How Does the Waste Water Treatment Plant Protect the Environment?

The wastewater treatment plant was built in 1954 and is adjacent to Bayfront Park and the Mill Valley - Sausalito multi-use path. The treatment facility takes in all the wastewater produced in the City of Mill Valley and the neighboring unincorporated communities. 

The Sewerage Agency of Southern Marin (SASM) was formed in 1979 to consolidate the wastewater collection, treatment, water reclamation and disposal needs of about 29,500 residents in Southern Marin County, including Almonte Sanitary District, Alto Sanitary District, Homestead Valley Sanitary District, Richardson Bay Sanitary District, and the Kay Park Area of the Tamalpais Community Services District. 

If you own a house in any of these areas,  wastewater from your kitchen sink goes down the drain and out through the plumbing probably located in your basement or under your land, to meet up with the municipally owned drainage pipes in the street. From there, often via a series of connections to other, larger pipes, all the wastewater winds up at the wastewater treatment plant.

Here the wastewater undergoes physical, biological and chemical treatment until it is 95% rid of solid and organic waste and 100% free of pathogenic bacteria. The final leg of the effluent's journey is conveyed via six miles of pipeline straight to Elephant Rock at Point Tiburon, one of the southernmost points of Tiburon. At that point, a right turn in the pipe sends the water 900 feet out to Raccoon Strait, between Tiburon and Angel Island, where it is discharged into the San Francisco Bay.

A small percentage of the water, instead of traveling to the Bay, undergoes further treatment, called water reclamation, to make it suitable for human contact. This water is used to irrigate some of southern Marin's parks, including Hauke and Bayfront Parks in Mill Valley.

The solid and organic waste extracted from the wastewater is treated further, and most of it is transported to the Redwood Sanitary Landfill in Novato, to cover the garbage dumped there daily. During the summer, some of this waste is also transported to a land-spreading site at Lakeville Highway and Highway 37, in Sonoma County.

The treated water is tested to ensure it meets all standards for dischargeable and irrigation water. Most verification is done with testing tools and chemicals, but one of the final testing processes uses small fish. Each month, wastewater treatment plant staff add twenty small fish to the treated water and the same number of similar fish to a control environment, to compare their survival rates.  The test runs for 96 hours and the idea is to see how many fish survive. The fish usually do very well in the effluent.

There are two equalization basins at the eastern side of the facility. On a typical dry day, the treatment plant can expect a peak of about five million gallons of wastewater flowing into the facility from Mill Valley and southern Marin sanitary districts. The system currently has a capacity of 25 million gallons per day and on successive days of heavy rains as much as 30 million gallons can flow into the wastewater facility. The equalization basins are used to temporarily hold excess sewage until the flow through the treatment plant ebbs, and the treatment plant can safely draw back the water from the equalization basin for treatment.

Water flowing into municipal outdoor drains in our streets, such as rainwater or the water you use to wash your car or irrigate your garden, is not treated before returning to natural waterways. That water flows directly into Richardson Bay, a good reason to eliminate the use of pesticides and chemical fertilizers in your garden and refrain from washing your car in your driveway. (Commercial car washes by law must treat water used in the washing process before sending it on its way.)

Why then must the wastewater treatment plant be prepared to handle extra wastewater during periods of heavy rain? Rainwater that does not flow into the street drains but instead follows its natural course underground is the culprit. The average age of Mill Valley's sewer pipes is 40 to 50 years, and the older pipes actually date back to the town's incorporation over a century ago. These aging pipes are breaking down, and many have leaks. Millions of gallons of rainwater per day that is soaked into the ground during storms seeps into the decaying pipes, joining the wastewater on its trip to the wastewater treatment plant.

The capture and treatment of sewage is extremely important to a municipality, its residents and its wildlife, and involves a complex series of tasks that lead to environmental protection.