drocarbons (benzenes and naphthalenes). Phoenix, Arizona, represents another large municipality with a variety of commercial activities.
The wastewater from Washington, D.C., comes largely from residential and government-related activities, and the above-noted organic contaminants are relatively low in concentration. It should be noted that the concentrations listed for the Washington, D.C., water are for a blend of biologically treated municipal wastewater and Potomac River water. The concentrations would be higher for many of the contaminants in the wastewater itself.
Table 2-5 also contains a comparison of data from Orange County for two different time periods to illustrate the effects of using different biological treatment processes and of segregating wastewaters to reduce the industrial contribution to reclaimed water. During the first period noted, all wastewaters from Orange County were treated by trickling-filter biological treatment, and this water was used as the influent to Water Factory 21, the AWT system. During the second period, the wastewaters were treated by the activated sludge process, and segregation reduced the industrial contribution in the water sent to Water Factory 21. These changes significantly reduced the chemical oxygen demand (COD) of the treated wastewater and the concentrations of both chlorinated and unchlorinated benzenes and naphthalene. However, the concentrations of trihalomethanes, which normally result from water disinfection, increased. As anticipated, better AWT effluent quality was achieved for contaminants when their influent concentration was lower—confirming that virtually any effort to improve the quality of incoming wastewater will improve the treated water's quality.
Disinfection ranks as the most important single process for inactivating microorganisms in water and wastewater treatment. However, in some cases, reactions of disinfectants with organic and inorganic constituents in the source water can create potentially harmful (in some cases, carcinogenic) disinfection by-products (DBPs) (Bull and Kopfler, 1991; ILSI, 1995). The most common disinfectants are chlorine-based oxidants, but ozone and ultraviolet light are also used. Other disinfectants, such as gamma radiation, bromine, iodine, and hydrogen peroxide, have been considered for disinfection of wastewater, but they are not generally used because of economical, technical, operational, or disinfection efficiency considerations.
A limited number of DBPs are regulated or being considered for regulation. Chief among these are the trihalomethanes, haloacetic acids