Chapter 11: Distribution System Water Quality
- Page ID
- 38935
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)After reading this section, you should be able to:
- Describe sampling procedures
- Describe water quality parameters
- Explain the causes behind water degradation in the distribution system
- Explain the risks of cross-connection
- Explain corrosion
- Describe data reporting
- Outline problem resolution
Sampling
All of the drinking water regulations apply to all public water systems.
Any water system that provides services for fewer connections or persons than these rules outline are not covered by the Safe Drinking Water Act. Certain other individuals and residences also are excluded, such as those whose water is supplied by an irrigation, mining, or industrial water system. However, regardless of size, all operators must strive to provide consumers with potable drinking water.
Drinking water regulations also consider the type of population served by the system and classify water systems as community or non-community systems. EPA defines a public water system as one which provides water for human consumption through pipes or other constructed conveyances to at least 15 service connections or serves an average of at least 25 people for at least 60 days a year. A public water system may be publicly or privately owned.
EPA further classifies public water systems according to the number of people they serve, the source of their water, and whether they serve the same customers year-round or on an occasional basis. EPA has defined three types of water systems1:
Community Water System: A public water system that supplies water to the same population year-round.
Non-Transient Non-Community Water System: A public water system that regularly supplies water to at least 25 of the same people at least six months per year. A small school would be an example of this.
Transient Non-Community Water System: A public water system that provides water in a place such as a gas station, or campground where people do not remain for long periods of time.
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Any water system that provides services for fewer connections or persons than these rules outline are not covered by the Safe Drinking Water Act. Certain other individuals and residences also are excluded, such as those whose water is supplied by an irrigation, mining, or industrial water system. However, regardless of size, all operators must strive to provide consumers with potable drinking water.
EPA has revised the Total Coliform Rule so that public water systems in the US take steps to ensure the integrity of the drinking water distribution system and monitoring for the presence of microbial contamination. Public water systems and the state and local agencies that oversee them must comply with the Revised Total Coliform Rule (RTCR) beginning April 1, 2016. The final RTCR requires the following:
- Public water systems are to notify the public if a test exceeds the MCL for E. coli in drinking water
- Public water systems that are at risk and therefore vulnerable to microbial contamination are required to identify and fix the problems, including the potential sources and the pathways of contamination
- Criteria will be used for public water systems that are well operated to qualify for and stay on reduced monitoring, which could reduce water system burden
- Provide incentives for better system operation, in particular small systems
- Require additional monitoring requirements for seasonal systems such as campgrounds and state and national parks
The RTCR establishes an MCLG and an MCL for E. coli and eliminates the MCLG and MCL for total coliforms, replacing them with treatment techniques for coliforms that require assessment and corrective action. EPA has established an MCLG of zero for E. coli, which is a more specific indicator of fecal contamination and those pathogens that are potentially more harmful than total coliform. EPA has removed the 1989 MCLG and MCL for total coliforms, although the acute total coliform MCL violation under the 1989 TCR has been maintained as the MCL for E. coli under the RTCR.
Under the new treatment technique, total coliforms serve as the indicator of a potential pathway of contamination into the system’s distribution system. Public water supplies that exceed a specified frequency of total coliform occurrence must conduct an assessment to determine if any sanitary deficiencies exist; if found, they must be corrected. A public water system that experiences an E. coli MCL violation must conduct an assessment and correct any sanitary defects that are identified.
Monthly notification requirements based only on the presence of total coliforms will no longer be required. Instead, the RTCR will require public notification when an E. coli MCL violation occurs, indicating a potential health threat or when a water system fails to assess and take corrective action.
Sampling Plan
The Total Coliform Rule requires each water supply system to develop and follow a written sampling plan. Each plan must specifically identify sampling points throughout the distribution system. Sampling plans must be approved by the state regulatory agency and it is necessary to check with the state agency to determine the details of their review process and what documents need to be submitted.
Data and Reporting
Monitoring Frequency
The routine monitoring frequency for community water systems is based on the population served. Routine monitoring frequencies for non-community systems are based on the source of supply and, in some cases, the population served. Whenever a routine sample tests positive for total coliform, two provisions of the Total Coliform Rule take effect: repeat samples (formerly referred to as check samples) must be taken and the original coliform-positive sample must be tested for the presence of fecal coliforms or E. coli to determine whether an actual or potential violation of the coliform MCL exists.
