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Controlling Nitrification in Public Water Systems with Chloramines

How a public water system that uses chloramines, can detect and respond to the degradation of drinking water quality in a distribution system caused by nitrification.

What Is Nitrification?

Nitrification is a natural, biological process where nitrifying bacteria convert ammonia and similar nitrogen compounds into nitrite (NO2–) and then nitrate (NO3–). Nitrification typically occurs in distribution systems that contain chloramines and results in the degradation of drinking water quality. The problem is greatest when temperatures are warm and water usage is low. For example, water systems in Texas experienced an increased occurrence of nitrification during the rainy summers of 2007 and 2015.

How Can Nitrification Be Prevented?

Key items to help prevent nitrification:

  • Optimize your chloramination process - Understand how chlorine and ammonia interact to form stable chloramine molecules. Training on this topic is offered by the TCEQ Water Supply Division at no expense to the water system.
  • Complete chloramine effectiveness sampling - Required by 30 TAC 290.110. The longer you wait to act, the harder it can be to bring nitrification under control.
  • Reduce water age - Keep water moving through your system by flushing distribution lines routinely and deep cycling storage tanks.
  • Follow your Nitrification Action Plan - To ensure the protection of public health, systems that have chloramines are required to develop and maintain a Nitrification Action Plan (NAP) per 30 TAC 290.46(z).
  • Complete preventive maintenance - Hard flush or mechanically pig your distribution system, and do valve sweeps to make sure valves are operational and set correctly.
  • Manage your disinfectants - If you have multiple sources that contain differing disinfectants, like chlorine and chloramines, managing their blending is essential to avoiding nitrification. Contact TCEQ for more information on obtaining blending approval. 
  • Replace aging infrastructure - Older pipes made of cast iron, ductile iron, or asbestos/cement are more likely to accumulate internal deposits called tuberculation. Tuberculation provides an environment for the biofilm that contains nitrifying bacteria to grow, increasing the likelihood of nitrification.

Optimize Your Chloramination Process

Before adding ammonia during your chloramination process, be sure you know whether your source water contains free available ammonia. If it does, take that into account when you consider how much ammonia to add to create stable chloramine.

Remember, the key to preventing nitrification is to ensure that nitrifying bacteria have no ammonia to consume. So, keep free ammonia levels low in your distribution system.

  • In many water systems, this means that free ammonia leaving the plant should be less than 0.1 mg/L—preferably, less than 0.05 mg/L.
  • Systems with pH levels in higher ranges (greater than 9.0) will be less susceptible to nitrification and may be able to safely operate with higher free ammonia levels.
  • Booster chlorination may be helpful to combine leftover free ammonia with chlorine to form more chloramines.
  • Systems that use chlorine dioxide may be less susceptible to nitrification because of the chlorite ion in distribution acts as a biocide.

These are general guidelines and may not apply to all public water systems. No two water systems are alike. You must know your water and adjust accordingly. If you need help, consider our free direct assistance to water systems.

Reduce Water Age

Disinfectant levels decrease as water ages. If you experience a reduction in water usage, a temporary solution is to flush your mains to keep new water moving into the system. Remember, all PWSs must flush all dead-end mains monthly as required in 30 TAC 290.46(l).

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

Follow Your Nitrification Action Plan

The purpose of a Nitrification Action Plan (NAP) is to ensure that chloramine disinfection is successful by preventing and/or responding to nitrification. TCEQ has developed a short  NAP Guidance which provides basic information about developing a NAP. The longer NAP Summary provides a more in-depth discussion of NAPs. The ExcelNAP template provides explanations, examples, and a template to help your system develop a NAP.

Complete Preventive Maintenance

Regularly scheduled maintenance should include measures to reduce biofilm and tuberculation.

  • Some systems have found that a unidirectional flushing plan once a year helps to keep nitrification in check.
  • If your system has cast iron pipes, mechanical pigging can remove deposits and tuberculation where nitrifying bacteria live.
  • When usage is low, consider rotating storage tanks out of service to clean the insides and remove sediment deposits. Just a quarter inch of sediment in a tank is enough to harbor nitrifying bacteria.
  • Some systems find it necessary to temporarily convert to free chlorine as a part of their periodic preventive maintenance routine. For more about free-chlorine conversions see How can I stop nitrification once it has begun?

Manage Your Disinfectants: Blending Chloraminated and Chlorinated Waters Together in One System

One of the most difficult scenarios to manage is blending chloraminated and chlorinated waters. Chloramines are formed by combining ammonia and chlorine at a ratio that creates monochloramine, the desirable species for disinfection. When blending of chloraminated and chlorinated waters occur, whether in the distribution system or in a storage tank, uncontrolled blending of chlorinated and chloraminated waters causes the chlorine to ammonia-nitrogen ratio to change uncontrollably, causing disinfectant residuals to drop, taste and odor issues, and nitrification. Therefore, blending of chloraminated and chlorinated waters is not recommended. If you have two sources of water and use different disinfectants for each source, you may be blending.

If your system is blending chloraminated and chlorinated waters, you must have a site-specific exception granted by the TCEQ Plan & Technical Review Section as required by 30 TAC §290.42(e)(3)(G) to ensure that blending will be managed and a compliant disinfectant residual will be maintained in all areas of the system’s distribution system. Read  Blending Chloraminated and Chlorinated Water to see what options are available for your water system to preferably eliminate blending. If your system chooses to continue blending but does not currently have an approved exception, see the exceptions web page and Blending Chloraminated and Chlorinated Waters Checklist for information about submitting an exception request for blending. Systems that are blending without an approved exception may be subject to enforcement action.

