Potential of hybrid NF-RO system to enhance chloride removal and reduce membrane fouling during surface water desalination

Abstract

The present study aims to investigate desalination processes of surface water of M’rirt city located in the Middle Atlas in Morocco. Membrane-based processes, including NF and RO single combinations and NF-RO hybrid system. The performances of these combinations were evaluated and compared in terms of TDS and chloride ion reduction. The membranes used are NF270 as NF membrane and BW30 as RO membrane. ROSA software was applied to simulate and to compare the filtration design of two proposed combinations in terms of specific energy consumption, water recovery rate, number of membranes and pressure tubes. The study also investigated the technico-economical performances of two coagulants such as ferric chloride and aluminum sulphate in terms of turbidity removal in the pretreatment step. The obtained results showed that the NF-RO hybrid system is a very flexible design and the application of NF upstream of RO process is favorable where the satisfactory permeate quality required for human consumption is obtained right after a slight post-remineralization to rebalance it again. The simulation results by ROSA software showed that the hybrid NF-RO system requires less membrane elements and lower specific energy consumption than RO process while single RO system requires 3 and 2 times more membranes than NF-RO hybrid system and single combination NF system respectively. On the other hand, the results of the coagulation pretreatment reveal that the ferric chloride is more efficient in terms of turbidity removal at lower doses in comparison with aluminum sulphate and with a cheaper cost estimated at 0.032 $/m3.