Sulfate disinfection, stabilisation and heavy metal removal from sludge (Sulfate Digestion process) is a three-stage process which relies on the different oxidation states of the sulfur in order to remove heavy metals and destroy pathogens in sludge. The main focus of this project was on the hybrid H2S Generator which consisted of an anaerobic continuously stirred tank reactor (CSTR) fed primary sludge as a carbon source and sulfate as an electron acceptor to stimulate H2S production. The amount of sulfate required to re-circulate around the Sulfate Digestion process was calculated at 5 g/1 for a primary sludge concentration of 2.6 g/1. This was equivalent to a Chemical Oxygen Demand:Sulfate (C0D:S04) ratio of approximately 5:1. At this ratio, the results showed that using sulfate as a salt had no detrimental effects on total COD (TCOD) destruction of the primary sludge; however, the total solids (TS) concentration in the effluent was significantly higher compared to a completely methanogenic reactor. This was accounted for by the higher biomass yield of the Sulfate Reducing Bacteria (SRB) in the hybrid H2S Generator.
The use of sulfuric acid instead of sodium sulfate in the range of 1.47-4.33 g/1 resulted in an increase in both TS and TCOD concentration in the final effluent with decreasing C0D:S04 ratio’s in the H2S Generators. This was shown to be as a result of a decrease in the rate of sludge particle hydrolysis, possibly due to the release of a fermentative and/or acidogenic bacteria (AcB)-specific toxin during sludge acidification with the sulfuric acid, since the final stages of digestion in tenns of methanogenesis and sulfidogenesis were not adversely affected. Kinetic studies on the seed sludge from the hybrid H2S Generators showed that a mixed microbial consortia of acetotrophic SRB (ASRB) and acetotrophic Methane Producing Bacteria (AMPB) were both responsible for acetate degradation at low sulfate concentrations (<120mg/l), while at higher concentrations, ASRB were no longer competitive. SRB were also shown to be the main oxidisers of propionate at all sulfate concentrations, whereas acetogenic bacteria (AB) rather than SRB were the main butyrate and valerate oxidisers.