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Wastewater/Recycling

History
The Scotts Valley WRF is owned and operated by the City of Scotts Valley to provide residents with efficient, reliable wastewater treatment services and high-quality recycled water for landscape irrigation and other potential beneficial uses.

The plant was originally constructed in 1964 as a 30,000 gallon/day package wastewater treatment plant to provide secondary level wastewater treatment. In 1972, an activated sludge treatment system was added and plant capacity was expanded to 120,000 gallons/day. Over the years plant capacity has been expanded several times to serve growth in the community and now has the capacity to treat 1.5 million gallons each day, enough to see the City through planned build-out.

Recent plant upgrades include a Tertiary Treatment Facility that treats up to 1 MGD of recycled water. In 2001, the City received a permit from the Regional Water Quality Control Board to produce recycled water for unrestricted irrigation use. The recycled water is used mainly for irrigation at local parks, schools, residences, landscape medians, and businesses. State-of-the-art ultraviolet disinfection kills any pathogens (disease causing bacteria) without the use of chemicals such as chlorine. Following disinfection, the tertiary treated water meets State Title 22 standards for water reuse in California and is safe for all permitted uses.

Treatment Process
When wastewater enters the plant, it passes through an efficient treatment process that delivers high quality effluent suitable for use as recycled water or for safe disposal in the ocean:

Step 1 – Influent Pump Station
Wastewater from the sanitary sewer collection system first enters the WRF at the Influent Pump Station. The wastewater passes through fine screens that remove larger inorganic material such as plastics, nylon, and fiber from the wastewater. These materials are rinsed and sent to a sanitary landfill. The remaining flow is pumped to the Flow Equalization Structure.

Step 2 – Flow Equalization Structure
The wastewater enters the WRF at varying rates, depending on the time of day and the season of the year. Whatever flows into the WRF must be treated and pumped out.  During wet weather, high flows can exceed the flow capacity of the pipeline that carries treated effluent from the WRF to the ocean. The Flow Equalization Structure is used to temporarily store wastewater at the WRF so that influent flows can be “equalized” into a more constant flow. This allows flows to be pumped through the downstream treatment process at a relatively constant rate through the effluent pipeline. An added benefit is that the treatment process is optimized by maintaining flows at a relatively constant rate.

The Flow Equalization Structure is divided into four tanks. The center tank is used to equalize typical flows. It is constantly mixed to help maintain a uniform mixture and prevent solids from settling out of the wastewater. Air is blown into the center tank to help minimize odors and start the treatment process. The outer tanks are used to store excess water during exceptionally wet weather. Wastewater stored in the Flow Equalization Structure is pumped to the Aeration Basins.

Step 3 – Aeration Basins
Wastewater treatment is mostly a biological process. For this reason, a primary focus of wastewater treatment is to maintain conditions in the wastewater so that microorganisms can live and feed on the organic material in the wastewater. Tiny air bubbles are released through specially designed panels at the bottom of the Aeration Basins to provide air for aerobic microorganisms that break down the organic material in the wastewater. Wastewater in the Aeration Basins flows by gravity to the Secondary Clarifiers.

Step 4 – Secondary Clarifiers
The Secondary Clarifiers separate the solid matter in the wastewater from the liquid.  Wastewater from the Aeration Basins flows into the center of the Secondary Clarifiers. As the wastewater flows from the center of each clarifier to the outer edge, the solids in the wastewater settle to the bottom of the clarifier. The “clarified” liquid (called “secondary effluent”) spills over the weir at the outer edge of each clarifier and flows by gravity to the Secondary Effluent Diversion Box. The settled solids (called “sludge”) are removed from the bottom of each clarifier by a slow-moving mechanism. Part of the sludge is returned to the Aeration Basins where it helps to maintain the biological process in the basins. The rest of the sludge is pumped to the belt filter press in the Sludge Dewatering Building where it is prepared for dewatering and disposal.      

Step 5 – Secondary Effluent Diversion Box
A portion of the secondary effluent flowing into the Secondary Effluent Diversion Box is pumped to the Tertiary Treatment Facility for added treatment and use as recycled water. The remaining secondary effluent is pumped to the Chlorine Contact Tank for disinfection and disposal in the ocean.

Step 6 – Tertiary Treatment and Recycled Water
During the irrigation season (typically 8 – 10 months per year), much of the secondary effluent is treated at the Tertiary Treatment Plant for use as recycled water. Secondary effluent entering the Tertiary Treatment Plant is first filtered in sand beds to remove any solids and then disinfected using ultraviolet lights. UV lights are specially designed to kill bacteria that might be present in the effluent without creating undesirable disinfection byproducts. After disinfection, the effluent is pumped to the Recycled Water Storage Facility where it is held until needed for irrigation.

Step 7 – Chlorine Contact Tank and Outfall Disposal
Secondary effluent not used for recycled water is pumped to the Chlorine Contact Tank where liquid Chlorine is mixed into the effluent to kill any bacteria that might be present. After passing through the Chlorine Contact Tank, the secondary effluent is pumped to Santa Cruz through a 7-mile long pipeline. There the effluent is mixed with effluent from the City of Santa Cruz Wastewater Treatment Facility and safely discharged to a deep-water outfall in Monterey Bay.

Step 8 –- Sludge Dewatering and Disposal
Sludge from the Secondary Clarifiers is pumped to the Sludge Dewatering Building, where liquid is removed from the sludge using a belt filter press. The liquid removed from the sludge is returned to the plant process for additional treatment. The dewatered sludge is discharged into a dump truck and hauled to a sanitary landfill daily.