Author Description

Sulfur oxyanions (e.g. sulfate, sulfite) can be removed from aqueous waste- and process streams by biological reduction with a suitable electron donor to sulfide, followed by partial chemical or biological oxidation of sulfide to elemental sulfur. The aim of the research described in this thesis was to make this biological process more broadly applicable for desulfurization of flue-gases and ground- and wastewaters by using the cheap chemical methanol as electron donor for the reduction step. Besides determining the selectivity and rate of reduction of sulfur oxyanions with methanol in bioreactors, also insight was acquired into the microbiology of the process. At pH 7.5 and thermophilic (65 °C) conditions (applicable for flue-gas desulfurization), sulfate-reducing microorganisms ultimately outcompete methanogenic consortia for methanol in anaerobic high-rate bioreactors. Methane formation from methanol was quickly inhibited by imposing slightly acidic pH-values (6.7 instead of 7.5). Acetate represented a side-product from methanol at 65 °C, accounting for up to 13 % of the methanol degraded. The rate of acetate formation was linearly correlated to the rate of sulfate and sulfite reduction with methanol. At a hydraulic retention time (HRT) of 10 h, maximum reduction rates of 6 g SO 32-.L -1.day -1 (100% elimination) and 4-7 g SO 42-.L -1.day -1 (40-70% elimination) were attained simultaneously in the reactors, equivalent to a sulfidogenic methanol-conversion rate of 6-8 g COD. L -1.day -1 (COD:Chemical Oxygen Demand). The resulting sulfide concentration of about 1800 mg S.L -1 (or the H 2 S concentration of 200 mg S.L -1 at pH 7.5) limited the rate of sulfate reduction at HRT=10 h.