The Operations Staff of the Napa Sanitation District's (NSD) is proud to showcase our award winning Water Recycling Facility (WRF). Carollo Engineers and Napa Sanitation District recently received the California Association of Sanitation Agencies (CASA) Achievement Award Winner (Associates Achievement Award).

Our Soscol Water Recycling Facility Operations staff consists of ten California State Water Resources Control Board certified operators. This includes one grade V, one grade IV, six grade III’s, one grade II, one grade I. There are also plant attendants and support staff that help keep the plant in great condition. There is also Equipment Maintenance Specialist (EMS) department staff of three that is responsible for the equipment maintenance and repair at the treatment facility as well as the pump stations around town. The EMS staff is part of the Operations and Maintenance Department.

Treatment Facility
The Soscol Water Recycling Facility (SWRF) is a secondary and tertiary biological physical-chemical treatment facility that treats a mixture of domestic and industrial wastewater. The SWRF currently receives wastewater from the City of Napa and unincorporated areas of the county of Napa. The District has completed upgrades to the SWRF, which include primary treatment, activated sludge facilities, and sludge digestion and solids de-watering facilities.

The facility has a number of treatment options that include preliminary treatment (screening), primary treatment (clarifiers), biological secondary treatment (340 acres of oxidation ponds and/or activated sludge facilities), secondary clarification or sedimentation, sand filtration, chlorination, sludge digestion and solids de-watering facilities. The SWRF has a dry weather design capacity of 15.4 MGD. The wastewater is treated and discharged in various manners, depending on the source of the wastewater and the time of year. From November 1 through April 30 (the wet season period), approximately 14.7 MGD of treated wastewater is discharged to the Napa River. From May 1 through October 31 (the dry season period), discharge to the Napa River is prohibited and wastewater is either stored in the stabilization ponds or treated and beneficially reused for landscape irrigation in industrial parks, golf courses, pasture lands, feed and fodder crops, and drip irrigation of vineyards. The SWRF produces high quality recycled water meeting Title 22 unrestricted use standards. The treatment facility at the Imola Avenue site has been decommissioned as well as demolished.

Energy Recovery-Cogeneration
Three forms of matter are recycled and reused at the treatment facility; the liquid, solid, and gas. That’s right! The water is used for reclamation, saving our precious water resource. The solids are plowed into fields as biosolids, enriching the soil with nutrients. But also, the gas is reused. During anaerobic digestion, digester gas is produced which is approximately 60-70 percent methane, a valuable fuel. This digester gas is used to co-generate electrical power using a gas engine generator housed in the Cogeneration Building. Waste heat, a by-product of power generation, is recovered from the engines and exhaust.

The electricity and heat, which are produced by the cogeneration system, are utilized in the plant to reduce purchased electricity and natural gas. Electricity produced by the generators is fed into the plant electrical distribution system for use where needed. Heat from the gas engines is recovered and used for heating the Digester and nearby buildings.

The average recoverable energy produced by the cogeneration unit is between 4,200 and 4,500 kilowatt-hours/day. According to PG&E, the average residential household in the PG&E service area uses 521 kilowatts-hours each month. At this rate, the energy from the District's cogeneration unit is enough to supply the energy requirements of about 250 homes each month. For every kilowatt-hour generated at this facility, that's one less kilowatt-hour of energy purchased, and that is a cost saving for the District's ratepayers.