Potable Waters

Drinking water is a major necessity of life many membrane processes have been developed to produce it and/or enhance its quality. Filtration [16] aims at the removal of  

Examples of the use of ceramic membranes in the production of potable water are quite numerous [42,60-65]. An interesting review is presented by Pouet et al. [60] of some 15 installations working with ceramic membranes for the production of drinking water. Sizes of these installations, installed in France between 1984 and 1990, vary from 5 to 100 m /h. 

Moncorge and Pascal [61] and Bauer et al. [42] describe the use of the carbon/carbon composite membranes of Le Carbone Lorraine in the filtration of drinking water. With 0.2 pm membranes the fluxes range between 1000 and 2000 1/m h at trans-membrane pressures from 1 to 2 bars. The use of Kerasep membranes [65] (Rhone-Poulenc s alumina/alumina membranes, 0.2 pm pore size) leads to fluxes of 600-12001/m h at 2 bar transmembrane pressure. 

Micro-organisms form a very important source of fouling various authors [66,67] report a strong decline in flux, even as much as 70% of the original value, in the presence of micro organisms. 

Moulin et al. [64] use a coagulant and ozone to enhance the flux of their 0.2 pm ceramic membrane (Membralox). Using ozone and a concentration of 45 ppm coagulant, flux is approximately 15001/m h, with 110 ppm coagulant flux increases to 20001/m h. The ozone treatment decomposes the organic material, so fouling is kept at a minimum. 

Please note that the proposed EPA drinking water standards reduces the coliform count from 1 to 0 per 100 ml. All types of water discussed from this point on will fall under the category process water. 

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The National Sanitation Foundation pubHshes a Hst of acetal resins which they find acceptable for use in potable water appHcations. 

Anionic and nonionic polyacrylamides effectively remove suspended soHds such as silt and clay from potable water. SuppHers provide special grades which meet EPA/FDA regulations for residual acrylamides. A recent pubHcation (102) states that hydrolyzed polyacrylamides with narrow interchain charge distributions provide better performance in flocculation of clay. These polymers were prepared by alkaline hydrolysis. (See Flocculating agents.)... 

Until 1990 the EPA maintained a Hst of chemicals suitable for potable water treatment ia the United States. Siace then the entire question of certification and standards has been turned over to a group of organi2ations headed by the National Sanitation Eoundation, which has issued voluntary standards. As of January 1992, standards had been issued for most of the principal inorganic products, but only for two polymers, poly(DADMAC) and Epi-DMA (epichl orohydrin dimethyl amine) polymers (78). Certifications for commercial products meeting specified standards are issued by the National Sanitation Eoundation, Underwriter Laboratories, and Risk Eocus/Versar (79). 

Water Treatment. Flotation in water treatment is used both for the removal of dissolved ions such as Cu ", Cr ", or (PO or surfactants and suspended soHds as in the case of sludge treatment. The final product in this case is purified water rather than a mineral concentrate. Furthermore, water is treated either for drinking purposes (potable water preparation) or safe disposal to the environment. 

Fluoridation of potable water suppHes for the prevention of dental caries is one of the principal uses for sodium fluoride (see Water, municipal WATER treatment). Use rate for this appHcation is on the order of 0.7 to 1.0 mg/L of water as fluoride or 1.5 to 2.2 mg/L as NaF (2). NaF is also appHed topically to teeth as a 2% solution (see Dentifrices). Other uses are as a flux for deoxidiziag (degassiag) rimmed steel (qv), and ia the resmelting of aluminum. NaF is also used ia the manufacture of vitreous enamels, ia pickling stainless steel, ia wood preservation compounds, caseia glues, ia the manufacture of coated papers, ia heat-treating salts, and as a component of laundry sours. 

A(jua.tlC Pollution. Aquatic pollution is of some concern from hydrothermal resources. The primary problem is the disposal of highly saline fluids from water-dominated reservoirs. This is generally overcome by pumping into deep reservoirs situated well beneath potable water sources. The fluid... 

Health and Safety Aspects. The U.S. EPA has significantly reduced the aHowed levels of antimony compounds in drinking water causing a toxicity cloud over the viabHity of this class of stabilizers. Presently, antimony products are no longer aHowed for use as potable water pipe stabilizers pending completion of NSE International s review (28). Eor these reasons, the future of this stabilizer technology appears limited. 

The first reverse osmosis modules made from cellulose diacetate had a salt rejection of approximately 97—98%. This was enough to produce potable water (ie, water containing less than 500 ppm salt) from brackish water sources, but was not enough to desalinate seawater efficiently. In the 1970s, interfacial composite membranes with salt rejections greater than 99.5% were developed, making seawater desalination possible (29,30) a number of large plants are in operation worldwide. 

Vacuum Treatment. Milk can be exposed to a vacuum to remove low boiling substances, eg, onions, garlic, and some silage, which may impart off-flavors to the milk, particularly the fat portion. A three-stage vacuum unit, known as a vacreator, produces pressures of 17, 51—68, and 88—95 kPa (127, 381—508, and 660—711 mm Hg). A continuous vacuum unit in the HTST system may consist of one or two chambers and be heated by Hve steam, with an equivalent release of water by evaporation, or flash steam to carry off the volatiles. If Hve steam is used, it must be cuUnary steam which is produced by heating potable water with an indirect heat exchanger. Dry saturated steam is desired for food processing operations. 

Magnitude of the residue in potable water, fish, irrigated crops Magnitude of the residue in food handling estabHshments... 

