Management of reject water in decentralized community RO plants by devising an integrated treatment scheme of NF and RO by pilot-scale analysis

Abstract

Community RO plants have been installed in the semi-arid state of Rajasthan in India to provide potable water to the scattered rural settlements by treatment of brackish groundwater. Presently, these are using standalone RO systems which are operating at low recovery along with the problem of early membrane scaling. To ensure sustainability and maximize the recovery of fresh water, hybrid configurations of membrane processes must be evaluated. In this work, it is aimed to design a conclusive hybrid scheme of NF and RO to deliver maximum freshwater recovery. Firstly, the individual performance of NF and RO in a two-pass NF-RO configuration is evaluated i.e., the removal of ions with respect to feed concentration, ionic radius and hydration radius. The removal efficiency was 85% for sulphate, 54% for calcium and 56% for magnesium by NF. The scaling potential of the water greatly reduced as indicated by the LSI and RSI values by NF pre-treatment. The characterization of RO and NF by FESEM-EDS and FTIR Spectroscopy showed numerous peaks in NF as compared to RO corresponding to inorganic scaling. The specific energy consumption for NF, RO and two-pass NF-RO was 0.13–0.27 kWh/m3, 0.04–0.08 kWh/m3 and 0.17–0.35 kWh/m3, respectively. Based on the performance of standalone NF, RO and two pass NF-RO, mathematical simulations were performed to derive best configurations for NF-RO integration. The resulting configuration, a two stage RO-NF with NF permeate blending to the raw water, resulted in a recovery of 70–80% which was ∼50% higher than the two-pass NF-RO scheme.