Treatment of sulfide-rich UASB effluent: comparison of poly-aluminium chloride and iron-salts

Coagulation treatment of sulfide-rich effluent from an upflow anaerobic sludge blanket (UASB) reactor in a full-scale Sewage Treatment Plant (STP) is reported. The STP treats combined wastewater from tanneries and sewage, however, the treatment in the existing UASB reactor was not optimal. This resulted in high concentrations of chemical oxygen demand (COD), 530–590 mg/L; biochemical oxygen demand (BOD), 180–210 mg/L; and sulfide, 142–161 mg/L in the treated effluent. The high concentrations of sulfide in the UASB effluent resulted in higher oxygen demand. Therefore, further treatment of UASB effluent was necessary to comply with the discharge standards. In the present study, polyaluminium chloride (PACl, liquid & solid-type), ferric alum and Fe-salts (ferrous sulfate and ferric chloride) were used as coagulants and their effectiveness was compared. Optimization of doses of the chosen coagulants/salts, were carried out with the objective of achieving discharge standards with respect to COD, BOD and sulfide. Application of optimized dose of Ecorite PACl-2018 (0.378 mmol/L Al) and PACl (other brand, OB, 0.684 mmol/L Al) resulted in 68.0–80.0% TSS (total suspended solids), 13.0–18.0% COD, 26.0–35.0% BOD and 58.5–59.2% sulfide removal. On the other hand, use of Fe-salts lead to enhanced removal of pollutants; 80.0–83.0% TSS, 57.0–60.7% COD, 88.0–89.0% BOD and 99.0–99.4% sulfide. The Fe-salts were found to be more effective for sulfide removal as FeS. Such efficient removal of sulfide leads to decrease in COD and BOD concentrations and ensures compliance of final treated effluent from STP with the discharge standards.