EVALUATION OF ALTERNATIVE TREATMENT METHODS FOR ACHIEVING COMPLIANCE WITH TRIHALOMETHANE WATER QUALITY CRITERIA

The City of Clearwater (FL) owns and operates three wastewater treatment plants (WWTPs) that have surface water discharges. Each facility has had difficulty complying with the annual average permit limits for certain trihalomethane (THM) compounds. The annual average permit limits for bromodichloromethane (BDCM) and chlorodibromomethane (CDBM) are 22 and 34 μg/L, respectively. THMs are by-products of chlorine disinfection utilized at the WWTPs. The primary goal of this study was to evaluate alternative treatment technologies for removing either the THM precursors before disinfection or the THMs themselves from the wastewater treatment plant effluents once they are formed. Granular activated carbon (GAC), powdered activated carbon (PAC), Magnetic Ion Exchange (MIEX) resin, metal salts (alum, ferric chloride, ferric sulfate), and membrane processes (ultrafiltration/nanofiltration) were evaluated for removal of THM precursors before disinfection. Air stripping for removing THMs following disinfection was also evaluated. Alternative disinfection processes that do not form THMs, namely chlorine dioxide and ultraviolet (UV) are also evaluated. The results showed that GAC, MIEX, membranes and UV disinfection processes were viable alternatives. MIEX treatment and UV disinfection were determined to be the two most economically feasible options from the viable alternatives. The capital costs associated with these two alternatives are nearly equivalent, with the MIEX costs slightly lower. However, the annual operation and maintenance costs associated with UV disinfection are considerably lower. Therefore, it is recommended that the City of Clearwater begin planning for the conversion to UV disinfection process which will eliminate the formation of THM compounds and thus allow the City to comply with its permit limits for THMs regardless of what level of surface water quality criteria (SWQC) is eventually established by the Florida Department of Environmental Protection (FDEP).