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Show prohibits the discharge of cooling water to the sewer system. In addition, the city has provided rebates on equipment purchases as a financial incentive to further water conservation. Water Conservation Water operative air conditioners can affect water resources in three ways. They consume a significant amount of water, contribute to the quantity of the wastewater stream and may impact the quality of the wastewater stream. Water Consumption The amount of water consumed depends largely on the type of cooling system. Once- through or single- pass systems consume the most water. For example, a 500- ton capacity once- through type unit requires 300 gpm. By comparison, an equivalent capacity recycling system requires only about 8.4 gpm. With evaporative coolers, water use may vary among types, but it is generally insignificant in comparison to once- through recycling type cooling systems. Water consumption may be less than one- tenth that of recycling systems. Table 1 summarizes typical water consumption for each of the three cooling system types. Wastewater Quantity and Quality The quantity of wastewater also varies by the type of cooling system. Once the thermal value of the water is used, a single- pass system discharges the entire stream ( 300 gpm from the previous example) to the wastewater collection and treatment system. For recycling type systems, part of the total water consumed is lost to evaporation and drift; the remainder is discharged to waste. The amount of blow down is about 30 percent of the makeup water or 2.4 gpm. This equates to a reduction of approximately 77 million gallons wastewater annually for a 500- ton capacity unit. While recycling reduces the amount of water sent to waste, it contributes to the pollutant loading of the wastewater stream. The pollutants come from biocides, dispersants and corrosion control chemicals which are added to the recirculating water stream of cooling towers to prevent scale formation and other maintenance problems. The two primary pollutants are copper and zinc. From a study performed for the city of San Francisco, the net increase in copper and zinc concentration of blow- down water was 0.21 mg/ 1 and 0.79 mg/ 1 reciprocity. Assuming that all once- through type cooling systems were converted to recycling systems with cooling towers, the wastewater stream would receive an additional 4,825 lbs. of copper and 18,095 lbs. of zinc each year. Costs The annualized cost of once- through systems was compared to recycling systems on a unit basis. As in the previous example, a unit cooling capacity of 500 tons was assumed. The costs are summarized in Table 2. The evaluation also assumes a useful life of 15 years and an annual interest rate of 5 percent. On an annualized basis, conversion to recycling type cooling equipment has a significant economic benefit. The evaluation did not include a comparison reflecting a conversion to, or incorporation of reuse rather than recycling. This is because the implementation of reuse is specific to the conditions of the application and may vary widely from installation to installation. Potential Regulations As previously mentioned, the industry appears to be headed in the direction of conservation, for the most part, without regulation. It is likely, however, that promulgation of appropriate standards or 75 |