&EPA

United States
Environmental Protection
Agency

Distribution System Water Quality

Protecting Distribution System Water Quality through
Control of Nitrification

Systems using chloramines for secondary disinfection may experience nitrification when the system has high source water
ammonia levels and/or when the system adds too much ammonia. The breakdown of chloramines, releasing free ammonia, can
also contribute to nitrification. Nitrification in distribution systems is undesirable because it can cause biofilm growth, water quality
degradation (e.g., disinfectant depletion, coliform occurrences, and nitrite/nitrate formation), and subsequent non-compliance
with existing regulations (e.g., disinfectant residuals below regulatory limits). This fact sheet is part of EPA's Distribution System
Toolbox developed to summarize best management practices that public water systems (PWSs), particularly smail systems, can use
to maintain distribution system water quality and protect public health.

Understand Nitrification Causes

•	Nitrification is a two-step biological process:
o First, ammonia-oxidizing microbes oxidize ammonia to nitrite,
o Second, nitrite-oxidizing bacteria oxidize nitrite to nitrate,
o Some microorganisms can perform both steps.

•	Distribution system nitrification occurs when ammonia-oxidizing microbes
grow faster than they are inactivated by the disinfectant residual.

•	Factors that favor nitrification: high free ammonia concentration, low
disinfectant residual concentration, high water age, high temperature,
sediments or deposits that consume disinfectant residual and protect bacteria.

•	In general, the following water quality progression occurs with nitrification:
o As chloramine residual decreases, free ammonia concentration will

initially increase. Chloramine residual will continue to decrease and
eventually approach zero,
o Free ammonia decreases as free ammonia is consumed by ammonia-

oxidizing microorganisms, forming nitrite,
o Increasing nitrite concentration favors the growth of nitrite-oxidizing
bacteria resulting in nitrate formation, indicating a nitrification event.

•	Comprehensive Nitrification Action Plans or Nitrification Control Plans are
used by water system operators to prevent and respond to nitrification in their
distribution systems.

•	Some states require systems using chloramination to have a Nitrification
Action Plan or Nitrification Control Plan to help maintain disinfectant residual.
Details on state requirements may be found at individual state web sites.

Nitrification Action or Control Plans

•	The major components and considerations included in a Nitrification Action
Plan or Nitrification Control Plan include:

o Description - historical nitrification occurrence in the system
o Objectives - regulatory compliance and specific water quality objectives
o System details - pertinent information about the system, including

treatment and monitoring locations
o Monitoring plan - sampling locations, parameters, and frequency
o Data analysis - process for analyzing collected data
o Water quality benchmarks (triggers)

Disclaimer: To the extent this document mentions or discusses statutory or regulatory authority, it does so for
information purposes only. It does not substitute for those statutes or regulations, and readers should consult
the statutes or regulations themselves to learn what they require. The mention of trade names for commercial
products does not represent or imply the approval of EPA.

Examples of Utility Actions

A PWS serving nearly 500,000 people in
the southern U.S. had two nitrification
episodes in the period of two years, each
resulting in a boil water notice for low
chlorine residuals and elevated coliforms.
In response to these nitrification
episodes, PWS staff improved their
Nitrification Action Plan. They performed
more water quality monitoring in the
distribution system and undertook
corrective actions: e.g., eliminating dead-
end mains and implementing a new tank
cycling strategy to reduce water age. At
the time of reporting, three years after
the corrective actions were implemented,
the PWS had not experienced any more
nitrification events.

A PWS serving 8,600 people in the
northeastern U.S. was having difficulty
maintaining chloramine residuals and
found evidence that nitrification was
occurring. The PWS conducted a study to
evaluate the age and quality of water
entering the distribution system. The PWS
determined that the chloramine residual
was unstable and rapidly decreased.
Coagulant dosing was modified, and the
coagulation/flocculation process was
improved to remove natural organic
matter more effectively. The PWS also
improved mixing during chloramine
formation and increased the chloramine
residual entering the distribution system.
The treatment changes led to improved
chloramine residual stability and higher
chloramine concentrations in the
distribution system.


-------
Nitrification Action or Control Plans (continued)

o Response strategies - actions to be taken when nitrification occurs
o Prevention strategies - actions to be taken to prevent nitrification

•	PWSs review and update their Nitrification Action Plan or Nitrification Control Plan on a regular basis to ensure the plan is
effective and representative of current system operations.

Nitrification Prevention Strategies

•	Nitrification prevention strategies will be specific to a given water system, but may include:
o Optimizing treatment processes to improve organic matter removal.

o Ensuring that the correct chlorine-to-ammonia ratio is used for secondary disinfection,
o Keeping the distribution system clean through regular flushing, pigging, swabbing, or other measures,
o Developing and implementing a Nitrification Action Plan or Nitrification Control Plan,
o Reducing water age in the distribution system.

o Optimizing storage tank operations to improve turnover and mixing performance,
o Conducting a seasonal (e.g., spring) conversion to free chlorine for secondary disinfection.

Nitrification Response Strategies

•	Once started, nitrification is difficult to stop. Therefore, it is desirable not only to take steps to prevent nitrification episodes,
but also to be prepared to detect and correct them quickly when they do occur.

•	The most common responses used to halt nitrification episodes include:

o Distribution system flushing - effective for limited, localized nitrification
o Draining and disinfecting storage tanks - effective if the nitrification source is the storage tank
o Temporary conversion to free chlorine - removes the food source (i.e., ammonia) and causes nitrification to stop,
although results may be short-lived.

•	Unintended consequences associated with nitrification response strategies should be considered: for example, conversion to
free chlorine could result in increased disinfection byproduct formation.

•	Once nitrification is brought under control, the underlying causes of the nitrification event should be understood and
corrected, or nitrification is likely to occur again.

Table 1. Resources and Guidelines for Distribution System Nitrification Prevention and Response

Resource Title and URL

Relevance to Nitrification Prevention and Response

TCEQ. 2021. Controlling Nitrification in Public Water Systems with
Chloramines.

https://www. tcea.texas.gov/drinkingwater/disinfection/nitrification. html

Provides information related to controlling nitrification in
drinking water systems, including detection and response
actions.



ASDWA. 2020. Distribution System Survey White Paper
https://www. asdwa.org/

Summarizes information about state requirements for
Nitrification Action Plans.

Ybanez, C.; Ramirez, G.; Becker, W.; and Santos, M. 2020. When the
Distribution System Needs a NAP: Tackling One City's Nitrification Action
Plan. Journal AWWA. 112(1), 40-51. https://doi.org/10.1002/awwa.1430.

Note: There may be a fee associated with obtaining this resource.

Illustrates an example where a Nitrification Action Plan was
implemented to address nitrification in a municipal drinking
water distribution system.

AWWA. 2017. M68 Manual of Water Supply Practices. Water Quality in
Distribution Svstems, 1st Edition, https://www.awwa.org/.

Note: There may be a fee associated with obtaining this resource.

Discusses nitrification prevention, monitoring, and response in
drinking water distribution systems, including an example
Nitrification Action Plan.

AWWA. 2013. M56 Manual of Water Supply Practices. Nitrification
Prevention and Control in Drinking Water, 2nd Edition.
https://www.awwa.org/.

Note: There may be a fee associated with obtaining this resource.

Summarizes information and best practices related to
nitrification prevention, nitrification monitoring, and responses
to nitrification episodes in drinking water distribution systems.

Office of Water (4606)
EPA 815-F-22-009
August 2023


-------