BACKGROUND: Previous research on a biotrickling filter for the removal of high loads of H2S showed that accumulation of elemental sulfur (S-0) when dealing with high H2S concentrations could lead to reactor clogging. Since S-0 can also serve as substrate for sulfur-oxidising bacteria, this study investigates the biological oxidation of S-0 as a remediation strategy. RESULTS: Results indicated that biological oxidation of S-0 inside a clogged biotrickling filter occurred at a comparable rate to those reported for stirred tank reactors. When biologically produced dried and powdered S-0 was manually added as a substrate in stirred tank reactor experiments, significantly lower S-0 oxidation rates were found compared to those for biological S-0 freshly produced in situ. It was speculated that either the powdered S-0 particle size or the surface properties hindered S-0 bioavailability even in a well-stirred environment. Respirometric experiments with the same powdered S-0 and acetone-dissolved S-0 confirmed that biological oxidation of S-0 was basically limited by the solid S-0 bioavailability. CONCLUSIONS: Therefore, results showed that S-0 oxidation basically depends on S-0 bioavailability and that Sulfate volumetric production rates as high as 3.48 mmol SO42- h(-1) L-1 can be achieved inside a clogged biotrickling filter probably due to the high biomass retention capacity. Overall, the results indicate that biological oxidation of S-0 can be considered a suitable strategy for unclogging bioreactors clogged with S-0. (C) 2010 Society of Chemical Industry