High rate biological treatment of sulfate-rich wastewater in an acetate-fed EGSB reactor

An expanded granular sludge bed reactor, inoculated with acclimated sulfidogenic granular sludge, was operated at 33 degrees C and fed with acetic acid as COD source and sulfate as electron acceptor. The bioreactor had a sulfate conversion efficiency of 80-90% at a high sulfate loading rate of 10.4 g SO42--S/l.d after only 60 days of start-up. This was achieved by implementing a dual operational strategy. Firstly acetic acid was dosed near stoichiometry (COD over sulfur ratio = 2.0 to 2.2) which allowed almost complete sulfate removal. Secondly the pH in the bioreactor was kept slightly alkaline (7.9 +/- 0.1) which limited the concentration of the inhibitory undissociated hydrogen sulfide H2S (pK(a) = 7). This allowed the acetotrophic sulfate reducing bacteria to predominate throughout the long term experiment. The limitations of the EGSB technology with respect to the sulfate conversion rate appeared to be related to the biomass wash-out and granule deterioration occurring at superficial upflow velocities above 10 m/h. Increasing the recirculation flow caused a drop in the sulfate reduction rate and efficiency, an increase of the suspended sludge fraction and a considerable loss of biomass into the effluent, yielding bare mainly inorganic granules. Elemental analysis revealed that a considerable amount of the granular sludge dry matter at the end of the experiment, at an upflow velocity of 20 m/h, consisted of calcium (32%), mainly in the form of carbonate deposits, while organic matter only represented 7%.