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suspended particles are altered to increase disposed of or regenerated either on-or off- <br /> their agglomeration, creating larger flocs,which site. Purchase of GAC can be costly, and GAC <br /> are more readily separated from solution by disposal may be difficult, particularly if the <br /> sedimentation and/or filtration. In drinking spent GAC is considered a hazardous waste <br /> water applications, chemical coagulants such as (McGuire et al, 1989). <br /> aluminum sulfate(alum),ferric chloride,and <br /> ferric or ferrous sulfate are typically added to Membrane Filtration <br /> increase floc formation, increasing <br /> sedimentation rates.This process can be used Membrane processes can remove TOC through <br /> to remove particulate carbon; in addition, filtration and adsorption. Organic molecules <br /> some coagulants react with dissolved organics, <br /> rendering them insoluble. greater in size than the membrane pores are <br /> rejected based on size exclusion. Membranes <br /> In drinking water applications,TOC removal is may also remove TOC through surface <br /> routinely increased through enhanced adsorption, though this mechanism is <br /> coagulation, in which chemical coagulants are undesirable as it has been shown to cause <br /> optimized for organics removal (U.S. irreversible fouling. Microfiltration (MF) and <br /> Environmental Protection Agency, 1999). ultrafiltration (UF) have shown limited efficacy <br /> in TOC removal due to their relatively large <br /> Figure 1 'presents a typical coagulation, <br /> pore size. <br /> flocculation, and sedimentation process. <br /> Coagulant <br /> SEDIMENTATION <br /> BASIN <br /> Treated " o <br /> E <br /> Wastewater - <br /> Effluent <br /> RAPID MIX FLOCCULATION FILTERS <br /> BASIN <br /> Figure 1. Typical Coagulation, Flocculation,Sedimentation Process <br /> Adsorption <br /> Activated carbon may be used to adsorb Treated <br /> soluble organics, including carbon and nitrogen <br /> Wastewater— <br /> compounds. Granular activated carbon (GAC) Effluent <br /> is a well-tested approach for TOC removal, <br /> traditionally used in drinking water <br /> applications. GAC can be used in either a <br /> down-flow or an upflow contactor. GAC GAC <br /> systems typically require pretreatment(such as CONTACTOR <br /> coagulation,filtration, or softening)to prevent <br /> filter clogging. Figure 2 presents a typical GAC Figure 2. Typical GAC Process <br /> adsorption process. <br /> The-GAC adsorption-effectiveness decreases Both reverse osmosis (RO)and nanofiitration <br /> over time, with effluent TOC concentrations (NF) membranes are effective at reducing TOC <br /> increasing over time. Once the effluent concentrations. In addition to TOC, both RO <br /> concentration has reached a threshold limit, and NF membranes reject most contaminants, <br /> the GAC column must be taken offline to allow though precise rejection fractions vary by <br /> the GAC to be replaced. Spent GAC may be membrane manufacturer. However, RO <br /> membranes are typically more effective at <br /> The NEWEA Journal Winter 2010 29 <br />