NSD's Wastewater Treatment Plant

Soscol Water Recycling Facility at a Glance

Dry Weather Treatment Capacity: 15.4 million gallons per day (mgd)
Recycled Water Produced Annually: 650 million gallons annually
Biosolids produced: 1,200 dry tons annually
Methane gas produced: 150,000 cubic feet daily
Energy produced: 2,975 MWH annually (36% of energy used by the water recycling facility)
Hours of operation: 24 hours/day, 365 days/year 

Suscol Water Recycling Facility  

Soscol Water Recycling Facility

The Soscol Water Recycling Facility (SWRF) is a state-of-the-art wastewater treatment plant that utilizes many complex processes to produce treated wastewater and recycled water. Wastewater undergoes primary, secondary and tertiary treatment and disinfection before being released to the Napa River or distributed for irrigation as recycled water.

Wastewater that enters the treatment plant (influent) is about 99% water and 1% solids. The wastewater flows through a series of treatment processes that screen out large solids, remove smaller solids that sink or float, and then removes smaller materials that are dissolved in the wastewater. This treatment process involves physical, chemical and biological treatment techniques.

From November 1 through April 30 (the wet season), an average of 13.7 million gallons per day of treated wastewater is discharged to the Napa River. From May 1 through October 31 (the dry season), discharge to the Napa River is prohibited by the District’s water quality permit and wastewater is either stored in the treatment ponds or treated and beneficially reused (recycled) for irrigation of landscaping, parks, golf courses, pasture lands and vineyards.

Here’s a summary of the treatment processes used by the District at the Soscol Water Recycling Facility (SWRF) to produce treated wastewater and recycled water. Take our virtual treatment plant tour to learn more details, or schedule an onsite tour of our treatment plant by calling (707) 258-6002.


primary clarifier  Primary Treatment:

 • Preliminary Treatment (physical): When wastewater arrives at the treatment plant, it contains many solids that cannot be removed by the wastewater treatment process. This can include rags, paper, wood, food particles, egg shells, plastic – even toys and money. To remove these solids, the wastewater enters a building called the Headworks and passes through large screen filters which removed this material. The solids are then placed in a dumpster and taken to the landfill. This is the only byproduct of wastewater treatment that is not recycled!
• Primary Clarifiers (physical): From the Headworks, the wastewater flows into two huge circular tanks called Primary Clarifiers. These tanks can hold 600,000 gallons of water each. Here the wastewater slows down and remains in the tanks for about two hours. This allows material suspended in the wastewater to either float to the surface or sink to the bottom. Large paddles rotate slowly over the surface and floor of the Primary Clarifier, removing these materials from the wastewater. There are two Primary Clarifiers at SWRF. Pictured at left, the clarifiers are covered to reduce odors!


aeration basins

 Secondary Treatment:

• Aeration Basins (biological): From the Primary Clarifiers, the wastewater flows into large, rectangular tanks called Aeration Basins, where a biological treatment called the “activated sludge process” occurs. The wastewater flows slowing through a series of chambers as large volumes of air are bubbled up through the water. There is so much air added that it looks as if the water is boiling. In these basins, the wastewater is mixed with the “activated sludge”: hundreds of millions of actively growing single-celled microorganisms (mostly bacteria and protozoa) referred to as “bugs”. The air is needed to help the organisms thrive and multiply. What do they eat? The waste in the wastewater! As the bugs eat, the wastewater is cleaned. While the wastewater is in the Aeration Basins, mixed with millions of bugs and air bubbling through it, it looks like boiling hot chocolate. When all of the food (waste) is gone – after about eight hours - the wastewater leaves the Aeration Basins. Some of the bugs are sent back to the beginning of the Aeration Basins to keep the process going. Some organisms move on with the wastewater to the next step in the treatment process, the Secondary Clarifiers. The Aeration Basins (pictured at left) can hold 1.1 million gallons at a time.

clarifier(1) • Secondary Clarifiers (physical): When the wastewater enters the two Secondary Clarifiers, it still contains lots of microorganisms from the Aeration Basins and looks brown and murky. The Secondary Clarifiers are identical to the Primary Clarifiers: materials in the wastewater sink and float and rotating arms remove this material from the water. After treatment in the Secondary Clarifiers, the wastewater is now ready to be released to the Napa River, or further treated to produce recycled water. The Secondary Clarifiers each hold 800,000 gallons of water.


