Reverse osmosis plant

Feed characteri2ation, particularly for nondesalination appHcatioas, should be the first and foremost objective in the design of a reverse osmosis plant. This involves the determination of the type and concentration of the main solutes and foulants in the stream, temperature, pH, osmotic pressure, etc. Once the feed has been characteri2ed, a reaHstic process objective can be defined. In most cases, some level of pretreatment is needed to reduce the number and concentration of foulants present in the feed stream. Pretreatment necessitates the design of processes other than the RO module, thus the overaH process design should use the minimum pretreatment necessary to meet the process objective. Once the pretreatment steps have been determined and the final feed stream defined, the RO module can be selected. 

Designing the membrane structure for a reverse osmosis plant is a difficult project, particularly in view of the fact that in addition to the pressure exposure, the presence of strong concentrations of dissolved minerals is a hostile environment for plastics. 

Reverse osmosis plants also are not immune from silica fouling, and where the raw water source naturally contains relatively high levels of silica, good pretreatment of the RO FW is a prerequisite. To reduce fouling of RO membranes by silica, pretreatment by acid adjustment, alum coagulation, and filtration usually is provided. 

Design Considerations for RO Reverse osmosis plants are typically assembled onto carbon steel or stainless steel frames using permutations of components from the hundreds of individual standard stock items commonly available, including a wide range of membranes, each with their own range of design features and applications. 

Reverse osmosis plant are always subject to an insidious and gradual loss of permeate volume output or quality deterioration due to membrane fouling. The rate of decline is strongly influenced by the input RW quality. Therefore, any and all features, such as those above, that can be employed to delay the onset and degree of fouling and extend membrane life are to be recommended. 

Figure 8.19. Outline flow-diagram of a large-scale reverse-osmosis plant for the demineralisation of brackish...

On account of the relatively low water regain of cellulose acetate, the molal concentration of ionic groups in the swollen material exceeds Smmolal. This is comparable to the concentration of 300 ppm sodium chloride, a typical reverse osmosis product solution. Our homogeneous membranes are believed to be very similar to the active layer of an asymmetric membrane as developed by Loeb and Sourirajan. It is evident therefore that the concentration of fixed charges in the membrane is sufficient to exercise a significant Donnan exclusion of co-ions on the downstream side of the membranes in a reverse osmosis plant. 

Reverse osmosis is now extensively used to reduce salt concentrations in brackish waters and to treat industrial waste water, for example, from pulp mills. Reverse osmosis has also proved economical (the cost can be as low as about 1 per 1000 liters) for large-scale desalination of seawater, a proposition of major interest in the Middle East, where almost all potable water is now obtained by various means from seawater or from brackish wells. Thus, at Ras Abu Janjur, Bahrain, a reverse osmosis plant converts brackish feedwater containing 19,000 ppm dissolved solids to potable water with 260 ppm dissolved solids at a rate of over 55,000 m3 per day, with an electricity consumption of 4.8 kilowatt hours per cubic meter of product. On a 1000-fold smaller scale, the resort community on Heron Island, Great Barrier Reef, Australia, obtains most of its fresh water from seawater (36,000 ppm dissolved salts) directly by reverse osmosis, at a cost of about 10 per 1000 liters. 

Desert countries like Saudi Arabia have built reverse osmosis plants to produce fresh water from seawater. Assume that seawater has the composition 0.470 M NaCl and 0.068 M MgCl2 and that both compounds are completely dissociated. 

Figure 5.19 Skid-mounted reverse osmosis plant able to produce 700 000 gal/day of desalted water. Courtesy of Christ Water Technology Group...

The relationship between brine solution concentration factor and water recovery rate is shown in Figure 5.20. With plants that operate below a concentration factor of 2, that is, 50 % recovery rate, scaling is not normally a problem. However, many brackish water reverse osmosis plants operate at recovery rates of 80 or 90 %. Salt concentrations on the brine side of the membrane may then be far above the solubility limit. In order of importance, the salts that most commonly form scale are ... 

