The Water Resource Recovery Facility stands as a model of innovation, sustainability, and community stewardship. What began over a century ago as a single clarifier and open digester has evolved into a state-of-the-art treatment facility that produces high-quality recycled water, protects San Luis Obispo Creek, and supports the City’s long-term water resilience goals.

With advanced biological treatment, expanded storm capacity, energy-efficient processes, and a strong focus on environmental health, the WRRF is among the most modern facilities of its size in California. It reflects the City’s ongoing commitment to innovation, public service, and environmental excellence.

The Next Generation of Water Recovery

From 2015 through 2025, the City of San Luis Obispo carried out the largest Capital Improvement Project in its history, transforming the WRRF into a modern, resilient, and environmentally responsible facility built to meet today’s highest standards of water quality.

Nearly every part of the treatment process saw improvement during this decade of renewal. The new bioreactor system is significantly more robust and efficient, incorporating the former aeration basin as part of a modern biological process that removes ammonia and converts nitrates through advanced denitrification. The system also uses ultrafiltration (UF) membrane technology at the end, which filters out fine solids and microorganisms down to 0.01–0.1 microns (µm) size, producing exceptionally clean effluent. These upgrades enable the WRRF to support both creek health and the City’s growing water reuse program.

The project also strengthened the facility’s infrastructure by expanding the existing equalization basin to allow for up to 9.5 million gallons of storm flow equalization capacity, enhancing reliability during heavy rain events. Odor control was also expanded through a new facility-wide neutralization network, improving air quality for surrounding neighborhoods and demonstrating the City’s commitment to being a good neighbor.

In conjunction, the WRRF upgraded its solids handling systems with a state-of-the-art screw thickening process and expanded digestion capacity by nearly 250,000 gallons, increasing sludge storage and loading capacity for more stable and efficient biosolids processing.

Finally, the City replaced chlorine disinfection with ultraviolet technology, eliminating hazardous chemicals and the formation of carcinogenic byproducts. This cleaner, safer, and more sustainable approach reduces costs, protects public health, and continues San Luis Obispo’s century-long tradition of innovation and environmental stewardship.

Investing in Sustainable Water Treatment

To further protect aquatic life in San Luis Obispo Creek, the WRRF introduced advanced biological treatment processes to remove ammonia from the effluent (liquid waste or sewage discharged). This included the activated sludge process, which uses diverse soil-based microorganisms to purify the water naturally.

During this period, the City also implemented biogas energy recovery, capturing methane from the digesters to power on-site microturbines. This reduced reliance on grid electricity and marked a major step toward sustainable, energy-efficient wastewater treatment.

 SLO Invests in Natural Treatment Solutions

In the decades that followed, additional clarifiers and biofilters were installed to improve treatment performance. These biological processes mimic a mountain stream ecosystem, using natural microorganisms to break down pollutants. The City also added new digesters to manage the increasing amount of stabilized sludge, now referred to as biosolids.

Wartime Growth and Expansion

  

* Images of the wastewater treatment plant - captured in 1945

World War II brought major population growth with the establishment of Camp San Luis Obispo, expanding the City’s population to roughly 20,000 residents. To meet the increased demand, the treatment facility was expanded with two additional clarifiers and two biological filters.

Effluent from the plant irrigated crops such as sugar beets grown on the sludge fields, and the income from those crops helped pay the facility’s electrical costs. During wet seasons, treated water was discharged to San Luis Obispo Creek. 

Pioneering Wastewater Treatment

* Wastewater treatment plant 1926 

In 1920, San Luis Obispo constructed one of the first sewage treatment plants in California. The original facility featured a single clarifier and an open digester, both of which have since been upgraded and incorporated into the WRRF’s modern operations. Sludge from the clarifier was pumped into the digester, and the digested sludge and effluent were dried and spread on nearby fields. This early system marked the beginning of the City’s long commitment to responsible wastewater management.

Before modern wastewater treatment, the City of San Luis Obispo had no centralized sewage system. Wastewater was spread on open fields and disked into the soil as fields were rotated. This early practice reflected the community’s first efforts to manage wastewater safely and locally.