Technical Features

The Calera Creek Water Recycling Plant can treat 4 million gallons of sewage per day (up to 20 MGD during storm events). The sequencing batch reactors are innovative, in that they combine the basins for aeration and clarification, which are separate at most older plants.

This project helped pioneer the use of ultraviolet disinfection for wastewater effluent in California. UV allows release of recycled water into wetlands because residual chlorine was not allowed in the permitting process. This technology was developed by Trojan Technologies and tested by the City of Pacifica and UC Davis.

This plant is one of the first fully automated plants in the San Francisco Bay Area designed for nutrient removal. This is also one of the first plants using ATAD technology (auto thermal thermophilic aerobic digesters) for the generation of Class A sludge in California.

Two diesel-powered generators maintain plant electricity in the event of a power outage. To minimize visual impact, the entire facility except for the filters and control building are buried in a hillside covered with native plants. The odor control scrubbers pull air from all process areas to neutralize odor-causing gases.

The plant has the first successfully restored wetlands using the Hydro Geomorphic model. This model for wetlands was developed for this project and is now the standard used by many resource agencies (including the Environmental Protection Agency and Regional Water Quality Control Board) for wetlands restoration design and monitoring. Thirty acres of wetlands, riparian vegetation and coastal scrub were restored in the process.

Pacifica’s wastewater flows by gravity to the Sharp Park and Linda Mar pump stations where it is screened to remove inorganic objects, then pumped to the Calera Creek Water Recycling Plant. The first stage of treatment at the plant is a vortex chamber that separates grit from the sewage.

Second, the influent goes to the SBR (sequencing batch reactors) basins, a combination of primary and secondary treatment and nutrient removal. Here, micro-organisms feed on the organic matter. Air is injected to assist the organisms and to mix the contents. After the organisms consume the sewage and solids, the remaining particles are allowed to settle to the bottom in the form of sludge.

Some of the sludge is left in the reactor basins because it contains the organisms that will multiply to feed on new sewage when it arrives. The rest of the sludge is pumped to a biosolids holding tank. The sludge is then thickened and pumped into the digesters. In the digesters, the ATAD organisms live at a high temperature (135 degrees), which kills both SBR organisms and other bacteria normally found in sewage. This disinfects the sludge so it can be recycled as topsoil.

After the sludge settles to the bottom of the SBR basins, water is drawn out from the middle and drained to the filters. The water’s passage through sand filters removes any remaining particles. The water then passes through the UV disinfection channel. Finally, clear, disinfected water is discharged into wetlands or used as utility water.