Distribution Systems & Their Maintenance

Introduction

Even if the water source for your small water system is of pristine quality, if the distribution system is not maintained or is in a state of disrepair, the quality of water may deteriorate before it reaches the customer.

The focus on this section is on the safe delivery of water. We will discuss the following:

  • delivery of water
  • some common issues and hazards that must be avoided 
  • sampling and monitoring
  • operations and maintenance
  • easements
  • leak detection and water loss
  • water metering
  • the importance of a cross connection control program
Glossary of Terms
average daily consumption

The total demand for water during a period of time divided by the number of days in that time period. Also called the average daily demand.

bacteriological

Related to the study of bacteria.

coliform

A group of bacteria found in humans, plants, soil, air, and water. Fecal coliforms are a specific class of bacteria that only inhabit the intestines of warm-blooded animals. The presence of coliform bacteria is an indication that the water is polluted and may contain pathogenic (disease-causing) organisms.

contamination

The introduction into water of micro-organisms, chemicals, toxic substances, wastes, or wastewater in a concentration that makes the water unfit for its  intended use.

disinfection

The process designed to kill or inactivate most microorganisms in water, including essentially all pathogenic (disease-causing) bacteria. There are several ways to disinfect, with chlorination being the most frequently used in water treatment.

distribution system

The equipment involved with the delivery of treated water from the treatment facility to the intended end-point user.

Escherichia coli

E. coli is the predominant coliform in feces and the only member of the coliform group exclusively associated with feces. Therefore, it is the most specific indicator of fecal pollution and the possible presence of pathogenic microorganisms.

gate valve

A type of valve that uses a flow control element shaped like a sliding gate to block flow, often used as isolation valves.

isolation valves

A valve designed for isolation purposes when equipment needs to be worked on.

PSI

Pound per Square Inch: An imperial unit of pressure. 1 psi = 1 pound per square inch.

potable water

Water that does not contain objectionable pollution, contamination, minerals, or infective agents and is considered satisfactory for drinking.

pressure

A force acting on a given area. The pressure is calculated by dividing the force by the area over which it is acting. The unit of pressure is the Pascal (metric system) or pound per square inch (psi).

reservoir

A pond, lake, basin, or other structure (natural or artificial) that stores, regulates, or controls water.

residual chlorine

The amount of free and/or available chlorine remaining after a given contact time under specified conditions.

sediment

Solid material that settles to the bottom of a liquid.

turnover rate

The time between when water enters and leaves a storage structure such as a reservoir.

valves

A device which is used to control isolate or flow in a piping system.

watermains

Large distribution pipes that run along road major roadways.

Delivery of Water

The primary function of any water distribution system is the transportation of drinking water safely to the consumer. The water source may originate from a well, river, lake or spring. In order to ensure the water is safe to drink (potable), it is usually treated to remove bacteria, viruses or parasites, and dissolved minerals that may cause illness in humans. 

As the water flows through the distribution system, there are a number of components that keep the system operational. These include reservoirs, pumping stations, air valves, gate valves and piping networks, each of which is described below.

The reservoir stores water for higher demand flows, such as for fire emergencies, and peak domestic flows, such as when people are getting ready for work in the morning and returning home later in the day. The reservoir also acts as a buffer in maintaining constant flow and pressure of water in the distribution system. For small water systems, pneumatic tanks are typically used instead of large reservoirs, as they, too, are capable of supplying and keeping up with the water demand. The tanks are generally not sized to provide fire flow. The minimum storage should be equal to the average daily consumption or the storage calculated to meet CT disinfection requirements, whichever is greater. A pressure switch regulates the amount of water to be stored in order to maintain a constant pressure in the distribution system.

Pumping stations are added to the distribution system to maintain pressure and delivery of water to uphill areas and reservoirs.

Air valves are devices that allow air to be introduced into the distribution pipe when a vacuum may be created. A vacuum can potentially damage the pipe or stop the water and is to be avoided. The air valves are located at high points in the distribution system.

Gate valves are added throughout the distribution system, so sections can be isolated for water main work and the water flow can be throttled for pipeline repair. They are a type of valve that uses a flow control element shaped like a sliding gate to block flow, often used as isolation valves.

Pipe networks’ efficiency is effected by both the materials used and the layout. Pipe material is crucial for the efficient delivering of water. The smoother the interior of the pipe, such as with PVC pipes and ductile iron, the less friction there is. Also, the less twists and turns in the pipe, the more efficient the delivery of water.

Common Issues & Hazards

The greatest concerns for the safe delivery of water are loss of pressure, loss of chlorine residual and cross contamination.

Loss of pressure may result from a water main break, fire flow or inoperable pumping stations, due to power failure.

Loss of chlorine residual can be caused by a number of factors:

  1. Source water quality: Water that is high in organic matter will use up the chlorine residual faster than water that is lower in organic matter.
  2. Residency time: The more time the water spends in storage and distribution, the more chlorine residual is used up. Long residency time can result from low water usage, dead ends in the distribution system and poor turnover in the reservoir.
  3. Reaction with pipe materials: Some pipe materials (e.g., iron) can react with chlorine, resulting in loss of the residual.
  4. Biofilm growth: Biofilm is a large colony of microorganisms that grown on pipe walls within the distribution system that will use up the chlorine residual.

If low chlorine residual is detected, water should be flushed through the system until the residual is reestablished. If chlorine levels continue to drop below an acceptable level, the cause should be investigated.

Cross contamination from the exterior environment into the water main may occur if there is a leak or opening in the pipe. Standard practice is to maintain a minimum water pressure of 20 psi to prevent the potential of cross contamination.

