oEPA

United States
Environmental Protection
Agency

Distribution System Water Quality

Protecting Water Quality Through Water Age
Management

Excessive water age can lead to a deterioration of distribution system water quality, including increased disinfection byproduct
(DBP) formation, disinfectant residual consumption, and microbial growth. Water age can be managed by using design and
operational practices that optimize flow through the distribution system while maintaining pressure requirements. This fact sheet
is part of EPA's Distribution System Toolbox developed to summarize best management practices that public water systems
(PWSs), particularly small systems, can use to maintain distribution system water quality and protect public health.

What is Water Age?

•	Water age is the time water spends in the distribution system prior to use.

•	Water age can become excessive when water usage is low, when piping or
storage tanks are larger than needed, or when water circulation is poor.

•	Poor water circulation can be a particular problem at dead-end water mains
and behind closed valves. It can also occur where accumulated air blocks
water flow at high points in the distribution system. Poor water circulation
can occur in storage tanks when thermal stratification limits vertical mixing,
increasing water age in portions of the tank.

How Does Water Age Affect Water Quality in the Distribution System?

Water quality can deteriorate in the distribution system when there is more time
for biological, chemical, and physical reactions to occur within the bulk water or
between the bulk water and wetted surfaces. For example:

•	Particles and corrosion products can settle in piping and tanks, creating
sediment.

•	Chlorine can react with organic and inorganic constituents in sediment and
on the pipe wall, or in the bulk water, causing the chlorine residual to be
consumed.

•	Low chlorine residuals provide conditions where microbial growth can
increase, and in some cases microbial corrosion can leach metals from pipe
scales.

•	In excessively aged water, chlorine may have more contact with natural
organic matter, increasing DBP formation.

•	At PWSs that use monochloramine for secondary disinfection, nitrification
can occur in the distribution system under conditions of high water age and
warm temperatures.

•	Water that is stagnant in storage tanks and dead-end mains can increase in
temperature, contributing to increased microbial growth and DBP
formation.

•	Changes in water chemistry associated with water age can enhance biofilm
growth on pipe and tank walls and allow opportunistic pathogens such as
Legionella to proliferate.

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 in the southern U.S. serving 2.2
million people violated the Total Coliform
Rule and continued to have high numbers
of positive total coliform results and low
disinfectant residuals for a four-week
period. In response, the city conducted
hydraulic modeling and valve checks and
found that 44 isolation valves that had
been closed during maintenance work had
not been re-opened, leading to excessive
water age in parts of the system. Some
valves could not be operated because of
poor condition. The city took corrective
actions including valve repairs and spot
flushing, and achieved compliance. To
prevent future violations, the city
implemented routine valve checks and
increased disinfectant residual monitoring.

Water age contributed to DBP violations at
a wholesale water system in the
southeastern U.S. directly serving 28,000
people and at three of its consecutive
systems, each serving fewer than 5,000
people. In response, the wholesale system
increased water quality monitoring at
dead ends, large mains, and tank outlets.
They also cleaned storage tanks, optimized
tank operations, conducted flushing to
reduce water age, and optimized
treatment practices to reduce the water's
total organic carbon concentration. As a
result, the DBP concentrations in the
wholesale system decreased by 43
percent. One consecutive system had a 36
percent decrease in DBP concentrations
after taking similar corrective actions.


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Strategies for Finding Areas of the Distribution System
with High Water Age

•	Review monitoring data to identify areas with low
disinfectant residual, high DBP levels, high bacterial
counts, or high nitrite/nitrate levels, and review customer
complaints.

•	Measure disinfectant residual at potential problem areas
such as dead-end water mains and tank outlets.

•	Measure water temperature at different depths in
storage tanks to identify thermal stratification that could
cause poor water circulation.

•	Construct a hydraulic or water quality model and use it to
evaluate planned or possible changes in water demand
and tank operating conditions.

•	Conduct a tracer study by adding a tracer chemical (e.g.,
calcium chloride) and measure it at various locations and
times to better understand water movement in the
distribution system.

Design Strategies for Water Age Management

•	Use hydraulic models to right-size water mains and
storage tanks.

•	Install tank mixers and/or optimize the placement of
inlets and outlets to improve water circulation in tanks.

•	Eliminate dead-end water mains: for example, by
installing new pipe to form a closed loop.

•	Optimize valve operations to promote water circulation.

•	Install sampling taps and automatic monitoring
equipment to monitor residual chlorine at finished water
storage tanks and other locations.

•	Install air valves at high points in the distribution system
to remove accumulated air, which can block pipelines
and reduce water circulation.

Use gentle bulk water turnover flushing to remove
stagnant water and temporarily restore disinfectant
residual.

Optimize pumping and tank filling/draining
operations.

Regularly measure disinfectant residual at dead-
ends, tank outlet lines, and low water usage areas.
Exercise valves to maintain valve operations, verify
that valves are set in the correct position (open or
closed), and identify repair needs.

Use hydraulic models to evaluate flow direction and
flow rate, determine optimal valve positions (open or
closed), and run water age analysis.

Make sure that valves are reopened after repair
work.

Table 1: Resources and Guidelines for Water Age Management

Resource Title and URL

Relevance to Water Age Management

ASDWA. 2020. State Drinking Water Distribution
System Survey.

http://www.asdwa.org/

Summarizes the results of a survey on topics that include state policies,
recommendations, and regulations related to maximum residence time,
valve exercising, flushing, and disinfectant residual monitoring.

GLUMRB. 2018. Ten States Standards.

https://www.mngovDublications.com/ Note: There
may be a fee associated with obtaining this resource.

Provides guidance on distribution system topics like tank sizing, location, and
appurtenances; water main sizing; provisions for adequate flushing; isolation
valve spacing; and air relief valve location.

AWWA. 2017. M68 Manual of Water Supply Practices.
Water Quality in Distribution Systems.

https://www.awwa.org/. Note: There mav be a fee
associated with obtaining this resource.

Describes how to properly size pipes, valves, and storage to provide
adequate capacity and avoid water age problems. Explains how to estimate
water age using hydraulic models, tracer studies, and system hydraulic and
water quality records.

Use of Low Velocity Flushing for Bulk Water Turnover
without Scouring

Operational Strategies for Water Age Management

Source: Confluence Engineering Group. Used with permission.

Office of Water (4606)
EPA 815-F-22-002
April 2022


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