Pulp manufacture uses sulphur in a variety of forms and these sulphur compounds ultimately end up in the effluent. Under anaerobic conditions, sulphite and sulphate are reduced to sulphide, presenting problems of toxicity, odour, corrosion, and reduced methane yields and treatment efficiencies. The fate of these inorganic sulphur compounds in a bleached chemi-thermomechanical pulp/thermomechanical pulp (BCTMP/TMP) effluent mixture was examined in two phase anaerobic reactors at 35°C and 55°C. The following sulphur management strategies were investigated: 1) 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 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.
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 ineffective in promoting wastewater treatment efficiency. Molybdenum addition to the wastewater at levels from 0.1 to 1.0 mM was effective at 1.0 mM in retarding sulphate reduction or sulphide formation. Hydrogen sulphide stripping, using ferric chloride scrubbed and recycled off gas, appeared to be the most effective means of sulphur management for this type of wastewater under these conditions.
Tbermophilic 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 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 organic carbon removal efficiency or on sulphate reduction. Molybdenum inhibition of sulphur 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 at 55°C. Stripping hydrogen sulphide from the reactor liquor helped to promote the treatment efficiency and lowered the sulphide and sulphate levels. Similar to the 35°C study, sulphide removal by gas stripping appeared to be the most effective means of sulphur management