Detoxification of wastewater containing five common pesticides by solar AOPs–biological coupled system

A mixture of five pesticides commonly used in intensive agriculture in the southeast of Spain, Methomyl, Dimethoate, Oxamyl, Cymoxanil and Pyrimethanil, has been completely mineralized in a combined solar photocatalytic–biological pilot plant. Two advanced oxidation processes (AOPs: TiO2 and photo-Fenton) were employed for enhancing the biodegradability of wastewater and an aerobic immobilised biomass reactor (IBR) was used for the following continuous biological treatment. TiO2 photocatalysis experiments were performed in a 35-L solar pilot plant made up of three compound parabolic collectors (CPCs), whereas photo-Fenton tests were carried out in a 75-L solar pilot plant with four CPCs units. The initial pesticide concentrations in the mixture were 50 mg L−1 each. The TiO2 catalyst concentration employed was 200 mg L−1, and two different Fe2+ concentrations, 20 mg L−1 and 55 mg L−1, were used in the photo-Fenton tests. Toxicity (Vibrio fischeri) and biodegradability assays (Zahn-Wellens test) were also performed to monitor toxicity and biodegradability of samples at different stages of photo-Fenton treatment. Biodegradable compounds generated during the preliminary oxidative process were mineralized in a 60-L activated sludge biological reactor filled with 30 L of propylene Pall Ring supports. Total disappearance of the parent compounds, more than 90% mineralization and complete nitrification were achieved by the combined system.