Replace Aging Infrastructure

Corroded pipes and equipment provide plenty of crevices for nitrifying bacteria to escape disinfectant. If you have to do excessive maintenance to keep your infrastructure clean and maintain a residual, consider replacing problematic components with newer pipes and equipment that is less susceptible to corrosion. 

How Can I Detect Nitrification?

TCEQ rule requires systems that use chloramines to perform specific monitoring, chloramine effectiveness monitoring, to ensure an adequate disinfectant residual is maintained, and that nitrification is not occurring in the distribution system. The monitoring required is summarized in this Chloramine Fact Sheet.

Nitrification will usually show up first in areas where water age is highest—for example, dead-end mains, storage tanks, and areas where pressure planes overlap. Monitor these areas carefully and look for the following indicators:

  • Disinfectant Residual. If disinfectant residual is low, nitrifying bacteria that live in biofilm may begin to multiply. As these bacteria thrive and consume ammonia they will reduce the disinfectant residual farther, perpetuating the problem. If disinfectant residual drops below your normal baseline, nitrification may soon follow. Collecting data and mapping levels of disinfectant residual 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. Systems are required to sample nitrate and nitrite, at least quarterly, in the distribution system. One variety of nitrifying bacteria oxidizes ammonia to produce nitrite, and another oxidizes nitrite to produce nitrate. By monitoring nitrite and nitrate levels, you will know at what stage nitrification may be occurring and when action is needed. Monitoring more frequently than the minimum requirement of quarterly will help to create a more detailed picture when trying to determine how to respond.
  • Ammonia levels. Ammonia is food for nitrifying bacteria. If ammonia levels are decreasing in at least part of your distribution system, nitrification could be the cause. Once you have determined baseline ammonia levels for your system, look more closely at the current data from any location for decreasing ammonia levels. If the ammonia levels in the system are less than the ammonia levels of the water leaving your treatment plant, or less than your system's baseline ammonia levels, nitrification may be occurring.

Frequently, but not always, systems that have active nitrification may 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, such as 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 temporarily convert your disinfectant from chloramine to free chlorine. See the Temporary Free-Chlorine Conversion webpage for detailed information. When you take this step, keep these key points in mind:

1) Notify the TCEQ Disinfection Byproducts Program by emailing DBP@tceq.texas.gov at least 30 days before you switch. TCEQ staff will discuss the possibility of delaying DBP sample collection for up to 30 days each calendar year. If there is an emergency which does not allow 30 days' notice, please notify us as soon as possible. Include the following information in your email:

  • PWS ID and name
  • PWS contact name, title, and phone
  • Estimated start and end date
  • PWS ID and names of customer systems (Be sure to remind your wholesale customers to tell any downstream PWSs about a temporary conversion to free chlorine)
  • Reason for change in treatment (routine preventive maintenance; corrective maintenance due to nitrification)

2) 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.

3) It is fairly common for customers to notice changes in taste and odor during a temporary change in disinfectant types, both during the change to free chlorine and changing back to chloramines. To minimize noticeable changes in taste and odor, increase flushing of your distribution lines following each stage.

4) Monitor your distribution system for total chlorine, monochloramine and free chlorine until results stabilize. When you have successfully made the conversion to free chlorine, total chlorine should equal free chlorine. When you have returned to chloramines, total chlorine should equal monochloramine.

5) Be sure to remind your wholesale customers to tell any downstream PWSs about the temporary conversion to free chlorine  and provide them with an estimated time of arrival of converted water, if able..

6) If you have a purchase water system and do not currently have the means to convert to free chlorine but wish to purchase and install chlorination equipment on a temporary or permanent basis:

You must submit plans, certified by a Professional Engineer (PE) licensed in the State of Texas, to TCEQ. Plans must include:

  • Specifications for chlorine feed equipment;
  • NSF certification for chlorine feed equipment;
  • NSF certification for chlorine;
  • Documentation regarding dosing; and
  • Proof that the chemical feed pump is properly sized to deliver the correct dose.

Advantages of permanent equipment versus temporary equipment:

  • The same equipment used for a free chlorine conversion can be used to properly boost chloramine residual coming into your system, reducing low residual problems and keeping the distribution system healthy.
  • Proper residual boosting and testing can prevent nitrification and reduce the need to conduct a free chlorine conversion.
  • Nitrification can happen during periods of hot summer temperatures. PWSs will typically need to do a free chlorine conversion periodically; therefore permanent equipment and approval will ensure timely action and reduce notification time with the TCEQ.

Where Can I Get More Help?

The 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.

Request the Chloramination Directed Assistance Module (DAM 5) and/or the Nitrification Action Plan Directed Assistance Module (DAM 8) for free, on-site training. To get more information or to make a training request, call the Water Supply Division at 512-239-4691 or go to the Assistance for Public Drinking Water and Wastewater Systems web page to learn about the FMT Assistance Program.

Where Can I Learn More?

For advice about process management, changing disinfectants, and other operational matters, contact our Water Supply Division staff. Additional resources are also available online: