City of Soledad Wastewater Treatment Plant, United States of America


The upgrade and expansion of the Californian city of Soledad's wastewater treatment plant (WWTP) was completed in January 2010. The plant capacity was elevated from 3.1mgd to 5.5mgd. Work on the $49m expansion began in June 2008 and was completed within 18 months. The plant serves a population of approximately 26,000.

The WWTP is one of two plants operated by the Public Works Department in Soledad. The upgrade and expansion was undertaken as part of a long-term wastewater management plan initiated in May 2006.

"The WWTP is one of two plants operated by the Public Works Department in Soledad."

Until 1995, the city plant operated at 650,000gal per day. In 1995, the plant's capacity was expanded to 3.1mgd, with 2.1mgd appropriated for the facilities of a local prison.

The upgrade and expansion was devised to meet the city’s rapid population growth (at 7.5% a year) and waste discharge requirement (WDR) standards.

Purpose of the project

Soledad was under pressure to reduce toxic pollutants such as ammonia and disinfection by-products that were discharged into the Salinas River.

With completion of the project, the plant meets the WDR effluent limits adopted by the State Water Resources Control Board of California. The WDR aims to reduce sanitary sewer overflows. The maximum effluent limits set by the WDR 2005 were 10 mg/L Biological Oxygen Demand (BOD), 10 mg/L Total Suspended Solids (TSS), 5 mg/L ammonia and 5 mg/L nitrate.

Plant design

The plant's upgrade has been designed to meet future population and industrial growth.

The plant has an operations building, a blower and an electrical building. It also has a UV disinfection canopy. New treatment equipment was installed and pre-engineered metal buildings were constructed.

Wastewater treatment process

Earlier, Soledad’s WWTP was using three sets of ponds – aerated ponds, secondary ponds and a polishing pond – to treat wastewater. The upgrade and expansion project converted the old pond system to a mechanical plant. The pond system mainly deals with domestic waste whereas mechanical plants are equipped to handle domestic waste as well as agricultural waste and industrial waste.

"The plant's upgrade has been designed to meet future population and industrial growth."

The water treatment process includes four stages: primary, secondary, tertiary and residual treatments.

The primary treatment process involves the pretreatment of perforated plate screening and vortex grit removal. The secondary treatment comprises four-phase biological nutrient removal processes and clarification. Tertiary treatment includes flocculation, filtration and ultra-violet disinfection. Residual treatment involves the treatment of sludge stabilisation and screw press sludge dewatering.

The treated wastewater will either be used for domestic purposees or will flow to the rapid infiltration basins (RIBs) for disposal. RIBs allow land treatment and the disposal of wastewater.