Degradation and COD removal of trichlorophenol from wastewater using sulfite anion radicals in a photochemical process combined with a biological reactor: Mechanisms, degradation pathway, optimization and energy consumption

This study proposes the combination of UV/sulfite with a biological reactor (USPB) for the degradation, dechlorination and mineralization of 2, 4, 6-trichlorophenol (TCP). The effects of pH, sulfite dosage, co-existing water anions and quenchers on the performance of the UV/sulfite process (USP) were investigated. The rate of degradation, dechlorination and mineralization of TCP under optimum experimental conditions of pH, 7.0; [sulfite]/[TCP] = 3.13; UV irradiation, 87 μW cm −2 ; dissolved oxygen (DO), 2 mg L -1 and an 80 min reaction time, were 100, 98 and 30.2%, respectively (initial TCP concentration was 250 mg L -1 and initial COD was 363 mg L -1 ). Investigation of the reaction mechanism revealed the superiority of sulfite radicals (SO 3 -) in the degradation and dechlorination of TCP. The co-existing water anions did not significantly affect the performance of TCP degradation. The intermediates derived from TCP degradation included: 1,3-cyclohexadiene, 2-chloro-1-benzoquinone, 2-chlorophenol, benzene, 2-hydroxy benzoquinone and cyclohexene . The (BOD 5 )/(COD) ratio (the solution biodegradability index) was obtained as 0.57 after 80 min of UV/sulfite reaction time. The effluent obtained from UV/sulfite after 80 min of reaction time (with COD of 253 mg L -1 ) was efficiently post-treated in a batch biological reactor (MLSS = 5000 mg L -1 ), and the COD reduced to 37 mg L -1 after 720 min (12 h) of aeration. Therefore, it was found that the combination of the UV/sulfite with a biological process might be an efficient method for the treatment of wastewater containing high concentrations of toxic compounds.