Fouling reduction in nanofiltration membranes in the treatment of municipal sewage - effect of coagulant type used for prior chemically enhanced primary treatment.

The limiting factor in wide-scale application of membranes for wastewater treatment is membrane fouling. Coagulation has emerged as an effective technique for fouling control. In this research, municipal wastewater was treated using a two-stage treatment. In stage-1, chemically enhanced primary treatment (CEPT) was rendered using an optimum dose of two coagulants, i.e. alum, ferric chloride and a 1:1 mix of both. The optimum doses for coagulants were determined using a jar test. In stage-2, a nanofiltration (NF) membrane was used to further treat the effluent from stage-1. In CEPT, the 1:1 mixture of coagulants showed maximum removals, i.e. 75-77% for the total suspended solids and 73-75% for the chemical oxygen demand (COD). Stage-2 provided 85-95% removals for turbidity (0.88 nephelometric turbidity units), COD (41 mg/L), total dissolved solids (101 mg/L), hardness (11 mg/L as CaCO3), chlorides (80 mg/L), and heavy metals (copper [0.03 mg/L] and lead [0.02 mg/L]). The operational time of the NF membrane was 46 min, 55 min and 70 min using alum, ferric chloride, and mix (1:1), respectively. Significant reduction was observed in membrane fouling for 1:1 mixture of coagulants. The effluent met the US Environmental Protection Agency guidelines for non-potable reuse.