Reverse osmosis of the retentate from the nanofiltration of secondary effluents

Wastewater reclamation and reuse are being widely implemented due to the lack of fresh water. When salinity is a limiting parameter to obtain the required water quality for agricultural uses, a desalination step is necessary to achieve it. Therefore, techniques such as nanofiltration (NF) or reverse osmosis (RO) can play an important role and have to be deeply studied in order to implement an economically feasible tertiary treatment. In this work, NF of a secondary effluent from a municipal wastewater treatment plant has been evaluated for increasing feed concentration in order to reach a final conversion of approximately 75%. Experiments were carried out with a laboratory plant containing a spiral wound membrane (DURASLICK-2540, General Electric) with an active area of 2.2 m2. Tests were performed increasing volume concentration factors (VCFs) up to almost 4 and replicated three times to confirm the results statistically. After NF experiments an RO step of the retentate was carried out in order to concentrate the final brine. In NF experiments, for the highest VCF values, fluxes recommended for the membrane maker (17 L m(2 h(1) were produced at a transmembrane pressure (TMP) of 3.2 bar. The TMP hardly increased with the VCF. NF membranes yielded a conductivity retention index around 42%, reaching nearly 99% for sulphate ions and only 23% for chloride ions.

A flux of 17 L m(2 h(1 was produced by the DURASLICK NF membrane at a TMP of 3.2 bar for a VCF of 3.25. Thus, divalent ions were removed from the secondary effluent with low energy consumption. SAR value of NF permeate was higher than 6; thus soils could have a great infiltration risk. A mixture with the RO permeate is proposed to avoid infiltration problems that could happen if permeates were used on their own (NF permeate because of the high relation between monovalent and divalent ions concentration and the RO permeate because of the salinity scarcity contribution). The combination of NF and RO yielded a final recovery of 95% obtaining a final brine with a conductivity value higher than 10 mS cm(1.