Brine treatment membrane processes

High recovery/brine treatment membrane processes... 

The majority of the discharge from a desalination processes is concentrated brine from the membrane process, and this may contain quantities of treatment chemicals used. Treatment of water is necessary in all desalination plans for variety of reasons feed water treatment, membrane protection, membrane cleaning, permeate treatment and concentrate treatment prior to discharge. Although non-chemical treatment is possible, chemical treatment is widely practiced. 

Electrodialysis is by far the largest use of ion exchange membranes, principally to desalt brackish water or (in Japan) to produce concentrated brine. These two processes are both well established, and major technical innovations that will change the competitive position of the industry do not appear likely. Some new applications of electrodialysis exist in the treatment of industrial process streams, food processing and wastewater treatment systems but the total market is small. Long-term major applications for ion exchange membranes may be in the nonseparation areas such as fuel cells, electrochemical reactions and production of acids and alkalis with bipolar membranes. 

Membrane processes for this analysis include SWRO, BWRO, low-pressure RO (LPRO), brine recovery RO (BRO), pressurised MF/UF (pMF/UF), immersed membrane bioreactor (iMBR), cross-flow membrane filtration (XMF) and electrodeionisation (EDI). Membrane process characteristics for water treatment are detailed in Table 5.1. Typical process flow schematics of RO membrane plants are shown in Figures 5.1 and 5.2. RO/NF systems are typically multi-stage and single-pass or multi-stage and double-pass, as shown in Figures 2.21-2.23. 

Finally, we consider the membrane cells in Fig. 6.5. The electrode processes are the same as those in the diaphragm cells (Eqs. 1 and 2). Anolyte processing is quite similar to that practiced with mercury cells. We saw above in the discussion on brine treatment that membrane cells had stricter requirements. The same is true regarding dechlorination of the depleted brine. After vacuum dechlorination, the residual active chlorine content is high enough to damage the ion-exchange resin in the brine purification... 

FIGURE 7.33. Guide to brine treatment processes. Basis membrane cells numbers in boxes correspond to sections in text. 

Early membrane cells operated at relatively low current efficiencies and were able to tolerate correspondingly higher concentrations of impurities. As membranes were improved and better results became possible, the requirements for brine purity became stricter. For a time, the addition of phosphate to the brine to sequester the hardness ions and prevent them from entering the membranes mitigated some of the effects of hardness. Finally, it became necessary to devise a process to increase the purity of the brine well beyond that obtained by chemical treatment, and ion exchange is now the standard technique. Several general reviews of the brine ion-exchange process itself are available [119-121]. 

Depleted brine will be physically saturated with chlorine, and some chlorine wUl react to form hypochlorite (Section 7.5.9.1). This chlorine value represents an economic asset to be recovered and, particularly in the case of membrane cells, an intolerable contaminant in the brine treatment system. There are several approaches to this problem [208], and we cover these below. We divide them into methods aimed at recovery of the bulk of the chlorine in a useful form (primary dechlorination Section 7.5.9.2) and those whose purpose is to reduce the active chlorine to chloride and safeguard the environment or other parts of the process (secondary dechlorination Section 7.5.9.3). Some of the hypochlorite that forms in the anolyte will continue to react to form chlorate. This is a much less harmful impurity in the cells, and higher concentrations are tolerable. Many plants keep the chlorate concentration under control by natural or deliberate purges from the brine system (Section 7.5.7.2A). In others, it is necessary to reduce some of the chlorate ion to chloride in order to maintain control (Section 7.5.9.4). 

The sulfate accompanying NaCl brine is sometimes recovered as a by-product. In the diaphragm-cell process, sulfate is available as a concentrated purge from the evaporators. In the membrane-cell process, there is no natural point of high sulfate concentration, but it is possible to isolate sodium sulfate in the brine treatment process (Section 7.5.7.2B). 

Mercury process. Pressure control is a bit more difficult than in the low-pressure membrane-cell process but not very much different. The chlorine gas usually is very low in oxygen and hydrogen content, unless a problem in brine treatment allows some metal contaminant to produce unsafe quantities of hydrogen in the chlorine. The presence of hydrogen is most frequently a problem with mercury cells. 

Caustic. Dilute solutions of caustic that are free of process contaminants can return to the process. They can be used to dilute caustic product for brine treatment or for general utility application. In membrane- and diaphragm-cell plants, they can be blended at a controlled rate with evaporator feed. In mercury-cell plants, they can similarly be used as part of the decomposer water feed. 

Abstract This chapter discusses the characteristics of membrane concentrate, and the relevance that the concentrate has on the method of disposal. Membrane concentrate from a desalination plant can be regarded as a waste stream, as it is of little or no commercial benefit, and it must be managed and disposed of in an appropriate way. It is largely free from toxic components, and its composition is almost identical to that of the feed water but in a concentrated form. The concentration will depend on the type of desahnation technology that is used, and the extent to which fresh water is extracted from the brine. Based on the treatment processes that are used, a number of chemicals may also be present in the concentrate, albeit in relatively small quantities. 

The cooling step removes most of the water vapor. The condensate, saturated with chlorine, requires treatment. Figures 6.4 and 6.S showed that membrane- and mercurycell plants require facilities for removal of dissolved chlorine from depleted brine. With proper design, those facilities can also accept the condensate from the chlorine cooling plant. A conunon practice in diaphragm-cell plants is to strip the condensate, after acidification, with steam. The chlorine, again, can return to the main gas header, and the stripped condensate becomes a process waste. 

Impurities in brine affect electrode reaction kinetics, cell performance, the condition of some cell components, and product quality. Treatment of brine to remove these impurities has always been an essential and economically significant part of chlor-alkali technology. The brine system typically has accounted for 15% or more of the total capital cost of a plant and 5-7% of its operating cost. The adoption of membrane cells has made brine specifications more stringent and increased the complexity and eost of the treatment process. Brine purification therefore is vital to good electrolyzer performance. This section considers the effects of various impurities in all types of electrolyzer and the fundamentals of the techniques used for their control. Section 7.5 covers the practical details of the various brine purification operations. 

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