GAC ADSORBER DESIGN FOR REMOVAL OF CHLORINATED PESTICIDES

The cost-effective removal of alachlor and heptachlor from drinking water by GAC adsorption was investigated. Alachlor and heptachlor represent two classes of chlorinated pesticides: the acetanilides and the cyclodienes, respectively. Transformation of these pesticides prior to and upon adsorption is an important aspect concerning their fate. In aqueous solution, heptachlor undergoes complete hydrolysis to 1-hydroxychlordene (1-HC) with a half-life of three days decreeing the use of 1-HC in GAC investigation. The dispersed-flow homogeneous-surface diffusion model (DFHSDM) was used for predicting the dynamic behavior of the fixed-bed adsorber for the target pollutants. Model parameters were obtained from bench-scale equilibrium, rate, and mini-column experiments. The model was able to successfully predict/simulate mini-column data in both organic free water and humic acid background solution. Significant reduction in adsorber performance was experienced in the presence of humic acid due to competitive interaction. Full-scale adsorbers were designed through up-scaling procedures to estimate carbon utilization rates. Cost for removal of alachlor was $0.113/1,000 gal ($0.03/m3) treated in a 10-mgd (38 x 10(3) m3/d) plant with 5-min EBCT, and 4 gpm/sq ft (235 m3/m2/d) surface loading for an influent concentration of 100-mu-g/L and treatment objective of 2-mu-g/L. The cost was increased to $0.135/1,000 gal in the presence of humic acid. The cost for removal of 1-HC was, in general, 30% higher than that for alachlor.