DDT degradation by bacteria from activated sludge

The degradation of DDT in aqueous phase was studied by treating it with microflora present in activated sludge developed in bench-scale reactors and in vitro by incubating it with mixed liquor from the activated sludge and with the bacteria obtained after enrichment of the mixed liquor culture through repeated transfers with increasing doses of DDT in standard inorganic medium. It was observed that several strains of bacterial isolates as well as the mixed culture were able to degrade DDT with hardly any formation of the stable metabolites DDD or DDE. The activated sludge degraded DDT (1 to 55 mg/L) almost completely within 23 h, as only some of the effluent samples contained DDTs up to 0.03 mg/L. The bacteria belonged to the genera Bacillus, Micrococcus, Pseudomonas, and Flavobacterium. The mixed liquor and individual bacteria, when incubated with DDT in a mineral medium for 42 days under culture conditions which were static, brought about 50% to 74% and 44% to 69% degradation of DDT respectively. Under shaked conditions, these bacteria, when incubated up to 1 month with 50,200 and 500 mg/L DDT, showed 5.3% to 76.6% degradation of DDT, respectively. Traces of DDE and DDD were detected in a few samples. The microorganisms were able to utilise DDT as a source of carbon and hence can be used for biotreatment of DDT.