Partial thiosulfate oxidation by steady-state continuous culture in a bioreactor-settler system

The design, characterization, and performance of a bioreactor-settler system that allows the partial biological oxidation of reduced sulfur compounds is described. The design incorporates the physical separation of the aeration from the bioreactor. The reactor has a dynamic flow zone and a static zone. Steady state thiosulfate-oxidizing cultures were established at different O2/S2O32− molar ratios. At O2/S2O32− molar ratios >1.0, sulfate was the main product of the biological oxidation of thiosulfate, while below O2/S2O32− molar ratios of about 1.0, formation of elemental sulfur was observed. The maximum sulfur formation (60%) was obtained at a molar ratio of 0.98. The elemental sulfur settled in the bottom of the bioreactor and was removed daily from the system. A recovery efficiency of 70% was obtained. The settled sulfur–biomass particles consisted of 90% sulfur and 6% biomass and had an average size of 150 μm and a sedimentation rate of 7.50 m h−1. The hydraulic retention time (HRT of 2 min) of the medium in the aerator vessel showed an influence over the system performance due to the oxidation of thiosulfate and intermediates under the high oxygen concentration found in this vessel.  2004 Society of Chemical Industry