Water Protection Program fact sheet
Division of Environmental Quality Director: Kyra Moore

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:

  • Reduction of total chlorine residual.
  • Reduction of pH and alkalinity.
  • Increase in HPC (heterotrophic plate count).
  • Decrease in DO (dissolved oxygen) [often coupled with an increase in temperature].

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:

  • Notify customers of the effective date of the temporary switch. The notification should include the intent of temporarily switching to free chlorine, the approximate time period expected for the conversion, and who to contact in case of concerns or questions. If the conversion to free chlorine is system-wide, coordinate with all consecutive water systems for their customer notification, water quality sampling and flushing protocols. Also notify the appropriate Missouri Department of Natural Resources regional office. For a sample public notice, please contact your regional office.
  • Switch to free chlorine disinfection, ensuring that at least 0.5 ppm is entering the distribution system. It is not advisable to adjust free chlorine residuals at the entry point greater than 4.0 ppm during the conversion process. The maximum residual disinfection level (MRDL) for chlorine is 4.0 ppm (10 CSR 4-4.055, Disinfection Requirements). Chlorine levels above this range may cause skin irritation for people with skin sensitivities, such as eczema, psoriasis, diabetes, etc.
  • Monitor the disinfection residual throughout all parts of the distribution system (coordinating with consecutive systems) paying particular attention to dead ends, water storage facilities, and farther reaches in the system.
  • Once chlorine levels have stabilized through all parts of the distribution system including consecutive systems, you may return to chloramine disinfection. Notify customers and the department when the free chlorine conversion is complete.

Additional information:

Nothing in this document may be used to implement any enforcement action or levy any penalty unless promulgated by rule under chapter 536 or authorized by statute.

For more information