Anaerobic sequencing batch reactors (ASBRs) treating slaughterhouse wastewater at 20degreesC were subjected to soluble organic, particulate organic and hydraulic shock loads. The normal organic loading rate (OLR) was increased by a factor ranging from 1.5 to 3.6. During the soluble organic shock load, effluent soluble chemical oxygen demand (SCOD), volatile fatty acid (VFA), and suspended solids (SS) concentrations increased from averages of 200, 58 and 249 mg/l, respectively, in control reactors to maxima of 5626, 1642 and 10 723 mg/l, respectively, in overloaded reactors. However, concentrations were rapidly reduced to pre-shock load levels shortly after normal operating conditions were resumed. The particulate and hydraulic shock loads had small and temporary effects on effluent SCOD and VFA concentrations, but high SS concentrations were observed in the effluent of most overloaded reactors. However, solids loss during effluent discharge did not significantly affect long-term performance of the ASBRs, probably because the sludge bed contained high initial volatile SS (VSS) concentration (21.9 +/- 3.2 g/l), and also a fraction of the VSS removed from the reactors during effluent discharge was composed of undegraded colloidal solids fed with the substrate, as opposed to bacterial flocs. Nevertheless, maintaining high VSS concentrations in the sludge bed will probably prevent long-term negative effect of biomass loss during sudden shock loads. (C) 2004 Published by Elsevier Ltd.