Hydrogen sulphide conversion to elemental sulphur in a suspended-growth continuous stirred tank reactor using Chlorobium limicola

A biological process employing green sulphur bacteria to remove sulphide (S2−) from industrial wastewaters and convert it to elemental sulphur was investigated. This research was unique in that dissolved sulphide was present in the liquid influent fed into a continuous-flow photosynthetic bioreactor. A suspended-growth once-through continuous-flow stirred-tank bioreactor was successfully operated under five different experimental conditions. For the first three experiments, concentrated nutrient solution and sulphide stock solution were pumped separately into a 13.7 litre reactor at a hydraulic retention time of 45 h and S2− loading rates of 2.1, 4.4, and 5.6 mg/h•l. At the lowest loading rate, nearly all influent S2− was oxidized to sulphate. The middle loading rate resulted in complete conversion of S2− to elemental sulphur. Steady state conditions were not achieved at the highest loading rate, and S2− accumulated in the bioreactor. In two more experiments, nutrient medium and S2− stock solution were separately fed into a 12.0 litre bioreactor at S2− loading rates of 3.2 and 2.7 mg/h•l and hydraulic retention times of 173 and 99 h respectively. In these trials, the loading rates were adjusted to maintain a residual of 20 to 30 mg S2−/h•l in the bioreactor, and consequently, there was nearly complete conversion of the consumed S2− to elemental sulphur. A parameter was developed to relate the results from these experiments to those reported in the literature, where smaller reactors and higher bacterial concentrations were used in batch reactors fed with H2S(g). This parameter described the capacity of the bioreactor to consume S2−, and was calculated as the product of the radiant flux per unit reactor volume and the bacteriochlorophyll concentration.