofi lm (22 per cent), Calcium Phosphate (5 per cent) and Organic Matter (27 per cent).

Clay minerals are typically formed over long periods by the gradual chemical weathering of silicate bearing rocks by carbonic acid and other dilute acidic solvents which migrate through the weathering rock. The transported clay minerals are deposited in surface water aquatic environments and from here can directly enter reverse osmosis feed water. The naturally occurring transportation of these minerals from the surface source makes clay minerals potentially problematic in all membrane feed waters.

These minerals are naturally resilient to breakdown by chemical attack and also high temperatures and pressures. This makes them fairly resistant to ÂnormalÊ cleaning products. Clays also have the ability to take up or lose water according to the surrounding temperature and amount of water present.

There are two main properties exhibited by clays which make them harder to remove from membranes First, Water Impermeability which means only the outside surface is available for attack by the Restoring Clay Fouled Membranes C cleaning solution as this property prevents water and cations in solution from entering the internal layers. Second, Plasticity is the deformation of a material undergoing a non-reversible change of shape in response to applied forces.

This characteristic allows the shape of the deposit to change and deform under applied pressure causing the deposit to form and block the pores at the membrane surface. Swelling of the clay on hydration also reduces porosity of the deposit preventing entry of cleaning solutions to the internal structure. This swelling effect also increases flux reduction at the membrane surface.

Effects of Clay Fouling in Membrane Operating Performance
Pretreatment processes such as coagulation, flocculation and low pressure membrane filtration (microfiltration and ultrafiltration) have been used in front of RO processes to remove particles and large colloids, but fouling by small colloidal matter (<2øm) and fine suspended particles is still a problem [1]. The principal consequence of membrane fouling by clay minerals is an increase in hydraulic resistance resulting in a greater energy requirement to operate the process.

The formation of highly impermeable deposits by clay minerals on the membrane surface will result in significant problems in maintaining permeate flux with frequent cleaning eventually being required to maintain system operation.

When untreated, fouling can gradually affect all the membranes resulting in reduced membrane flux, increased salt passage and also an increase in P. Membrane can also be damaged through abrasion due to the compression of the crystalline structure against the membrane surface by increased operating pressures.

If we accept that due to inadequate chemical and physical pre-treatment systems clay particles will continue to enter the membrane system then it is important to reduce the frequency of required cleaning. In order to reduce cleaning frequency and minimise membrane damage the operator can achieve optimum deposit removal by cleaning with a technically correct product.

Genesol 703 - New Cleaning Approach Cleaning agents can affect fouling materials present on a membrane surface in three ways: (i) foulants may be removed, (ii) morphology of foulants may be changed (swelling, compaction) and/or (iii) surface chemistry of the deposit may be altered, such that the hydrophobicity or charge is modified [2]. Reported foulant-cleaning agent reactions are hydrolysis, peptisation, saponification, solubilisation, dispersion (suspension) and chelation. If an inappropriate cleaning agent is chosen negative effects can appear and membrane performance can be adversely affected.

In normal operation of an RO system the pressure provided by the High Pressure Pump (HPP) overcomes the osmotic pressure of the feed water. During cleaning, the Genesol 703 solution is introduced to the system at a cleaning pressure below four bar. The feed water salinity will increase. It is possible that at the membrane surface the local osmotic pressure may become higher than the Net Driving Pressure (NDP) of the feed water. If this were the case then potentially there may be some localised forward osmosis taking place. Any movement of permeate water through the membrane to the feed water may assist lifting of the clay deposits around the membrane pores. This in turn would allow greater access to the surfactant cleaning chemicals to remove deposits. The removal of deposits away from the membrane into the concentrate stream is likely to help minimise membrane abrasion. This phenomenon may go some way to explain the effectiveness of the cleaning formulation.

In addition to the effectiveness of Genesol 703 in removing clay deposits its application also serves as a means of „shock treatment‰ of a reverse osmosis system to reduce the biofouling potential through lysis of microorganisms; in turn this helps to prevent further system contamination. Cell lysis occurs due to the semi permeable nature of the membrane surrounding the microorganism; the cleaning solution creates the movement of water from the cell cytoplasm resulting in the eventual removal of the membrane from the cell wall. Laboratory tests (as indicated below) proved this product to be much more effective at removing clay mineral deposits than conventional acid and alkaline cleaning products.

