The main objective of this study is to assess the achievability of stringent discharge criteria i.e. BOD5 < 15 mg/L, TSS < 15 mg/L and NH4-N < 1 mg/L during the treatment of tomato processing wastewater with COD of 2800-15,500 mg/L, BOD5 of 1750-7950 mg/L, TKN of 48-340 mg/L and NH4-N of 21-235 mg/L. Two treatment systems, a UASB-aerobic system and a UASB-anoxic-aerobic system were tested. Furthermore due to alkalinity deficiency, in the raw wastewater, the study explored varying UASB effluent recirculation flowrates to the UASB influent to reduce additional alkalinity requirements. The UASB-anoxic-aerobic system was effective in treating tomato canning wastewater at an overall HRT of 1.75 days while achieving 98.5% BOD5, 95.6% COD, 84% TSS and 99.5% NH4-N removal producing effluent BOD5, COD, TSS, NH4-N, TKN, NO2-N, NO3-N and PO4-P of 10, 70,15,0.5,3,0,60 and 4 mg/L, respectively. The biogas yield was 0.43 m(3)/kg COD removed. (c) 2005 Elsevier Ltd. All rights reserved.