[MANAGEMENT & OPERATIONSI Evaluating GAC for Trihalomethane Control Michael J. McGuire, Marshall K. Davis, Carol H. Tate, E. Marco Aieta, Elaine W. Howe, and John C. Crittenden The primary objectives of this study were to optimize granular activated carbon (GAC) treatment to meet more stringent trihalomethane (THM) standards, to estimate the costs of optimized GAC treatment, to identify by-products of the final disinfection of GAGtreated water, and to assess the ability of the rapid small-scale column test (RSSCT) to simulate total organic carbon (TOC) breakthrough from pilot-scale carbon contactors. The lowest THM concentration achievable by means of GAC treatment was found to be 10 pg/L. It was concluded that this type of treatment is expensive for the control of THMs, the predominant by-products of postdisinfection of GAC effluents with chlorine or chloramines. The RSSCT was shown to be an economical means of estimating pilot-scale TOC breakthrough, which can be used in making preliminary estimates of carbon utilization rates and cost estimates. The 1986 amendments to the Safe Drinking Water Act identify granular activated carbon (GAC) as a feasible treatment technique for synthetic or- ganic chemicals.* Trihalomethanes (THMs) may be included among the compounds for which GAC could be considered a feasible treatment tech- nology. The current drinking water standard for total THMs is 0.10 mg/L (100 pg/L). Several recent actions by the US Environmental Protection Agency (USEPA) suggest that this maximum contaminant level (MCL) for THMs will be lower in the future. On Jan. 22,1988, the USEPA issued a final priority list of disinfection by-prod- ucts (DBPs), including THMs.‘ Proposed Figure 1. Metropolitan’ s pilot plant at La Verne, Calif. 38 MANAGEMENT AND OPERATIONS MCLgoals and actual MCLs are expected to be issued in the early 1990s. At the present time, it is not known what these MCLs will be. The USEPA may choose to regulate THMs on an individual basis rather than combined as a total of the four compounds. The MCL could be as low as 5 pg/L and probably not higher than 50 pg/L (for a total of the four THMs). The THM treatment objectives assessed as part of this study were 5,10, 20, and 5Opg/L; thesegoals were selected because they span the range of possible new THM regulations. The purpose of this article is to present the significant findings of a study en- titled “ Optimization and Economic Eval- uation of Granular Activated Carbon for Organic Removal.“ 3 The full report of this study and other related publications are cited throughout this article and should be consulted for important meth- odological details. Objectives The primary objectives of this study, which were directed toward the needs of the Metropolitan Water District of Southern California (MWD) and the water industry as a whole, were l to optimize GAC treatment for meeting lower THM standards, l toestimate thecosts associated with optimized GAC treatment for MWD and for six other water utilities across the United States, l to identify DBPs resulting from the final disinfection of GAC-treated water with chlorine or chloramines, l to assess the ability of a mathemat- ical model or bench-scale column test such as the rapid small-scale column test (RSSCT) to simulate total organic carbon (TOC) breakthrough curves from pilot-scale carbon contactors, and l to develop helpful information and evaluation techniques that can be used A full report of this project (catalog no. 90550) is available from the AWWA Research Foundation, 6666 W. Quincy Ave., Denver, CO 80235. JOURNAL AWWA Copyright (C) 1991 American Water Works Association