*»EPA

United States
Environmental Protection
Agency

Distribution System Water Quality

Protecting Water Quality with Online Water Quality Monitoring

Many public water systems (PWSs), including some small and medium systems, implement online water quality monitoring
(OWQM) in their distribution systems for regulatory compliance and operational surveillance. While grab samples are most often
used for compliance monitoring in their distribution systems, online analyzers are typically used for continuous disinfectant
residual monitoring at entry points and storage facility outlets, and for detecting water quality changes (e.g., red water incidents,
nitrification) to provide system personnel information for them to optimize operations. This fact sheet is part of EPA's Distribution
System Toolbox developed to summarize best management practices that PWSs, particularly small systems, can use to maintain
distribution system water quality and protect public health.

What Types of Water Quality Monitoring are used in a Distribution
System?

•	Grab samples are single volume samples collected at one time from one place.
Grab samples are collected for compliance with SDWA regulations such as the
Revised Total Coliform Rule and the Disinfectants and Disinfection Byproducts
Rules. Systems serving <3,300 people may collect grab samples for disinfectant
residual measurements at entry points to meet Surface Water Treatment Rule
requirements.

•	Composite samples are a mixture of several grab samples collected from
different locations at the same time, or samples collected from the same
location at different times. Composite samples are allowed in regulatory
frameworks, typically for select chemical groups (e.g., volatile organic
compounds) when a PWS has multiple entry points. This can help the PWS
reduce analytical laboratory costs. Under the SDWA, composite samples are not
allowed for bacteriological analytes or DBPs.

•	Continuous sampling with online analyzers (i.e., OWQM) generates real-time
data that can be used for regulatory compliance monitoring of disinfectant
residual at entry points and for operational monitoring throughout the
distribution system. Online analyzers are commercially available for parameters
such as turbidity, pH, alkalinity, orthophosphate, ammonia, nitrate, nitrite,
chlorine, monochloramine, oxidation reduction potential (ORP), total organic
carbon (TOC), temperature, disinfection byproducts, and specific conductance.

Design and Operational Considerations for an OWQM Station in the
Distribution System

•	Setting up monitoring objectives ensures that an OWQM system will provide
useful information to a utility; possible monitoring locations and parameters
should be selected based on established monitoring objectives.

•	Compiling and assessing distribution system informational resources prior to
designing the OWQM system can be helpful.

•	Including all individuals that will be involved with OWQM system operation and
maintenance (equipment technicians, IT specialists, water quality managers)
from the beginning of system design will help the system meet its objectives.

•	Installing monitoring systems at distribution system entry points, finished water
storage facility outlets, booster pump stations and meter or valve vaults, or in
other protected areas, such as fire or police stations, town halls and parks can
give a broad sense of water quality throughout the distribution system.

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 8,600 customers in the
mid-Atlantic had difficulty maintaining
a chloramine residual throughout the
distribution system. In response, the
PWS conducted continuous disinfectant
residual monitoring at tank outlets,
collected grab samples to measure
disinfectant residual at other locations,
and worked with the state to evaluate
water age in finished water storage
tanks and the distribution system. As a
result, the PWS modified tank
operations, expanded flushing
operations, and improved disinfection
practices, and the chloramine residual
increased to 1.0 mg/L at all locations
and tank water age decreased by 50
percent.

A PWS serving approximately 130,000
people in the northeast found that
water quality monitoring with the use
of grab samples was not able to detect
distribution system contamination in a
timely manner. The PWS installed 15
OWQM stations in the distribution
system, including at entry points,
storage tanks, and booster stations, to
monitor free chlorine, pH, specific
conductance, temperature, turbidity,
and UV-254. By integrating OWQM
data with the supervisory control and
data acquisition (SCADA) system and
establishing water quality alert levels,
the PWS improved abnormal water
quality event identification and
reduced the corrective action response
time.


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Online water quality monitoring station at a
distribution system booster pumping station

•	Developing an information management and analysis system
(e.g., SCADA) helps a utility to achieve the selected monitoring
objectives. This system provides users with useful data and
information to monitor real-time system conditions and
generate notifications when anomalies are detected. Standard
operating procedures for alarms and triggers based on OWQM
data can help detect potential anomalies; such procedures can
be configured based on utility objectives.

•	Routine instrument maintenance and calibration needs to be
implemented to ensure data quality and equipment
performance.

Applications where OWQM May Be Helpful in the
Distribution System

•	Improve operations by using OWQM data when corrective actions
are needed such as changing the disinfectant dosage rate, flushing
water mains, or cleaning storage tanks.

•	Manage water age by monitoring disinfectant residual at entry points, dead-end locations, and tank outlets and triggering
corrective actions such as optimizing tank operating levels or opening valves to improve water circulation.

•	Identify the onset of nitrification by monitoring pH, monochioramine, free and total chlorine, temperature, ammonia,
nitrate, and nitrite.

•	Assist in diagnosing causative factors for increased risks of waterborne disease by monitoring disinfectant residual and
temperature.

•	Evaluate the effectiveness of booster disinfection or flushing operations by continuously monitoring disinfectant residual.

•	Identify areas where the system may have trouble maintaining disinfectant residuals by continuously monitoring
disinfectant residual.

•	Detect contamination incidents or unusual water quality conditions by establishing a historical record of OWQM data and
using the data to establish setpoints for alerts and alarms.

•	Planning and justifying capital improvement projects such as tank mixers, piping projects that eliminate dead-end water
mains, and replacement of unlined cast iron water mains.

Table 1. Resources and Guidelines for Online Water Quality Monitoring

Resource Title and URL

Relevance to Online Water Quality Monitoring

USEPA. 2021. Online Water Quality Monitoring Resources.

https://www.epa.gov/waterqualitvsurveillance/online-water-
guality-monitoring-resources

Provides resources and guidance documents to help PWSs
select online water quality analyzers.

Urnberg and Rogers. 2017. Take a Cost-Effective Approach to
Online Water Quality Monitoring. Opflow, www.awwa.org
Note: There may be a fee associated with obtaining this
resource.

Presents applications for online water quality monitoring
and describes how to develop a monitoring program and
design a monitoring station.

AWWA. 2017. M68 Manual of Water Supply Practices. Water
Quality in Distribution Systems, https://www.awwa.org/
Note: There may be a fee associated with obtaining this
resource.

Describes monitoring programs for various water quality
applications in distribution systems. Summarizes
regulatory monitoring requirements. Includes an example
nitrification monitoring plan and case study examples.

National Academy of Sciences. 2006. Drinking Water
Distribution Systems. Assessing and Reducing Risks.

www.nap.edu

Explains how monitoring can be used to maintain the
water quality integrity of a distribution system. Lists
important parameters and describes available online
sensors. Describes how to use customer complaint data.

Office of Water (4606)
EPA 815-F-22-012
August 2023


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