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Sulphur management strategies for anaerobic treatment of a mechanical pulping effluent Stephenson, Robert John


Pulp manufacture uses sulphur in a variety of forms and some form of these sulphur compounds ultimately appears in the effluent. Under anaerobic conditions, sulphate, sulphite and thiosuiphate are reduced to sulphide. This results in problems of toxicity, odour, corrosion, and inhibition of the wastewater treatment microorganisms. The fate of these inorganic sulphur compounds in a bleached chemithermomechanical puip/thermomechanical pulp (BCTMPITMP) effluent mixture was examined in two phase anaerobic reactors at 35 °C and 55 °C. The following sulphur management strategies were investigated: 1) shifting the sulphide to the less toxic form by controlling the pH of the acidogenic reactor, 2) inhibiting the sulphur reducing bacteria via molybdenum addition to the feed tank, and 3) stripping the hydrogen sulphide dissolved in the methane phase reactor liquor by recycling hydrogen sulphide-free scrubbed off gas. The laboratory scale experimental apparatus consisted of upflow anaerobic sludge bed pre-treatment or acidogenic reactors followed by hybrid upflow anaerobic sludge bed/fixed film methanogenic reactors. The sulphur management strategies which were investigated demonstrated significantly improved treatment efficiencies. At 35 °C, controlling the pH of the acidogenic reactors with sodium carbonate from 5.5 (uncontrolled) to 8.0 in order to shift the formed sulphide species to the less toxic ionic form appeared to be effective in promoting the wastewater treatment efficiency. Sulphate reduction efficiencies were typically 75 to 90% irrespective of acid phase reactor pH. Maximum total organic carbon (TOC) removals of 55 and 63% were observed at an acid phase reactor pH of 7.5 for total hydraulic retention times (HRTs) of 1.2 and 1.8 days respectively. Molybdate was added to the wastewater at levels from 0.1 to 1.0 mM. At 35 °C, it was effective in two out of three effluent batches at the 1.0 mM level in decreasing sulphate reduction from 90% down to 23 to 40%. Maximum TOC removal efficiencies of 42% were observed at 0.5 mM molybdate. Molybdate additions greater than 0.5 mM resulted in reduced TOC removals and gas production rates. Hydrogen suiphide stripping, using ferric chloride scrubbed and recycled off gas, resulted in lower dissolved suiphide levels and increased TOC removals. Sulphate reduction was unaffected by the varying concentrations of dissolved suiphide. Stripping resulted in TOC removal efficiencies of up to 57%, a significant improvement over the unstripped control runs where the TOC removals were only approximately 24%. This sulphur management strategy at 35 °C appeared to be the most effective means of sulphur management for sulphur rich mechanical pulping effluents. Thermophilic 55 °C anaerobic treatment was also studied using the same effluent, inocula and sulphur management strategies. Overall, both the treatment efficiency and the sulphate reduction were considerably lower for the thermophilic runs compared to the mesophilic runs. Raising the acidogenic phase reactor pH from 7.0 to 7.5 to 8.0 appeared to have no significant effect on the organic carbon removal efficiency (maximum 24%) or on sulphate reduction efficiency where a maximum of only 51% was realized. Thermophilic molybdate inhibition of sulphate reduction was not as marked as for the 1.0 mM level at 35 °C, perhaps due to the already low baseline sulphate reduction efficiency (maximum of 50%) at 55 °C. Molybdate addition of up to 1.0 mM improved the TOC removal efficiency, perhaps by decreasing the suiphide inhibition of the methanogenic bacteria. Stripping hydrogen sulphide from the reactor liquor at 55 °C helped to promote the treatment efficiency to a maximum of 39% and was effective in lowering the suiphide levels. Similar to the 35 °C study, suiphide removal by gas stripping appeared to be the most effective means of sulphur management for the thermophilic experiments. Very high acetate concentrations and minute gas production rates were recorded throughout the experimental program. Since the experiments resulted in the various combinations of high and low levels of both sulphate and suiphide, this study demonstrated that compounds in addition to sulphate and suiphide inhibited the methane producing bacteria from using acetate. Various wood extractives and chelating agents are suspected since they are present in high concentrations in this effluent and they are known to inhibit the anaerobic wastewater treatment microorganisms. iv

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