Summary of Issues Strategies Benefits & Costs Key Uncertainties Additional Resources KEY POINT: Appropriate pretreatment can prevent fouling, optimize membrane performance and extend membrane life. SUMMARY OF ISSUES Effective pretreatment is an essential component of every desal process (particularly seawater open intake desal) and can have very significant implications for overall treatment process performance. The three primary objectives of desal pretreatment are: 1. Optimize reverse osmosis (RO) performance 2. Prevent fouling 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, which can be caused by particulate matter, biological constituents, natural organic compounds, and scalants: Particulate fouling is caused by the deposition of turbidity, colloids, suspended solids, etc. in the RO membrane elements, which increases the required operating pressure and/or reduces membrane 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 1
fouling limits membrane flux and can necessitate an increase in operating pressure. Organic fouling can be irreversible. Scaling on the membrane surface is caused by the precipitation of sparingly soluble salts or the deposition of amorphous solids [e.g., silica (SiO2) 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. Conventional pretreatment for RO (i.e., coagulation and filtration) is a mature technology and has been proven effective and used almost exclusively by large seawater desal plants with surface water intakes. Non-conventional pretreatment systems, including microfiltration (MF) and ultrafiltration (UF), can produce a superior quality of water compared to conventional pretreatment processes. These systems are beginning to gain more popularity. High-quality pretreatment will be of greater importance with the advent of high-flux (high-permeability) 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. Source water from subsurface intakes typically requires minimal pretreatment compared to open intake systems. STRATEGIES The following sections outline strategies and technologies available for assessing and preventing/controlling the different types of membrane fouling identified above. Pretreatment processes for facilities with subsurface intakes are also discussed. Particulate fouling The potential for particulate fouling is best gauged through water quality analyses and careful source water monitoring (e.g., turbidity, particle counts, silt density index testing, etc). Conventional treatment. The prevention of particulate fouling involves varying degrees of prefiltration. Conventional pretreatment, including coagulation and sedimentation followed by media filtration, is the most commonly used method for the treatment of open intake feedwater (for seawater facilities). Existing seawater desal plants, such as Tampa Bay, Perth, and Ashkelon, 2
almost exclusively employ conventional pre-treatment processes. Figure 1 provides a simple schematic of a conventional RO treatment system. Seawater Intake Coagulation/ Filtration Cartridge Filter RO Figure 1. A conventional RO pretreatment system Conventional pretreatment typically includes the addition of chemicals such as ferric chloride or polyelectrolytes to enhance the coagulation of suspended solids prior to settling and filtration. 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 Microfiltration (MF) and ultrafiltration (UF) membranes are increasingly being used to treat feedwater with more challenging prefiltration needs. MF membranes are used to reduce turbidity and remove suspended particles, algae, and bacteria. UF membranes are used for removal of contaminants that affect color, high-weight dissolved organic compounds, bacteria, and some viruses (NRC 2008). With both types of membranes water is pushed (or pulled) through the membrane at very low pressures. Particles larger than the membrane pore size (0.03-10 μm for MF and 0.002-0.1 μm for UF) are removed. The benefits of MF/UF pretreatment compared to conventional pretreatment technologies include: Production of feedwater to the RO system of constant and high quality regardless of source water fluctuations; Reduced RO fouling, which results in less cleaning and longer membrane life; Smaller footprint; and Lower consumption of chemicals. Potential disadvantages include higher costs and negative environmental impacts of concentrate from these membranes. 3
Biofouling The propensity for biofouling is not necessarily straightforward. For example, the use of RO to desalinate 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). The occurrence of biofouling may be detected by a comparative assessment of the feed and concentrate streams. Increased biological activity in the concentrate may indicate a potential problem. Biofouling control includes the use of standard oxidants such as chlorine or hypochlorite. However, the membranes commonly used in RO desal have a low tolerance for these oxidants. Chlorine needs to be removed in pretreatment by addition of a reducing agent such as sodium bisulfite. Sodium bisulfite and copper sulfate can also be used as biocides in membrane systems. Higher chlorine tolerance is one characteristic of membranes made from cellulose acetate material and can be considered for applications with significant biological fouling potential. Caveat: these membranes must be operated within a relatively narrow ph range, and are subject to microbial degradation). Ultraviolet (UV) and ozone treatment are being considered potential replacements for chlorinebased biological growth control. Both UV and ozone have merit and have been successfully used in small to midsized drinking water and water reuse applications. UV will not cause problems with oxidant-sensitive membranes. Ozone is a much more effective disinfectant, but it poses a problem to the oxidant-sensitive RO membranes (Cotruvo 2005; Glater et al. 1983). Organic fouling Total organic carbon (TOC) is the primary indicator of the propensity for organic fouling. TOC may not necessarily correlate with fouling potential in all cases due to the range of potential organic compounds that can be found in feedwater, including 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. Processes known to be effective for removing TOC such as conventional treatment or granulated activated carbon (GAC) can be used as pretreatment. This can add a significant cost to the overall desal process. Scaling The potential for scaling is a function of the solubility of the constituent ions in the feed water. Higher concentrations of sparingly soluble salts will increase the potential for scaling. As a 4
