Flushing and Treating Your Water Distribution System
A critical component in the delivery of safe potable water to the end users of your water system is
ensuring the distribution system is kept clean. Even water systems that are currently not meeting the
requirement to deliver potable water will benefit from routine flushing followed by a dose disinfectant to
clean their system. The information presented in this document is intended to assist small water system
operators with the procedure of flushing and treating their water distribution systems. Contact your
Environmental Health Officer if you have any questions or require more details.
Why is it done?
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Removes the bulk of poor quality water out of the system - less demand on the chlorine to do its job.
Over time sediment from surface systems builds up in distribution systems and needs to be flushed out.
Bacteria and other micro organisms adhere to the pipe walls and begin to grow impacting water quality.
Provides a cleaner environment for the transportation of water improving both the aesthetic and health
aspects of the water.
When should one flush and treat a water system?
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Water system has not been used for some time (seasonal facilities)
Work has been performed on the system (repairs, upgrades)
Water treatment has malfunctioned (Emergency Response Plan)
Water suspected to be contaminated (Emergency Response Plan)
Water sampling results confirm the presence of contamination (Emergency Response Plan)
Routinely during the course of a year as preventative maintenance (Operations Guidebook)
It is important that the water used to flush the system is in the best condition possible.
 For systems with treated water, that means the water treatment unit is fully operational.
 For systems or portions of the distribution that carry non-potable water, need to conduct the flushing
program when the water is free from turbidity or known contamination.
What is used to do the job?
Most common disinfectant used is unscented liquid bleach at a concentration of 50 parts per million.
How much bleach is needed to obtain this concentration will be dependant on: 1) the estimated total
volume of water held in all of the cold water supply lines and holding/pressure tanks in the system:
 1 teaspoon (1/6oz) or 5 ml of bleach for every 100ft of ½ inch pipe
 ½ oz or 15 ml of bleach for every 100 ft of 1 inch pipe
 2 oz or 65 ml of bleach for every 100 ft of 2 inch pipe
 13 oz or 375 ml of bleach for every 100 US Gallons holding tank capacity or;
 15 oz or 450 ml of bleach for every 100 Imperial Gallons of holding tank capacity or;
 17 oz or 500 ml of bleach for every 500 Litres of holding tank capacity
 Contact your health department if your source is a well
And also dependant on: 2) what type of bleach you have access to:
 Commercial grade is rated at 12% chlorine content per volume,
 Regular household bleach is near 6% chlorine content per volume
 Inexpensive no name brands are about 3% chlorine content per volume
The volumes and calculations given above are for regular domestic bleach. You will need to use ½ the
required volumes if using commercial bleach, and double the required volumes if using no-name brands.
Bleach also loses its strength over time so ensure your supply is fresh, not left over from last year. Will
also need to obtain chlorine test strips to confirm the level of chlorine in the system during this procedure.
How is it done ?
 As previously stated if a water treatment unit is present it must be fully operational prior to performing
this process.
 Water must not be used during this period apart from sanitary needs – MUST NOTIFY ALL USERS
NOT TO USE THE WATER IN WRITING. An alternate source of potable water will need to be on
hand.
 Flush each water fixture (sinks, toilets, taps, ice machines, dishwasher, washing machines) for a minute
starting with the ones closest to the beginning of the distribution system and work outwards. Aerators
at the end of the taps must be removed during the flushing period as the restrictors in them don’t
permit the water to flow fast enough to flush out any sediment or slime (called scouring velocity)
 If there is a hydro-pneumatic pressure tank in the system, consult the manufacturer regarding release
and re-establishment of air pressure.
 Add the volume of required chlorine in one shot into the system. Depending upon your design, it may
be possible to add the chlorine at the intake, the holding tanks or a filter housing. The more of the
system you treat, the better your final results.
 Return the aerators to the taps and flush each fixture again until you can smell chlorine, hold for 24
hours.
 After 24 hours, take a chlorine residual reading at a number of locations – you should still detect a free
available chlorine residual in the water. If no chlorine is detected the demand for chlorine was higher
than the amount added to the system. Therefore the process will need to be repeated.
 Flush each fixture again allowing the excessive chlorine levels out of the system. Flushing needs to
continue until the residual chlorine levels are below 1ppm. It is recommended that you obtain sodium
thiosulfate to help neutralize the high chlorine levels in the wastewater.
 Discard all ice produced during this period as well as the next batch.
Anything else that can be done?
Backflow prevention is a program to eliminate the chances of potentially dangerous liquid material from
entering into the distribution system by means of a cross connection. This is an excellent time while
visiting all of the various components of the water system when flushing to ensure no cross-connections
are present in your system. The attached guidebook “50 Cross-Connections Questions and Answers”
details the risks associated with cross connections and methods to eliminate the risk.
Hot water tanks benefit greatly from being drained slightly once a month and being turned off and fully
drained when not in use. The hot temperature of these units usually takes care of any biological
contamination when they are kept above 49c so chlorine normally isn’t required. Sediment build up in the
tanks can contribute to sediment loading in the hot water distribution system and also accelerates the
erosion/rusting of the tank itself.