Profiles and removal efficiency of organochlorine pesticides with emphasis on DDTs and HCHs by two different sewage treatment works

Abstract The removal efficiencies of nine organochlorine pesticides (OCPs) were investigated in two sewage treatment works (STWs; Stonecutter Island [SCI] and Shatin [ST]). The levels of OCPs in sewage samples collected from SCI (chemically enhanced primary treatment) and ST (secondary treatment) were determined with gas chromatography–mass spectrometry. The crude sewage in the two STWs generally had low levels of total hexachlorocyclohexane (HCHs; total of alpha-, beta- and gamma-HCH; SCI, 19.4 ± 4.62 ng/L; ST, 12.1 ± 4.27 ng/L) and total dichloro-diphenyl-trichloroethane (DDTs; total of p,p’ and o,p’ -DDE, -DDD and -DDT; SCI, 6.31 ± 2.83 ng/L; ST, 6.09 ± 1.67 ng/L). High total removal efficiencies for total HCHs (SCI, 79.9 ± 7.03%; ST, 82.5 ± 2.51%) and total DDTs (SCI, 96.1% ± 3.37%; ST, 99 ± 0.501%) were observed. In particular, ST achieved outstanding performance in the removal of heptachlor (100% ± 0.00%), heptachlor epoxide (94.8 ± 3.96%), hexachlorobenzene (99.7 ± 0.387%), gamma-HCH (100 ± 0.00%) and total DDTs (99.2 ± 0.644%). There was no observable correlation between the removal efficiencies and the log K o w values of individual OCPs, which suggests that the removal mechanisms may not be related to sorption by particulate matter. Removal of OCPs seems to rely more on other mechanisms such as volatilisation, advection and biotransformation. This study is the first to investigate the fate and distribution of OCPs throughout the entire sewage treatment process. The results provide valuable reference for potential modification of sewage treatment processes with regard to OCPs.