HAA removal by GAC adsorption

AND YUEFENG F. XIE aloacetic acids (HAAs) and trihalomethanes (THMs) are the major disinfection by-products (DBPs) found in chlorinated drinking water. Nine species of HAAs are commonly found in chlorinated water: monochloroacetic acid (ClAA), monobromoacetic acid (BrAA), dichloroacetic acid (Cl2AA), bromochloroacetic acid (BrClAA), dibromoacetic acid (Br2AA), trichloroacetic acid (Cl3AA), bromodichloroacetic acid (BrCl2AA), chlorodibromoacetic acid (ClBr2AA), and tribromoacetic acid (Br3AA). Five HAAs (HAA5)—ClAA, BrAA, Cl2AA, Br2AA, and Cl3AA—are regulated under the Stage 1 Disinfectants/Disinfection Byproducts Rule (D/DBPR) with a maximum contaminant level of 60 μg/L (USEPA, 1998). Four species of THMs (THM4)—chloroform (CHCl3), bromodichloromethane (CHBrCl2), chlorodibromomethane (CHBr2Cl), and bromoform (CHBr3) are also regulated under the D/DBPR at 80 μg/L. The best available technologies for DBP control are enhanced coagulation/softening and granular activated carbon (GAC) adsorption for precursor removal (USEPA, 1998). In an adsorption isotherm study for THMs, the Freundlich constants (Ks) for CHCl3, CHCl2Br, CHBr2Cl, and CHBr3 were 92.5, 241.0, 585.0, and 929.0 (μg/g)(L/μg)1/n, respectively (Speth & Miltner, 1990). Preformed THMs can be removed using GAC adsorption, but this process requires frequent GAC regeneration and is not economically feasible (Lykins et al, 1988). Adsorption isotherm studies for HAAs have also been conducted (Liu & Andrews, 2001; Speth & Miltner, 1998). The diverse experimental conditions found in these studies make it difficult to use these adsorption data to assess the capacity of GAC for HAA removal in field applications. In a laboratory column study, Xie and Zhou (2002) observed that complete carbon saturation occurred in approximately one month for Cl2AA. Another study (Xie et al, 2004) reported that the bed lives (for 50% carbon saturation) for HAAs in a GAC pilot-filter study were 8 and 26 days for Cl2AA and Cl3AA, respectively. These column/filter study results suggested that the GAC adsorption capacities for some HAAs were much lower than those for THMs. The objective of this study was to investigate the role of GAC adsorption in the early stage of the filter run, especially with respect to the overall perforH