Treatment for Dissolved Solids

The traditional way to free water of dissolved solids is to distil it, either at atmospheric pressure or by multistage flash evaporation at reduced pressure. Distillation removes virtually all solutes but is wasteful of energy unless the low grade heat can be economically recovered from the condensers. Flash evaporation is attractive in countries such as Saudi Arabia where energy is inexpensive and the only plentiful source of water is the sea, but problems usually arise with deposition of CaCOa, Mg(OH)2, and CaS04 scales. 

Alternatively, potable water can be extracted from seawater by freezing salts, which depress the freezing point of water, remain in the liquid phase. Generally, though, it is more practical to remove the relatively small amount of solutes (typically, 0.02% for river water) from the great excess of water, rather than vice versa. Seawater is an exceptional case, with about 3.5% dissolved solids. Water softening is concerned primarily with removal of Ca + and Mg +, but for some purposes removal of all dissolved solids deionization or demineralization) is necessciry. 

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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). 

Treatment of brackish waters in the production of potable supplies has been the largest application of electrodialysis. Costs associated with electrodialysis processes depend on such factors as the total dissolved solids (TDS) in the feed, the level of removal of TDS (percent rejection), and the size of the plant. In brackish water treatment, operating costs for very large ED installations (on the order of millions of gallons a day) have been between 40 cents to 50 cents per 1,000 gallons for brackish feed waters, which compares favorably with RO costs. 

Boiler feedwater may be from various supplies. If it is from a main water supply, further filtering prior to treatment is unlikely, but for other supplies such as borehole, lakes, rivers and canals, filters may be required. Impurities in water may be classed as dissolved solids, dissolved gases and suspended matter and suitable treatment is required. 

When steam in the cycle is lost or used in a process, the reduced volume of returning condensate is compensated for by introducing some level of MU water. The loss of water or steam from a steam system cycle may vary from 1 to 100%. The supply of MU (and to a lesser degree the addition of chemical treatments) provides a source of dissolved solid contaminants that can concentrate in the boiler until some predefined limit is reached. At this point, BD is required, the loss of which is also compensated for by the addition of further MU water. 

Although the presence of hardness is reported as calcium carbonate, in reality, for most water supplies the most common major contributors to total dissolved solids are calcium and magnesium bicarbonates. These dissolved solids most readily produce crystalline scales and thus predominantly contribute to boiler system deposits unless removed by some form of pre-boiler, external treatment process. 

This formulation is designed as a single-drum treatment for boilers operating at up to 300 psig and with a total hardness (TH) in the FW of up to 30 ppm CaC03. Initial feed is 15 ppm product per 1 ppm TH (as CaC03) in the FW. Control is based on maintaining 200 to 300 ppm hydroxide (OH) alkalinity and a maximum of 3,000 ppm total dissolved solids (TDS) in the BW. 

Applications RO is primarily used for water purification seawater desalination (35,000 to 50,000 mg/L salt, 5.6 to 10.5 MPa operation), brackish water treatment (5000 to 10,000 mg/L, 1.4 to 4.2 MPa operation), and low-pressure RO (LPRO) (500 mg/L, 0.3 to 1.4 MPa operation). A list of U.S. plants can be found at www2.hawaii.edu, and a 26 Ggal/yr desalination plant is under construction in Ashkelon, Israel. Purified water product is recovered as permeate while the concentrated retentate is discarded as waste. Drinking water specifications of total dissolved solids (TDS) < 500 mg/L are published by the U.S. EPA and of < 1500 mg/L by the WHO [Williams et ak, chap. 24 in Membrane Handbook, Ho and Sirkar (eds.). Van Nostrand, New York, 1992]. Application of RO to drinking water is summarized in Eisenberg and Middlebrooks (Reverse Osmosis Treatment of Drinking Water, Butterworth, Boston, 1986). 

A reference solution is prepared by a dilution method. A known quantity of sample is dissolved in a known volume of the system buffer of known pH the amount of sample is X times less than in the above case in order to avoid precipitation in the formed solution. The spectrum is immediately taken by the UV spectrophotometer, to take advantage of the possibility that solution may be supersaturated (i.e., solid should have precipitated, but because not enough time was allowed for the solid to precipitate, the solution was temporarily clear and free of solid). Mathematical treatment of the spectral data yields the AUC of the reference sample solution, AUQ . The ratio R = AUCS/AUCS is used to automatically recognize the right conditions for solubility determination when the reference has no precipitate, and the sample solution is saturated with precipitate. Under these conditions, solubility is determined from the expression... 

Efficiencies for removal in the wastewater treatment plant were estimated for total and soluble BOD, total COD, soluble COD, color, total suspended and dissolved solids, and total solids. The removal efficiencies summarized in Table 21.14 are high for total BOD, soluble BOD, and suspended solids, at 96%, 96%, and 95%, respectively. The removal efficiencies for total and soluble COD were significantly lower at 76% and 66%, respectively. The removal efficiency for color was only about 38%. This value is typical for biological treatment of pulp and paper wastewater, and may be due, at least partially, to the formation of new colored groups when the bleach effluents are oxidized in the treatment system. 

Sepaflot A process for removing solids and oil from wastewater by a combination of Activated Sludge treatment and dissolved air flotation. Offered by Lurgi. 

The waste from this treatment scheme should be suitable for discharge or reuse with a possible need for removal of inorganic dissolved solids if a "zero discharge" system is used. 

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