Chemical storage

Tertiary Treatment

• Filtration (physical/chemical): Wastewater leaving the Secondary Clarifiers looks as clean as drinking water! Depending on conditions, this water can go directly to the Disinfection process to produce recycled water, or it can go the Filtration Building. The Filtration Building contains a series of sand filters that are 27 feet tall, and can filter almost 14,000 gallons of water per hour. The filters remove very tiny solids (“suspended solids”) from the wastewater before it moves on to disinfection. Polymers are added at this step to cause the suspended solids to clump together, making them easier to filter out.

Pictured at left is the chemical storage area where the polymers are stored safely.


contact basin


• Chlorination (chemical): The final step in the treatment process is disinfection using chlorine. The Chlorine Contact Basins are long, snake-like channels where chlorine is added to the flowing treated wastewater for disinfection. Disinfection is the process where disease-causing organisms (mostly bacteria) are killed by the chlorine. It’s the same process used in swimming pools (with the same chlorine smell), but the chemicals used are much stronger. The long channels slow down the flow, allowing the chlorine enough time – at least two hours - to kill all the disease-causing organisms. Because chlorine can also kill fish and other aquatic organisms, the chlorine is neutralized with sodium bisulfite whenever the treated water is discharged to the Napa River. This process of removing the remaining chlorine is called dechlorination.



Solids Treatment:

• Digester: Where did all the solids go that were removed from the wastewater in the Primary and Secondary Clarifiers? To the Digester, a giant egg-shaped building that is sometimes referred to as “the giant stomach”. Even though it doesn’t look like a person’s stomach, it works in a similar way. All the solids – called sludge - from the Primary and Secondary Clarifiers are pumped to the egg-shaped Digester to be digested. The sludge in the Digester is kept at 95.5 degrees Fahrenheit, about the same temperature as a human stomach. In the digester, anaerobic bacteria (bacteria that do not use oxygen) break down the sludge. The sludge remains in the digester for 16 days to as long as 50 days. The Digester is LARGE - it stands 74 feet tall and contains 1.3 million gallons of sludge. After the sludge is thoroughly digested, it’s called biosolids. As a result of digestion, gases such as methane and carbon dioxide are also produced in the Digester, just as in the human stomach. The methane gas (or biogas) is actually captured and used to produce electricity in a process called cogeneration. This energy is used to help heat the digester and power the wastewater treatment plant.
• Solids Handling Building: From the Digester, the biosolids are pumped to the Solids Handling Building where they’re pressed to remove extra water. The de-watered biosolids are then trucked to surrounding fields where it is plowed into the ground as a soil conditioner. The biosolids contain nitrogen, phosphorus and potassium, three elements that plants need to grow.


  Additional Treatment Facilities:
• Ponds: The water-cleaning processes in the wastewater treatment plant are essentially the same as what occurs naturally in a lake or stream. A treatment plant simply speeds up the natural water purification process. During big storms, the amount of wastewater entering the treatment plant can exceed the plant’s capacity. When that happens, the District sends some of the wastewater directly to the treatment ponds. In the ponds, the water undergoes natural purification processes, resulting in clean water after about a month. The same water processed through the treatment plant would be cleaned in one day!

There are four ponds linked together by gate valves, with a total area of 342 acres and a capacity of about 665 million gallons. Water enters at Pond 1 and leaves after slowly meandering through all four ponds. In the ponds, bacteria use the oxygen to metabolize the solids and chemicals in the water and release carbon dioxide. Algae use the carbon dioxide released by the bacteria as well as nutrients in the water to grow, releasing oxygen back into the water. The water in the first pond is a greenish gray, turning greener with algae in each following pond. By the time the water gets to the fourth pond it looks a lot like green pea soup. Hundreds of ducks and geese nest here in the spring and thousands of them, including pelicans, seagulls, and other migrating birds stay here in the winter. Water and algae from Pond 4 is pumped to the flocculating clarifiers (see below) for further treatment. 
   • Flocculating Clarifiers (physical/chemical): Water that has been treated in the ponds is sent to the Flocculating Clarifiers to remove algae from the water. Flocculation is a physical/chemical process whereby chemicals called polymers are mixed with the algae and water from Pond 4, causing the algae to clump together and form what is called floc. The algae floc is heavier than water and settles to the bottom of the clarifier. A slowly rotating paddle on the bottom collects the solids that settled, which is known as sludge. This algae sludge is pumped back into Pond 1.
The clean water that leaves the Flocculating Clarifiers is piped either to the sand filters or directly to chlorination, depending on the water quality.



