Methanol as electron donor for thermophilic biological sulfate and sulfite reduction

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 ₃^2- .L ^-1 .day ^-1 (100% elimination) and 4-7 g SO ₄^2- .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.At a hydraulic retention time of 3-4 h, maximum reduction rates of 18 g SO ₃^2- .L ^-1 .day ^-1 (100% elimination) and about 12 gS O ₄^2- .L ^-1 .day ^-1 (50% elimination) were attained, equivalent to a sulfidogenic methanol-conversion rate of 19 g COD.L ^-1 .day ^-1 . At this HRT, the sulfate reduction rate was limited by the biomass concentration of 9 to 10 g VSS.L ^-1 that maximally was retained in the reactor. The time needed to reach maximum process performance amounted to 40-60 days. From one of the reactors a thermophilic sulfate reducing bacterium, Desulfotomaculum strain WW1 was isolated, that probably represented the most abundant sulfate reducer. In the reactor, strain WW1 is not confined to the use of methanol, as it also grows on methanol degradation products like acetate, formate and H ₂ /CO ₂ . The presence of high numbers of methanol-oxidizing, hydrogen-producing bacteria in the sludge indicated that hydrogen may represent an important electron donor for sulfate reduction in the sludge. In the cultures in which the presence of these species was demonstrated, the formation of acetate (about 15% of the methanol degraded) seemed to be strictly coupled to growth of the methanol-oxidizing species. This might explain the coupling of sulfide and acetate formation from methanol in the reactors. Methanol was not a suitable electron donor for mesophilic (30 °C) sulfate reduction, relevant for bio-desulfurization of cold or slightly heated ground- or wastewater. Under mesophilic conditions, methanol was primarily degraded to methane