Feed water, quality

Feed water quality and its tendency to foul has a significant impact on the design of an RO system. Selection of the design flux, feed water and reject flows (and hence, the array), and salt rejection is influenced by the feed water quality. 

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In the recommended feed water quality guidelines, McCoy notes that for modem industrial boilers, which have extremely high rates of heat transfer, the concentrations for iron should be essentially zero. Similarly, total hardness should not exceed 0.3 ppm CaC03, even at the lowest pressures suspended solids in the feed water should be zero, if possible. 

Table 12.17 (McCoy BW table 6.11) Recommended feed water quality... 

Membrane pretreatment can improve the feed water quality and reduce particulate matter to a greater degree than other pretreatment methods. This reduces the required cleaning frequency of the downstream membranes and the amount of cleaning chemicals that are required and subsequently disposed of (Pearce 2007). 

The critical issue for a successful RO plant is pretreatment. Long-term operating experience proves the viability of continuous MF/UF pretreatment of RO for the desalination of a wide variety of water sources. MF/UF has proven to simplify and reduce the costs of traditional pretreatment, comprised of deep-bed media filters combined with chemical treatment. MF/UF produces filtrate of a consistent quality almost irrespective of fluctuations in feed-water quality. In the last five years, RO-membrane improvements, combined with the use of membrane filtration for pretreatment, have halved the cost of advanced treatment and are now more widely used for the reuse of municipal wastewater. 

Figure 5.7 shows a double-pass RO system. The design principles for the second pass are generally the same as for the first pass. However, because of the low concentration of dissolved and suspended solids in the influent to the second pass, the influent and concentrate flows can by higher and lower, respectively, than for the first-pass RO system (see Chapters 9.4 and 9.5, and Tables 9.2 and 9.3). Because the reject from the second pass is relatively clean (better quality than the influent to the first pass), it is virtually always recycled to the front of the first pass. This minimizes the waste from the system and also improves feed water quality, as the influent to the first pass is "diluted" with the relatively high-quality second-pass reject. 

Feed water source also influences the design array of the RO unit. This is because the feed water flow and concentrate flow rates are also determined based on feed water quality. Higher feed water quality allows for higher feed flows and lower concentrate flows to be employed. Higher feed water flows and lower concentrate flows reduce the number of membrane modules required in the RO system. 

Table 9.4 Hydranautics recommendation for beta values as a function of feed water quality.2...

Fouling Factor The designer can input the rate of fouling of the membrane based on the feed water quality. A fouling factor of 1.00 is used for new membranes. For... 

Flux decline per year input is located in the upper left of the screen. Recommended percentage flux decline per year as a function of feed water quality is shown in Table 10.2. Note that the flux decline increases as the water quality gets worse. 

Hence, traditional spiral wound NF membranes require the same level of pretreatment as spiral-wound RO membranes, as well as the same flux and flow rate considerations with respect to feed water quality (see Chapters 9.9 and 9.4). 

A significant amount of pretreatment is required to minimize fouling and scaling of the membranes in a CEDI system. Table 16.7 lists general feed water quality requirements for CEDI systems.15 21 Due to the stringent feed water quality requirements, most CEDI systems are preceded by RO. Common configurations used to pretreat CEDI feed water include the following 15... 

Table 16.7 General feed water quality requirements for CEDI systems. Adapted from specification sheets for various manufacturers.

Despite efforts to comply with the limitations on feed water quality, CEDI systems can still foul and scale with microbes, organics, iron and manganese, and calcium- and silica-based scales. This usually occurs due to upsets in the pretreatment system or a deficiency in the system design that result in excursion in feed water quality to the CEDI system. Organics, metals, hardness, and silica problems are usually found on the membranes and sometimes on the resin (as is the case with organics). Biofouling is typically found on the... 

A chelant—polymer program can produce clean waterside surfaces, contributing to much more reliable boiler operation. Out-of-service boiler cleaning schedules can be extended and, in some cases, ehminated. This depends on operational control and feed-water quality. Chelants with high complexing stabilities are "forgiving" treatments they can remove deposits that form when feed-water quality or treatment control periodically deviates from standard. 

Studies on in-line filtration showed that effluent turbidity of less than 2 NTU is achievable with alum dosage of 0-8 mg/L and cationic polymer dosage of 0-0.5 mg/L (73). However, the performance of this system is dependent on feed water quality. 

While size reduction or blockage of pores may be considered to increase the resistance of the membrane (Rm) to permeate flux, accumulation of materials in the cake and concentration-polarization layers (so-called polarised solids) presents additional resistances to permeation (denoted here as R c and Rep respectively). These resistances var) as a function of the composition and thickness of each layer, which in turn are determined by the feed water quality and the characteristics of mass transfer in the membrane module. In most instances encountered in water and wastewater treatment, it appears that the concentration-polarisation layer, if it is formed, contributes negligible resistance to permeate flux i.e. Rep << Rc and, therefore. Rep may be neglected (Mallevialle et al (1996)). While this is in reality not always the case (as shown in later sections of this chapter and in Chapter 7) for the filtration of... 

Feed water quality must be analysed every 8 h and must meet specifications for TDS, pH, conductivity, etc. 

Most brackish water RO plants require extensive pre-treatment as discussed in Chapter 2. Feed water quality ofbrackish waters in the US southwest is given in Table 3.4. The intensity of pre-treatment depends on the quality of feed water as detailed in T able 3.6. Feed water constituents that affect pre-treatment design are summarised below [62,63] ... 

The amount of product water (permeate) is generally dependent on (i) total area of membrane within each pressure vessel, (ii) membrane pressure supplied by the high-pressure pump(s), (iii) reject flow rate, and (iv) feed water quality, as discussed in Chapter 2. 

Caustic purity The recommended feed water quality is a resistance of greater than 200,000 ohm-cm (or less than 0.5 ppm calcium). However, impurities entering the catholyte compartment generally impose fewer problems than those entering the anolyte. 

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