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 mu W 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 (SO3 center dot-) 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 (BOD5)/(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. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.