Influence of recirculation on the performance of anaerobic sequencing batch biofilm reactor (AnSBBR) was studied in the process of treating hypersaline (total dissolved inorganic solids (TDIS) similar to 26 g/l) and low biodegradable (BOD/COD similar to 0.3) composite chemical wastewater. Significant enhancement in the substrate removal efficiency and biogas yield was observed after introducing the recirculation to the system. Maximum efficiency (COD removal efficiency - 51%; SDR -3.14 kg COD/cum-day) was observed at recirculation to feed (R/F) ratio of 2 (OLR - 6.15 kg COD/cum-day; HLR - 2.30 cum (liquid)/cum day; UFVA - 0.023 m/h). Subsequent increase of RIF to 3 (OLR - 6.15 kg COD/cum-day; HLR - 3.07 cum (liquid)/cum-day; UFVA - 0.035 m/h) resulted in reduction in COD removal efficiency (32%; SDR - 1.97 kg COD/cum-day). The enhanced performance of the system due to the introduction of recirculation was attributed to the improvement in the mass transfer between the substrate present in the bulk liquid and the attached biofilm. The hydrodynamic behavior due to recirculation mode of operation reduced the concentration gradient (substrate inhibition) of substrate and reaction by-products (VFA) resulting in mixed flow conditions. (c) 2006 Elsevier Ltd. All rights reserved.