The reason repeat samples are required is to investigate whether the original coliform-positive sample was caused by a contamination problem that exists throughout the distribution system or if it is a localized problem that exists only at that one sampling point. With this information, appropriate corrective action can be taken to eliminate the problem as quickly as possible.
The Total Coliform Rule specifies how many repeat samples must be taken, when, and from what locations. It also directs the water utility to collect a specific number of samples the following month, based on the number of routine samples ordinarily collected.
Non-community systems serving more than 1,000 people have the same requirements as community water systems. Non-community systems serving fewer than 1,000 people are required to collect one routine sample per quarter. When the routine sample tests positive, four repeat samples are required in the same quarter and five samples are required the following quarter.
Whenever a routine or repeat sample tests positive for total coliform, the water agency must collect a set of three or four repeat samples within 24 hours of receiving the laboratory results. At least one of the repeat samples must be taken from the same tap as the original coliform-positive sample; the remaining repeat samples in the set must be collected at nearby taps (within five service connections of the original sampling point), upstream and downstream of the original.
Repeat samples will be taken until no coliforms are detected or until the MCL is exceeded and the state is notified.
Determining Compliance
The MCLG for total coliforms (including fecal coliforms and E. coli) is zero. The MCL, based on the presence-absence concept, is as follows:
- For water systems analyzing at least 40 samples per month, no more than 5.0 percent of the samples (including routine and repeat samples) may be positive for total coliforms.
- For water systems analyzing fewer than 40 samples per month, no more than one sample per month may be positive for total coliforms.
The Total Coliform Rule makes another significant change in the way compliance is calculated. All valid coliform-positive samples, routine and repeat samples, must be counted when calculating compliance with the monthly MCL. Under previous regulations, check or repeat samples were not included in the monthly MCL calculation. On a case-by-case basis, the state may declare a sample invalid for one of several reasons, including interference by heterotrophic bacteria during laboratory analysis, as previously discussed.
Total-coliform-positive samples may be invalidated by the state under any of the following conditions:
- The analytical laboratory acknowledges that improper sample analysis caused the positive result
- The state determines that the contamination is a local plumbing problem
- The state has substantial grounds to believe that the positive result was unrelated to the quality of drinking water in the distribution system
Reporting and Notification Requirements
Reporting frequencies for coliform test results increase in step with the urgency of the problem. A water agency must report the results of monthly coliform testing to the state regulatory agency within the first 10 days of the following month.
Any time a water agency fails to collect a sample as required; the state must be notified within 10 days after the system learns of the violation. An invalid sample result is considered a failure to monitor and must be reported.
If the MCL is exceeded, the state must be notified no later than the end of the next business day and the public within 14 days. This requirement could occur when the water agency exceeds its monthly coliform-positive limit or when test results show the presence of fecal coliforms or E. coli in any sample.
The most critical situation exists when either of two situations occurs:
- A routine sample tests positive for total coliforms and for fecal coliforms or E. coli,
and any repeat sample tests positive for total coliforms.
- A routine sample tests positive for total coliforms and negative for fecal coliforms or E. coli, and any repeat sample tests positive for fecal coliforms or E. coli.
These situations are considered an acute risk to health. This occurrence is a Tier 1 violation, which requires that the state and the public be notified within 24 hours.
Water Quality
Section 304 (a)of the Clean Water Act directed EPA to develop water quality criteria not only for human health, but also aquatic life criteria, biological criteria, and microbial/recreational criteria. But they all point towards protecting public health.
Water agencies and utilities utilize local or imported surface water and groundwater sources and then convey it to treatment plants and distribution systems composed pipes, pumps, and storage reservoirs. Sometimes the storage reservoirs are small impoundments to large lakes with recreation. In all cases, primary standards set by the Federal, State, and tribal governments must be met. This extends to monitoring the health of aquatic life, water bodies in terms of the kinds of organisms present as well as bacteria and other pathogens. It is quite a responsibility. And it is a dynamic process. Regulations may become more stringent, and it will be the responsibility of engineers, scientists, and operators to ensure that a safe supply of water is always maintained for the public and environment.
Furthermore, the drinking water public judges the quality of their drinking water from Secondary (aesthetic) standards. You may be providing a safe supply of water to a customer, but if it slightly discolored from natural minerals in a groundwater supply, no one is going to trust it.
The quality of treated water put into the water distribution system from finished water treatment plants is high. The most pervasive water quality problem that affects most American communities results from the deterioration of water quality within water distribution conveyance systems.
Distribution Systems
Conditions exist in the distribution system that could result in degradation of water quality in the distribution system. The system operator must be aware of them and take corrective or protective measures where possible.
