Water Protection Program fact sheet
Division of Environmental Quality Director: Ed Galbraith

Public water systems that use chloramine as the primary disinfectant should monitor all parts of the distribution system and water storage facilities on a routine basis to check for signs of nitrification. Nitrification is the process by which nitrogen compounds (primarily ammonia) are oxidized first to nitrite and then to nitrate, and can result in loss of disinfection capabilities of the finished water. The reaction is caused by chloramine-resistant organisms (non-pathogens) that exist naturally in distribution system biofilms.  Ammonia-oxidizing bacteria (AOB), primarily Nitrosomonas, and Nitrobacter, are responsible for the two-step oxidation reaction. The most noticeable signal of nitrification is a drop in total chlorine residual in areas of the distribution system. Signs of nitrification may be indicated by the following in the distribution system:

Various conditions can promote nitrification: pH of 7.5 to 8.5, water temperatures between 25 to 30 degrees Celsius, excessive levels of free ammonia in the finished water, low flow conditions, water age, a dark environment, etc. For this reason, particular attention should be placed on water storage facilities and dead ends, or areas of low water usage in the distribution system.   Systems that feed chloramines should conduct routine checks of their distribution network and document trending data used to monitor for signs of nitrification. The data can be helpful in determining the best solution for the system. A practice commonly employed to combat nitrification is to temporarily convert system disinfection from chloramine to free chlorine, with the intent of eliminating nitrifying organisms from the biofilms.  Some public water systems conduct annual free chlorine conversions as standard operations and maintenance practices. During this switch, the public water system must conduct testing of all reaches of the distribution system on at least a daily basis to monitor the free and total chlorine residual. Free chlorine conversions can last for several weeks or longer depending on when the free and total chlorine levels have stabilized. Once the residual disinfectant levels have stabilized throughout all parts of the distribution system, the public water system may recommence chloramination. During the free chlorine conversion, water customers may experience taste, odor and discolored water issues. Although this is a normal effect of temporarily switching disinfectants and flushing, the public water system should give advance notice to their customers and purchasing systems of any planned activities along with assurances that it is not a threat to public health. During a free chlorine conversion, the public water system will need to initiate flushing through all reaches of its distribution system to adequately distribute the free chlorine disinfectant throughout the water system. In addition, consecutive systems purchasing water should also coordinate to flush their mains and monitor the disinfectant residual as well.

How to conduct a free chlorine conversion:

Additional information:

Nitrification by US EPA
Basic Information about Chloramines and Drinking Water Disinfection by USEPA
Disinfection with Chloramine by Centers for Disease Control and Prevention