BACKGROUND: This research has assessed the impact of sulfate accumulation on the performance of two biofilters treating CS2. Both biofilters were packed with a pelletized organic fertilizer and operated in downflow mode (Biofilter A) and flow reversal mode (Biofilter B). The performance of the biofilters after an excessive accumulation of sulfate has also been evaluated. RESULTS: The excessive accumulation of sulfate and the reduction in the packing material's moisture content resulted in a decrease in the initial high removal efficiency recorded during the first 50 days (approx.) of operation in both biofilters. A recovery strategy based on mixing and washing the packing material led to a sustained RE of 40 +/- 9% and 62 +/- 11% for Biofilters A and B, respectively. Both biofilters recovered their initial performance after irrigation. A high CO2:EC ratio suggested that secondary oxidation reactions related to heterotrophic bacteria occurred in both bioreactors. The maximum elimination capacity (ECmax) recorded in Biofilters A and B was 11.6 and 16.6 g CS2 m-3 h-1, respectively. CONCLUSIONS: This study confirmed the capacity biofilters treating CS2 have to recover after sulfate accumulation episodes and the importance of optimized irrigation strategies. The benefit of the flow reversal operating mode was highlighted. Copyright (c) 2012 Society of Chemical Industry