Primary Sedimentation Project

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Primary Sedimentation Expansion and Improvements

 Construction workers perform pre-drilling to prep the ground for sheet pile driving for the Primary Sedimentation project.
Construction workers perform pre-drilling to prep the ground for sheet pile driving in May 2019 for the Primary Sedimentation project.
Increasing efficiency, enabling the primary sedimentation tanks to be able to treat more wastewater, and improving the wastewater treatment process will be the benefits of the Primary Sedimentation Expansion and Improvements Project at DSRSD’s Regional Wastewater Treatment Facility in Pleasanton.

Contractors began work in April 2019 and will continue through fall 2021 on the $19 million project, the largest capital project to be built by the District since 2000.

The project involves constructing a new primary sedimentation tank and partially demolishing and replacing an existing one (including deepening it from 10 feet to 14 feet), adding an additional grit tank, replacing internal mechanisms of the three remaining primary sedimentation tanks, and replacing the motor control center.

Treating wastewater is a biological process. After bar screens remove large objects from the wastewater (such as pieces of wood, cloth, and condoms), grit tanks remove smaller inorganic material (sand or gravel) and non-degradable organic material (coffee grounds, eggshells, and hard-shelled seeds) from the wastewater. This protects equipment and prevents clogged pipes at the plant. The wastewater continues to the primary sedimentation tanks. This is where scum is skimmed from the water’s surface and solids are scraped from the bottom of each tank.

 The emptied primary sedimentation tank sits ready for construction.
The emptied primary sedimentation tank sits ready for construction.

The primary treatment splits the sewage into two streams: solids and liquid. The solids are thickened and transferred to a digester where bacteria decompose the organic solids and destroy pathogens. The solids are then sent to facultative sludge lagoons for about six years, after which they are scraped from the bottom of the lagoon and injected 18 inches under the surface into the dedicated land disposal site.

The liquid stream continues on from primary treatment to secondary treatment, which includes aeration and clarification. The five aeration basins (208 feet long, 30 feet wide, 15 feet deep) provide oxygen for the millions of beneficial microscopic organisms that break down remaining dissolved organic material in the liquid stream. The four cylindrical secondary clarifiers (three are 90 feet in diameter, and one is 110 feet diameter) work similarly to the primary tanks, skimming scum from the surface and scraping solids from the bottom. As the liquid stream moves through the process, it gets cleaner and clearer. The treated wastewater is then disinfected and either discharged to the San Francisco Bay or advanced treated to be used as recycled irrigation water. Learn more about how long it takes to turn wastewater into recycled water.

Currently, the primary treatment capacity is undersized for the facility’s average dry weather flow of more than 10 million gallons a day. According to the Wastewater Treatment and Biosolids Facilities Master Plan, the plant needs additional primary treatment capacity in preparation for future flows. Insufficient primary treatment capacity can make the next steps in the treatment process difficult.

Having more effective primary treatment will both reduce energy use in secondary treatment and enable the plant to send more solids to the digesters to create biogas, a renewable fuel used to generate electricity and heat to power the plant. The expansion and improvements will provide needed primary treatment capacity for current and future buildout flows.

View a YouTube time-lapse video showing construction progress from April 25 to May 24, 2019


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