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 Expande d Granular Sludge Bed (EGSB) reactors operated at 65 degreesC and pH 7.5 wi th methanol as the sole carbon and energy source for the sulfate- and sulfi te-reducing bacteria. At a hydraulic retention time (HRT) of 10 h, maximum sulfite and sulfate elimination rates of 5.5 gSO(3)(2)- L-1 day(-1) (100 % elimination) and 5.7 gSO(4)(2-) L-1 day(-1) (55% elimination) were achieved , resulting in an effluent sulfide concentration of approximately 1800 mgS L-1. Sulfate elimination was limited by the sulfide concentration, as strip ping of H2S from the reactor with nitrogen gas was found to increase the su lfate elimination rate to 9.9 gSO(4)(2-) L-1 day(-1) (100 % elimination). A t a HRT of 3 h, maximum achievable sulfite and sulfate elimination rates we re even 18 gSO(3)(2)- L-1 day(-1) (100% elimination) and 11 gSO(4)(2)- L-1 day(-1) (50% elimination). At a HRT of 3 h, the elimination rate was limite d by the biomass retention of the system. 5.5 +/- 1.8% of the consumed meth anol was converted to acetate, which was not further degraded by sulfate re ducing bacteria present in the sludge. The acetotrophic activity of the slu dge could not be stimulated by cultivating the sludge for 30 days under met hanol-limiting conditions. Omitting cobalt as trace element from the influe nt resulted in a lower acetate production rate, but it also led to a lower sulfate reduction rate. Sulfate degradation in the reactor could be describ ed by zeroth order kinetics down to a threshold concentration of 0.05 g L-1 , while methanol degradation followed Michaelis-Menten kinetics with a K-m of 0.037 gCOD L-1.