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Controlling Nitrification in Chloraminating Water Systems

How to detect and prevent the formation of nitrites and nitrates from residual chloramine disinfectant or free ammonia in a water distribution system.

What Is Nitrification?

Nitrification is a microbial process that converts ammonia and similar nitrogen compounds into nitrite (NO2) and then nitrate (NO3). Nitrification can occur in water systems that use chloramine for their residual disinfectant.

The problem is greatest when temperatures are warm and water usage is low. For example, a number of water systems in Texas saw their first episodes of nitrification during the rainy summer of 2007.

How Can It Be Prevented?

Simply stated, the key points to preventing nitrification are:

Also, monitor for nitrification regularly. The longer you wait to act, the harder it can be to bring nitrification under control.

Optimize Your Chloramination Process

Before you calculate how much ammonia to add in chloramination, be sure you know whether your treated water already contains some FAA (free available ammonia). If it does, take this concentration FAA into account when you consider how much ammonia to add to balance the chlorine.

Remember: One key to preventing nitrification is to ensure that the biofilm has no ammonia to work with. So keep FAA levels low in your post-chloramination water:

  • In many areas, this means you should make sure FAA leaving the plant is less than 0.1 mg/L—preferably, less than 0.05 mg/L.
  • In some areas, particularly in East Texas, you can allow FAA levels of up to 0.2 mg/L due to competition in the water for the ammonium ion.
  • Systems with pH levels in higher ranges (> 9.0) will be less susceptible to nitrification and might be able to safely operate with higher FAA levels.
  • These are guidelines. No two water systems are alike. Know your water and make adjustments accordingly. If you need help, consider our direct assistance to water systems.

Reduce Water Age

Disinfectant levels fall when water stands still in your system. If usage falls, flush your mains to keep new water moving into your system.

When usage is low for extended periods, consider whether to reduce the holding volume of water in your reservoirs to ensure low residence times.

Do Preventive Maintenance

Be sure to include measures to reduce biofilms and tuberculation in your regularly scheduled maintenance:

  • Some systems have found that a hard flush once a year helps to keep nitrification in check.
  • Especially if your system has cast iron pipes, mechanical pigging can remove deposits and tuberculation where nitrifying bacteria can set up shop.
  • When usage is low, consider rotating reservoirs out of service to clean their interiors vigorously.
  • Some systems find it necessary to periodically switch to chlorine as disinfectant as a part of their preventive maintenance routine. For more about switching to chlorine see “How can I stop nitrification once it has begun?

Replace Aging Infrastructure

Corroding pipes and equipment provide plenty of crevices for nitrifying bacteria to escape your residual disinfectant. If you find your system having to do excessive maintenance to keep your infrastructure clean, consider replacing the problematic components with newer, less corrodible equipment.

How Can I Detect Nitrification?

Nitrification will usually show up first in areas where residence time (or “water age”) is highest—for example, dead-end mains, storage tanks, and areas where pressure planes overlap. Watch these areas especially carefully.

To be sure nitrification is not happening, watch these indicators well enough to know your system's baseline for each:

  • Residual disinfectant. Low levels of residual disinfectant can allow bacteria in the biofilm to multiply. When residual disinfectant drops below your normal baseline, nitrification may soon follow. So monitoring and mapping levels of residual disinfectant proves to be a quick and inexpensive tool you can use to pinpoint affected areas and focus your mitigation efforts.
  • Nitrite and nitrate levels in distribution. Not all systems routinely monitor for these contaminants in distribution. But one variety of nitrifying bacteria oxidize ammonia to produce nitrite, and another will oxidize nitrite to produce nitrate. By monitoring nitrite and nitrate levels, you can know when action is needed.
  • FAA levels. FAA is food to nitrifying bacteria. So if FAA levels are low in at least part of your distribution system, nitrification could be the cause. Look more closely at the data from any location that shows FAA levels below those of either the water leaving your treatment plant or other locations in your distribution system.

Frequently, but not always, systems that have nitrification occurring also have coliform-positive test results, increases in heterotrophic plate counts (HPC), or both.

If you see symptoms of nitrification, first check to be sure they aren’t masking other problems. For example, check the nitrogen balance in your system. With all values in mg/L, the nitrogen balance of your system is:

FAA (as N) + NO2 (as N) + NO3 (as N) + (0.27 × NH2Cl) = Nitrogen balance

This number will fluctuate somewhat under normal operating conditions. If you find that certain areas in your distribution system have high nitrate + nitrite levels and an increased nitrogen balance, then you should check for and rule out cross connections, line breaks, and other sources of contamination before you determine that nitrification is the culprit.

Finally, if you are sure an event has begun, act quickly to bring nitrification under control.

How Can I Stop Nitrification Once It Has Begun?

The key to stopping nitrification is to starve the nitrifying bacteria of nitrogen. The most effective way to do this is to switch disinfectant from chloramine to free chlorine. When you take this step, keep these key points in mind:

  1. 30 days before you switch, you must notify our Public Drinking Water Staff by letteror by e-mail to of this planned change in treatment method. Include this information in your notice to us:
    • PWS ID and name
    • PWS contact name, title, and phone
    • Estimated start and end date
    • PWS ID and names of customer systems
    • Reason for change in treatment (routine preventive maintenance; corrective maintenance due to nitrification)
  2. Coordinate this treatment change with appropriate adjustments in your disinfection byproduct (DBP) sampling. In other words, your DBP sampling should accurately represent standard operating procedure.
  3. As a courtesy, notify your customers before the change occurs. Be sure to mention these points:
    • A temporary change has been made to the treatment process to improve the quality of water being served to our customers.
    • Some taste and odor changes may briefly occur, but there are no associated health risks.
    • The name and phone number of the person customers can contact at your water system if they have any questions.
  4. It is fairly common for customers to notice (and complain about) changes in odor whenever you change disinfectants. Some customers will notice the different taste of free chlorine; others may notice when you switch back and the chloramine wavefront hits the chlorinated water in the distribution system. To minimize noticeable changes in odor, increase flushing of your distribution lines following each change. To determine where odors are most likely to be noticeable, monitor your distribution for both free and total chlorine until levels stabilize.

Where Can I Get More Help?

Our Financial, Managerial, and Technical (FMT) Assistance Program helps public water systems stay informed of the best ways to deal with today’s problems. There is no cost to water systems for the training itself, although each system is responsible for having the appropriate materials and equipment on hand.

A new round of this directed assistance will be available this September. To participate, contact our Utilities and Districts staff and ask about the FMT Assistance Program.

Where Can I Learn More?

For advice about process controls, changing disinfectants, and other operational matters, contact our Public Drinking Water staff. These helpful resources are available online: