Reverse osmosis permeate quality

The combined hybrid systems met criteria for reuse in the textile industry. Differences in performance are discussed The high-quality of the reverse osmosis permeate meets the demands of the rinsing processes for industrial laundries... 

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. 

An UF system utilizing hollow-fiber (FIF) membranes has been successfully used as pretreatment prior to seawater reverse osmosis (SWRO) desalination without any chemical treatments [8]. The quality of UF permeate was good and satisfied the need of SWRO feed water [8]. 

There are two other types of technologies—reverse osmosis (RO) and electrodeionization (EDI)—that remove dissolved solids from a liquid stream. Both are widely used in the water purification industry and have potential for use in the treatment of CMP wastewater. RO is a method by which water is forced through a semipermeable membrane that does not allow ions to cross. EDI removes ions from a liquid stream by means of an applied voltage. Both the EDI or RO are very effective in removing anions and cations, but the tradeoff is that the feed to the EDI or RO must be preconditioned to prevent damage to the equipment. In particular, the feedwater to an RO should not have a silt density index (SDI) greater than 3.0, which may require additional filtration to ensure all the solids are removed from the liquid stream. Some EDI manufacturers recommend that the feedwater to the EDI be RO permeate or of better quality. 

For radioactive effluent treatment, the relevant membrane processes are microfiltration, ulfrafiltration (UF), reverse osmosis, electrodialysis, diffusion, and Donnan dialysis and liquid membrane processes and they can be used either alone or in conjunction with any of the conventional processes. The actual process selected would depend on the physical, physicochemical, and radiochemical nature of the effluents. The basic factors which help in the design of an appropriate system are permeate quality, decontamination, and VRFs, disposal methods available for secondary wastes generated, and the permeate. 

It is understood that the economical success of any membrane process depends primarily on the quality of the membrane, specifically on flux, selectivity and service lifetime. Consideration of only the transport mechanisms in membranes, however, will in general, lead to an overestimation of the specific permeation rates in membrane processes. Formation of a concentration boundary layer in front of the membrane surface or within the porous support structure reduces the permeation rate and, in most cases, the product quality as well. For reverse osmosis. Figure 6.1 shows how a concentration boundary layer (concentration polarization) forms as a result of membrane selectivity. At steady state conditions, the retained components must be transported back into the bulk of the liquid. As laminar flow is present near the membrane surface, this backflow is of diffusive nature, i.e., is based on a concentration gradient. At steady state conditions, the concentration profile is calculated from a mass balance as... 

On the other hand, the quality of the permeate is independent of the feed concentration. Whereas in seawater desalination reverse osmosis is strongly affected by the osmotic pressure of the (highly) concentrated feed solutions, membrane disdilation can handle even higher salt concentrations without a substantial decrease in membrane performance. The removal of volatile organic components (VOCs). such as chlorinated hydrocarbons or aromatics, from an aqueous solution is another application. These volatile contaminants are often present in very low concentrations in surface water or industrial effluent. 

Table 3.3.15 shows that thermal desalination of sea water or brackish water by multistage flash distillation is more energy intensive than membrane desalination, but can better deal with more saline water and delivers even higher permeate quality, although reverse osmosis usually fulfills the requirements of drinking water (Table 3.3.16). 

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