Performance of a thermophilic sulfate and sulfite reducing high rate anaerobic reactor fed with methanol

Thermophilic sulfate and sulfite reduction was studied in lab-scale Expanded Granular Sludge Bed (EGSB) reactors operated at 65 degrees C and pH 7.5 with methanol as the sole carbon and energy source for the sulfate- and sulfite-reducing bacteria. At a hydraulic retention time (HRT) of 10 h, maximum sulfite and sulfate elimination rates of 5.5 g SO3(2-) L(-1) day(-1) (100% elimination) and 5.7 g SO4(2-) L(-1) day(-1) (55% elimination) were achieved, resulting in an effluent sulfide concentration of approximately 1800 mg S L(-1). Sulfate elimination was limited by the sulfide concentration, as stripping of H2S from the reactor with nitrogen gas was found to increase the sulfate elimination rate to 9.9 g SO4(2-) L(-1) day(-1) (100% elimination). At a HRT of 3 h, maximum achievable sulfite and sulfate elimination rates were even 18 g SO3(2-) L(-1) day(-1) (100% elimination) and 11 g SO4(2-) L(-1) day(-1) (50% elimination). At a HRT of 3 h, the elimination rate was limited by the biomass retention of the system. 5.5 +/- 1.8% of the consumed methanol was converted to acetate, which was not further degraded by sulfate reducing bacteria present in the sludge. The acetotrophic activity of the sludge could not be stimulated by cultivating the sludge for 30 days under methanol-limiting conditions. Omitting cobalt as trace element from the influent resulted in a lower acetate production rate, but it also led to a lower sulfate reduction rate. Sulfate degradation in the reactor could be described by zeroth order kinetics down to a threshold concentration of 0.05 g L(-1), while methanol degradation followed Michaelis-Menten kinetics with a Km of 0.037 g COD L(-1).