The approximate operating costs for brackish and seawater reverse osmosis plants are given in Table 5.3. These numbers are old, but improvements in membrane technology have kept pace with inflation so the costs remain reasonably current. 

Table 5.3 Operating costs for large brackish water and seawater reverse osmosis plants [49]. Capital costs are approximately US 1.25 per gal/day capacity for the brackish water plant and US 4-5 per gal/day capacity for the seawater plant...

A simplified flow scheme for a brackish water reverse osmosis plant is shown in Figure 5.24. In this example, it is assumed that the brackish water is heavily contaminated with suspended solids, so flocculation followed by a sand filter and a cartridge filter is used to remove particulates. The pH of the feed solution might be adjusted, followed by chlorination to sterilize the water to prevent bacterial growth on the membranes and addition of an anti-sealant to inhibit precipitation of multivalent salts on the membrane. Finally, if chlorine-sensitive interfacial composite membranes are used, sodium sulfite is added to remove excess chlorine before the water contacts the membrane. Generally, more pretreatment is required in plants using hollow fiber modules than in plants using spiral-wound modules. This is one reason why hollow fiber modules have been displaced by spiral-wound systems for most brackish water installations. 

Figure 5.24 Flow schematic of a typical brackish water reverse osmosis plant. The plant contains seven pressure vessels each containing six membrane modules. The pressure vessels are in a Christmas tree array to maintain a high feed velocity through the modules...

I. Nusbaum and D.G. Argo, Design and Operation of a 5-mgd Reverse Osmosis Plant for Water Reclamation, in Synthetic Membrane Processes, G. Belfort (ed.), Academic Press, Orlando, FL, pp. 377-436 (1984). 

Dalvi, A. G. I., Al-Rasheed, R., and Javeed, M. A. (2000) Studies on organic foulants in the seawater feed of reverse osmosis plants of SWCC. Desalination 132,217-232. 

FIGURE 8.2 Bank of modules at the Sanibel-Captiva reverse osmosis plant, Florida. 

Abdel-Jawad, M., et al. (1997). Pretreatment of the municipal wastewater feed for reverse osmosis plants, Desalination. 109, 2, 211-223. 

Prabbakar, S., Panicker, S.T., and Misra, B.M., Design aspects of reverse osmosis plants for radwaste treatment. Paper presented at the 10th National Conference of Indian Membrane Society, Mumbai, January, 1993. 

Membrane installations operated in nuclear industry are pressure-driven systems majority of them are reverse osmosis plants. Uncontrolled growth of operation pressure may result in module damage and valves leaks resulted in contamination hazard. The selection of appropriate pumps and security devices can avoid the danger of pressure overgrowth and its detrimental implications. The security valves outlets have to be connected with existing waste distribution systems to direct the eventual leaks to the waste collecting tanks. 

Bourns, W.T. and Le, V.T., The Reverse Osmosis Plant in CRNL Waste Treatment Centre-Description Design and Operation Principles, Report CRNL-2352, Atomic Energy of Canada Ltd., Chalk River, 1984. 

The authors, in collaboration with LAINSA company, developed the project for decontaminating the radioactive liquids, by means of a reverse osmosis plant. The aim of the treatment was to remove the Cs radioisotope from the treated liquid and to reduce the volume of the solution for further immobilization. 

As early as in the 1970s, DOS reverse osmosis plants were installed in the Toten sulfite pulp mill in Norway and the Reed Lignosol mill in Canada. RO membranes were used to concentrate spent sulfite liquor to help the overloaded evaporation systems of the mill [1]. 

The waters in question are pumped to a membrane bioreactor equipped with an air injection system, where part of the feed is recycled, making it move across a membrane ultrafiltration system, to prevent the presence of suspended microelements in the later phase of reverse osmosis. From the ultrafiltration process, two streams are obtained a concentrated stream of salts and microbial mass, which is recycled to the bioreactor, and a permeate stream that passes to the reverse osmosis plant. 

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