 Water Quality Laboratory

In order to protect public health and the Napa River, the District is required to meet stringent water quality regulations. At our treatment plant, we operate a water quality laboratory that constantly monitors the wastewater as it undergoes treatment and after treatment is complete. The District laboratory also uses commercial laboratories with high tech equipment that perform analyses capable of detecting some pollutants down to parts per trillion.

The lab conducts over 26,000 tests each year to identify a variety of wastewater constituents, including conventional pollutants, metals, toxic organic compounds, and pathogens.

The Soscol Water Recycling Facility is designed to remove solids (grit, rags, plastic) and organic materials (paper, food particles, human waste). Many toxic materials are not removed by the wastewater treatment process and are capable of disrupting the treatment process. To protect the wastewater treatment plant and ultimately the river, the District’s Pollution Prevention Program works closely with businesses and the community to prevent toxics from entering the collection system in the first place.  



Energy Cogeneration & Recovery

Wastewater treatment requires a lot of energy, and energy is expensive. In an effort to reduce costs and greenhouse gas emissions, the District implements projects at the treatment plant to increase energy efficiency and generate our own electricity.

When the solids that are removed from the wastewater are treated (digested), it produces a lot of methane gas. Lucky for us, methane gas is a great energy source. The District installed generators that can burn this methane to produce electricity that then powers the wastewater treatment process. This process, called cogeneration, produces electricity that replaces the electricity and natural gas that would normally be purchased from PG&E.
Waste heat, a by-product of the electricity generation, is recovered from the cogeneration engines and exhaust. This heat is used for heating the digester where the solids are treated, and for heating nearby buildings at the plant.
The average amount of energy produced each month by the cogeneration unit is equivalent to the amount of electricity used monthly by about 185 homes. For every kilowatt-hour generated at this facility, that's one less kilowatt-hour of energy purchased, and creating a cost savings for the District's ratepayers.


FOG receiving station

Fat, Oils, and Grease Receiving Station

Generating our own energy is great news, which made District engineers wonder: how can we produce even more? It turns out that the answer is grease. Each month, thousands of gallons of fat, oils and grease (FOG) are hauled away from local food service facilities. Beginning in August 2012, that waste can be hauled to the Soscol Water Recycling Facility where it is pumped directly into the digester from a grease receiving station. The FOG provides high-energy food for the digester bacteria, bumping up gas production that allows the District to produce more electricity.

This increased energy production could cut the district’s $750,000 annual energy bill by $100,000 to $200,000. Besides that cost savings, the District will generate income by charging a tipping fee to FOG haulers. The grease receiving station can accept over 200,000 gallons each month. Revenue generated by this service can be used to offset other costs and help keep rates down for District sewer customers.

The benefits of this program ripple out beyond the Napa Sanitation District and its customers. Before the District opened its grease receiving station, haulers had to take FOG loads all the way to Oakland for disposal. A more convenient disposal location on Napa means fewer trucks on the road and reduced carbon dioxide emissions.  

Wastewater Treatment Plant Master Plan

A wastewater treatment plant is a complex network of processes that work together to clean wastewater. In order to be effective in protecting public health and water quality, the treatment plant has to meet the challenges of serving an increasing population while keeping costs down.
In April 2011, the District completed a Wastewater Treatment Plant Master Plan (Master Plan) to insure that the plant is ready to meet these challenges. The Master Plan, which covers the period through 2030, determines the capacity of existing facilities, estimates future wastewater loads and regulation impacts, and develops a recommended plan for upgrading existing facilities. Increasing recycled water production was also considered in the planning process.
To view sections of the report, click on the links below.

Wastewater Treatment Plant Master Plan (entire document)
Table of Contents, Abstract, and Frequently Asked Questions
Chapter 1: Introduction
Chapter 2: Basis of Planning
Chapter 3: Existing Facilities and Performance Assessment
Chapter 4: Capacity Analyses
Chapter 5: Alternatives Screening
Chapter 6: Business Case Evaluation of Alternatives
Chapter 7: Description of Recommended Project
Appendix A: References
Appendix B: Abbreviations
Appendix C: NPDES Permit
Appendix D: Title 22 Requirements
Appendix E: Existing Facilities Descriptions and Design Criteria
Appendix F: Performance Assessment
Appendix G: Technical Memoranda
Appendix H: One Page Summaries