Watermains for Small Water Distribution Systems

Introduction

A watermain is an underground pipe that delivers a steady supply of fresh, clean drinking water to residents and businesses. The design of a water distribution system should follow the Master Municipal Construction Documents (MMCD) Standard Specifications and Drawings and local Subdivision and Development Servicing Bylaws.

Glossary of Terms
bacteriological

Related to the study of bacteria.

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.

AWWA – American Water Works Association

The American Water Works Association is an international, nonprofit, scientific and educational society dedicated to providing total water solutions assuring the effective management of water. Founded in 1881, the Association is the largest organization of water supply professionals in the world, with 50,000 members representing the full spectrum of the water community: public water and wastewater systems, environmental advocates, scientists, academicians, and others who hold a genuine interest in water. www.awwa.org

corrosion

The gradual decomposition or destruction of a material due to chemical action, often an electrochemical reaction. Corrosion starts at the surface of a material and moves inward.

cross-connection

A connection between a drinking (potable) water system and an unapproved water supply that may lead to contamination of the water supply. For example, if you have a pump moving non-potable water and you use the drinking water system to supply water for the pump seal, a cross-connection or mixing between the two water systems can occur.

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.

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.

peak demand

The maximum momentary load placed on a water treatment plant, pumping station, or distribution system. This demand is usually the maximum average load in one hour or less but may be specified as the instantaneous load or the load during some other short time period.

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).

purveyor

A supplier or provider of water.

watermains

Large distribution pipes that run along road major roadways.

Unlike a conventional municipal water distribution system, a small distribution system does not provide water for fire protection, where the pipes are sized based on the pressure and flow at the period of peak demand. Therefore, special design considerations should be given to the pipe sizing, watermain looping, separation, and provision of adequate valves for flow control and flushing.

In BC, a Construction Permit is required before commencing construction, installation, alteration or extension of a watermain. The purpose of this requirement is to ensure that the design complies with applicable standards and reduces public health risks from contaminated distribution systems.

Design Considerations for Small Water Distribution Systems

A distribution network should be designed to ensure flexibility of operation and to minimize the area of the community affected by shutdowns during watermain network repairs. Watermains should be looped to reduce dead ends, prevent stagnation of water and minimize service disruption during repairs. If dead ends or low points are present, blow-offs or hydrants should be provided for flushing. For small water systems, “yard-hydrants” are normally used for this purpose. To prevent water pipes from freezing, a minimum depth of cover of 1.8m is required in BC. Design the depth of cover by calculating the depth of frost penetration or by referring to the depth of successful existing similar water pipes in the area.

Watermain sizing is determined by the hydraulic model, considering cost, pressure, and the current and future water demand. In general, the distribution lines are required to maintain a minimum pressure of 140 kPa (20 psi) at ground level at all points in the system. The minimum acceptable watermain size of is 50 mm (2 in) in diameter, while the minimum acceptable service line is 25 mm (1 in).

Shut-off valves are used to isolate reasonably sized sections of the system for repairs or maintenances. Shut-off valves are typically located at no more than 150 metres intervals in commercial areas and at no more than 240 metre intervals in other areas. Consider the nature of the development and future expansion when spacing the isolation valves.

As air pockets can collect at high points in a distribution line, air release valves can be installed to prevent surges or water hammers. Use concrete thrust blocks, to protect the pipe by providing support, at valves, bends, tees, reducers, plugs, caps, valves, hydrants, and blow-offs for watermains 100mm or larger.

Watermain Materials

Both metallic and plastic pipe are permitted in the construction of watermains. Common materials include the following:

  • Ductile Iron and Steel
  • Polyvinyl chloride (PVC)
  • Polyethylene/High Density Polyethylene (PE or HDPE)
  • Concrete Pressure Pipe

Select pipe material and fittings that can withstand all surge and transient pressures and conform to the applicable AWWA or NSF standards (e.g., NSF 61, AWWA C906, AWWA 900, etc.). To prevent pipeline corrosion, also consider the anticipated water quality and bedding soil characteristics when choosing the pipe material and fittings.

Watermain Separation from Contamination Sources

The watermain should be designed and constructed away from any contamination sources. Consider the following factors in providing adequate separation:

  • Materials and type of joints for water and sewer pipes
  • Soil conditions
  • Service and branch connection into the water main and sewer line
  • Compensating variations in the horizontal and vertical separations
  • Space for repair and alterations of water and sewer pipes
  • Off-setting of pipes around manholes

Parallel Installation: A watermain should be laid at least 3 m (10 ft) horizontally and 450mm (18 inch) vertically, measured edge to edge, from any existing or proposed sanitary or storm sewers. If conflicts occur, additional protective measures should be implemented. The water supplier can contact the regional Public Health Engineer to confirm the requirements.

Watermain Separation Illustration from Northern Health

Crossings: Where a watermain crosses a sanitary or storm sewer, the watermain should be laid to provide a minimum vertical distance of 0.45 metres between the outside of the watermain and the outside of the sewer. While the watermain may be either above or below the sewer, it is preferred that the water main be above the sewer. Locate one full length of water pipe at the crossing, so both joints will be as far from the sewer as possible. Special structural support for the water and sewer pipes may be required. If conflicts occur, provide additional protective measures.

Watermain Crossing Illustration from Northern Health

Where possible, the above separation practice should also apply to service connections. In addition, have no water pipe pass through or come in contact with any part of a sewer manhole.  Maintain a 3 meters (10 ft)—preferably 7.6m (25 ft)—setback between the watermain and septic tanks, waste stabilization ponds, or open sewage discharge locations. Under no circumstances extend a waterline through a septic tank, disposal field, or feedlot.

To avoid cross-connections, there shall be no connection between the distribution system and any pipes, pumps, hydrants, or tanks whereby unsafe water or other contaminating materials may be entered into the system.

Disinfection of Water Pipelines After Construction, Alteration, Extension or Repair

Disinfect water pipelines following construction, alteration, extension or repair and prove they are bacteriologically safe prior to operation. Disinfection should follow the most current version of C651-92 American Water Works Association (AWWA) Standard for Disinfecting Water Mains, which contains options for rapid disinfection.

After disinfection, the chlorinated water in the pipeline should be completely removed by thorough flushing. If there is no sanitary sewer manhole to dispose of the chlorinated water, the water purveyor must dechlorinate the water with sodium thiosulphate prior to discharging it to the approved outlet. The new watermains should be kept isolated from the existing water distribution system, using a physical separation, until satisfactory bacteriological testing is approved by the local Environmental Health Officer.

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