Treated water

Water has been treated for thousands of years, but only when the connection between bacteria in sewage and severe epidemics was made were procedures developed for safe water in the nineteenth century [8]. Today the World Health Organization (WHO), European Union (EU), and the United States Environmental Protection Agency (U.S. EPA), along with many other organizations, have a well-defined set of limits and standards for microorganisms and toxic substances in drinking water. 

The resistivity of potable water is usually between 1000 and 5000 Q cm. By definition, certain mineral constituents are also restricted. 

For example, the chlorinity will be not more than 250 ppm chloride ion in the United States or 400 ppm on an international basis. 

The total mineral content of water can be removed by either distillation or mixed-bed ion exchange. The level of purification maybe described qualitatively (e.g., triple-distilled water). However, the most accepted description for both distilled and demineralized water is in terms of its specific conductivity or resistivity. Demineralization actually removes the dissolved minerals and represents the most efficient treatment of hard water. The two most common and cost-effective methods are deionization and reverse osmosis. 

Reverse osmosis forces water through a semi permeable membrane under high pressure and varying flow conditions. This process can remove as much as 95 percent of dissolved minerals. Reverse osmosis units are often used in conjunction with a water softener pretreatment stage to reduce the demand on the osmosis process itself. Filters need to be flushed and replaced as necessary. Microbiological contamination also needs to be regularly monitored. 

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The treated water containing sodium chloride, cyanides, phenols and traces of H2S and NH3 is recycled to the crude desalting unit and used as wash water for the hydrotreaters and FCC units. 

A city s water supply is fluoridated by adding NaF. The desired concentration of F is 1.6 ppm. How many milligrams of NaF should be added per gallon of treated water if the water supply already is 0.2 ppm in F ... 

DeioniZa.tlon, The removal of cations and anions from water and replacement of them with hydrogen and hydroxide ions is called deionization. The completeness of the ionic removal is dependent on resin selection, design of the system, operating conditions, and the quaUty of treated water required. In general, systems become more complex as quaUty requirements increase. 

Sodium alumiaate is used ia the treatment of iadustrial and municipal water suppHes and the use of sodium alumiaate is approved ia the clarification of drinking water. The FDA approves the use of sodium alumiaate ia steam generation systems where the steam contacts food. One early use of sodium alumiaate was ia lime softening processes, where it iacreases the precipitation of ions contributing to hardness and improves suspended soHds removal from the treated water (17). Sodium alumiaate reacts with siHca to leave very low residual concentrations of siHca ia hot process water softeners. Sodium alumiaate is often used with other chemicals such as alum, ferric salts, clays, and polyelectrolytes, as a coagulant aid (18,19). 

Table 1 Hsts a number of common inorganic coagulants. Typical iron and aluminum coagulants are acid salts that lower the pH of the treated water by hydrolysis. Depending on initial raw water alkalinity and pH, an alkah such as lime or caustic must be added to counteract the pH depression of the primary coagulant. Iron and aluminum hydrolysis products play a significant role in the coagulation process, especially in cases in which low turbidity influent waters benefit from the presence of additional colHsion surface areas.

To improve magnesium reduction, which also improves siHca reduction in cold process softening, sodium aluminate may be used. The sodium aluminate provides hydroxyl ion (OH ) needed for improved magnesium reduction, without increasing calcium hardness in the treated water. In addition, the hydrolysis of sodium aluminate results in the formation of aluminum hydroxide, which aids in floe formation, sludge blanket conditioning, and siHca reduction. 

Alkalinity Reduction. Treatment by lime precipitation reduces alkalinity. However, if the raw water alkalinity exceeds the total hardness, sodium bicarbonate alkalinity is present. In such cases, it is usually necessary to reduce treated water alkalinity in order to reduce condensate system corrosion or permit increased cycles of concentration. 

Treated Water Quality. Predicted analyses of a typical raw water treated by various lime and lime—soda softening processes are presented in Table 3. Treatment by lime converts the sodium bicarbonate in the raw water to sodium carbonate as follows ... 

Add required treated water, withholding a quantity sufficient to rinse all containers to remove all traces of ingredients. 

Drinking water suppHed to carbonated soft drink manufacturing faciUties from private or municipal sources must comply with all regulatory requirements. Treated water must meet all U.S. Environmental Protection Agency primary maximum contaminant levels and may also be subject to additional state requirements. Treated water is routinely analyzed for taste, odor, appearance, chlorine, alkalinity, iron, pH, total dissolved soHds, hardness, and microbiological contamination. 