result, the higher the percent recovery of the desal process, the higher the TDS concentration of the concentrate, and thus, the greater the potential for scaling The potential for scaling is typically evaluated using any one of the proprietary modeling programs available from 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. To prevent scaling, a range of options exist (see Table 1): The most economical option is to limit the recovery of the system, which reduces the levels of scale forming ions in the concentrate. This option is typically 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. The most expensive options include (1) Combining oxidizing scale forming species and then pre-filtering the precipitates prior to RO, or (2) Ion exchange (IX). The most common method is to add acid (e.g.sulfiuric acid H2SO4) to lower the ph, thus increasing the solubility of sparingly soluble salts, or to add any one of the numerous proprietary chemicals designed to interfere with the precipitation process (e.g. antiscalants, sequestering agents, and dispersants). Table 1. Pretreatment options for scale prevention Method ph reduction (acid) Anti-scalants Sequestrants Dispersants Oxidation / prefiltration Limiting recovery Ion exchange Function Increase solubility Interfere with crystal growth Ion binding / prevent precipitation Maintain scalants in suspension Eliminate scalants upstream Reduce scaling ion concentrations Selectively remove scaling ions Click here to return to top of page 5
Pretreatment of subsurface intake water Facilities with subsurface intakes often require minimum pretreatment due to reduced suspended solids, turbidity and SDI in the source water (compared to open intake systems). Typical treatment systems may include simple cartridge filtration, microfiltration (MF) or ultrafiltration (UF), with ph adjustment and antiscalant addition. Feedwater from subsurface intakes can contain elevated concentrations of dissolved iron and manganese which can complicate the selection of pretreatment processes. Dissolved iron and manganese can cause RO membrane scaling and permanently damage membranes, particularly if the water is exposed to any oxidizer, such as air. A common approach for the removal of iron and manganese particles is oxidation followed by granular media filtration. Membrane technology using MF/UF is also capable of providing consistent water quality but has higher capital and O&M costs than conventional greensand filtration (MWDOC 2007). BENEFITS & COSTS Pretreatment can constitute a significant component of the overall cost of a desal project, both in terms of capital and operations. 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. Click here to return to top of page KEY UNCERTAINTIES Pretreatment process design often relies on average source water quality, taking into account the variation over several years. However, unexpected events including factors, such as oil contamination and the red tide event that occurred in California during the summer of 2005, can challenge and cause a complete failure of the desal process. 6
Some believe that media filtration can be designed to endure challenging water quality events. Such a design was implemented and performed successfully at the Point Lisas SWRO plant in Trinidad and Tobago (Trussell et al. 2004, Irwin and Thompson 2003). The pretreatment system consists of coagulant mixing, clarification, single-stage media filtration and cartridge filters. However, this treatment train has not been challenged by such a red tide event to validate the design. Click here to return to top of page ADDITIONAL RESOURCES Allam, J., G. K. Pearce, and K. Chida. 2003. Ultrafiltration to RO: Trials at Kindasa Water Services, Jeddah, Saudi Arabia. Paper presented at the 2003 IDA Conference in Bahamas. Cotruvo, J. A. 2005. Water desalination processes and associated health and environmental issues. Water Conditioning and Purification, January:13-17. Galloway, M., and J. Mahoney. 2004. Ultrafiltration for seawater reverse osmosis pretreatment. Membrane Technology 2004(1):5-8. Glater, J., M. R. Zachariah, S. B. McCray, and J. W. McCutchan. 1983. Reverse osmosis membrane sensitivity to ozone and halogen disinfectants. Desalination 48:1-16. Irwin, K.J. and J.D. Thompson. 2003. Point Lisas SWRO Plant Trinidad First year success. International Desalination & Water Reuse Quarterly, 13(3):12-16. Latorre, M. 2001. Surface open intake SWRO pilot plant with MF pretreatment. Presentation at 2001 IDA World Congress, Bahrain. MWDOC (Municipal Water District of Orange County). 2007. Final Draft of Feasibility Report: Dana Point Ocean Desalination Project. March 2007. Available: http://www.mwdoc.com/documents/finaldraftreport4-6-07.pdf. [Cited July 1, 2007]. NRC (National Research Council), Committee on Advancing Desalination Technology. 2008. Desalination: A National Perspective. Washington, D.C.: National Academy Press. Available: <http://www.nap.edu/catalog/12184.html>. [cited August 12, 2008]. Pearce, G. K. 2007. The case for UF/MF pretreatment to RO in seawater applications. Desalination 203:286-295. 7
Taniguchi, Y., K. Ohta, T. Okabe, M. Hirai, and T. Goto. 1995. Pretreatment with membranes for sea water reverse osmosis process. Proceedings of the International Desalination Association 2:135-146. Trussell, R.R., J. Jacangelo and R. Cass. 2004. Design and performance of the pretreatment for the Point Lisas Desalter. In Proc. of the 2004 AWWA Annual Conference. Orlando, FL.: Denver, Colo.: AWWA Zhang, J., S. Gao, H. Zeng, F. Zhang, C. Li, Y. Liu, D. Fu, and C. Ye. 2006. Pilot testing of two inside-out UF modules prior to RO for highturbidity seawater desalination. Desalination 196:66-75. Click here to return to top of page 8