CONTROL OF S-TRIAZINE HERBICIDES IN A SURFACE WATER TREATMENT PLANT BY GAC FILTER-ADSORBERS

摘要

s-triazine herbicides are applied extensively for weed control in the corn-belt of the mid-west United States and subsequently appear in drinking water supplies. Atrazine and cyanazine are the most frequently detected s-triazine herbicides in the surface water supplies of Missouri. Powdered and granular activated carbon (PAC and GAC) adsorption are designated by the USEPA as the best available technology for removal of s-triazines from drinking water. Higginsville, a rural community in north central Missouri, obtains its drinking water from an impoundment that collects agricultural runoff periodically containing elevated levels of s-triazines. In 1994, ten rural Missouri water systems, including Higginsville Water Treatment Plant (HWTP), received notices of violation for atrazine compliance. The use of PAC at doses of 10 to 30 mg/L was the most common corrective action to regain compliance. In 1996, Higginsville became the first water plant in Missouri to use GAC filter-adsorbers for herbicide control. Two of the four multi-media gravity filters at Higginsville were retrofitted with 24 inches of GAC (Calgon~?F-300) by replacement of the anthracite caps. The resulting empty-bed contact time (EBCT) was about 7.5 minutes and the surface hydraulic loading was 2.1 gpm/ft2. After more than two years service, the atrazine levels of the GAC filter-adsorber effluent were still below the Maximum Contaminant Level (MCL) of 3 μg/L. Therefore, in consideration of multiple water quality benefits, the other two filters containing anthracite caps were also retrofitted with GAC (Calgon~?F-300) in March, 1999. After GAC installation, regular use of PAC in the treatment processes was discontinued. GAC filter-adsorber effluents showed significant reductions of UV_(254) immediately after service begins (approximately 45% for the first two months of service). Then, 15 to 20% removal of UV_(254) was sustained over the long-term. In the first six months after GAC installation, the filter-adsorber effluents had no detection or very low levels of s-triazine herbicides. Aged GAC filter-adsorber effluents had higher s-triazine herbicide levels, but still less than 3 μg/L, and their removal of striazines ranged from 30 to 60%. Over time, the carbon was heavily loaded with calcium