Can nitrite contamination be more of a problem in water distribution systems that have switched from chlorine to chloramines for secondary disinfections?

Maybe, depending on other conditions that could allow cultures of ammonia-oxidizing bacteria (AOB) to thrive in the water storage and distribution system. With the presence of AOB, also called nitrifying bacteria, any residual ammonia in the system left over from formation of chloramines, could be converted to nitrite. Accumulated sediment in cracks, crevices or on the bottom of storage tanks are potential locations for AOB to thrive.

What are the benefits to using chloramine instead of chlorine in the distribution system? The benefits of chloramine compared with chlorine for distribution system disinfections are:

• longer lasting disinfectant and ability to reach remote areas,
• effectiveness as a disinfectant for biofilms,
• tendency to form lower levels of regulated DBPs (e.g., THMs and HAAs), which are probable carcinogens (USEPA, 1998), and
• ability to minimize chlorinous or other objectionable taste and odors. Chloramine is more stable and lasts longer in the water in the distribution system because it is less reactive than chlorine.

The water agencies that have converted to chloramine report that customers note an improvement to the odor of the water. Research on the taste-and-odor quality of drinking water has demonstrated the benefits of monochloramine over chlorine. The San Francisco Public Utilities Commissions (SFPUCs) change to chloramine helps ensure compliance with more stringent federal and state drinking water quality regulations. In San Francisco, chloramination has virtually eliminated the presence of Legionella species in large building hot water heaters (Flannery et al., 2006).

What are the drawbacks to using chloramine instead of chlorine in the distribution system? The drawbacks of using chloramine compared with chlorine for distribution system disinfection are:

• potential temporary deleterious effects on older elastomeric materials sometimes used in some home appurtenances and plumbing fixtures,
• vulnerability to the microbiological process known as nitrification,
• potential formation of chloraminerelated DBPs if precursor material is present in the source water (Kirmeyer et al., 2004). The treatment precautions for hemodialysis clinics and fish cultures must be taken both with chlorine and with chloramine (Amato, 2005). Certain natural rubber products and their derivatives used in household appliances (e.g., toilet tank valves, hot water heater dip tubes) will deteriorate faster with chloramine than with chlorine (Reiber, 1993).

If such effects are experienced, replacing these items with alternative materials available in the plumbing and hardware stores will eliminate this temporary nuisance rubber deterioration. Vulnerability of chloramine to nitrification can be remedied by several practices, including:

• reducing the detention time of water in the drinking water storage reservoirs and low-use pipelines,
• keeping the system clean of deposits, which may harbor bacteria,
• flushing when necessary, and
• monitoring the system.

All these actions have an additional benefit for customers by providing fresher, shorter "shelf age" water. Typically, a change to chloramine has been preceded and followed by distribution system capital improvements aimed at decreasing water age such as: seasonal or permanent outages of water tanks, improving mixing within the tanks, redesign of pressure zones for better interconnectivity, changing pumping schedules to improve stored water turnover, or installation of new water quality monitoring stations (Wilczak et al., 1996, Odell et al., 1996; AWWA 2006a).

What is nitrification and how does it impact water quality? Nitrification is a microbial process by which ammonia is sequentially oxidized to nitrite and nitrate ions. Nitrification causes depletion of chloramine disinfectant thus allowing bacterial regrowth. Every utility using chloramine needs to assess nitrification potential and implement proper control measures. Nitrite and nitrate ions produced due to nitrification are of no significance to SFPUC. Other impacts of nitrification may include some decrease in alkalinity, pH, and dissolved oxygen (Wilczak et al., 1996; Kirmeyer et al. 2004). Nitrification is a utility operational issue and does not post any health concerns. Nitrification results from metabolism and growth of harmless non-pathogenic nitrifying bacteria that are ubiquitous in soils and water. Utilities implement operational control measures, including decreased water age and enhanced monitoring to limit the extent of nitrification (AWWA, 2006a).

After this optimization period, the customers benefit from fresher water that was stored for a shorter period of time in the distribution system. SFPUC has implemented a vigorous nitrification monitoring and control program and has been successful in controlling the nitrifying bacteria.