Primary & Secondary Treatment

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Primary treatment consists of physical separation using a bar screen and settling tanks.

When raw sewage enters the Headworks a bar screen removes large materials that could plug our pumps and interfere with treatment. It also removes items which are inorganic which would not break down in our biological processes. The bar screen removes rags, cleaning wipes, wrappers, feminine hygiene products, condoms, and other debris larger than 1/8”.

After the headworks, the flow is slowed down in Primary Clarifiers where gravity takes over. This is where solids which sink and float are removed. Primary treatment removes 70-75 percnt of the solids. The removed solids are decomposed naturally in Anaerobic Digesters to produce biosolids and methane gas. Methane gas byproduct is used as fuel to generate onsite power in the Co-generation facility. The biosolids are reused at the neighboring landfill where they are composted then used as landfill daily cover and for bank stabilization.

Secondary treatment utilizes microscopic organisms found naturally in the environment such as in rivers and streams. Instead of leaves and rocks, our microbes live on Trickling Filters. As the primary treated water passes over the microbes on the filters, they feed on the organic matter. The Regional Treatment Plant also uses a Secondary Treatment Bioflocculation Basin process. These basins contain millions and millions of microbes which decompose the organic matter in the water. To maximize biological efficiency, air is bubbled into the basins to increase the dissolved oxygen content in the water.  The microbes are then settled out in Secondary Clarifiers and reused.

During the winter, when agricultural irrigation water is not needed, secondarily treated wastewater is safely discharged two miles into the Monterey Bay through a 60-inch diameter outfall pipe. The treated water meets and exceeds all State discharge requirements.

Approximately 60 percent of all intake water is recycled each year (see Slowing Seawater Intrusion page). Recycling water reduces the discharge of treated wastewater into the Monterey Bay. Understanding that wastewater is a valuable resource to be reused where feasibly possible, the Board adopted a “Zero Discharge” goal Ordinance in 2012.

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Monterey, CA 93940

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