Nanofiltration Process for Enhanced Treatment of RO Brine Discharge

Mohamed E.A Ali

doi:10.3390/membranes11030212

Abstract

Brine discharge of reverse osmosis (RO) desalination plants represents a challenge for both inland and coastal desalination plants. Zero-liquid discharge (ZLD) can be accomplished by using additional stages of RO, which can recycle that brine water, but the key challenge is the high concentration of divalent salts. These divalent salts (especially calcium and magnesium salts) forms a scaling layer on the RO membrane surfaces and hence shorten the life-time of the membranes. In this study, the nanofiltration (NF) procedure was used to remove divalent ions from the brine discharge to minimize the load on additional stages of RO membranes. One of the most critical considerations influencing the selection of an effective NF is the water type, which is expected here by calculation of some hydrochemical parameters (major ions, hypothetical soluble salts (electrolyte), and saturation indices). NF experiments were undertaken on a lab-scale using a low-pressure hand-made system of 4–7 bar. Synthetic single salts solutions and two real brine water discharge (brackish (BWRO) and seawater (SWRO) desalination plants) were used as a feed solution for NF system. The chemical characteristics of the RO-feed, RO-brine, NF-permeate, and NF-reject in were investigated. Electrolyte concentrations and saturation indices were determined based on the concentration of the major ions and the NETPATH software package, respectively. Calculations reveal that the brine concentrate samples contained mostly MgSO4 and MgCl2 soluble salts. The results show that 79–89% of the total dissolved salts (TDS) and 96–98% of the total hardness (TH) were retained using the NF process. The salt rejection of the NF membrane follows the order of CaSO4, Na2SO4, MgSO4, MgCl2, and NaCl with a percent of 97.4, 97.3, 95.2, 93.4, and 79%, respectively.

Highlights

Reverse osmosis (RO) is the most effective technology for desalinating brackish and saline groundwater because it has a higher recovery when the feed water contains low salt concentrations [1]
NF membrane process plays an important role in water treatment; because it can operate at low operating pressure with lower permeate flux and high divalent and multivalent ion salts retention [29,30]
A nanofiltration (NF) procedure was used in this study for the removal of divalent ions from the brine discharge

Summary

Introduction

Reverse osmosis (RO) is the most effective technology for desalinating brackish and saline groundwater because it has a higher recovery when the feed water contains low salt concentrations [1]. NF membrane process plays an important role in water treatment; because it can operate at low operating pressure with lower permeate flux and high divalent and multivalent ion salts retention [29,30]. It is the most technical and economical solution for removing sulfates from the brine. The salt scale formation on the membrane surface is supposed to be low because NF could work at a high recovery level; its use in the pre-treatment of RO brine could substantially render the NF permeated water ideal for a further RO step, which minimizes energy usage and the overall total costs. To achieve the goal of this work, a commercial a NF membrane element with high divalent salt rejection was used

Materials

Chemical Characterization of RO Feeds and Brines

Experimental Set-Up and Membrane Performance Evaluation

Evaluating the Performance of the NF Process

Findings

Conclusions