In this study, a promising system consisting of up-flow anaerobic sludge blanket (UASB) reactor followed by down-flow hanging sponge (DHS) reactor was investigated for onion dehydration wastewater treatment. Laboratory experiments were conducted at two different phases, i.e., phase (1) at overall hydraulic retention time (HRT) of 11 h (UASB reactor: 611 and DHS reactor: 5 h) and phase (2) at overall HRT of 9.4 h (UASB reactor: 5.2 h and OHS reactor: 4.2 h). Long-term operation results of the proposed system showed that its overall TCOD, TBOD, TSS, TKN and NH(4)-N removal efficiencies were 92 +/- 5, 95 +/- 2, 95 +/- 2, 72 +/- 6 and 99 +/- 1.3%, respectively (phase 1). Corresponding values for the 2nd phase were 85.4 +/- 5, 86 +/- 3, 87 +/- 6, 65 +/- 8 and 95 +/- 2.8%. Based on the available results, the proposed system could be more viable option for treatment of wastewater generated from onion dehydration industry in regions with tropical or sub-tropical climates and with stringent discharge standards. (C) 2011 Elsevier Ltd. All rights reserved.