Nanofiltration-reverse osmosis hybrid process for hardness removal in brackish water with higher recovery rate and minimization of brine discharges

Abstract

In this paper, the feasibility of hybrid nanofiltration-reverse osmosis (NF-RO) process to remove hardness in brackish water at higher recovery rate and concentrate minimization is investigated. Two nanofiltration (NF) membranes are tested in the first stage for selecting the appropriate membrane on the basis of lower applied pressure and higher permeate flow. Then the evaluation of the performances of hybrid NF-RO system of each stage is studied in terms of recovery rate and Langelier saturation index (LSI) values to prevent the corrosion potential. The blending strategy is adopted with NF brine and reverse osmosis (RO) permeate in order to bring the RO permeate characteristics in accordance with drinking water standards and to minimize the volume of NF brine. Also, energy consumption performance of conventional nanofiltration (NF-NF) and NF-RO hybrid system in hardness removal is compared. The results show that the NF-RO hybrid system combined to blending strategy provides the simplest process but most energy efficient process compared to a single conventional NF-NF. Moreover, including NF upstream of RO is responsible of the increasing of the overall water recovery rate (95 %) and of the permeate quality with lower total dissolved solids (TDS). However, less water with lower salinity treated by RO increases significantly the recovery rate in RO stage up to 80 %. The portion rate of NF brine blending with RO permeate is equal to 32.45 %, corresponding to 240L/h, which meet the water quality regulations for drinking water. Hybrid NF-RO system has a lower energy cost per m3 than NF-NF, it is around of 0.055 $/m3. At the same time, this procedure minimizes the fraction of brine disposal dumping to sewage, making it environmentally friendly.