Bacteriological contamination and microbiological growth is also a concern. It may be introduced through a cross connection (more on cross connections at the bottom of this section) or failure to adequately treat the water at source.

Sampling & Monitoring

In order to ensure drinking water is bacteriological free, routine sampling is required under the BC Drinking Water Protection Regulation Schedule B, with the number of samples required determined by population. For most small water systems, which serve a population under 5,000, the minimum number of samples to be taken per month is four (4).

In order to determine the best sampling locations, it is best to consult with your local Drinking Water Officer. Typically, these sites include sampling at source, mid-point in the distribution system and at the far end. Once set, these locations remain permanent, in order to compare the latest samples with past results.

Schedule A of the Drinking Water Protection Regulation establishes the bacteriological standard that must be maintained according to the standards in the table below:

Schedule A

Water Quality Standards for Potable Water

(sections 2 and 9)

Parameter:

 

Standard:

Fecal coliform bacteria

 

No detectable fecal coliform bacteria per 100 ml

Escherichia coli

 

No detectable Escherichia coli per 100 ml

Total coliform bacteria

 

 

(a) 1 sample in a 30 day period

 

No detectable total coliform bacteria per 100 ml

(b) more than 1 sample in a 30 day period

 

At least 90% of samples have no detectable total coliform
bacteria per 100 ml and no sample has more than 10 total
coliform bacteria per 100 ml

For monitoring mineral content, chemical samples are obtained from the source water both prior to treatment and after treatment. The comparisons determine how much of the mineral is removed. This is important for monitoring health-related parameters, such as arsenic, nitrates, total organic carbon, pesticide residues, etc.  

Operations & Maintenance

It is essential that all equipment for operating and maintaining the distribution system is exercised (i.e., tested or used) on a regular basis. This ensures the water flow is minimally disrupted in the event of an emergency or during regular repair work. For example, in the event of a water main leak, valves can be easily operated if they were previously located and exercised. If a valve is not exercised, it may seize up or become difficult to turn, making the repair work much more difficult. Other challenges that may occur if the equipment is not regularly maintained include the failure of a backup generator to start in the event of a power failure or of a hydrant to deliver adequate water flow in an emergency.

Timing of Maintenance

  • Hydrants and valves should be evaluated and exercised at a minimum once per year. If a hydrant is used, it should be evaluated afterwards.
  • Backup generators should be tested each month.
  • Pumps should undergo maintenance as per the manufacturer’s recommendation.
  • Dead end mains should be flushed on a routine basis to maintain water quality.
  • Water reservoirs should be evaluated annually and cleaned every 3 to 5 years, based on sediment collection on the bottom.
Easements

An Easement or Right of Way (ROW) is a strip of land on private property that acts as a corridor for water mains, sanitary sewers and/or storm mains. With an easement, a legal agreement exists between the landowner and the utility to provide 24/7 access for operating, maintaining or repairing any component that requires attention. The minimum widths of these ROWs are generally 3 metres. Typically, the width is determined by 2 X depth of pipe + width of trench excavated. For example, if the top of the pipe is 1 m from the ground surface and the trench width is 1 m, the result is 2 X 1m depth of pipe + 1 m excavation trench = 3 m ROW width.

Leak Detection & Water Loss

Most water systems experience approximately 10% leakages and/or unaccounted water use. This may be from water main leaks, unaccounted water use from a property, or illegal use of a fire hydrant.

Leaks may have originated from weakened joints or fitting connections or from a damaged or corroded part of the pipe. If unresolved, leaks may undermine pavement or other structures, resulting in damage. Perhaps the greatest concern is that the leak will soak the ground surrounding the pipe and, in the event that pressure is lost in the pipe, the water, combined now with dirt and other contaminates, may backflow into the pipe.

If a water system is metered, leak detection is easier to detect. Once an area for the water loss is determined, specially trained personal use leak detection equipment to pinpoint the area. They typically use sound-intensifying equipment in a systematic fashion to locate leaks. Preliminary methods of locating leaks include damp spots or water seepage in the vicinity of mains or services.

Reference: Water Distribution System Operation & Maintenance, 5th Edition, A Field Study Training Program, California State University, Sacramento California, 2005, p.211.

Water Metering

Water metering establishes a user pay system, which ensures equity and fairness for water consumers. If a water system is unmetered and users pay a flat rate, there may be inequity if a neighbour uses more water for their green house and swimming pool and another one uses only water within the home.

The cost/benefit for establishing a metering program may be more beneficial for a large municipal system versus a smaller system, due to the lower dollar cost averaging for installation and maintenance of the meters. For smaller systems, it may be more beneficial to maintain a flat rate system (unmetered) and ensure users use water responsibly.

Cross Connections

In order to ensure the safe delivery of water to users, any potential cross connections with contaminated sources need to be addressed. This includes simple threats, such as leaving the garden hose in a pool or hot tub or leaving it connected to a pesticide dispenser. Any loss of pressure (negative pressure) from water delivery may turn the hose into a vacuum and draw the contaminated water into the plumbing system. More complex threat include a direct connection of an irrigation system to a water supply without the barrier of a backflow preventer, or the direct connection of the water to a chemical supply. The backflow preventer contains a spring-loaded valve that closes if the water flows in reverse, hence isolating the contaminated source from the water supply. In order to protect drinking water from all potentially contaminated sources, a cross connection control program should be instituted. The acceptable backflow preventer devices (BPDs) should be testable and meet the relevant CSA standards (i.e., CSA B64) or equivalent. Operation and maintenance of the water pipeline should include regular testing and maintenance of testable BPDs.

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