Water main flushing is the release of water at a high flow rate to clean the interior of the water mains which supplies water to houses, businesses and fire hydrants. This is done as part of a maintenance schedule to ensure that drinking water quality does not deteriorate.
Water main flushing is also undertaken after fault repair and service and network testing.
Flushing out the mains creates a scouring action that removes sediment, debris, and biofilm (layer of microorganisms) in the system, which improves the carrying capacity of the pipes and maintains proper operation of the complex and costly water distribution system, besides maintaining water quality.
Understanding the water main flushing process
Potable water is forced at a high velocity into the water mains and discharged through fire hydrants and scour valves. The force of the water at a high flow rate scours the insides of the pipes, removing debris and sediment while dislodging biofilm.
The water is tested and continues to be flushed until it appears clear or until the normal chlorine residual is present for chlorinated water or until required turbidity levels are achieved.
The discharged water is dechlorinated before it enters storm water drains to reduce environmental impact. If the water is discharged into the environment, then it will soak back into the groundwater.
Different flushing techniques
When performing water main flushing, there are three main techniques:
A. Conventional Flushing
This traditional technique involves discharging water through fire hydrants at specific locations to clean out water from the pipes. The idea behind conventional flushing is to replace the water in the pipes with fresher water. Flushing is stopped when the water discharged is clear or appears to have the required turbidity. The discharged water can also be tested using sensors to ensure that flushing was successful and that the water quality is good. Although conventional flushing doesn’t require much planning, it uses more water and large volumes of potable water gets discharged into stormwater drains. It may also not clean the pipes adequately as velocities may be too low.
B. Unidirectional Flushing
Unidirectional flushing (UDF) is when a section of the pipeline is isolated by closing key valves with the aim of creating a single directional flow at a velocity of at least 1.5 m/s. This velocity is the minimum requirement to remove impurities and can vary according to pipe size and amount of debris in the pipelines. This technique requires more preparation and planning. UDF produces better quality water with less water utilization at a higher velocity and is often the recommended method of flushing the mains.
C. Continuous Flushing
Continuous flushing or continuous blow-off is mainly used in low water circulation locations, especially at dead ends. With this technique, some water is extracted in a low velocity flow through blow-off valves. Continuous flushing does not remove sediment but can be helpful in some situations, such as stabilising disinfectant levels by removing stagnated water.
Water discoloration during flushing
Sometimes water can appear dirty, discoloured, or cloudy after the flushing process. This is caused by sediment and debris that gets dislodged during the cleaning.
If there is discolouration, simply run a cold water tap at the lowest point of your property for up to 15 minutes or till the water runs clear. It’s important not to run a hot water tap as this will draw the sediment into the hot water tank. Do not run a tap with a filter attached, as the sediment will clog the filter.
Why regular water main flushing is important?
Water main flushing can be done anytime, especially after large water outages when the increased flow rate can stir up debris and sediments in the pipe. It can also be done as part of a regular maintenance program, which is important and necessary for the following reasons:
- Maintains the quality and taste of water.
- Removes old water.
- Cleans the water distribution system.
- Ensures proper functioning of the system.
- Reduces costly repairs.
- Identifies broken or inoperable valves and hydrants.
- Allows the assessment of flow rate for fire-fighting purposes.
As you can see, water main flushing is one of the most important practices carried out on the public drinking water distribution system. City councils will usually inform residents a week before the scheduled mains cleaning program, advising them of disruptions to the water supply and likely water discolouration.