Coagulation–flocculation studies of tannery wastewater using combination of alum with cationic and anionic polymers

Abstract

A study was conducted to treat the tannery wastewater through coagulation–flocculation–sedimentation. Alum was used as coagulant with cationic and anionic polymers as coagulant aid. The results were subsequently compared with the study in which alum was used alone for the treatment. Jar test apparatus was used to conduct research work. The results of the study revealed that the combination of alum with cationic polymer C-492 [molecular weight (MW) = 6 million Dalton; charge density (CD) = 40%] resulted in effluent turbidity removal of 97%, total suspended solids (TSS) removal of 93.5%, total chemical oxygen demand (TCOD) removal of 36.2% and chromium removal of 98.4%. Sludge production was 40 mL/L and cost of chemicals to treat one cubic meter of wastewater was $ 0.07. For this combination the optimum dose of alum was 100 mg/L as Al 2(SO 4) 3 with 5 mg/L of C-496. The combination of alum with suitable anionic polymer A-100 (MW = 15 million Dalton; CD = 16%) resulted in effluent turbidity removal of 99.7%, TSS removal of 96.3%, TCOD removal of 48.3% and chromium removal of 99.7%. Sludge production was 30 mL/L and cost of chemicals to treat one cubic meter of wastewater was $ 0.08. The results of the above combinations were compared with those when alum was used alone for the treatment. The comparison revealed that use of coagulant aid reduced sludge volume by 60–70% and cost of chemicals by 50% for comparable removal efficiencies.