Ali Mohamed E A
Egypt Desalination Research Center of Excellence (EDRC) & Hydrogeochemistry Department, Desert Research Center, Cairo 11753, Egypt.
Membranes (Basel). 2021 Mar 18;11(3):212. doi: 10.3390/membranes11030212.
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 MgSO and MgCl 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 CaSO, NaSO, MgSO, MgCl, and NaCl with a percent of 97.4, 97.3, 95.2, 93.4, and 79%, respectively.
反渗透（RO）海水淡化厂的浓盐水排放对内陆和沿海海水淡化厂来说都是一项挑战。零液体排放（ZLD）可以通过增加RO阶段来实现，这样可以循环利用浓盐水，但关键挑战在于二价盐的高浓度。这些二价盐（尤其是钙盐和镁盐）会在RO膜表面形成结垢层，从而缩短膜的使用寿命。在本研究中，采用纳滤（NF）工艺从浓盐水排放中去除二价离子，以最小化RO膜额外阶段的负荷。影响有效NF选择的最关键因素之一是水的类型，这里通过计算一些水化学参数（主要离子、假想可溶性盐（电解质）和饱和指数）来预期。NF实验在实验室规模上使用4 - 7巴的低压手工制作系统进行。合成单盐溶液和两种实际浓盐水排放（苦咸水（BWRO）和海水（SWRO）淡化厂）用作NF系统的进料溶液。研究了RO进料、RO浓盐水、NF渗透液和NF截留液的化学特性。电解质浓度和饱和指数分别基于主要离子浓度和NETPATH软件包确定。计算表明，浓盐水浓缩样品主要含有MgSO和MgCl可溶性盐。结果表明，使用NF工艺可截留79 - 89%的总溶解固体（TDS）和96 - 至98%的总硬度（TH）。NF膜对盐的截留顺序为CaSO、NaSO、MgSO、MgCl和NaCl，截留率分别为97.4%、97.3%、95.2%、93.4%和79%。
