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Analysis M ost public water suppliers are interested in findings ways to comply with existing regulation on disinfection byproducts (DBPs) at minimal treatment cost. The Stage-2 DBP Rule is pushing water suppliers to find alternatives for maintaining the level of trihalomethanes (THMs) and haloacetic acids (HAAs) below regulatory standards. Several water suppliers preferred to switch to chloramine for secondary disinfection. Although chloramine is a less powerful disinfectant than chlorine, it is adequate to inhibit microbial growth in finished drinking water. Water suppliers love the chloramine approach because it helps them keep the level of THMs and HAAs below regulatory limits and creates low temporal-spatial variability in the distribution system (Parvez et al., 2011). This makes more economical sense to water suppliers than treating water with high levels of THMs and HAAs. However, chloramine is blamed for producing toxic nitrosamines and extracting lead from old distribution pipes. This is particularly true for water systems with higher pH and lower redox potentials. Some of these problems can be controlled by frequent flushing of the system, minimizing the use of free chlorine, and reducing the water retention time, but they cannot be eradicated. Also, chloramine does not prevent DBP formation in the preceding disinfection stage (primary disinfection) where chlorine reacts with organics (natural organic matters) to form high concentrations of THMs and HAAs. To eliminate or minimize DBP formation, water engi- neers need to focus on two fundamental things: First, find ways to incorporate technology for the removal of organics; and second, identify a good alternative disin- fectant for chlorine. Fortunately, there are technology and alternative disinfectants available to address both issues. However, it is important to note that no single, stand-alone technology or disinfectant can prevent DBP formation. The important key is to use them in the best possible combination to meet the individual water supplier's treatment needs. Let me discuss some of the important treatment options and disinfectants that can be useful to minimize DBP formation. Options For Organics Removal The removal of precursor organics before disinfection prevents DBP formation. Enhanced coagulation (EC), powdered activated carbon (PAC), and granulated acti- vated carbon (GAC) are effective for removing organic precursors. EC is quite affordable and commonly used by water suppliers. However, this has low organic removal efficiency (45 to 50 percent) and works best when remov- ing negatively-charged, large-molecular-weight organics. Therefore, most water suppliers prefer PAC to boost removal in wet seasons when high organic load is expected in raw water. PAC in general has better removal efficiency than EC, but it works best for low-molecular- weight and uncharged organics. A combined EC and PAC approach works best for maximizing removal efficiency (75 to 80 percent) for both high- and low-molecular- weight organics (Kristiana et al., 2011). To achieve superior organic removal efficiency, GAC filters work best and are preferred by water suppliers with high organic load in raw water. Iodine and molasses num- bers are typically used to characterize GAC filters. These numbers describe the quantity of small- and large-pore volumes in a sample of GAC. For GAC filters, a minimum iodine number of 500 is recommended by the American Water Works Association (AWWA). The granular volume of the filter allows higher flow rates and provides good molec- ular adsorption for a range of contaminants. It also improves the taste and odor of drinking water by removing chlorine. However, high installation, operation, and maintenance costs make GAC unaffordable to many water suppliers. It is also susceptible to biological growth, which helps to degrade organics and other biodegradable compounds, but reduces the performance of the filter. To control biological growth, frequent backwashing (once a week) is required. Disinfection is recommended after GAC adsorption of organics to prevent biological growth in the distribution system. In addition to the technologies discussed above, other treatment technologies include ion exchange and nanofiltration. However, engineers 29 Disinfection Byproducts: Treatment Options And Challenges For Public Water Suppliers R e g u l a t e d a n d e m e r g i n g d i s i n f e c t i o n b y p r o d u c t s ( D B P s ) p o s e s e v e r a l c h a l l e n g e s f o r w a t e r s u p p l i e r s , b u t t h e r e i s a v a r i e t y o f Regulated and emerging disinfection byproducts (DBPs) pose several challenges for water suppliers, but there is a variety of c o s t - e f f e c t i v e c u r e s . cost-effective cures. By Shahid Parvez wateronline.com ■ Water Online The Magazine 2 9 _ V E R T _ 0 5 1 4 C l e a n w a t e r _ I n d i a n a _ D G . i n d d 1 29_VERT_0514 Cleanwater_Indiana_DG.indd 1 4 / 2 2 / 2 0 1 4 9 : 3 3 : 4 2 A M 4/22/2014 9:33:42 AM