Summary of Issues Strategies Benefits & Costs Key Uncertainties Additional Resources

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1 Summary of Issues Strategies Benefits & Costs Key Uncertainties Additional Resources KEY POINT: Appropriate pre-treatment can prevent fouling and scaling, optimize membrane performance, and extend membrane life SUMMARY OF KEY POINTS The primary objectives of pretreatment are: 1. Optimize reverse osmosis (RO) performance 2. Prevent membrane fouling and scaling 3. Protect RO membranes / extend membrane life The most common issue associated with pretreatment is the potential for reversible (i.e. able to be removed via chemical cleaning) and irreversible (i.e. cannot be removed) membrane fouling. Membrane fouling occurs when solutes or particles deposit onto the surface of the RO membrane or the membrane pores. Membrane fouling results in the deterioration of water flux and quality (i.e., solute rejection), shortened membrane life, and increased optional costs. Typical membrane foulants include biological materials and colloidal particles. As discussed below, scaling on the membrane is also a primary concern. Particulate matter, biological constituents, natural organic compounds, and scalants can cause reversible and irreversible membrane fouling: Particulate fouling is caused by the deposition of turbidity, colloids, suspended solids, etc. in the RO membrane elements, increasing the operating pressure and/or reducing the flux. Particulate fouling may be irreversible because RO elements cannot be backwashed. Biological fouling, also called biofouling, is the result of either the growth of microorganisms on the membrane surface or the secretions of these organisms (i.e., biofilm). Microbial growth on the membrane can lead to head loss across the membrane surface, reducing the flux of water or necessitating an increase in the pressure to maintain constant flux. Biofouling can also reduce salt rejection and damage the membrane. Once biofouling becomes established, it can be difficult to reverse in some cases. Organic fouling occurs via the interaction of organic compounds with the membrane material. Examples of organic foulant include naturally occurring humic and fulvic acids, as well as pollutants such as oil and grease. Organic fouling limits membrane flux and can necessitate an increase in operating pressure. Organic fouling can be irreversible. 1

2 Scaling on the membrane surface is caused by the precipitation of sparingly soluble salts or the deposition of amorphous solids [e.g., silica (SiO 2 ) and aluminum hydroxide (Al(OH) 3 )] as the feed water gets increasingly concentrated throughout the treatment process. The presence of scale on the membrane surface can inhibit the flux of water, reducing permeate production. Another issue is the potential for oxidants to damage the RO membranes, which can rapidly degrade the polymer structure and increase salt passage to the permeate water. High-quality pretreatment will be of greater importance with the advent of high-flux (highpermeability) RO membranes, as the propensity for fouling is significantly increased. To realize the benefits of high-flux RO membranes, more effective pretreatment systems will be needed. STRATEGIES Effective pretreatment is an essential component of every desal process and can have very significant implications for overall process performance. The following outlines strategies for preventing/minimizing membrane scaling and fouling. Particulate fouling Potential for particulate fouling is best gauged by water quality analyses and careful monitoring (e.g., turbidity, particle counts, silt density index testing, etc). Prevention of particulate fouling involves varying degrees of prefiltration depending on the feed water quality and application: Cartridge filtration is standard pretreatment. The rating (pore size) of the filters varies from 1 to 25 um, depending on the expected size of the particulate matter and the hydraulic considerations (e.g., greater head loss is associated with filters that have smaller ratings). More robust processes (such as media filters) may be necessary for applications with the potential for increased solids loading, such as surface water, and tertiary wastewater for reuse. Caveat: conditioning chemicals used in conjunction with filtration (e.g., coagulants and polymers) can foul and/or damage membranes. If these chemicals are used, precision process control is important to prevent carryover onto the membranes. 2

3 Most challenging pre-filtration cases may employ microfiltration (MF) or ultrafiltration (UF). Rule of thumb. NF/RO feed water should have turbidity <0.3 NTU and SDI <4. Biological fouling Biofouling potential The propensity for biofouling is not necessarily straightforward. The water with higher organic concentration and higher biological activity often has higher membrane fouling potential. For example, the use of RO to desalinate surface water and tertiary wastewater is much more likely to result in membrane biofouling than when RO is applied to desalinate a brackish aquifer (typically a much less biologically active environment). Biofouling may be detected by comparing the feed and concentrate streams. Increased biological activity in the concentrate may indicate a potential problem. Biofouling prevention Chlorine and other stronger disinfectants can only be used sparingly because RO membranes can be damaged by oxidants unless followed by a dechlorination step prior to the membrane process Higher chlorine tolerance is one characteristic of membranes made from cellulose acetate material, so these are an option to consider for applications with significant biological fouling potential. Caveat: these membranes have other limitations (e.g., must be operated within a relatively narrow ph range, are subject to microbial degradation). Disinfectant or oxidant carryover into the membrane feed water could be quenched via the application of a reducing agent. Optimized process control for the upstream treatment can prevent this carryover more economically. Weaker disinfectants, such as chloramines, can be applied continuously at low doses. 3

