Feed water quality rejection

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. 

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. 

The boron problem still exists due to the low rejection of boric acid through the membranes, yet several other solutions exist, as described below. Final mixing of the water is advisable in some cases to increase salt concentration slightly. Small organic compounds dissolved in the feed water may also find their way into the water produced. Salt content depends on feed quality (brackish or seawater) and may vary between 50-600 ppm of TDS. A secondary stage may improve quality with only a... 

Process Water Purification Boiler feed water is a major process apphcation of RO. Sealants and colloids are particularly well rejected by membranes, and TDS is reduced to a level that makes ion exchange or continuous deionization for the residual ions very economic. Even the extremely high quahty water required for nuclear power plants can be made from seawater. The ultra-high quality water required for production of electronic microcircuits is usually processed starting with two RO systems operating in series, followed by many other steps. 

Once the pretreatment study had been completed, it will be possible to decide on the type of elements to be used in the reverse osmosis unit. If the SDI of the pretreated feed is 3.0 or less, then either the spiral wound or hollow fine fiber elements can be used. The choice will depend on economics (element price) and desalination characteristics (flux and rejection). If the pretreated feed SDI is more than 3.0, then the spiral wound element should be used. When the decision as to element type is made, then it is appropriate to forward a copy of the pretreated feed water analysis to reverse osmosis element manufacturers to obtain a prediction of product water quality, recommended type of element, total number of elements required, possible problems with sparingly soluble compounds in the feedwater, allowable recovery, and price and delivery. 

The amount of concentrate produced is calculated for a plant capacity of 1000 m /d and recoveries from Table 8.12. The loading of the concentrate streams for a water quality as used in the experiments is calculated as a function of rejection and recovery. This leads to the concentrate concentration c c as shown in equation (8.7), where Vf is the feed volume, cf the feed concentration, R recovery as defined in equation (8.2) and the rejection R as defined in Chapter 3. Rejection values for the different solutes and membranes are tabulated in Table 8.11. 

Operate the RO units systems at 50% recovery instead of70-80%. This reduces the frequency of membrane cleanings substantially, increases reliability, and results in consistent product water quality. In addition, the reject water is better suited for reuse apphcations such as for cooling tower make-up. It can also be used as feed for WFI vapour compression stills and clean steam-generators, provided the silica content is less than 15 mg/1. 

The rejection against As(V) versus the operation pressure of the above composite NF membrane is shown in Figure 11.6. The rejection for the As(V) was about 97% at a transmembrane pressure of 0.3 MPa and a water flux around 32 L m" h At a feed As(V) concentration of 360 xg, one throughout using the nanofiltration membrane results in a permeate water quality slightly higher than the WHO MCE limit of 10 xg level. At elevated operation transmembrane pressure, the rejection increases, leading to a lower permeate arsenic concentration. At 0.4 MPa, the permeate As(V) concentration is lower than 10 xg and the water flux reaches about 43 L m h . These... 

The steam turbine of AHWR is designed for a steam quality of 99.75%. During normal operation or in a bypass mode of operation the steam from the turbine exhaust is condensed in a condenser, which rejects the heat to seawater. The condensate is heated in heat exchangers by the moderator system. The feed water temperature is finally raised to 403 K through LP (low pressure) heaters and de-aerators using the steam bled from the turbine. The feed water pumps then pump the feed water into the steam drum where it mixes with the water separated from steam-water mixture. 

Use of warmer water may result in lower boron rejection and require feed water pH adjustment to meet stringent boron water quality targets. 

The removal of PhCs by NF membranes occurs via a combination of three mechanisms adsorption, sieving and electrostatic repulsion. Removal efficiency can vary widely from compound to compound, as it is strictly correlated to (a) the physicochemical properties of the micro-pollutant in question, (b) the properties of the membrane itself (permeability, pore size, hydrophobicity and surface charge) and (c) the operating conditions, such as flux, transmembrane pressure, rejections/recovery and water feed quality. 

Reverse osmosis is a cross-flow membrane separation process which separates a feed stream into a product stream and a reject stream. The recovery of a reverse osmosis plant is defined as a percentage of feedwater that is recovered as product water. As all of the feedwater must be pretreated and pressurized, it is economically prudent to maximize the recovery in order to minimize power consumption and the size of the pretreatment equipment. Since most of the salts remain in the reject stream, the concentration of salts increases in that stream with increased recovery. For instance, at 50% recovery, the salt concentration in the reject is about double that of the feed and at 90% recovery, the salt concentration in the reject is nearly 10 times that of the feed. In cases of sparingly soluble salts, such as calcium sulfate, the solubility limits may be exceeded at a high recovery. This could result in precipitation of the salt on the membrane surface resulting in decreased flux and/or increased salt passage. In addition, an increase in recovery will increase the average salt concentration in the feed/reject stream and this produces a product water with increased salt content. Consequently, the recovery of a reverse osmosis plant is established after careful consideration of the desired product quality, the solubility limits of the feed constituents, feedwater availability and reject disposal requirements. 

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