Municipal wastewater and industrial wastewaters like those effluents from brewery, citric acid production, tannery, pulp and paper industry, and mussel processing contain sulphate ranging from 20 mg.L-1 to 11400 mg.L-1. When these wastewaters are treated in an anaerobic system like prefermentors or anaerobic digesters the sulphate is reduced to sulphide by sulphate reducing bacteria (SRB). The presence of sulphate reduction is not desirable as it may reduce methane yield due to partial substrate utilisation by SRB, causes system toxicity and the production of malodor H2S in the gas phase. In this thesis, the effects of operational conditions on sulphate transformation and assimilation was studied in a laboratory scale anaerobic wastewater treatment system. The laboratory scale system consisted of two reactors the first one a well-mixed fermentor (referred to as an acidogenic reactor) and the second an expanded granular sludge blanket reactor (referred to as a methanogenic reactor) with pH and temperature control. Two sets of studies were performed; in the first set both reactors were connected serially to represent a two-stage high-rate anaerobic treatment system. The system was fed molasses and operated at temperature of 35oC. The acidogenic reactor was controlled at pH of 6 while the methanogenic reactor was controlled at pH of 7.2 by automatic addition of caustic. In the second set of experiments only the first reactor was used to represent a prefermentor and the first stage of the two stage. The reactor was fed with glucose at various concentrations, operated at pH of 6 and temperature of 35oC. Information gained from these studies was encapsulated in a mathematical model to describe sulphate reduction in anaerobic treatment systems. This model was also validated using data generated from the experiments. The experimental study showed that · At low sulphate concentrations of about 250 mg.L-1 and COD concentration of 10,000 mg.L-1 in feed, relatively high percentage (up to 35 %) of produced sulphide was assimilated by biomass, while the rest of the sulphur was distributed as unconverted sulphate, dissolved sulphide, H2S gas and to a lesser extent as metallic sulphide precipitates. · The major electron donor for sulphate reduction in both the acidogenic and the methanogenic reactor was hydrogen gas. Therefore, sulphate reduction not only competed with hydrogen utilising methanogens for the available hydrogen, but also changed the distributions of organic acids, which were directly or indirectly influenced by the H2 partial pressure. · Sulphide concentrations of up to 6.5 mM free hydrogen sulphide) at pH of 7.2 was not inhibitory to methanogens · Sulphate reducing bacteria were able to grow even at a low hydraulic retention time of 1.2 hours in the well-mixed acidogenic reactor. It was estimated that the maximum specific growth rate (m) and half saturation constant (ks) of SRB was 1.31 h-1 and 3.8 mg S.L-1, respectively. These values were higher than those reported in literature. · Sulphate reduction was suppressed at high concentration of carbon in the feed. Accumulation of high concentration of volatile organic acids at high feed-carbon concentrations had little effect on sulphate reduction. However, extent of sulphate reduction had a negative correlation with total concentration of biomass. A non-competitive biomass inhibition function was proposed to model the correlation. From this fit it was estimated that a biomass concentration of about 3300 mg-COD.L-1 will completely inhibit sulphate reduction. · Sulphate reduction was affected by redox potential control and pH in the acidogenic reactor. High pH and low redox potential values were essential for sulphate reduction to proceed. At redox potential control of -300 mV, sulphate reduction was inhibited more at pH of 6 than it was at pH of 7. At redox potential values of -250 mV or higher, about 90 % inhibition of sulphate reduction was observed at both pH of 6 and 7. An existing model describing carbohydrate degradation was extended to include sulphate reduction processes. Despite experimentally observing that sulphate reduction only took place from hydrogen, all possible substrates for sulphate reducion was considered. These included: lactic acid, butyric acid, propionic acid, acetic acid and hydrogen. Kinetic parameters for sulphate reduction processes were obtained from documented literature. Inhibition of sulphate reduction by biomass and sulphur assimilation by biomass were included in the model. A new approach to calculate caustic consumption at given pH values was also included. A modification to hydrogen regulation function was also made to better predict product distributions as a function of gas-phase hydrogen concentration. Model validation was performed using data from dynamic experiments. Comparison to actual data was undertaken on several key variables in the acidogenic and methanogenic reactors such as: organic acid concentrations, gas compositions, gas production rates, sulphate and sulphide concentrations and caustic consumption rates. The model satisfactorily predicted sulphate and sulphide concentrations in both reactors. However, discrepancy between predicted and experimental data on organic carbon concentrations was seen, especially during organic carbon concentration step changes.
| Identifer | oai:union.ndltd.org:ADTP/253737 |
| Creators | Haris, Abdul |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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