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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 needs to 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 disinfection byproducts. 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 treatment plants to 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. After several years of use by the City, 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), and the Water Division may be assisted by street paving staff since asphalt paving is weather dependent and cannot be completed during most of the winter months.
The City’s overall water system has been divided into three (3) flushing areas, therefore over a three (3) year period the whole system will have been flushed and the cycle will start over again. This year, we are conducting hydrant flushing in Pressure Zone 2, which includes regions of central and southwest Napa as shown on the map: Full City Flushing Area (PDF). Areas not included in this year’s program will be flushed in the winter of 2023-24 or 2024-25.
The flushing program may cause periods of 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.
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. The Water Division will update the City’s web site daily with maps identifying where we are currently flowing hydrants and where we will move to next. To limit customer impacts of discolored water and lower pressure, the flushing program is conducted during the winter months when water demands are at their lowest.
The water used for this program is not waste. It performs an essential water quality function 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 just one tenth of one percent of Napa's total annual demand, 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.
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 Napa River. Due to the high volume 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 river.
Customers may call the Water Division at (707) 257-9521. Customers may also visit our office at 1700 Second Street, Suite 100 between the hours of 8am and 5pm, Monday through Thursday (except for holidays).