Consider how water is delivered to most of Southern California. Surface water is conveyed hundreds of miles in an aqueduct system, possibly blended with a groundwater supply, treated, and then delivered to a retail customer. The biggest threat to water quality degradation is typically in the last several miles of the distribution system pipeline conveyance system from the treatment plant.
Distribution System Integrity2
There are three components to distribution system integrity.
1. Physical Integrity of the system – the maintenance and isolation of the infrastructure (pipes, pumps, various valves including air release and blowoff valves, tanks) from the external environment
2. Hydraulic Integrity – the maintenance of a desirable water flow, and pressure
3. Water quality integrity – the maintenance of water age and disinfection residual in the system to prevent internally derived contamination. This involves routine continual water quality monitoring at various locations in the distribution system, particularly at unique and vulnerable locations such as area of lower flows and dead ends times. This will be discussed in the next section.
The causes of water quality degradation are due to breaches in distribution system integrity. It should be noted that minor water quality changes such as fluctuations in temperature, pH, alkalinity, or disinfectant residual are quite normal in the day-to-day operation of a well maintained distribution system and treatment plant.
Causes of Quality Degradation to Distribution System
Breaches to distribution system integrity were discussed in an EPA issue paper on the Total Coliform Rule. Various causes of major water quality degradation requiring an assessment and improvement in operations and maintenance procedures include:
Water main breaks, repairs, installation
Water distribution systems under pressure are susceptible to contamination if a break occurs. It is quite common for utilities like natural gas and sewer lines to share common corridors (rights of way) such as streets, A major break, particularly in those areas, create a pathway of contamination to the distribution system.
Contamination from sediment and high groundwater during repair or new installation is always a concern. Maintenance of the systems requires it to be properly, isolated, disinfected and flushed before the line is placed back into service.
Operation and Maintenance Deficiencies
Pipeline flushing is a routine maintenance practice often conducted within the distribution system to address consumer complaints and to reduce the retention time of water to improve water quality. Utilities have typically manually flushed water from the system using fire hydrants or flushing hydrants to control microbial growth.[2]
These practices can affect distribution system water quality in a negative manner if not conducted properly. Improper flushing can result in moving a contaminant further into the distribution system. The amount of time (age) water stays in a distribution system can affect its disinfectant residual and lead to bacterial growth. Stagnant water can occur in dead-end pipes or storage facilities that are over-sized or have periods of limited use.
Stagnant water provides an opportunity for suspended particulates to settle into pipe sediments, for biofilm to develop, and for biologically mediated corrosion to accelerate (Brandt et al., 2004). Long-term water storage in finished water reservoirs (lasting weeks or several months) can result in waterborne heterotrophic bacteria growing in sediments, attaching to inner walls, and spreading a biofilm over surfaces (Geldreich, 1996). Long retention time can also result in reduction in disinfectant residual and cause the release of ammonia through the decay of chloramines (Brandt et al., 2004). Loose deposits are susceptible to entrainment and suspension under normal hydraulic scenarios, such as flow reversals and velocity changes (Friedman et al., 2004a). Sudden flow increases (USEPA, 1992) or hydraulic disturbances (USEPA and CA DHS, 1989) can cause accumulated biofilm, scales, sediment, or tubercles to shear or slough, resulting in release to the water column. Also, if the distribution system is fed by multiple sources with varying water quality, the release of biofilms, scales, or sediments may occur at the interface between the sources.
Permeation
Non-metallic pipelines may be susceptible to contaminants passing through the pipeline walls from the exterior. This is referred to as permeation. Modern PVC pipes exhibit lower permeation, but some organic solvents are capable of permeating through plastic pipe.
Storage tanks for treated water
Storage tanks without screens for vents, or any hatch or opening that is not locked is a pathway for contamination. Storage tank walls must be properly lined (coated) on the inside as well.
Cross-connections
A cross-connection is typically defined as a connection between the drinking water distribution and another connection with different source water, or any other source. It is not uncommon for water agencies to share cross connections in the event of an emergency. But if the disinfection methods are different, or one source is not treated, then a pathway for contamination exists.
Backflow is the “undesirable reversal of flow of water or mixtures of water and other liquids, gases or other substances in the distribution pipes of the potable water supply from any source or sources.”[3] Backflow occurs when a pressure differential exists between the cross connection and the distribution because of a hydraulic change in the distribution. If the pressure in a water users connection becomes greater than the pressure in the distribution system, backflow will occur.
