Napa Sanitation District (NSD), located in the Napa Valley, provides wastewater collection, treatment and disposal services to the residents and businesses in the City of Napa and surrounding unincorporated areas of Napa County. NSD has been serving the public since it was organized under the California Health and Safety Code in November 1945.

To collect, treat, beneficially reuse, and dispose of wastewater in an effective and economical manner that respects the environment, maintains the public's health and meets or exceeds all local, state and Federal regulations.

As a Special District, NSD is an independent local agency governed by 3 elected officials from the City and County, as well as 2 public appointees. There are over 33,000 connections within NSD's Sphere of Influence of approximately 23 square miles of service area. Through a network of approximately 250 miles of underground sewer pipelines, assisted by a system of six lift stations, the sewage makes its way to the Soscol Water Recycling Facility (SWRF) for treatment. The SWRF is a secondary and tertiary biological physical-chemical treatment facility that treats a mixture of domestic and industrial wastewater. NSD has completed upgrades to the SWRF, which include primary treatment, activated sludge facilities, and sludge digestion and solids de-watering facilities. The SWRF has a dry weather treatment design capacity of 15.4 Million Gallons Day (MGD). The wastewater is treated and discharged in various manners, depending on the source of the wastewater and the time of year.

NSD's regulating body, the Regional Water Quality Control Board, permits discharge to the Napa River from November 1 through April 30 (the wet season period). The average discharge of treated water to the Napa River is approximately 14.7 MGD. NSD provides full secondary treatment at its wastewater facility whenever discharging 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 stabilization ponds or treated and beneficially reused for landscape irrigation in industrial parks, golf courses, pasturelands, and vineyards. High quality, Title 22 unrestricted use recycled water is provided to all recycled water users. The treatment facility at the Imola Avenue site ceased operation in 1997 and was recently demolished due to the Napa County Flood Control District Project. NSD's Administration and Collection System offices are still located at the Imola Avenue site.