Treatment of fatliquoring effluent from a tannery using membrane separation process: experimental and modeling

Treatment of fatliquoring effluent generated from a tannery, using a hybrid separation process involving gravity settling, two step coagulation, nanofiltration and reverse osmosis is presented in this study. The optimum dose of coagulation, i.e., 0.5% (w/v) of ferrous sulfate followed by 0.15% (w/v) calcium oxide resulted in reduction of chemical oxygen demand from 13,688 to 4921 mg/l. Low pressure nanofiltration of the supernatant was carried out in the range of 828-1242 kPa. Chemical oxygen demand of the nanofiltration permeate varied from about 1300-2700 mg/l depending upon the operating conditions. To bring the chemical oxygen demand value less than the allowable permissible limit in India (250 mg/l), nanofiltration permeate was subjected to reverse osmosis (operating pressure range from 1313 to 1724 kPa). The final treated effluent, i.e., reverse osmosis permeate had chemical oxygen demand values in the range of 117-174 mg/l. The membrane filtration experiments included flow in laminar, laminar with turbulent promoter and turbulent flow regimes. Using a combination of osmotic pressure and solution diffusion model for both nanofiltration and reverse osmosis, three transport coefficients, namely, the effective osmotic coefficient, solute diffusivity and solute permeability through the membrane were obtained by comparing the permeate flux and permeate concentrations using the model calculated values and the experimental data. The calculated data agreed closely with the experimental values.