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Pilot Study How To Best Reduce DBPs: A Comparison Of Centralized And Decentralized Treatment Air stripping and granulated activated carbon were applied at different points in the distribution system to evaluate effective removal of disinfection byproducts (DBPs). By Chandra Mysore, Ph.D., James Fletcher, Bill Roberts, and Mark Xerxis B ackground and objectives — Stage 2 of the eral major tasks: Disinfectants and Disinfection Byproduct Rule (D/DBPR) — require total trihalomethanes • Conduct pilot-scale studies to compare the cen(TTHMs) and haloacetic acids (HAAs) to be tralized (GAC) and decentralized (air stripping or below 80 parts per billion (ppb) and 60 ppb, respectively, GAC) approach at each monitoring location in the distribution system. As • Bench-scale studies to determine DBP reformation an alternative to treating the entire flow at a centralized potential after decentralized treatment facility, many utilities are considering treating only a partial • Develop lifecycle costs for centralized and decenflow in the distribution system to be in compliance with tralized treatment. the Stage 2 D/DBPR requirements. The City of Scottsdale uses Central Arizona Project (CAP) as the source water Air stripping pilot studies: and uses granular activated carbon (GAC) treatment for The decentralized treatment focused on conducting air reduction of DBPs. Over the years, the cost of this centralstripping pilot studies with units supplied by various ized GAC treatment (treating the entire flow ~ 40 MGD) manufacturers. These studies were conducted in the has increased and water distribution is not effective at system at the reducing TTHM Desert Mountain levels at distant location (Site locations within the 92B) that has water distribution historically system. Localized experienced or decentralized higher TTHM treatment at the levels. Each unit point of non-comwas operated at pliance is a costair:water ratios effective option, of 30:1, 40:1, as only the flow 60:1, and 120:1. that is necessary is Several scenarios treated, to be in were tested that compliance with included blending the Stage 2 regulaof treated Figure 1: TTHM removal percentages for the two air stripping units tions. The focus of and untreated this project was to water in treated compare and contrast the merits and demerits of centralwater:untreated water ratios of 25:75, 50:50, and ized versus decentralized treatment for the reduction of 75:25. Water quality data collected included pH, DBPs through bench- and pilot-scale studies. temperature, total organic carbon (TOC), alkalinity, The objective of the project was to identify the most chlorine residual, bromide, TTHMs, and HAA5s (the reliable and cost-effective treatment to meet the requiresum of five HAAs: monochloroacetic, dichloroacetic, ments of Stage 2 of the D/DBPR. trichloroacetic, monobromoacetic, and dibromoacetic acids). Figure 1 shows the results from the two air Approach And Results stripping units. At an air:water ratio of 30:1, a TTHM To meet the objectives, the approach consisted of sevremoval of 68 percent was observed. The effect on 18 wateronline.com ■ Water Online The Magazine