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Controlling Nitrification in Public Water Systems with Chloramines

How a public water system (PWS) can detect nitrification and prevent the degradation of drinking water quality 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 contain chloramines.

The problem is greatest when temperatures are warm and water usage is low. For example, a number of water systems in Texas saw episodes of nitrification during the rainy summers of 2007 and 2015.

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How Can It Be Prevented?

Simply stated, the key points to preventing nitrification are:

  • Optimize your chloramination process. Know your water and the chemicals you use to treat it.
  • Reduce water age. Keep water moving through your system by flushing routinely and deep cycling your storage tanks.
  • Do preventive maintenance. Hard flush or mechanically pig your distribution system, and do valve sweeps to make sure valves are operational and set correctly.
  • Replace aging infrastructure. Older pipes made of cast iron, ductile iron, or asbestos/cement are affected by deposits called tuberculation. Tuberculation allows biofilm to accumulate.
  • 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 our rules in Title 30, Texas Administrative Code (30 TAC), Subsection 290.46(z).
  • Manage your disinfectants. If you have source water that contains chlorine and other sources that contain chloramines, managing their blending is essential to avoiding nitrification.

Also, monitor for nitrification regularly, at least as much as required in 30 Tac, Section 290.110. The longer you wait to act, the harder it can be to bring nitrification under control.

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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 free available ammonia. If it does, take the source ammonia concentration 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 ammonia levels low in your post-chloramination water:

  • In many areas, this means you should make sure ammonia leaving the plant is less than 0.1 mg/L—preferably, less than 0.05 mg/L.
  • Systems with pH levels in higher ranges (> 9.0) will be less susceptible to nitrification and might be able to safely operate with higher ammonia levels.
  • Booster chlorination may be helpful in combining leftover 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.

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.

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Reduce Water Age

Disinfectant levels drop when water stands still in your system. If your water usage drops, a temporary solution is to flush your mains to keep new water moving into your system. Remember all PWSs must flush all dead-end mains monthly as required in 30 TAC, Subsection 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.

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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 temporarily convert to free chlorine as disinfectant 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?

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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.

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 Two-page NAP Guidance which provides basic information about developing a NAP. A longer NAP Summary document provides a more in-depth discussion of NAPs. Finally, this ExcelNAP template provides explanation, examples, and a template to help your system develop a NAP.

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

One of the most difficult scenarios to manage is blending chloraminated and chlorinated waters. Whether this occurs in the distribution system or in a storage tank, uncontrolled blending can cause a loss in disinfection residual, taste and odor issues, and nitrification.

To protect public health, if you blend chloraminated and chlorinated water, you must have site-specific approval from the TCEQ through an "exception" granted with conditions showing how the blending will be managed. If you have two sources of water and use different disinfectants for each source, you may be blending.

Please see the guidance  Blending Chloraminated and Chlorinated Water to see what options are available for your water system to safely blend or eliminate blending. If your system chooses to continue blending, please see the exceptions web page and the Word Blending Chloraminated and Chlorinated Waters Checklist for information about submitting an exception request for blending.

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How Can I Detect Nitrification?

TCEQ rule requires systems that use chloramines to perform specific monitoring to ensure an adequate disinfectant residual is being 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 residence time (or “water age”) is highest—for example, dead-end mains, storage tanks, and areas where pressure planes overlap. Watch these areas carefully.

To be sure nitrification is not happening, watch these indicators carefully 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. Under the new rule, systems are required to sample nitrate and nitrite quarterly in the distribution system. One variety of nitrifying bacteria oxidizes 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. You may find more frequent monitoring is beneficial for your system.
  • Ammonia levels. Ammonia is food for nitrifying bacteria. So 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 system 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 nitrification occurring 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. For example, check the nitrogen balance in your system. With all values in mg/L, the nitrogen balance of your system is:

Free ammonia (as N) + NO2 (as N) + NO3 (as N) + 0.20 × Monochloramine (as Cl2) = 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.

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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) 30 days before you switch, you should notify our Public Drinking Water Staff by letter or by e-mail to of this planned change in treatment method. They will discuss DBP sampling schedules with you. If there is an emergency which does not allow 30 days' notice, please still contact us. 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 (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 (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.

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

5) Be sure to remind your wholesale customers to tell any downstream PWSs about the temporary conversion to free chlorine.

6) If you are a purchase water system and do not currently have the means to convert to free chlorine:

PWS that wishes to purchase and install chlorination equipment on a temporary or permanent basis:

Plans must be submitted by Professional Engineer (PE) licensed in the State of Texas. 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.
  • For PWSs that purchase water, 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.

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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.

Request the Chloramination and/or Nitrification Action Plan Directed Assistance Modules to get free, on-site training. To participate, 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.

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Where Can I Learn More?

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

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