HIGH-RATE BIOLOGICAL TREATMENT OF SULFATE-RICH WASTE-WATER IN AN ACETATE-FED EGSB REACTOR

An expanded granular sludge bed reactor, inoculated with acclimated su lfidogenic granular sludge, was operated at 33 degrees C and fed with acetic acid as COD source and sulfate as electron acceptor. The biorea ctor had a sulfate conversion efficiency of 80-90% at a high sulfate l oading 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 ace tic 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 H2 S (pK(a) = 7). This allowed the acetotrophic sulfate reducing bacteria to predominate throughout the long term experiment. The limitations o f the EGSB technology with respect to the sulfate conversion rate appe ared to be related to the biomass wash-out and granule deterioration o ccurring at superficial upflow velocities above 10 m/h. Increasing the recirculation flow caused a drop in the sulfate reduction rate and ef ficiency, an increase of the suspended sludge fraction and a considera ble loss of biomass into the effluent, yielding bare mainly inorganic granules. Elemental analysis revealed that a considerable amount of th e granular sludge dry matter at the end of the experiment, at an upflo w velocity of 20 m/h, consisted of calcium (32%), mainly in the form o f carbonate deposits, while organic matter only represented 7%.