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

Abstract

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. HIGHLIGHTS Coagulants (alum and ferric chloride) give best results when used in a 1:1 mix.; Up to 95% removal of turbidity, COD, TDS, and heavy metals using chemically enhanced primary treatment and nanofiltration.; CEPT is an effective technique for NF membrane fouling control.; NF membrane run time is 1.5 and 1.3 times higher when coagulants are used in a mix rather than when used separately.; Effluent met the USEPA guidelines for non-potable reuse.;

Keywords

• alum
• coagulants
• ferric chloride
• membrane fouling
• nanofiltration
• wastewater