Water Main Cleaning

Cleaning water mains helps maintain water quality and protect public health.



As recommended by the California Department of Public Health, MMWD cleans approximately 20 percent of its water mains each year to remove sediment buildup in transmission and distribution pipelines because this buildup can impair water quality. Sediment buildup can be particularly high in areas with dead-end pipes and in neighborhoods with older cast iron mains, and MMWD wants to ensure customers in these areas are also delivered high quality water.

To clean a water main, selected hydrants along a section of the main are opened. The high velocity of the released water scours the main, removing any collected sediment. The water is released onto the street, the chloramine is removed by dechlorination tablets, and the water enters the storm drain system. To conserve water, MMWD performs unidirectional flushing of the water mains, which uses significantly less water than conventional flushing. Flushing takes about 15 minutes for each hydrant.

During the cleaning process, customers may notice water in the street as it enters the storm drain system. In addition, customers may notice changes in their own plumbing, including slight pressure drops, air in the pipes, or “dirty” water. To counter these effects, we recommend restricting water use (especially of hot water) while our staff is cleaning the main and running the cold water for a short time afterward to remove any loosened sediment in the pipeline. Be assured the discoloration is not a health concern and is comprised predominantly of rust.

Take a look at the video below to see water main cleaning in action.


2018 Water Main Cleaning Schedule


Jan. 10: Mill Valley 
Jan. 24: Mill Valley
Jan. 31: Mill Valley
Feb. 7: Tam Valley
Feb. 14: Tam Woods
Feb. 28: Sausalito
Mar. 7: Sausalito/Marin City
Mar 14: Sausalito