4 Biological inhibitors such as sodium bisulfate (NaHSO3) may be used. Copper sulfate can be applied as a pretreatment if the possibility of algae cells in the RO feed is a concern. Organic fouling An indicator of the propensity for organic fouling is total organic carbon (TOC) in the feed water. Although TOC may not necessarily correlate with fouling potential in all cases (because the range of organic compounds includes a variety of complex substances with diverse properties), ongoing research has narrowed the range of organic matter components (e.g., polysaccharides) that contribute most significantly to organic fouling. To control organic fouling, processes known to be effective for removing TOC such as conventional treatment or granulated activated carbon (GAC) can be used as pretreatment. These pre-treatments add a significant cost Scaling Potential for scaling is typically evaluated using any one of the proprietary modeling programs available from each of the RO membrane manufacturers. These programs are generally available for free public download at the manufacturers web sites. Accurate and thorough data for a wide variety of dissolved species will improve the modeling results. Preventing scaling. The most commonly used methods are: ph adjustment. Add either acid (e.g., sulfiuric acid H 2 SO 4 ) to lower the ph, thus increasing the solubility of sparingly soluble salts. And/or Scale formation control chemicals. Add any one of the numerous proprietary chemicals designed to interfere with the precipitation process such as: Anti-scalants (i.e., threshold inhibitors, scale inhibitors): interfere with crystal growth Sequestering agents: ion binding / prevent precipitation 4

5 Dispersants: maintain scalants in suspension Limit the recovery of the system, which reduces the levels of scale forming ions in the concentrate (see Table 1). This is probably not practical since the value of potable quality water is continually increasing and utilities will likely operate their systems to maximize the recovery in order to produce as much water as possible. Removal of sparingly salts. If ph adjustment and addition of antiscalants could not control membrane scaling. The following processes can be employed, which can add treatment cost significantly. Softening using lime or NF membrane Oxidizing scaling forming species and then pre-filtering the precipitates prior to RO Ion exchange (IX). BENEFITS & COSTS Benefits Effective control of membrane fouling can decrease operating costs by reducing energy consumption, reducing chemical uses for cleaning and extending the life of the membrane. Costs can constitute a significant component of the overall cost of a desal project, both in terms of capital and operations (this is especially true for surface water surfaces rather than groundwater sources). Thus, although in some cases it may seem prudent to utilize the most economical means of pretreatment at the sacrifice of some measure of performance, it is critical to understand that inadequate pretreatment can be extremely detrimental to the overall efficiency of a desal plant, possibly resulting in costly repairs and significant facility down time. Care should be taken to account for the trade-offs between capital and operating costs for different pretreatment systems. For example, the increase in the capital costs of a system with an ultrafiltration (UF) or microfiltration (MF) pretreatment process compared to a conventional pretreatment process is relatively high. The significant benefit of the UF/MF-based pretreatment is realized through reduced operating costs. 5

6 KEY UNCERTAINTIES The chemicals added during pretreatment may cause membrane degradation, fouling and scaling, including oxidants and disinfectants, coagulants and polymers. Online precision process control is important to prevent carryover of these chemicals onto the membranes. Chlorination followed by dechlorination has been found to contribute to RO fouling occasionally. Chlorination may break down organic material into assimilable organic carbon, which acts as a food source for the re-growth of bacteria on membrane surface. ADDITIONAL RESOURCES USBR (United States Bureau of Reclamation) Desalting Handbook for Planners. 3rd Edition. Desalination and Water Purification Research and Development Report #72. Denver, CO: United States Department of the Interior, Bureau of Reclamation, Water Treatment and Research Group. USBR The Desalting and Water Treatment Membrane Manual: A Guide to Membranes for Municipal Water Treatment. Water Treatment Technology Program Report No. 29 (R-98-5). USEPA Membrane Filtration Guidance Manual. Report No. 815-R , November Wagner, J Membrane Filtration Handbook: Practical Tip and Hints. 2 nd Ed., Rev. No. 2. Osmonics, Inc., November WHO Desalination for Safe Water Supply: Guidance for the Health and Environmental Aspects Applicable to Desalination. Geneva: WHO. Available: Xu, P., Cath, T, Wang, G., Drewes, J.E. and Dolnicar, S Critical assessment of implementing desalination technology. AwwaRF Project Published by American Water Works Association Research Foundation, Denver, CO. 6