Nanofiltration based water reclamation from tannery effluent following coagulation pretreatment

• Chromium(VI) removal from tannery effluents by coagulation–nanofiltration process.
• Pretreatment was performed by coagulation and process parameters were optimized.
• Coagulation–nanofiltration hybrid process achieved about 99% removal efficiency.
• Membrane fouling was reduced with pre-treated wastewater.
• Treated water was recycled back to the plant for reuse.

Coagulation–nanofiltration based integrated treatment scheme was employed in the present study to maximize the removal of toxic Cr(VI) species from tannery effluents. The coagulation pretreatment step using aluminium sulphate hexadecahydrate (alum) was optimized by response surface methodology (RSM). A nanofiltration unit was integrated with this coagulation pre-treatment unit and the resulting flux decline and permeate quality were investigated. Herein, the coagulation was conducted under response surface-optimized operating conditions. The hybrid process demonstrated high chromium(VI) removal efficiency over 98%. Besides, fouling of two of the tested nanofiltration membranes (NF1 and NF3) was relatively mitigated after feed pretreatment. Nanofiltration permeation fluxes as high as 80–100 L/m2 h were thereby obtained. The resulting permeate stream quality post nanofiltration (NF3) was found to be suitable for effective reuse in tanneries, keeping the Cr(VI) concentration (0.13 mg/L), Biochemical Oxygen Demand (BOD) (65 mg/L), Chemical Oxygen Demand (COD) (142 mg/L), Total Dissolved Solids (TDS) (108 mg/L), Total Solids (TS) (86 mg/L) and conductivity levels (14 mho/cm) in perspective. The process water reclaiming ability of nanofiltration was thereby substantiated and the effectiveness of the proposed hybrid system was thus affirmed.