Reverse osmosis technique

Kev>erse Osmosis (RO) Membranes A type of membrane system for treating oily wastewater is currently undergoing commercialization by Bend Research, Inc. The system uses a tube-side feed module that yields high fluxes while being able to handle high-sohds-content waste streams (Ref. 25). Another type of reverse osmosis technique is being designed to yield ultrapurified HF recovered from... 

Solid sorbent procedures are currently being compared to reverse osmosis techniques f25) for accumulating organic materials for bioassays but results are not yet available. The apparent and unexplained correlation between the accumulation of mutagenic and gas chromatographable material from water is also being investigated. 

Concentration of Selected Organic Pollutants Comparison of Adsorption and Reverse-Osmosis Techniques... 

In the early applications of the reverse osmosis technique, the membranes available were the polysaccharide membranes snch as the CA membrane. As new membranes were developed, they were differentiated from the saccharide membranes by calling them nonpolysaccharide membranes. Thus, the NS-200 is a nonpolysaccharide membrane. 

Bou-Hamad, S., et al. (1997). Performance evaluation of three different pretreatment systems for seawater reverse osmosis technique. Desalination Int. Symp. Pretreatment of Feedwater for Reverse Osmosis Desalination Plants, March 31-April 2, 110, 1-2, 85-92. Elsevier Science B.V., Amsterdam, Netherlands. 

Conventional softening treatment plant may not remove the impurities in water to the recommended permissible level. Demineralised or reverse osmosis technique is needed for removal of TDS from water but is costly. Water purification in the process house normally consists of floculation, sedimentation, filtration and ion-exchange. Hard water is normally softened using one or combination of methods the details of each process is given in many text books. 

A number of authors have described the use of ultrafiltration, sometimes combined with reverse osmosis techniques, to concentrate and size-fractionate organic phosphorus in... 

Reverse osmosis is used for desalination of seawater, treatment of recycle water in chemical plants and separation of industrial wastes. More recently the technique has been applied to concentration and dehydrogenation of food products such as milk and fruit juices. See ultrafiltralion. 

Although these composite fibers were developed for reverse osmosis their acceptance in the desalination industry has been limited due to insufficient selectivity and oxidative stabiUty. The concept, however, is extremely viable composite membrane fiat films made from interfacial polymerisation (20) have gained wide industry approval. HoUow fibers using this technique to give equivalent properties and life, yet to be developed, should be market tested during the 1990s. 

Cellulose acetate Loeb-Sourirajan reverse osmosis membranes were introduced commercially in the 1960s. Since then, many other polymers have been made into asymmetric membranes in attempts to improve membrane properties. In the reverse osmosis area, these attempts have had limited success, the only significant example being Du Font s polyamide membrane. For gas separation and ultrafUtration, a number of membranes with useful properties have been made. However, the early work on asymmetric membranes has spawned numerous other techniques in which a microporous membrane is used as a support to carry another thin, dense separating layer. 

Interfdci l Composite Membra.nes, A method of making asymmetric membranes involving interfacial polymerization was developed in the 1960s. This technique was used to produce reverse osmosis membranes with dramatically improved salt rejections and water fluxes compared to those prepared by the Loeb-Sourirajan process (28). In the interfacial polymerization method, an aqueous solution of a reactive prepolymer, such as polyamine, is first deposited in the pores of a microporous support membrane, typically a polysulfone ultrafUtration membrane. The amine-loaded support is then immersed in a water-immiscible solvent solution containing a reactant, for example, a diacid chloride in hexane. The amine and acid chloride then react at the interface of the two solutions to form a densely cross-linked, extremely thin membrane layer. This preparation method is shown schematically in Figure 15. The first membrane made was based on polyethylenimine cross-linked with toluene-2,4-diisocyanate (28). The process was later refined at FilmTec Corporation (29,30) and at UOP (31) in the United States, and at Nitto (32) in Japan. 

Membranes made by interfacial polymerization have a dense, highly cross-linked interfacial polymer layer formed on the surface of the support membrane at the interface of the two solutions. A less cross-linked, more permeable hydrogel layer forms under this surface layer and fills the pores of the support membrane. Because the dense cross-linked polymer layer can only form at the interface, it is extremely thin, on the order of 0.1 p.m or less, and the permeation flux is high. Because the polymer is highly cross-linked, its selectivity is also high. The first reverse osmosis membranes made this way were 5—10 times less salt-permeable than the best membranes with comparable water fluxes made by other techniques. 

Transport Models. Many mechanistic and mathematical models have been proposed to describe reverse osmosis membranes. Some of these descriptions rely on relatively simple concepts others are far more complex and require sophisticated solution techniques. Models that adequately describe the performance of RO membranes are important to the design of RO processes. Models that predict separation characteristics also minimize the number of experiments that must be performed to describe a particular system. Excellent reviews of membrane transport models and mechanisms are available (9,14,25-29). 

The most significant application of reverse osmosis has been in the field of desalination to produce drinking water. Other important apphcations include the treatment of industrial waste water, concentration of fruit juices, and concentration of weak solutions such as aqueous ethanol [3-6]. The rest of the chapter will focus almost entirely on semi-permeable membranes used for reverse osmosis based applications. We chose this focus in view of the importance of reverse osmosis as a rather efficient separation technique for separating a wide range of solutions, especially very dilute solutions—which are usually notoriously difficult to handle using conventional techniques such as distillation. 

The use of reverse osmosis (RO), electrodialysis reversal (EDR) and other membrane techniques, and evaporation or demineralization may be applicable when a reduction in the TDS content of the MU water source is required. 

Reverse osmosis can be used to purify water, because the liquid passing through the semipermeable membrane is pure solvent. A water purifier that uses reverse osmosis requires semipermeable membranes that do not rapture under the high pressures required for reverse osmosis. Recall that seawater has an osmotic pressure of nearly 28 atm and that red blood cells rupture at 7 atm. Nevertheless, membranes have been developed that make it feasible to purify water using this technique. Reverse osmosis currently supplies pure drinking water to individual households as well as entire municipalities. 

Membrane filtration processes, such as reverse osmosis, and micro and ultra filtration, are used to filter out dissolved solids in certain applications see Table 10.9. These specialised processes will not be discussed in this book. A comprehensive description of the techniques used and their applications is given in Volume 2, Chapter 8 see also Scott and Hughes (1995), Cheryan (1986), McGregor (1986) and Porter (1997). 

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