Carollo first began working for the Napa Sanitation District (NSD) in 1982 with a study of required wastewater treatment plant modifications and the viability of wastewater recycling. This study’s major purpose was to evaluate alternative sites and outline facilities necessary to develop a recycled water program using wastewater effluent for agricultural irrigation. Following the study, Carollo designed water pumping and transmission facilities to deliver recycled water to ranches for pasture irrigation.

Carollo then completed a facility plan and designed a 10-mgd tertiary water recycling facility for NSD. Effluent from the new facility is discharged to the Napa River during the winter months and recycled for California Title 22 unrestricted irrigation uses during the summer months.

Phase 1 of the project, completed in 1996, includes tertiary filtration, sodium hypochlorite chlorination, and bisulfite dechlorination. Tertiary treatment for recycling includes three-stage flocculation; a 2,000-foot, 4-cell upflow continuous backwash filtration sys tem; a two-hour chlorine contact basin; and a two-cell, 20-acre-foot recycled water storage reservoir. Recycled water is pumped using three high-pressure vertical turbine pumps that up to 8 mgd each at pressures exceeding 140 psi. Each pump is fitted with a high-efficiency 600-hp motor. The pump station includes a cost-effective "pump can" wet well design and a valving and flow metering structure which is used to divert and meter recycled water to the "north" and "south" distribution systems.

Energy management was a key concern in design. The system includes a 10,000-gallon hydropneumatic surge tank that is used to deliver flow demands less than 1 mgd. For flow demands greater than 1 mgd, the pumps are speed controlled through VFD drives. A control system sets the pump speed based upon the flow meter reading and a pressure set point algorithm.

The recycled water delivery system is designed to pump recycled water to pasture land, golf courses, airport runway bound aries, local parks, business parks, and vineyards. Upon full implementation, over 2 billion gallons of Title 22 unrestricted use recycled water will be recycled each year.

As part of this project, Carollo helped NSD to develop an effective way inform/educate plant workers and the public on the rules and regulations governing the production and use of recycled water. Carollo produced a recycled water educational video which addresses three different and important issues: basic worker training, recycled water marketing to potential customers, and public education.