About 264,000 metric tons of elemental capacity is available in North America, plus another 79,000 t (P equivalent) of purified wet phosphoric acid (14). About 85% of the elemental P is burned to P2 5 hydrated to phosphoric acid. Part of the acid (ca 21%) is used direcdy, but the biggest part is converted to phosphate compounds. Sodium phosphates account for 47% calcium, potassium, and ammonium phosphates account for 17%. Pinal apphcations include home laundry and automatic dishwasher detergents, industrial and institutional cleaners, food and beverages, metal cleaning and treatment, potable water and wastewater treatment, antifree2e, and electronics. The purified wet acid serves the same markets. 

Water. Water mains should be connected to plant fire mains at two or more poiats, so that a sufficient water supply can be deHvered ia case of emergency. The plant loop and its branches should be adequately valved so that a break can be isolated without affecting a principal part of the system. If there is any question of maintaining adequate pressure, suitable booster pumps should be iastaHed. Any connection made to potable water for process water or cooling water must be made ia such a manner that there can be no backflow of possibly contaminated water check valves alone are not sufficient. The municipal supply should faH freely iato a tank from which the water is pumped for process purposes, or commercially available and approved backflow preventers should be used. 

F. Rubel, Jr. and F. S. Williams, Pilot Study of Fluoride and Arsenic Femoralfrom Potable Water, EPA-600/2-80-100, U.S. Environmental Protection Agency, Research Triangle Park, N.C., 1980. 

Desalination. Desalination of seawater and brackish water has been and, as of the mid-1990s, is the primary use of RO. Driven by a need for potable water in areas of the world where there is a shortage, this industry has developed. Desalination involves the reduction of the total dissolved soHds (IDS) concentration to less than 200 mg/L. RO offers several advantages over other possible desalination processes such as distillation (qv), evaporation (qv), and electro dialysis. The primary advantage of RO over the traditionally used method of distillation is the energy savings that is afforded by the lack of a phase change in RO. 

Distillation processes typically reduce the IDS concentration to levels weU below the required specifications. Because the product water from the two processes is combined, the RO process can produce water at higher I DS concentrations and stiU meet the potable water specifications. In addition, the power produced from the MSF process can be used in the RO process, cutting energy costs. 

Stringent OSHA composition limits exist for appHcations of brazing filler metals and solders. For example, only no-lead solders are permitted for joining parts that may come in contact with potable water. 

Carbon Adsorption. Carbon adsorption is a well estabflshed and widely used technology for the removal of organics from wastewaters and gaseous streams. Carbon adsorption is a proven technology for potable water treatment and capable of reducing organic concentrations to very low or nondetectable levels. 

Specific Conductance. The specific conductance depends on the total concentration of the dissolved ioni2ed substances, ie, the ionic strength of a water sample. It is an expression of the abiUty of the water to conduct an electric current. Freshly distilled water has a conductance of 0.5—2 ]lS/cm, whereas that of potable water generally is 50—1500 ]lS/cm. The conductivity of a water sample is measured by means of an a-c Wheatstone-bridge circuit with a null indicator and a conductance cell. Each cell has an associated constant which, when multiphed by the conductance, yields the specific conductance. 

Saline Water for Municipal Distribution. Only a very small amount of potable water is actually taken by people or animals internally, and it is quite uneconomical to desalinate all municipally piped water, although all distributed water must be clear and free of harmful bacteria. Most of the water piped to cities and industry is used for Htfle more than to carry off small amounts of waste materials or waste heat. In many locations, seawater can be used for most of this service. If chlorination is requited, it can be accompHshed by direct electrolysis of the dissolved salt (21). Arrayed against the obvious advantage of economy, there are several disadvantages use of seawater requites different detergents sewage treatment plants must be modified the usual metal pipes, pumps, condensers, coolers, meters, and other equipment corrode more readily chlorination could cause environmental poUution and dual water systems must be built and maintained. 

Some inland municipalities now distribute water with salt content exceeding 1000 ppm, water so brackish as to be unpleasant to the taste, even though it is distributed as potable water. Each home may produce or purchase the very small requirement of freshwater for drinking and cooking. Small... 

Alternatively, small amounts of potable water may be deUvered by tmck to distribution centers or to tanks on house roofs. This system exists in Kuwait, which has many filling stations from which tank-tmck operators buy water at 1.00/m for distribution at about 3.00/m. Although much water is direcdy piped to residences in Kuwait today, 12% of the people still get their water by tmck. In Khartoum, Sudan, families that buy from vendors, who dehver sacks of water by donkey, pay an average of 16 per month. 

Abundant supplies of fresh water are essential to the development of industry. Enormous quantities are required for the cooling of products and equipment, for process needs, for boiler feed, and for sanitary and potable water supply. 

Silicates. For many years, siUcates have been used to inhibit aqueous corrosion, particularly in potable water systems. Probably due to the complexity of siUcate chemistry, their mechanism of inhibition has not yet been firmly estabUshed. They are nonoxidizing and require oxygen to inhibit corrosion, so they are not passivators in the classical sense. Yet they do not form visible precipitates on the metal surface. They appear to inhibit by an adsorption mechanism. It is thought that siUca and iron corrosion products interact. However, recent work indicates that this interaction may not be necessary. SiUcates are slow-acting inhibitors in some cases, 2 or 3 weeks may be required to estabUsh protection fully. It is beheved that the polysiUcate ions or coUoidal siUca are the active species and these are formed slowly from monosilicic acid, which is the predorninant species in water at the pH levels maintained in cooling systems. 

Use dictates the quaUty required. Potable water must be bacteriologicaHy safe, and toxic substances must be present at levels that are accepted as safe (1—3) (see Table 1). In addition, the water must be aesthetically acceptable. Water that is suitable for drinking may be iaadequate for many iadustrial processes. On the other hand, many iadustrial processes can use water that is not pure enough to drink. 

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