Wex has studied numerous fouled membranes in India and it was observed that a majority of them had fouled due to clay and biofilms. Very few had „scales‰ of CaCO3 or CaSO4 indicating that the anti-scaling treatment had worked as designed. The membranes had over a period of time accumulated clay and microbial slime and the slime had further accelerated the fouling process. Conventional cleaning with alkaline solutions containing Na4-EDTA, DDBS followed by acid cleaners (HCl, HCl + Citric Acid etc.) has given limited success in restoring flows. Many commercial formulations had also not proved to be very successful in removing these foulants. Wex has investigated a new cleaning agent - Genesol 703 - introduced by its principal Genesys International Ltd., UK.

Genesol 703 was developed and tested as an effective cleaning compound for the removal of clay deposits. Genesol 703 is a 100 per cent active chemical powder based on a combination of high pH phosphate cleaners, a blend of chelants, surfactants and other active compounds. The product is approved under NSF/ANSI 60 guidelines. This combination of products has a detergent and surfactant effect on the colloidal foulants. Due to the relevance of biofouling problems detected in Genesys Membrane Products laboratories (detected in 35 per cent of membranes autopsied) and synergistic properties of this formulated cleaner; trials against biofilm removal under laboratory conditions were conducted and that showed Genesol 703 to be more effective.

The Genesol 703 mode of action can be described as follows: the first stage of attack occurs at the water/surface inter-phase of the biofilm and is due to the synergistic action of the combined speciality ingredients. This process works by reducing the surface tension of the deposit allowing the surfactant to become more effective in overcoming the impermeability of the EPS material; this allows the cleaning solution to penetrate into the biofilm structure. The foulant layer then becomes more porous increasing the permeability to water and consequently increasing the surface area of the deposit allowing more active chemical to penetrate and disrupt the ÂbodyÊ of the deposit. Genesol 703 provides a secondary physical action, which increases cleaning efficiency at the membrane surface allowing a Âdouble edgedÊ approach to deposit removal. This action removes blockages from the membrane pores caused by the biofilm layer.

This paper presents our experience at three typical sites where flow was restored after cleaning with Genesol 703.

In order to check Genesol 703 efficiency, several cleaning programs were designed according to the membrane manufactures guidelines for removing this type of foulant. Established limits of pH and temperature have also been applied. In order to achieve comparative results, contact time in each trial has been set up in 2 hours. The results obtained in the different cleaning tests and the conditions applied (temperature, pH and contact time) are summarised in Table 2.

Results and Conclusions
Clays are a common foulant occurring primarily on the membrane elements in the first position and they can occur in all RO feed waters.

The structural characteristics of clay minerals mean that operational increases in pressure can compress the deposit resulting in further loss of membrane flux. The test data supports the fact that clay deposits on a membrane surface can produce irreversible damage in terms of salt rejection due to abrasion processes. Improvements in pre-treatment can be considered although practical experience shows that small particulate/colloidal matter (< 2 øm) can not be completely removed from feed streams. In order to prevent membrane damage remedial action should be taken immediately when the first symptoms of fouling are detected. The cleaning carried out on the actual plant indicate that Genesol 703 is more effective at removing biofouling from membrane surfaces at a dosage rate of 1-2 per cent. This dosing rate is significantly lower than traditional cleaning chemicals. In these analyses cleaning regime was carried out on plant membranes, which have been affected by biological fouling during their natural operation. The effectiveness of the product has been proven in terms of the results of significant increases in flow and foulant layer removal.

The product has been used in over fifty operating plants with biofouling problems with encouraging results. Cleaning protocols we have designed are with appropriate contact times and combining Genesol 703 with additional cleaning steps (biocides, acidic cleaners) according to each plantÊs conditions. Rejection values have been reported to either improve or be maintained. In some cases, rejection has decreased after the cleaning procedure. Behaviour in salt rejection values depends on the type of foulant and condition and how they affect membrane rejection properties. The product has been tested in line with the membrane compatibility testing protocol established by DOW Filmtec and results confirm no effects on membrane rejection properties.

Further studies will be conducted and the authors hope to present additional case study information in the future.