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Central Marin Sanitation Agency
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The CMSA wastewater treatment plant is a vital component of the central Marin wastewater management system. In full compliance with federal, state, and regional water pollution control laws and regulations, CMSA processes and disposes an average of 8.5 million gallons of wastewater a day during dry weather flows and 13 million gallons of wastewater per day during wet weather flows, and has treated in excess of 120 million gallons per day during peak rainfall periods.

Click on the button below to go on a virtual tour of the treatment plant! You can also view and download a map of the facility below. And if you would like to take a tour of the real thing, just give us a call.

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    A drop of water takes several hours to travel through the plant, from when it's received at the Headworks to when it enters the Outfall for transport to the Bay.

    The Average Dry Weather flow is approximately 7 MGD (million gallons per day), and a drop of water takes about 15 hours to move through the plant.

    Our Peak Wet Weather flow is approximately 100 MGD, and a drop of water takes approximately 4 hours to move through the plant.

    Click on the "next" link above to start the tour and find out what happens to it!


    In the wastewater collection system, far upstream from the treatment plant, chemicals are injected into the sewer pipeline system to control odors and reduce corrosion.

    The Headworks (A) is where the wastewater enters the plant. It first flows through mechanical screens where large material that should not have been sent down household drains--such as cloth, wipes and plastic--is removed.

    The wastewater then enters Aerated Grit Chambers (B) where granular materials, such as sand and silt, settle to the bottom and are removed.

    The collected materials are then washed and hauled to landfill for disposal.


    Wastewater moves slowly through settling tanks called Primary Clarifiers (C). In these long rectangular tanks, grease, oil, and other floating material rise to the top and are scrapped off at one end, while heavier materials settle to the bottom and are collected at the other end. Both materials are pumped to anaerobic digesters for further treatment.


    Biotowers (D) are where the first stage of a dual biological treatment process takes place. Wastewater from primary treatment is pumped to the biotowers, and then trickles down through a dense milk crate-like plastic grid. Natural bacteria grow on this grid and consume the organic material in the wastewater as it trickles down through the plastic media. This bacterial growth is called biomass.

    In the Aeration Tanks (E), fine air bubbles are released from the bottom of the tanks and provide air for microorganisms that consume most of the remaining organic matter. These microorganisms are called activated sludge.

    Secondary Clarifiers (F) (pictured) settle out the activated sludge from the aeration tanks. Rotating arms gently move the settled sludge to the center of the tank where it is removed. Some sludge is returned to the aeration tanks to maintain a useful population of microorganisms. The rest is "wasted," meaning it is removed, thickened, and then sent to the anaerobic digesters for further treatment.

    By balancing the amount of returned and wasted sludge, the optimum age and type of microorganisms can be selected that will both treat the wastewater and settle well in the secondary clarifiers.


    The treated wastewater has to be fully disinfected before discharge into the San Francisco Bay. The disinfection process occurs in the Chlorine Contact Tanks (G), where sodium hypochlorite (similar to household bleach) is added to the water as it enters the tank to kill any harmful bacteria as the water flows through.

    Since chlorine is toxic to the environment, it is then removed from the water through the dechlorination process.


    Final effluent (wastewater that has been treated, disinfected and dechlorinated) is discharged into San Francisco Bay through a large pipe called an Outfall (H). During most of the year, the effluent flows out by gravity.

    At the end of the outfall, the final effluent is disbursed and mixed with the bay water through 176 diffusers along a 1,000-foot section of the pipeline which runs approximately two miles from the treatment plant.

    When wastewater flows are significant and occur during high tide, the Effluent Pump Station (I) is used to pump the effluent through the Outfall. The station has five pumps that are fully automated.

    The Effluent Storage Pond (J) is used for temporary storage of disinfected wastewater during plant maintenance procedures, and can hold up to 7 million gallons.


    Materials that are removed from the primary and secondary treatment processes (described in Steps 2 and 3) are pumped to the Anaerobic Digesters (K). The digesters are heated to approximately 98.6 degrees F and anaerobic microorganisms consume the organic matter to produce methane gas (biogas). The biogas is used as fuel in the engine/generator that supplies electricity and heat to the facility.

    CMSA also accepts Fats, Oils and Grease (FOG) and commercial food waste from private haulers at the Organic Waste Receiving Facility (L). These materials are processed and pumped to the digesters to produce additional biogas for cogeneration.


    After the digestion process, treated solids (biosolids) are pumped to the Solids Handling Building (M). Chemicals, such as a diluted polymer, are added to the biosolids to make them coagulate. The biosolids are then dewatered in a high speed centrifuge (similar to the spin cycle on a washing machine) to reduce the water content.

    Dewatered biosolids are stored in large hoppers, and then loaded into trucks for delivery to beneficial reuse sites. During the dry weather season, the biosolids are used as fertilizer for growing hay and alfalfa for horse and livestock feed. During wet weather, the biosolds are hauled to Redwood Landfill and used as alternate daily cover for the solid waste.


    CMSA produces biogas in two anaerobic digesters. This power is created using an engine/generator system that uses both the biogas and natural gas as fuel that provideds up to 95% of the Agency’s energy needs.

    Additional biogas is being produced from the FOG and Foodwaste supply, and will eventually eliminate the need to purchase natural gas. The exclusive use of biogas to fuel the Agency’s cogeneration engine will enable the Agency to achieve its long-term goal of attaining full energy self-sufficiency. In the future, CMSA may even be able to export electricity to the local utility.

CLICK HERE to download the Basic Treatment Plant Design Data Sheet


1301 Andersen Drive, San Rafael, CA 94901
Phone: 415-459-1455 / 415-223-7793
Fax: 415-459-3971


8:00 am - 4:00 pm  Monday - Thursday
8:00 am - 3:00 pm  Friday


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