TREATMENT OF SULPHATE RICH WASTEWATER USING CHEMICAL AND BIOLOGICAL PROCESSES

Abstract

Sulphate is a common constituent of wastewater from various industries like mining, paper and pulp, textile processing, fatty acid production, tannery, sea food processing, alcohol production, silica production. Discharging untreated high sulphate wastewaters can increase the salinity of the receiving water and disturb its ecology. Sulphate rich wastewater interferes in methanogenesis, resulting in lower biogas production at common effluent treatment plants. Chemical precipitation and biological sulphate reduction are two effective methods for treatment of sulphate in wastewater. The objective of this work is to investigate the efficiency of the above treatment methods, to treat the effluent from a silica producing industry with a sulphate concentration of 13000mg/L. At this concentration of sulphate, chemical precipitation using Lime yielded about 45 percent removal efficiency. The lime dosage was optimized through batch precipitation tests. A laboratory scale upward flow anaerobic bioreactor packed with Polyvinyl Alcohol (PVA) gel beads was fabricated and operated in continuous mode. The inlet sulphate concentration was increased in stepwise manner to acclimatize the mixed bacterial culture to the high sulphide condition and prevent bioreactor failure due to shock loading. As excessive sulphide production would present additional treatment and handling challenges, the biological treatment study was performed till an inlet sulphate concentration of 9000 mg/L at pH 7.7 which could be easily attained through the lime based chemical precipitation of process waters. The biological treatment removed around 60 percent of the sulphate entering the bioreactor achieving a final sulphate concentration of 3600 mg/L. An overall sulphate removal efficiency of at least 76 percent can be achieved by using both the processes in combination. As the sulphate concentration remained high after initial chemical precipitation, this method was found to be suitable as a pre-treatment stage to the biological sulphate reduction process. Further enhancement in biological sulphate removal shall be observed when operated at a lower inlet concentration of sulphate, i.e. around 7000 mg/L as attained during chemical precipitation of final effluent from the silica production process. A final post-treatment processing of the sulphide rich effluent by various pre-existing technologies like controlled aeration resulting in production of elemental sulphur and precipitation as iron sulphide using ix metallic iron sponge or salts is suggested for meeting the general standard for effluent discharge.