The greatest concerns for the safe delivery of water through a distribution system are loss of pressure, disinfection system failure, loss of chlorine residual, contamination due to intrusions (of water mains and reservoirs) and cross connections.
This may occur when unusually high amounts of water are discharged from a water main break or a fire hydrant (during firefighting or hydrant flushing) or when a pumping station becomes inoperable due to a power outage.
To help prevent contamination from cross connections, the BC Design Guidelines for Rural Residential Community Water Systems recommend that a water system should maintain a minimum water pressure of 140 kPa (20.3 psi) throughout the distribution system. However, to reduce customer complaints, a minimum pressure of 275 kPa (40 psi) is suggested.1
1 Appendices of the BC Water System Assessment User’s Guide (page 9) re minimum water pressures
This can occur when there are holes or leaks in pipes (water main breaks, etc.), valves or reservoirs. These leaks can allow the entry of pathogens or chemicals into the distribution system. Water suppliers can use leak detection equipment to detect leaks in water mains and reservoirs.
A cross connection can also permit biological or chemical contamination to enter the distribution system. A cross-connection is any actual or potential connection between a potable (drinking) water system and any source of pollution or contamination. Backflow is a flowing back of water or reversal of the normal direction of flow. There are two types of backflow:
This is a major concern for water suppliers with surface water sources and groundwater sources at risk of containing pathogens. Water supply systems with these sources should have, in addition to filtration, at least one treatment process that provides disinfection (e.g. chlorination and/or ultraviolet light radiation, etc.). If the disinfection system malfunctions or fails, untreated or partially treated water containing pathogenic microorganisms could travel through the distribution system to the consumer’s tap.
This can be caused by a number of factors:
- Source water quality: Water that is high in organic or inorganic matter will use up the chlorine residual faster than water that is lower in organic or inorganic matter.
- 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.
- Reaction with pipe materials: Some pipe materials (e.g., iron) can react with chlorine, resulting in loss of the residual.
- Biofilm: A biofilm is a layer of microorganisms contained in a matrix (slime layer), which forms on surfaces in contact with water in water pipes. Their presence can be responsible for a wide range of water quality and operational problems. Biofilm on the inside of pipes can harbour pathogenic microorganisms which can be released into the distribution system when the biofilm is disturbed.
If a low free chlorine residual is detected, you should flush the system until the residual is re-established. In some cases, you may need to issue a notification to your water users about the reduced drinking water quality. If chlorine levels continue to drop below an acceptable level, the cause should be investigated. Operators may consider increasing the disinfectant dose if that is not effective. A minimum free chlorine residual of 0.2 mg/L is recommended throughout the distribution system.
This is backflow caused by a negative pressure (i.e. a vacuum or partial vacuum) in a water supply system. Back siphonage can occur when there is a drop in water pressure due to nearby firefighting, a break or repair of a water main, high velocities in pipe lines (venturi effect), hydrant/water main flushing and reduced supply pressure on the suction side of a booster pump.
This is pressure that is greater than the water system supply pressure. It can happen when there is a connection to a non-potable supply operating at a higher pressure than the water distribution system. Increases in pressure can be created by booster pumps, temperature increases in boilers, interconnections with systems operating at higher pressures, and elevated piping (e.g., 9 metres above finished grade).
A backflow preventer contains a spring-loaded valve that closes if the water flows in reverse, which isolates a contaminated source from the water supply. It prevents backflow which can contaminate your water supply. There are five types of backflow preventers: air gap, vacuum breaker, dual check valve, double check valve and reduced pressure principle.
There are several factors when choosing a backflow preventer device (BPDs). They should be testable and must 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. Loss of pressure (negative pressure) from water delivery may turn a hose into a vacuum which could draw contaminated water into the plumbing system. To protect drinking water from all potentially contaminated sources, a cross connection control program should be implemented.
Common cross-connections found in plumbing and water systems include:
- leaving the garden hose in a pool or hot tub
- leaving the garden hose connected to a pesticide dispenser
- a water softener drain or other type of water conditioning equipment directly connected to a sanitary sewer
- a high pressure washer or car wash using soaps and cleaners connected to a hose bib without a BPD
- an irrigation system installed without a BPD
Guidance on Monitoring the Biological Stability of Drinking Water in Distribution Systems, Health Canada, Feb. 2022
Maintaining Distribution System Water Quality, Government of Nova Scotia
Drinking Water Chlorination Facts, HealthLinkBC, June, 2022
Drinking Water Chlorination, Health Canada
Water Distribution System Operation & Maintenance, 7th Edition, A Field Study Training Program, California State University, Sacramento California, 2018.
Water Disinfection with Chlorine and Chloramine, Centers for Disease Control and Prevention, USA
Leak detection in water distribution networks: an introductory overview by Samer El-Zahab & Tarek Zayed.
Drinking Water Protection Act: Drinking Water Protection Regulation, Province of BC, Reg. 200/2003, including amendments up to B.C. Reg. 122/2013.
Small Water System Guidebook, BC Ministry of Health, Feb. 2017