Application of nanofiltration membrane for treatment of chloride rich steel plant effluent

• Effluent was treated to reduce total solid, ion-concentration and salinity.
• Charged membrane rejects ions through charge–charge repulsion.
• Enhancement in transmembrane pressure and cross flow rate reduces rejection.
• Membrane can be washed in-situ, with tap water.
• A resistance in series model predicts polarization resistance and permeate flux.

Composite charged nanofiltration membrane of polysulfone and zinc chloride was used to treat the effluent from TATA Steel, India. The effluent contained high amount of total solid and various ions, namely chloride (1000–1200 mg/l), fluoride (140 mg/l), nitrate (37 mg/l) and phosphate (1730 mg/l). The effluent was pre-treated by passing over granular activated carbon and total solid was reduced by 38%. The membrane containing 1 wt% ZnCl2 was found to be optimized one. Chloride concentration was reduced below 800 mg/l via charge–charge interaction by nanofiltration. Similarly fluoride, nitrate and phosphate ions were rejected around 32%, 27% and 70%, respectively. Effect of different operating conditions (transmembrane pressure, cross-flow rate and solution pH) was studied. Rejection of various ions and solid content was reduced with increase in transmembrane pressure and cross flow rate. Based on permeate flux and quality, 1380 kPa and 80 l/h cross flow rate were selected as optimum operating conditions. Simple in-situ washing of the membrane with tap water could recover permeate flux upto 97%. A resistance in series model was used to quantify the effects of polarization on system performance. This system can be scaled up in spiral wound configuration for final deployment.

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