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Report technique in turn increases the discharge volume. This may or may not be a problem, depending upon the environmental authorities in charge of permitting. For new plants, this factor could be very important with regard to plant location and the quality of the water available for makeup. The emerging restrictions on sulfate in the discharge may, in some cases, eliminate makeup water acid treatment as a scale-control mechanism. An alternative method to reduce the threat of calcium carbonate scaling, albeit at considerable expense and increased system complexity, is makeup water cold-lime softening. This technique will lower both the calcium and bicarbonate alkalinity of the stream, but not without proper operator control and monitoring of the unit. A factor that may favorably influence softening for some applications is the ability to also include magnesium feed for reduction of silica. Silica chemistry is quite complex, but a general rule of thumb suggests 150 ppm as the silica limit in the cooling tower recirculating water, with possibly an upper limit of 200 ppm or so with some of the newer chemical treatment programs. This is often not an issue with surface water supplies, as these may contain only a relatively small silica concentration. However, groundwater is a fairly common source for cooling tower makeup, and some supplies, particularly in the West, may contain 30 to 50 ppm of silica. Thus, the high makeup silica will greatly limit the cycles of concentration in the tower, unless some of it is removed from the makeup. If phosphate discharge is prohibited, one possible alternative for recirculating water treatment is an allpolymer treatment program.2 A simple polymer, often a polyacrylate, typically is included in a phosphate/ phosphonate program to act as a sequestering agent and/or crystal modifier. An all-polymer program, on the other hand, relies on what are known as co- and ter-polymers that have more than one functional group to act as sequestering agents/crystal modifiers. One helpful technology that has been enhanced over the last decade or so is that of tagged polymers. The chemicals contain a functional group that can be monitored by fluorescence or other techniques, the data from which can then be used for automatic chemical control of cooling water chemistry. Another potential option is blowdown treatment by clarification, with primary coagulant being a compound, such as ferric chloride, that will cause the phosphate to precipitate and be removed with the clarifier sludge blowdown. With regard to the copper issue mentioned above, for new plants straightforward solutions include no 20 wateronline.com ■ copper in the condenser and a selection other than wood for the cooling tower material. FRP (fiberglass reinforced plastic) is becoming a common choice as a tower material. For existing plants with wood towers and a copper discharge issue, the solution is more problematic. Replacement of the tower is, of course, one possibility, but obviously at considerable cost. One potential wastewater technology is treatment of the stream in a clarifier that utilizes a coagulant with sulfide active groups. This treatment method has been implemented at some coal plants for mercury (Hg) removal from wet flue gas desulfurization streams. Copper also reacts quite readily with sulfide, so this technology may represent a control technique for that element as well. Backend Discharge Reduction Several methods are possible to reduce the volume of plant discharge, but I would like to focus on one rapidly emerging technology, a generic diagram of which is outlined below. One version of this process is licensed for various markets as HEROTM by such firms as Aquatech, GE, Figure 2: Generic outline of an emerging wastewater treatment technology and U.S. Water, while Veolia supplies their OPUSTM technologies for this purpose. Keys to the process are: • • • • • Micro- or ultrafiltration (UF) to remove suspended solids in the waste stream Sodium bisulfite (NaHSO 3) feed to remove residual oxidizing biocides A sodium softener to remove calcium and magnesium Sodium hydroxide injection to elevate the pH above 10 (The combination of hardness removal and pH elevation keeps silica in solution.) Two-pass reverse osmosis (RO) treatment to recover 90 percent of the water While the process appears straightforward, my colleagues and I have directly observed operating Water Online The Magazine