Comparative evaluation of nanofiltration and reverse osmosis treatment of an effluent from dairy industry

Nanofiltration (NF) and reverse osmosis (RO) membranes were evaluated for the treatment of the effluent of chemical-biological treatment plant and the original effluent of dairy industry, respectively. A thin film composite type of spiral wound membrane (2 m(2) area) with the NF (TFC-S) and the RO (TFC-HR) was used. The runs were conducted at approximately 16 and 30 % water recovery of a single membrane at different pressures and flowrates for the RO and NF, respectively. Permeate flux declined rapidly at the beginning of the run and then appeared to remain approximately constant after 50 h of operation at different pressures. At a pressure of 10 bar, the permeate fluxes of the NF and RO decreased from 46 l/m(2)h to 23 l/m(2)h after 88 h of operation and from 21 l/m(2)h to 10 l/m(2)h after 68 h of operation, respectively. The specific energy consumption increased as a function of time and decreased with increasing pressure for each membrane. The average specific energy consumption was obtained at 6 and 10 kWh/m(3) for the NF and RO units, respectively. Therefore, the NF and RO operational costs were estimated at U.S. $0.45/m(3) and U.S.$ 0.75/m(3) respectively. Also, the operational cost for chemical-biological treatment was found to be U.S. $ 0.30/m(3). Consequently, the operational costs for the treatment of the dairy industry effluent with the chemical-biological process followed by NF membrane and the RO membrane alone were nearly equal. Effluents with high hardness, the total hardness of the NF and RO feeds were reduced from 1425 to 280 mg CaCO3/l (80% removal) and from 480 to 15 mg CaCO3/l (97% removal), respectively. The conductivities of the NF feed, permeate were approximately equal to the RO feed, permeate. The COD of the NF feed and RO feed were also reduced from 470 to 9 mg/l (98% removal) and from 10500 to 80 mg/l (99% removal), respectively.