Watermain Flushing
To ensure clean, safe drinking water throughout Millbrae, The City of Millbrae practices Unidirectional Watermain Flushing as a water quality service.
This routine maintenance helps remove sediment and maintains water clarity in our distribution system.
Hydrant flushing is a periodic maintenance activity to clean and flush the City water mains.
Over time, particles and minerals can build up in the water mains. Chlorine is added to the water at the treatment plants to ensure the water is safe to drink when it reaches our customers.
As more deposits build up, additional chlorine may be added to the water, which results in higher treatment costs and a less appealing taste for our customers. The deposits that build up also react with chlorine and increase the potential formation of other solids.
Flushing the fire hydrants mobilizes and removes the particles and minerals that have built up in the water mains, assisting the City in delivering higher quality water from the source to the customers’ taps.
In the Unidirectional Hydrant Flushing method, City staff isolate the flushing areas by closing pre-selected system valves.
With the closure of these valves, water is directed to flow in one direction (“unidirectional”) and increase the velocity of the water in the pipes, thus more effectively removing particles and minerals from the pipe than simply operating hydrants without closing valves.
This method of system maintenance has proven to be an efficient and optimal way to maintain the quality of water within water mains.
Unidirectional Hydrant Flushing requires additional staff on site to open and close valves and monitor residual pressures (water pressure remaining in the system while a fire hydrant is flowing.
The City’s overall water system has four pressure zones see map (LINK) . The State of California (California Code of Regulations Title 22) contains technical standards for flushing, but not mandatory frequency. Best management practices recommend that most water mains be flushed at least every two years.
The City alternates flushing of the water mains every two years, typically Zones I, II, & III in one year and Zone IV the following year and repeating this program on an ongoing basis. Zone IV is a larger geographical area than the other individual zones and is also in the lower, flatter portion of the City.
The flushing program may temporarily cause discolored water and reduced water pressure in localized areas where flushing is being conducted.
The discoloration is due to particles and minerals that have settled in the water main being mobilized by the high velocity water before being discharged through the flowing fire hydrants. The water will remain safe to use and generally clears within a few hours after flushing has been completed. Residents should monitor their water prior to starting laundry, especially when washing white clothes.
Due to temporary reductions in water pressure in the vicinity of flushing operations, the reduced pressures may cause low-pressure alarms to activate on automatic fire sprinkler systems for commercial properties.
If you notice discolored water, run cold water from one faucet for 2-3 minutes to see if it clears. If it does not clear, wait an hour, and try again.
The water used for this program is not waste. It performs an essential cleaning function to improve water quality and is a planned investment for maintaining the water system.
The amount of water used is dependent on the amount of particles and minerals to be removed and the size and length of pipe to be cleaned. With fewer particles and minerals in the pipes thanks to previous unidirectional flushing, the volume of water needed to remove them this year will be reduced compared with previous years.
The program is expected to consume less water than all customers' combined use for irrigation on a single summer day, minimizing the impact on our water supplies while improving water quality.
It is neither practical nor cost effective to capture high velocity flows in a mobile operation.
There is equipment available today that is essentially a mobile treatment plant that treats the water and injects it back into the system, but the upfront capital cost for these systems is more than $500,000 and it is not a cost-effective investment for the City’s Water Fund.
In addition to these capital costs, the logistics, time delay, and manpower required to coordinate the collection, storage and treatment at numerous locations is not practical.
The hydrant flushing staff will be using dechlorination tablets to remove chlorine in the water before the water goes into the storm drain system and eventually into the Bay.
Due to the high velocity of water flowing from the hydrants, the dechlorination tablets may cause some bubbling of the water, giving it a soapy appearance.
The tablets are environmentally safe and are being used to protect the Bay.
This year’s flushing operations have been divided into many smaller individual flushing areas which are isolated from the rest of the water system by enforcing the direction of water flowing in the pipes.
This method reduces impacts to areas not inside the active flushing area.
To limit customer impacts of discolored water and lower pressure, the flushing program will be planned during the winter months when water demands are lower.
More questions on watermain flushing?
Contact Millbrae Public Works (650) 259-2339 or visit City Hall at 621 Millbrae Avenue with any questions or concerns.