Feed water desalination

The conditions utilized in the above development of minimum energy are not sufficient to describe electrodialysis. In addition to the desalination of water, salt is moved from a saline feed to a more concentrated compartment. That free-energy change must be added to the free energy given in Eq. (20-107), which describes the movement... 

Abstract This chapter discusses the characteristics of membrane concentrate, and the relevance that the concentrate has on the method of disposal. Membrane concentrate from a desalination plant can be regarded as a waste stream, as it is of little or no commercial benefit, and it must be managed and disposed of in an appropriate way. It is largely free from toxic components, and its composition is almost identical to that of the feed water but in a concentrated form. The concentration will depend on the type of desahnation technology that is used, and the extent to which fresh water is extracted from the brine. Based on the treatment processes that are used, a number of chemicals may also be present in the concentrate, albeit in relatively small quantities. 

The majority of the discharge from a desalination processes is concentrated brine from the membrane process, and this may contain quantities of treatment chemicals used. Treatment of water is necessary in all desalination plans for variety of reasons feed water treatment, membrane protection, membrane cleaning, permeate treatment and concentrate treatment prior to discharge. Although non-chemical treatment is possible, chemical treatment is widely practiced. 

NF has been also been studied as a potential form of pretreatment for reverse osmosis desalination processes (Hassan et al. 1998, 2000). Based on the feed water, it may be a suitable pretreatment method that allows for operation with little or even no use of antisealants. 

The concentrate volume and plant location will tend to be the two major factors that determine the suitability of a particular option. The concentration of the brine is also important, as the severity of potential environmental impacts generally increases with increasing concentration. The siting of a desalination plant must take into account the availability of disposal options, along with potential sources of feed water for the plant and the proximity to the end user. 

If the concentrate is discharged to the same body of water as the feed water source, both the intake and outfall should be located so as to not to interfere with one another. The presence of any other local desalination or wastewater discharge is an important factor, as these may increase the salinity of the receiving water or reduce its quality, thereby reduce the compatibility of the concentrate. 

Long term experiments of one pass seawater desalination using our module have been carried out at Chigasaki Laboratory of Water Reuse Promotion Center under the supervision of Ministry of International Trade and Industry, Japan (4). A continuous long term test for 12,000 hours was successfully conducted with the m-value of 0.02. Another demonstration plant with an 800 m3/D capacity has also been operating over 3,000 hours at the recovery ratio of using the feed water of F.I. value of about 4. 

Reverse osmosis desalination plants consisting of 8 long tubular membranes prepared from commercial CA of D.S. 2.5 were set up for supply of drinking water and boiler feed water at two different locations. 

As Figure 5.12 shows, Toray s PEC-1000 crosslinked furfuryl alcohol membrane has by far the best sodium chloride rejection combined with good fluxes. This explains the sustained interest in this membrane despite its extreme sensitivity to dissolved chlorine and oxygen in the feed water. Hollow fine fiber membranes made from cellulose triacetate by Toyobo or aromatic polyamides by Permasep (Du Pont) are also comfortably in the one-stage seawater desalination performance range, but the water fluxes of these membranes are low. However, because large-surface-area, hollow fine fiber reverse osmosis modules can be... 

Different sources of wastewater may contain different materials, so every type of wastewater must be tested in order to choose the right conditions. Flarussi et al. [28] compared the alternative costs of feed-water desalination for cities to wastewater desalination. The first alternative may be cheaper but the second is necessary in order to be compatible with the environment. 

The critical issue for a successful RO plant is pretreatment. Long-term operating experience proves the viability of continuous MF/UF pretreatment of RO for the desalination of a wide variety of water sources. MF/UF has proven to simplify and reduce the costs of traditional pretreatment, comprised of deep-bed media filters combined with chemical treatment. MF/UF produces filtrate of a consistent quality almost irrespective of fluctuations in feed-water quality. In the last five years, RO-membrane improvements, combined with the use of membrane filtration for pretreatment, have halved the cost of advanced treatment and are now more widely used for the reuse of municipal wastewater. 

Drak, A., Glucina, K., Busch, M., Hasson, D., Laine, J.M. and Semiat, R. (2000) Laboratory technique for predicting the scaling propensity of RO feed waters. Desalination, 132, 233-242. 

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]. 

Historically, a classic example of an evaporation process is the production of table salt. Maple syrup has traditionally been produced by evaporation of sap. Concentration of black liquor from pulp and paper processing constitutes a large-volume present application. Evaporators are also employed in such disparate uses as desalination of seawater, nuclear fuel reprocessing, radioactive waste treatment,preparation of boiler feed waters, and production of sodium hydroxide. They are used to concentrate stillage waste in fermentation processes, waste brines, inorganic salts in fertilizer production, and rinse liquids used in metal finishing, as well as in the production of sugar, vitamin C, caustic soda, dyes, and juice concentrates, and for solvent recovery in pharmaceutical processes. 

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. 

Provide usable water where none available Brackish water desalination Seawater desalination Pure water production Industrial rinse waters Food industry Electroplating Power plant boiler feed Beverage production Medical... 

A hollow-fiber membrane module similar to the one described in Example 2.14 is used for water desalinization. The feed water flows on the shell side at a superficial velocity of 5 cm/s, 298 K, 70 bar, and 2 wt% NaCl. The permeate flows in the fibers lumen at a pressure of 3 bar and a salt content of 0.05 wt%. For this particular membrane, a water permeance of 1.1 x 10-5 g/cm2-s-bar, and a salt rejection of 97% have been measured. 

During a study of the applicability of "spray" or "fog" evaporation to sea water desalination, it was found that this technique was particularly useful for scale deposition studies. Thus, test conditions are reproducible and heat transfer coefficients are very high, so that the effect of scale formation is readily apparent. Three novel methods for the control of scale deposits on the evaporating surfaces of a spray evaporator were explored. One involves the addition of small quantities of low molecular weight polyacrylic acid to the feed water, which prevents the formation of adherent scale. The methods are applicable under certain conditions to scales formed from sea water containing substantial amounts of calcium sulfate in addition to alkaline scale-forming substances. While spray evaporation appears to be of limited application in water desalination, the scale-control methods developed are probably applicable to other types of evaporator, particularly of the long-tube type. 

For a certain plant capacity, the required membrane area is directly proportional to the feed water concentration. This is illustrated in Figure 12. For brackish water of ca. 3(XX) ppm TdS and an average current density of 12 mA/cm, the required membrane area for a plant capacity of 1 m product per day is ca. 0.4 m of cation- and anion-exchange membrane. Other items such as pumps, electric power supplies, etc. depend on plant size. For desalination of brackish water with a salinity of ca. 3000 ppm the total capital costs for a plant with a capacity of 1000 m /d will be in the range of US 200,000.- to US 300,000.-. The costs of the actual membrane is less than 30 % of the total capital costs. Assuming a useful life of 5 years for the membranes and 10 years for the rest of the equipment, a feed water salinity of 3000 ppm and a 24-hours operating day, the total amortization of the... 

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