On the other hand, it has been demonstrated that the addition of CO2 to greenhouses iacreases the growth rate of plants so that an iacrease ia the partial pressure of CO2 ia the air could stimulate plant growth making possible shorter growing seasons and iacreased consumption of carboa dioxide from the air. CO2 is also used ia water-treatmeat appHcatioas. Because it is significantly safer than mineral acids, it can be used to reduce the alkalinity of treated water. 

As a weak acid (in aqueous solution) carbon dioxide neutralizes excess caustic in textile manufacturing operations. It does not injure fabrics and is easy to use. Carbon dioxide is also used for neutralizing alkaline wastewaters, treating skins in tanning operations, and carbonating treated water to prevent scaling. 

The conditions for chlorate formation are high pH, low reactant concentrations, and the presence of excess chlorine or hypochlorous acid. Thus, the addition of free chlorine or hypochlorite to chlorine dioxide treated water, which contains chlorite as a by-product of the chlorine dioxide treatment, predominandy forms chlorate in the pH 5—8 range typically used in water treatment (140). 

Water Consumption Jigs require much water. In most installations, the Harz-type jig uses 0.006 to 0.01 m water/kg (1500 to 2500 gal/ton) material treated. Water requirements for treating minus 10-mm (%-in) iron ore in a Wemco-Bemer rougher-cleaner jig circuit are approximately 0.005 m water/kg (1200 gal/ton) of material processed. 

The outer crust is composed of rust (hematite), precipitate, and settled particulate. Treatment chemicals may also deposit preferentially atop tubercles in response to associated corrosion. It is common to find several percent of zinc and phosphorus compounds in tubercles that grow in zinc- and phosphate-treated waters. Silicates also can be found in... 

The treated water contains sufficient concentration of surface film-forming agents if cold water spends about 12 min and warm water at least 20 min in the tank [19]. Sudden temperature variations over 10°C must be prevented because the active form of Al(OH)3 is sensitive to them [20]. If mixing with cold water or subsequent warming cannot be avoided, a short-term electrolytic aftertreatment must be provided in a small reaction tank. The development of undisturbed protective films in the tubing assumes continuous water flow with forced circulation by pumps [20]. 

Fig. 30-1. Integrated environmental control for electric generating plants (5). APH, air preheater ESP, electrostatic precipitator TREAT, water treatment system.

The injection of steam in the compressor discharge has been utilized over the years and has been found to be very effective. The amount of steam to be injected can vary from 5-15%. The injection of steam created from properly treated water does not affect the life of the hot section of the turbines. This is based on a large number of units where steam injection has been used. Steam injection, with an evaporative cooling inlet system would be best suited for hot humid areas this application based on the efficiency and cost as shown in Figure 2-45. 

Utility Baiances. The operating company should also require a balance for each plant utility. The most involved of the utility balances is usually the supply/demand steam tabulation showing all levels of steam and condensate and their interactions. The steam balance is almost always required at this stage for any required side studies. The steam balance influences many design parameters, such as boiler size and contingency, treated water makeup rates, blow -down disposal rates, chemicals usage, and surface condenser size. 

It is important for crosstied systems that a sufficient condensate network is provided for balancing the mix of condensate return and makeup treated water as required. The author has seen a system designed with process area and utility area fired boilers of the same pressure. Periodically, the utility area was required to supply makeup steam to balance a shortage in the process area, but no provisions were made to return equivalent condensate from the process to the utility area. The earlier such a mistake is caught, the better. 

For wet ESPs, consideration must be given to handling wastewaters. For simple systems with innocuous dusts, water with particles collected by the ESP may be discharged from the ESP system to a solids-removing clarifier (either dedicated to the ESP or part of the plant wastewater treatment system) and then to final disposal. More complicated systems may require skimming and sludge removal, clarification in dedicated equipment, pH adjustment, and/or treatment to remove dissolved solids. Spray water from an ESP preconditioner may be treated separately from the water used to wash the ESP collecting pipes so that the cleaner of the two treated water streams may be returned to the ESP. Recirculation of treated water to the ESP may approach 100 percent (AWMA, 1992). 

For PM applications, wet scrubbers generate waste in the form of a slurry or wet sludge. This creates the need for both wastewater treatment and solid waste disposal. Initially, the slurry is treated to separate the solid waste from the water. The treated water can then be reused or discharged. Once the water is removed, the remaining waste will be in the form of a solid or sludge. 

Eliminates high cost of water including expense of treating water. 

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