Membrane fouling dechlorination

To overcome membrane scaling, the operating pH of the feed brine to the unit was lowered to a range between 4 and 7. A simple modification was made to the plant to control the pH of the plant feed brine by mixing acidic dechlorinated brine with alkaline dechlorinated brine. This modification has proven to be effective and no further membrane fouling has occurred over the last two years. 

Pretreatment For most membrane applications, particularly for RO and NF, pretreatment of the feed is essential. If pretreatment is inadequate, success will be transient. For most applications, pretreatment is location specific. Well water is easier to treat than surface water and that is particularly true for sea wells. A reducing (anaerobic) environment is preferred. If heavy metals are present in the feed even in small amounts, they may catalyze membrane degradation. If surface sources are treated, chlorination followed by thorough dechlorination is required for high-performance membranes [Riley in Baker et al., op. cit., p. 5-29]. It is normal to adjust pH and add antisealants to prevent deposition of carbonates and siillates on the membrane. Iron can be a major problem, and equipment selection to avoid iron contamination is required. Freshly precipitated iron oxide fouls membranes and reqiiires an expensive cleaning procedure to remove. Humic acid is another foulant, and if it is present, conventional flocculation and filtration are normally used to remove it. The same treatment is appropriate for other colloidal materials. Ultrafiltration or microfiltration are excellent pretreatments, but in general they are... 

Microbial fouling is best dealt with before biofilm becomes mature. Biofilm protects the microorganisms from the action of shear forces and biocidal chemicals used to attack them. Microbes can be destroyed using chlorine, ozone, ultraviolet radiation, or some non-oxidizing biocides (see Chapters 8.2.1,8.2.2, 8.1.8, and 8.2.5, respectively). An effective method to control bacteria and biofilm growth usually involves a combination of these measures. Specifically, chlorination or ozonation of the pretreatment system, followed by dechlorination to protect the membranes, or UV distraction followed by periodic sanitation with a non-oxidizing biocide used directly on the membranes. 

Membrane pretreatment includes microfiltration (MF), ultrafiltration (UF), and nanofiltration (NF). Microfiltration and UF membrane processes can remove microbes and algae. However, the pores of MF and UF membranes are too large to remove the smaller, low-molecular weight organics that provide nutrients for microbes. As a result, MF and UF can remove microbes in the source water, but any microbes that are introduced downstream of these membranes will have nutrients to metabolize. Therefore, chlorination along with MF and UF is often recommended to minimize the potential for microbial fouling of RO membranes. The MF or UF membranes used should be chlorine resistant to tolerate chlorine treatment. It is suggested that chlorine be fed prior to the MF or UF membrane and then after the membrane (into the clearwell), with dechlorination just prior to the RO membranes. See Chapter 16.1 for additional discussion about MF and UF membranes for RO pretreatment. 

Dechlorinated and softened water flows to the RO skid through a 5.0-nm (nominal pore size) cartridge filter. The cartridge filter removes resin fines, particles and complexed colloids necessary to protect the RO membranes from particulate fouling. The RO membranes are thin-film composite (TFC) polyamide RO membranes (20 cm diameter X 100 cm long spiral wound elements) with rejection 99%. 

The filtration membranes are sensitive to fouling as well as to free chlorine. This situation is least troublesome in a membrane-cell plant, where the problem components already have been removed from the brine. In mercury-cell plant applications, an installation in the brine recycle loop should include some means of dechlorination. The usual choice is treatment with activated carbon, which is covered in Section 7.5.9.3B. The membranes are in spiral-wound modules placed in cylindrical housings and assembled as on the skid shown in Fig. 7.81. Figure 7.82 shows the construction of a modular element. The low-sulfate permeate flows through the membranes into spacer channels... 

Chlorine (or other disinfectant) is required to minimize the potential for fouling the membranes with microbes (see Chapters 8.2.1, 8.2.2, and 8.5.2.1). Once membranes are fouled with microbes, it is very difficult to remove them. A free chlorine residual of about 0.5 to 1.0 ppm in the pretreatment system is desirable. Feed water to the RO must be dechlorinated prior to the membranes because the membranes are sensitive to oxidizers, which will degrade the membrane. Sodium bisulfite is the preferred method to dechlorinate unless the RO feed water has a high organic concentration, in which case, carbon filtration at a flow rate of 2 gpm/ft is recommended. (see Chapters 8.1.4 and 8.2.3) Sodium metabisulfite is typically about 33% active, and the stoichiometic dosage of sodium metabisulfite is about 1.8 ppm per ppm free chlorine. So, the stoichiometric dosage of 33% active sodium metabisulfite is 5.4 ppm. For safety, a factor of 1.5 is used to increase the dosage of sodium metabisulfite to ensure complete elimination of free chlorine. 

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