Permeate water

Salt flux across a membrane is due to effects coupled to water transport, usually negligible, and diffusion across the membrane. Eq. (22-60) describes the basic diffusion equation for solute passage. It is independent of pressure, so as AP — AH 0, rejection 0. This important factor is due to the kinetic nature of the separation. Salt passage through the membrane is concentration dependent. Water passage is dependent on P — H. Therefore, when the membrane is operating near the osmotic pressure of the feed, the salt passage is not diluted by much permeate water. 

NOTE Generally speaking, the higher the TDS of the RW source to the RO, the higher the applied pressure required to produce a constant permeate water TDS. Also, for practical reasons, the rate of permeate recovery tends to decrease with increase in source water TDS. 

Permeate flush tank, from which an initial fill of permeate water is used to flush out the system on shutdown... 

An ASTM standard recommends the use of 0.005 normal calcium sulfate as the standard permeating water, because of its medium range electrolyte concentration. Calcium sulfate, with divalent calcium, will usually not reduce hydraulic conductivity. 

Feed water (Cp, Qp) is allowed to enter into the module from its left side. Permeated water at each membrane section is expressed simply by Solution Diffusion model (Eq (1), (2) in Table 3) Concentrated water (Cg, Qg) is discharged from the right side of the module. Summation of product water at each membrane section results in total product water (Cp, Qp). At each membrane section, material balance is taken on each component (Eq (7), (8) in Table 3). 

Figure 1. Exploded view of RO cell (to scale). The various components of the cell fit together, are compressed by machine bolts, and are sealed with Viton O-rings. The membrane (effective diameter = 3.8 cm) is compressed against a porous steel plate (1/16 in., porosity = 25 pm) and flushed with feed solution. A certain amount of water penetrates the membrane and is collected as the permeate water (D). The feed solution enters the cell (A) and washes across the membrane (B) before being forced out of the cell (C).

Maximized recovery of as many different organic compounds as possible is a primary goal of the RO concentration process. Potential losses of compounds can occur through (1) the permeate water stream, (2) volatile headspaces, (3) adsorption and absorption onto system components, and (4) binding or coprecipitation to or with other compounds such as humic acids or insoluble inorganic salts. Control of most if not all of these factors can be attained through the manipulation of process variables. 

Acrylonitrile grafted poly(vinyl alcohol) membranes were found to be capable of permeating water in preference to acetic acid from aqueous acetic acid mixtures. The permeation rate of water increases whereas permeation rate of acetic acid decreases as the water content of the feed increases. It was also determined that increase in temperature increased the permeation rate without affecting the separation factor much. As the downstream pressure increased permeation rate increased whereas separation factor decreased and from the PSI values of the membranes it could be said that especially at high acetic acid concentrations membranes behaved more separable. 

In the case of desalination of water by reverse osmosis illustrated in Figure 4.3(a), the salt concentration cio adjacent to the membrane surface is higher than the bulk solution concentration c, because reverse osmosis membranes preferentially permeate water and retain salt. Water and salt are brought toward the membrane surface by the flow of solution through the membrane J,.1 Water and a little salt permeate the membrane, but most of the salt is rejected by the membrane and retained at the membrane surface. Salt accumulates at the membrane surface until a sufficient gradient has formed to allow the salt to diffuse to the bulk solution. Steady state is then reached. 

In the case of reverse osmosis, the enrichment factors (E and Ea) are less than 1.0, typically about 0.01, because the membrane rejects salt and permeates water. For other processes, such as dehydration of aqueous ethanol by pervaporation, the enrichment factor for water will be greater than 1.0 because the membrane selectively permeates the water. 

Pervaporation - photocatalysis In the described systems the membrane usually permeates water and rejects the reactants, enhancing their residence time in the photoreactor. However, it is known that some intermediate products of the photo-catalytic degradation of organic compounds can negatively affect the reaction rate, therefore, in some cases it is useful to eliminate these by-products in order to improve the thermodynamic and/or the kinetics of the reaction. 

Reverse osmosis Pressure Difference in membrane permeation Water—retains virtually all ions Symmetrical porous Water desalination... 

Ultrafiltration Pressure gradients Difference in membrane permeation Water and salts—retains macromolecules Asymmetrical porous (1-100 nm) Separation of high and low MW compounds... 

The by-product of sap concentration by an RO, permeate, is also used in maple operations as a source of very clean water. Due to the low mineral concentration of permeate water, it is used for cleaning tubing and evaporator equipment, as well as the RO membrane itself, which should be run through a wash and rinse cycle after each use. The chemicals and dosage to be used are specified by the membrane and RO manufacturers, and should be carefully followed to avoid damaging the membrane or contaminating the sap concentrate. 

Higher recovery results in the need to dispose of less reject water. However, higher recovery also results in lower-purity permeate. Consider the example shown in Figure 3.1. At the influent end of the membrane, the influent concentration is 100 ppm, while the recovery is 0%, and the membrane passes 2% total dissolved solids (TDS) (see Chapter 3.3). The permeate right at this spot would be about 2 ppm. As the influent water passes across more and more membrane area, more water is recovered. At 50% recovery, the concentration factor is 2, so the influent water now has a concentration of about 200 ppm. The permeate water at this point would now have a concentration of 4 ppm. At 75% recovery, the concentration factor is 4, so the influent water now has a concentration of about 400 ppm. The permeate water at this point would have a concentration of 8 ppm. Hence, higher recovery results in lower product purity. 

First, when membranes come off line, they should be flushed with either permeate water or low-pressure feed water (see Chapter 13.1.1). This will reduce the concentration of ions and any suspended solids on the feed side of the membrane, thereby minimizing the potential for fouling or scaling the membrane while idle. The next step(s) depends on how long the membranes will be off line. 

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