Water Quality Flushing

The Hillsborough Department of Public Works flushes water lines in order to:

  • Maintain water quality - The disinfectant chloramine becomes less effective as water collects at dead end lines and becomes stagnant. The Town must therefore flush hydrants in areas where there is low water demand.
  • Clean water mains - Over time, small particles of rust and clay build up inside the Town's main water pipes and can harbor colonies of bacteria that may harm water quality. Water quality staff use unidirectional flushing to pull water at a high velocity from a specific point in the system to a discharge point. The rapid movement of water scours the insides of the pipes. Often, the water will appear brown as it flows from the hydrant, but clears up after some time. Over the years, Hillsborough has greatly reduced the amount of water used in these unidirectional flushing procedures by regularly maintaining the pipes.
  • Conduct fire flow tests - Each fire hydrant must be tested periodically to ensure and document proper functioning in case of an emergency. Hillsborough must test the flow of its hydrants much more often than neighboring cities because it is a hilly community - the Town must ensure that its hydrants at high elevations have sufficient pressure to fight fires.

In the past, this flushing was done by opening water hydrants and letting water flow out for an hour or more. It was necessary to flush millions of gallons of water down storm drains each year in order to ensure that the water remained clean and safe to drink. In an effort to "Lead by Example in Water Conservation," the Town of Hillsborough is testing an innovative way to flush its water system without wasting those vast amounts of water.


Hillsborough received a grant from the California Water Resources Control Board to help purchase an innovative truck-mounted unit called NO-DES (Neutral Output Discharge Elimination System). The NO-DES truck unit flushes, filters, and re-circulates the water within the water distribution system.

Did You Know - Fire Hydrants
NO-DES Truck

The NO-DES method is simple: the truck unit is parked between two fire hydrants with hoses connected from each hydrant back to the NO-DES unit, which results in a temporary loop being created in the water distribution system. A pump on the NO-DES unit then circulates water through that loop at a velocity fast enough to scour the inside of the water main. This water is filtered through the NO-DES unit (filtered down to one-micron absolute - approximately 100 times smaller than the diameter of a human hair.), which removes the sediment and biofilm that has been intentionally stirred up. The NO-DES unit can also add disinfectant during the process to further improve water quality and ensure its safety, something former methods cannot achieve.

NO-DES Diagram

Hillsborough was one of the first water agencies to purchase and use this innovative new system, with oversight from the California Department of Public Health. Rigorous water testing and reporting protocols will ensure that the water filtered and disinfected by the NO-DES system is clean and safe for consumption. For more information on the NO-DES system, visit the NO-DES website.

Flushing and You

Flushing operations are conducted Monday through Thursday from 8 a.m. to 4:30 p.m. The flushing crews will set out signs notifying residents if flushing operations will be conducted in the area.

Water is safe to use during the flushing process. However, the Town recommends that customers do not shower, do laundry or run unnecessary water during water flushing operations. There may be small pressure fluctuations or minor aesthetic effects like cloudy or discolored water. This is not dangerous; running a faucet outside of the house or running cold water inside the house for a few minutes should resolve the problem. Residents who experience continued water quality problems should call the Public Works Department at 650-375-7444. To report a water quality problem after hours, please call the Police Department at 650-375-7470.

Did You Know - De-Chlorinating Tablets

If your clothes have become stained as a result of using water during a flushing operation, please keep them wet. You can call the Public Works Department at 650-375-7546 to receive a cleaning agent to remove the stain at no charge.