The effect of influent substrate concentration on the performance of down-flow anaerobic fixed bed reactors (AFBR) treating piggery wastewater was studied at hydraulic retention times (HRTs) in the range of 1-6 days at tropical temperatures (24.2-30.5 degreesC). Six down-flow anaerobic fixed bed reactors of 6-1 total volume and 5-1 effective volume operated in parallel at influent strengths of 2, 4, 6, 8, 10 and 12 g total COD (TCOD) l(-1). The highest substrate removal efficiencies were obtained in reactors that operated at influent strengths in the range of 4-8 g TCOD l(-1) at HRTs of 1 and 2 days. At higher influent strengths, the efficiency of the process deteriorated. The removal rates of TCOD, soluble chemical oxygen demand (SCOD), biological oxygen demand (BOD) and total suspended solids (TSS) were increased with the influent strength up to 8 g TCOD l(-1), decreasing further for higher influent substrate concentrations. In addition, the concentration of microorganisms within the reactors increased with the influent strength. The results obtained demonstrated that the substrate removal rate was correlated with the effluent substrate concentration through a second-order kinetic model for multicomponent substrate degradation. The values of the kinetic constants obtained increased with the influent strength and were found to be 0.59, 0.83, 0.88, 1.09, 1.17 and 1.26 g TCOD g(-1) volatile suspended solids (VSS) per day for influent strengths of 2, 4, 6, 8, 10 and 12 g TCOD l(-1), respectively, for a probability level of 95 % (P less than or equal to 0.05). (C) 2004 Elsevier Ltd. All rights reserved.