| Water availability represents a growing concern for meeting human needs in the future. It is estimated that more than 60% of the world's population will lack access to an adequate supply by 2025, largely in Asia, Africa, and Latin America (Feeley et al., 2008). The U.S. is also not immune from water supply problems. As water use is increasing every year, at least 36 states are anticipating local, regional, or statewide water shortages by 2013, even under non-drought conditions (EPA, 2008). In response to the water scarcity issue, wastewater reuse is recognized as an environmentally sound approach for sustainable water management.;Among the major freshwater users in the US, thermoelectric power generation has recently become the top user (201 Bgal/day or 49% of total water withdrawals). Each kilowatt-hour (kWh) of the thermoelectric generation requires the use of approximately 25 gallons of water, which is primarily for cooling purposes (Feeley et al., 2008). As population growth and economic development continue to increase the demand for electric power, it is necessary to ensure a reliable and abundant water source for thermoelectric power generation. However, freshwater shortages and the competing demand from other water uses will increase pressure on power plants to reduce water consumption and adopt wastewater reuse. Several cases in arid areas, such as Arizona and Texas, have shown that lack of available cooling water sources can result in suspension in operation of existing power plants and delay construction of new power plants (Feeley and Ramezan, 2003; Dishneau, 2007).;Power plant cooling demands large quantities of relatively low quality water when compared to other uses of water (e.g., drinking and food production). Therefore, alternative sources to replace freshwater for cooling system operation are likely to be in great demand in the near future. Among all the alternatives, treated municipal wastewater (MWW) is a promising candidate for power plant cooling due to its widespread availability and consistent quality (Li et al., 2011a; EPRI, 2008). Use of treated MWW as make-up water for cooling in power plants has been in full-scale operation for several decades (Osborn, 1970; Humphris, 1977; Rebhun and Engel, 1988; Wijesinghe et al., 1996). However, these power plants typically use treated MWW only as a fraction of the total makeup water needed or only after significant additional treatment before addition to the recirculating cooling systems (Wijesinghe et al., 1996; Li et al., 2011a). Few studies have focused on the feasibility of using treated MWW as the dominant makeup water with or without additional prior treatment.;The main challenges when using MWW as cooling system makeup are scaling, corrosion, and biofouling (biological growth) due to impaired water quality (Wijesinghe et al., 1996; Selby et al., 1996; Puckorius, 2003; Vidic, 2009). The term "scaling" is generally used to describe the collection and growth of unwanted inorganic salts which increases both pressure drop and resistance to heat transfer in the cooling systems (Neufeld et al., 1985). The scaling problem would be exacerbated under typical recirculating cooling tower operations (i.e., elevated temperature and evaporative loss of water that leads to concentration of minerals). Because of the negative environmental impacts, traditional once-through cooling water systems are discouraged and recirculating cooling water systems are the only option for new or repermitted thermoelectric power plants (Reynolds, 1980).;Mineral scaling could be prevented in a number of ways. For mineral deposits that are pH-dependent, like calcium carbonate, acid addition can reduce the pH and alkalinity in the recirculating systems and thus lower the formation potential of some mineral scales. However, increased corrosion rates would occur at lower pH (Troup and Richardson, 1978). Physical water treatment (PWT) methods, including magnetic fields, electric fields, alteration of surface charges of water, and mechanical disturbance for scaling control have been reported in bench-and pilot-scale studies (Cho et al., 2004). However, the effect of the PWT on the mineral scaling is still questionable in real practice (Troup and Richardson, 1978). Up to now, addition of chemicals that serve as antiscalants is still the most effective approach for mitigating mineral scaling (Al Nasser et al., 2011).;Although there is abundant experience with scaling inhibition in freshwater, scaling control for treated MWW used as makeup for cooling systems is rather challenging due to its complicated water chemistry. Most antiscalants that have been proven effective in freshwater may not be as effective in treated MWW (Li et al., 2011b). In order to advance the reuse of the treated MWW as cooling water in power plants, it is necessary to develop and implement sound scaling control technologies for different types of treated MWW. |
