How To Best Reduce DBPs: A Comparison Of Centralized And Decentralized Treatment

By Chandra Mysore, Ph.D., James Fletcher, Bill Roberts, and Mark Xerxis, GHD Inc.

Air stripping and granulated activated carbon were applied at different points in the distribution system to evaluate effective removal of disinfection byproducts (DBPs).

Background and objectives — Stage 2 of the Disinfectants and Disinfection Byproduct Rule (D/DBPR) — require total trihalomethanes (TTHMs) and haloacetic acids (HAAs) to be below 80 parts per billion (ppb) and 60 ppb, respectively, at each monitoring location in the distribution system. As an alternative to treating the entire flow at a centralized facility, many utilities are considering treating only a partial flow in the distribution system to be in compliance with the Stage 2 D/DBPR requirements. The City of Scottsdale uses Central Arizona Project (CAP) as the source water and uses granular activated carbon (GAC) treatment for reduction of DBPs. Over the years, the cost of this centralized GAC treatment (treating the entire flow ~ 40 MGD) has increased and is not effective at reducing TTHM levels at distant locations within the water distribution system. Localized or decentralized treatment at the point of non-compliance is a costeffective option, as only the flow that is necessary is treated, to be in compliance with the Stage 2 regulations. The focus of this project was to compare and contrast the merits and demerits of centralized versus decentralized treatment for the reduction of DBPs through bench- and pilot-scale studies.