Removal, transformation and ecological risk assessment of pesticide in rural wastewater by field-scale horizontal flow constructed wetlands of treated effluent

Constructed wetlands (CWs) are widely used in sewage treatment in rural areas, but there are only a few studies on field -scale CWs in treating wastewater -borne pesticides. In this study, the treatment and metabolic transformation of 29 pesticides in rural domestic sewage by 10 field -scale horizontal flow CWs (HF-CWs), each with a treatment scale of 36-5000 m(3) /d and operated for 2-10 years, in Guangzhou, Southern China was investigated. The risk of pesticides in treated effluent and main factors influencing such risk were evaluated. Results demonstrated that HF-CWs could remove pesticides in sewage and reduce their ecological risk in effluent, but the degree varied among types of pesticides. Herbicides had the highest mean removal rate (67.35 %) followed by insecticides (60.13 %), and the least was fungicides (53.22 %). In terms of single pesticide compounds, the mean removal rate of butachlor was the highest (73.32 %), then acetochlor (69.41 %), atrazine (68.28 %), metolachlor (58.40 %), and oxadixyl (53.28 %). The overall removal rates of targeted pesticides in each HF-CWs ranged from 11 %-57 %, excluding two HF-CWs showing increases in pesticides in treated effluent. Residues of malathion, phorate, and endosulfan in effluent had high -risks (RQ > 5). The pesticide concentration in effluent was mainly affected by that in influent ( P = 0.042), and source control was the key to reducing risk. The main metabolic pathways of pesticide in HF-CWs were oxidation, with hydroxyl group to carbonyl group or to form sulfones, the second pathways by hydrolysis, aerobic condition was conducive to the transformation of pesticides. Sulfones were generally more toxic than the metabolites produced by hydrolytic pathways. The present study provides a reference on pesticides for the purification performance improvement, long-term maintenance, and practical sustainable application of field -scale HF-CWs.