Water Treatment and Delivery


The surface water from the Mt. Tamalpais Watershed is protected naturally from many kinds of contamination. Even so, this water still requires treatment to remove viruses, bacteria, and other naturally occurring pathogens. Water treatment starts in the reservoirs, where we seasonally aerate the water to maintain a proper oxygen balance. Next, the water goes to either the Bon Tempe Treatment Plant on Mt. Tam or the San Geronimo Treatment Plant in Woodacre for further processing. Suspended matter is removed in clarifiers; microscopic particles are removed in deep-bed, multi-media filters; and bacteria and pathogens are inactivated by disinfectants. The water is then treated to control corrosion. Since passage of a voter referendum in 1972, fluoride has been added to reduce tooth decay.  
The water we import from the Russian River is naturally filtered in the deep sand and gravel below the river bed and requires no further clarification. This water enters the MMWD system at our Ignacio Treatment Facility, where water quality is monitored continually. Final treatment is similar to that used at our two reservoir water treatment plants.

Delivery - Water Distribution

Since people use water every hour of the day, we operate around the clock.

On average, it takes 24 hours for water to travel from a reservoir to a treatment plant and then through miles of underground mains into your faucet. Control of our water-distribution system is by computer, enabling our operators to coordinate the operation of more than 200 district facilities and direct the flow of water through our pipelines.

With two treatment plants for drinking water and one treatment plant for recycled water, we can process up to 61 million gallons per day. Our pipeline system pumping units can deliver up to 38,000 gallons per minute. Water may be piped directly to you or into a tank until needed. Because of Marin's hilly terrain, about 90% of the water must be pumped at least once before it reaches the tap. Some water is pumped up to six times.

Our pipes range in size from the 3/4-inch pipe that connects your water meter to our main to the 42-inch transmission pipes that carry source water to the treatment plants. The pipes are made of various materials, depending on when and where they were installed. Since the late 1970s, however, we've installed only welded steel and polyvinyl chloride (plastic) mains due to their expected long life spans.

In recent years, we've extended pipeline life by using techniques to control corrosion. These will give newer pipelines a life span of more than 100 years. Through our capital improvement program we replace pipeline as needed, with priority based mostly on facility condition. Currently we replace about 40,000 feet of pipeline per year, at an average cost of $200 per foot. We also install larger mains through the fire flow program, a separately funded capital program developed cooperatively with local fire chiefs to improve Marin's fire-fighting capacity.