Rehabilitation Underway for Aging Clarifiers



Built on what was once a field surrounded by sugar beets, the SLO Water Resource Recovery Facility (WRRF) houses critical wastewater treatment infrastructure that has served the community for more than 100 years. As several pieces of equipment near the end of their life, the SLO Water Plus upgrade will modernize and repurpose the equipment with designs that save money, reduce waste and improve operational efficiency. Most notably, rehabilitation of two primary clarifiers kicked off in July.

During World War II, the SLO WRRF underwent a facility expansion to accommodate the City’s rising population. Much of the equipment that was added in the 1940s is still in use today, including the two primary clarifiers (shown above in the 1945 photo). Recognized by their wide cylindrical shape, the clarifiers are the first step of the treatment process. Here, solids settle out, are removed, and are then sent to the digesters for energy production. 

“Since 1945, the clarifiers have handled every drop of wastewater that the City of San Luis Obispo and Cal Poly have produced, running 24/7, 365 days a year,” said Patrick McGrath, WRRF Operator. “They are workhorses!”

Major upgrades have begun starting with the replacement of the support column, skimmers, rake arm and drive mechanisms which are at the end of their useful life. Upgrades to the clarifier will improve the efficiency of daily operations, decrease energy usage and improve safety. All of the original piping will be replaced, and new pumps and drive assemblies will be installed to improve the equipment’s ability to stay operational even in a natural disaster or other major disruption. Automated controls on new pumps will optimize their ability to remove sludge and scum. The pumps will run on timers at varying speeds, reducing their overall electricity demand. The clarifiers will pump less water to the WRRF’s digesters, which in turn will enhance biogas production and allow the city to generate more electricity. New railings, catwalks and site lighting will also improve safety and access. Finally, new covers and foul air ducting will be added as part of comprehensive design improvements to better control odor.

Despite the long life that the clarifiers have had, the tanks themselves are still in good condition and will be reused. As part of the SLO Water Plus upgrade, the tank bottoms will undergo minor concrete repairs to extend their life. By rehabilitating rather than replacing the tanks, the project team will save money and reduce unneeded waste.

“Instead of building a whole new clarifier, we are breathing fresh life into it,” described Patrick.

Rehabilitation of the clarifiers began in July 2020 and will be completed in Spring of 2021. Careful thought has been given to ensure regulatory compliance and to minimize operational impacts during construction.

© City of San Luis Obispo 2019