Feed water quality system design

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 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. 

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. 

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... 

Membrane fouling is the main cause of permeant flux decline and loss of product quality in reverse osmosis systems, so fouling control dominates reverse osmosis system design and operation. The cause and prevention of fouling depend greatly on the feed water being treated, and appropriate control procedures must be... 

A two- or three-stage RO system will usually remove about 96% to 98% of the total dissolved solids (TDS) in the water. The effluent water quality is generally high enough to send to low- to medium-pressure boilers without additional polishing. However, each application and feed water is different, and, therefore, water sampling and design projections should be conducted for every application to determine what the projected water quality will be (see Chapters 7 and 10). 

The program calculates total dissolved solids (TDS), conductivity, and the Langelier and Stiff-Davis Saturation Indexes (see Chapter 3.9). The screen allows the designer to use sodium, calcium, magnesium, chloride, sulfate, or bicarbonate to balance the water analysis. Water quality for up to five feed streams can be entered and blended together to make the total, combined feed water to the system. 

The quality of feed water required depends on boiler operating pressure, design, heat transfer rates, and steam use. Most boiler systems have sodium zeohte softened or demineralized makeup water. Feed-water hardness usually ranges from 0.01 to 2.0 ppm, but even water of this purity does not provide deposit-free operation. Therefore, good internal boiler water treatment programs are necessary. 

EDI units are designed to operate with little or no downtime. EDI produces a consistent quality water without the problems and costs of regenerating IX resins and waste neutralisation [25,26]. High-purity water of up to 16.0 MQ-cm resistivity can be produced with an EDI system using RO permeate of conductivity 1.0 iS/cm as feed water... 

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