Biofilm configured system with sequencing/periodic discontinuous batch mode operation was studied in anaerobic metabolic function for the treatment of composite chemical wastewater (low-biodegradable [low BOD/COD ratio similar to 0.31] and sulfate content, 1.75 g/l). Inert stone chips were used as biofilm supporting material. Reactor was operated with a total cycle period of 24 It (FILL: 15 min; REACT (aeration with recirculation): 23 h; SETTLE: 30 min; DECANT: 15 min) and the performance of the system was studied at various organic loading rates (OLR) of 1.15 kg COD/cum-day, 2.3 kg COD/cum-day, and 4.76 kg COD/cum-day. Substrate utilization showed a steady increase with increase in OLR and reactor showed sustained performance at higher loading rates also. The reactor yielded effective sulfate reduction with concomitant increase with organic loading rate. Rapid startup and stabilization tendency was evidenced during the operation. Periodically operated biofilm configured reactor showed comparatively effective performance over the corresponding continuously operated biofilm system studied with same wastewater with respect to rapidity in startup and stabilization, higher substrate removal efficiency, effective sulfate reduction, robust performance at higher loading rates and low operating retention time (24 h). Biofilm configured system coupled with periodic discontinuous batch mode operation imposes regular variations in the substrate concentration on biofilm organisms as a result, organisms throughout the film achieve maximum growth rates resulting in improved reaction potential leading to stable and robust system which is well suited for treating highly variable wastes. Periodic discontinuous batch mode operation of the biofilm reactors results in a more even distribution of the biomass throughout the reactor and was able to treat large shock loads than the continuous flow process. (c) 2006 Elsevier Inc. All rights reserved.