Enforceable regulations requiring back flow prevention devices for service connections have been adopted by State and Federal agencies.
Significant pressure reduction can occur in the event of line break. Backflow in this case is often referred to as backsiphonage.
Backsiphonage is a form of backflow caused by a negative or below atmospheric pressure within the water supply piping. Backsiphonage can develop from such causes as main breaks, an inadequate source of supply capacity, undersized mains, unusual water demands, planned shutdowns for maintenance or repair, or the use of on-line booster pumps. Backflow caused by back pressure can occur when the user’s water system is at a higher pressure than the public water system.
Chemical and microbial contamination from cross connections and backsiphonage have been responsible for many distribution system related illnesses. [4]
Corrosion and leaching
Corrosion is the gradual deterioration of metal pipe, metal fixtures, cement mortar lining in pipe, or other substances because of a reaction with the water.[5] Corrosion can be the result of physical actions that erode the coating of a pipe, chemical dissolution that leaches a pipe’s lining or wall material, or electrochemical reactions that remove metal from the wall of the pipe.[6 Corrosion can result in the leaching of certain metals, such as lead and copper.
Biological growth within the distribution system can also cause corrosion by providing an environment in which physical and chemical interaction can occur. Leaching is defined as the dissolution of metals, solids, and chemicals into drinking water (Symons et al., 2000). Some of the factors that influence corrosion and leaching are water velocity, pipe material, and water quality within the distribution system, such as pH, alkalinity, temperature, chlorine residual, and hardness of the water.
Contaminants from pipe linings, tank coatings, fittings, or other materials can sometimes leach into the drinking water, causing contamination. Cement-lined pipes and storage tanks can leach calcium carbonate into the water, which may significantly increase the alkalinity and pH of the water. This is especially true when the cement lined material is new, but also depends on the type of cement used, the contact time between the water and cement material, and the diameter of the pipe.
BiofilmBiofilms were introduced in Chapter 8. Contaminants, including total coliforms and some pathogens, may attach to or become enmeshed in biofilms on pipe walls in distribution systems. Many pathogens have been found to survive, if not grow, in these pipe biofilms where they are protected from disinfectants. Over time, coliform bacteria may detach or slough from the biofilm, causing persistent total coliform detections. Pathogens may also be included in the detached material and may result in waterborne disease. The biofilm can result in total coliform positive detections and other contamination events if disturbed. Organisms that have been found in biofilms include bacteria, viruses, protozoa, invertebrates, algae, and fungi.[7] Less efficient treatment of source water during runoff or changing water quality conditions may cause a change in the organic matter of treated water, which in turn may enable increased biofilm growth in the distribution system [8]
Key Terms
- appurtenances – small (but important) parts of a water distribution system, such as valves or meters, which help control flow
- Community Water System – a public water system that has at least 15 regular service connections and regularly serves at least 25 all year residents
- Cross connection - an unprotected connection between a part of a water system used or intended to be used to supply water for drinking purposes and any source or system containing water or a substance that is not or cannot be approved as safe drinking water. It is a connection between a regulated and approved drinking source and an unregulated drinking water source.
- Public Water System (PWS) – any system with at least 15 connections and regularly serves an average of 25 individuals daily at least 60 days out of the year
[1] https://www.epa.gov/sites/default/files/2021-05/documents/issuepaper_tcr_indicators_posted.pdf
[2] Brandt, M., J. Clement, J. Powell, R. Casey, D. Holt, N. Harris, and C. Ta. 2004. Managing Distribution Retention Time to Improve Water Quality - Phase I. AwwaRF. Denver, CO.).+
[3] USC-FCCCHR (University of Southern California Foundation for Cross-Connection Control and Hydraulic Research). 1993. Manual of Cross-Connection Control, Ninth Edition. University of Southern California. Los Angeles, CA.
[4] Craun, G. F. and R. L. Calderon. 2001. Waterborne Disease Outbreaks Caused by Distribution System Deficiencies. Journal AWWA 93:9:64-75
[5] AWWA. 1999a. Water Quality and Treatment, Fifth Edition. McGraw-Hill, Inc. New York, NY.
[6] AWWA. 2000. Water Distribution Systems Handbook. McGraw-Hill, Inc. New York, NY.
[7] AWWA. 2000. Water Distribution Systems Handbook. McGraw-Hill, Inc. New York, NY.
[8] USEPA, 2002. Health Risks from Microbial Growth and Biofilms in Drinking Water Distribution Systems.