For the purpose of comparison, water quality characteristics of estuarine water and reclaimed water are included in Table 3.2. Water quality associated with freshwater sources, such as surface water or groundwater, is dependent on the local geohydrology, but would normally be similar to reclaimed water. As shown, there are differences in the content of nutrients, organics, and salts among the water sources. Seawater has higher levels of dissolved minerals than estuarine water.
The suitability of a water source for use in recirculating water systems as compared to once-through non-contact cooling is related to water quality and water availability. High levels of dissolved minerals and/or can demand additional treatment to prevent scale formation. For example, cooling systems that rely on estuarine or seawater tend to be non-recirculating, unless it is economical to provide on-site treatment for control of dissolved solids.
water, alternative sources such as surface water, groundwater, estuarine, or seawater may also be available locally. Thus the incentives for using reclaimed water are specific to the situation. When reclaimed water is used in lieu of surface or groundwater, the higher quality water sources can be preserved for other applications, such as drinking water. On the other hand, when reclaimed water is used in lieu of estuarine or seawater the impetus is usually related to discharge limitations.
Depending on the water source, water quality can vary seasonally. Groundwater tends to have fairly consistent characteristics, whereas the quality of surface water, estuarine water, seawater, and reclaimed water can be influenced by patterns of rainfall, runoff and evaporation, leading to significant seasonal variations in oxygen demand and suspended material (Fig. 3 . 5 ) , nutrient levels (Fig. 3.6) and chloride (Fig. 3.7). Although water quality is source specific, the variation in wastewater characteristics and treatment alternatives means that similar trends arise with all reclaimed water sources. Levels of dissolved minerals associated with estuarine water are shown for sodium and chloride in Fig. 3.8 and for calcium, magnesium, potassium, and sulphate in Fig. 3.9. Dissolved solids levels in estuarine waters are almost two orders of magnitude higher and sulphate and magnesium levels one order of magnitude higher than those levels associated with freshwater or reclaimed water. Typically, the mineral content of seawater can be two- to three-fold higher than that associated with estuarine waters. These water quality characteristics influence the extent of treatment required to allow for use of recirculating systems.
Another characteristic of reclaimed water that is different from fresh or saline water sources is the potential presence of a disinfectant residual. Reclaimed water is treated to meet requirements pertaining to microbiological safety. As such, disinfection is a key component of the treatment system. When chlorine is used for this duty, residual chlorine is usually present in the reclaimed can act as a biocide and help to prevent biological fouling of the cooling system. There is also little seasonal variation in total residual chlorine (Fig. 3.10), provided chlorine is automatically dosed on demand through feedback control. Chlorine levels associated with reclaimed water thus depend on the operating practices of a given facility and, therefore, may differ from the trends shown in Fig. 3.10. In cases where disinfection is accomplished using ultraviolet (UV) irradiation or ozonation, residual disinfectants will not be present in the reclaimed water (Levine etal., 2002).