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can be used to reduce the occurrence of nitrification (Skadsen et al. 1996). However, many other <br />factors contribute to the viability of nitrifying bacteria and as a result, nitrification episodes have <br />been observed at pH levels ranging from 6.6 to 9.7 (Odell et al. 1996). <br />0.7 <br />0.6 <br />0.5 <br />F <br />oq <br />0.4 <br />0.3 <br />z <br />m <br />0.2 <br />0.1 <br />- - - 1 Nitrosomonas - - - - <br />0.. , <br />6.0 6.7 7.1 <br />0 <br />Z 1.5 <br />0o <br />N <br />'O 1 <br />0 <br />z <br />E <br />Nitrobacier <br />7.3 7.5 7.8 8.0 8.3 8.7 6.0 6.5 6.8 7.1 7.5 7.7 8 8.3 8.8 <br />PH pH <br />Source: Grady and Lim 1980 <br />Figure 3 <br />Effects of pH on Nitrosomonas and Nitrobacter enrichment cultures <br />According to Wilczak (2001) pH appears to be the most important factor controlling the rate of <br />chloramine autodecomposition. Thomas (1987) stated that the rate of chloramine decay <br />approximately doubles for a drop in pH of 0.7 units. <br />3.1.4 Disinfection Practices <br />Chemical control or treatment of nitrifying bacteria typically involves either the maintenance of <br />high distribution system disinfectant residuals (greater than 2 mg/L) or periodic breakpoint <br />chlorination. Analytical survey results of ten U.S. utilities showed that greater than 90% of <br />distribution system samples with increased nitrite and nitrate levels, indicative of nitrification, <br />occurred in water with disinfectant residuals less than 2 mg/L (Wilczak et al. 1996). Many <br />utilities have found that increasing disinfectant residuals by increasing chemical doses or <br />managing water age has helped to control nitrification. <br />Utilities can use booster chlorination in the distribution system to increase disinfectant residuals. <br />This practice is generally not employed in chloraminated distribution systems because <br />chloramines are normally more stable than free chlorine (Woolschlager et al. 2001, Valentine et <br />al. 1998). In addition, uncontrolled blending of chlorinated and chloraminated water could occur <br />near a chlorine booster station; in some cases uncontrolled blending has been shown to cause <br />unintended breakpoint chlorination, increases in DBP levels, or decreases in disinfectant <br />residuals (Mahmood et al. 1999 and Muylwyk et al. 1999). In 2001, one utility installed a <br />chloramine booster station that adds both sodium hypochlorite and aqueous ammonia to increase <br />residuals in a remote area of a distribution system (Potts et al. 2001). The booster station was <br />Prepared by AWWA with assistance from Economic and Engineering Services, Inc. <br />10 <br />