Major Overhaul of San Luis Obispo Water Treatment Plant Flows Along

New digesters, bioreactors, UV systems and other upgrades will fortify San Luis Obispo's water supply.

Photo courtesy of PCL

A high-tech upgrade of a century-old water treatment plant in San Luis Obispo, Calif. is nearing completion after four years of construction that itself followed a decade of planning. Crews have built a new membrane bioreactor (MBR) system, a new ultraviolet (UV) disinfection system, new digesters and other infrastructure within the same footprint as the old system, which had to stay operating 24/7 during construction.

Over the years, “we did a few upgrades, triggered by the need for city to expand or to meet discharge permit requirements,” says Miguel Barcenas, utilities deputy director with the city. “The first major expansion was in 1964.” When preparing for the current upgrade to the Water Resource Recovery Facility (WRRF), "we dealt with a lot of the electrical stuff from 1964” along with other unknown underground obstructions.

Dubbed SLO Water Plus, the project has WSC and HDR as program managers, with Jacobs providing design and engineering services, and Carollo for construction management.

With extensive “potholing,” as-built drawings, and consulting with seasoned operators, “we mapped out where we thought stuff would be,” says Jon Merryman, project manager with PCL Construction, which holds an approximately $110-million contract. Regarding underground piping and utility routing, “having the ability to pivot was mandatory—and we had to pivot a lot.”

Navigating a maze of pump lines, some gravity-fed, and electrical duct banks, the team had to sequence work to fit in new utilities while taking old structures offline, and retrofitting or replacing them, he says.

The WRRF serves the city, California Polytechnic State University and the regional airport, treating 4.5 million gallons of wastewater daily. New infrastructure includes solids-thickening equipment that lowers the plant’s energy demand, digesters that increase onsite biogas production for future conversion to electricity and heat, an odor control system, an upgraded electrical system with room for future onsite solar power, an expanded equalization pond to improve the plant’s capacity to treat storm flows, the MBR system that removes solids from water and improves water quality, and the UV system to destroy pathogens, eliminating need for chemical disinfection and its byproducts.

“We did a full plant bypass, taking everything coming in and bypassing existing structures for a full rebuild during the pandemic,” says Merryman. “Right out the gate, these plants have redundancy built into them.”

During the pandemic, another pivot involved sending new equipment from Germany to the U.S. for testing, says Barcenas. “Normally, extensive testing would be [done] at the factory." With Covid-era travel restrictions, engineers had that $20 million worth of equipment sent to the U.S. for testing instead. “Half came to California, some went to South Carolina.”

The MBR treatment process reduces chemical demand by about 80%, and the plant is upgraded to handle a 100-year storm event.

Two heavy rain storms this past winter have already tested the new equipment's mettle. “All the new gear stayed dry and clean, so that's a testament to preparing for the future,” says Merryman.

Work on the 90-acre site included installing 25,000 linear ft of underground pipe, 8,000 linear ft of electrical duct banks, 25 bypasses and relocations, 6,000 tons of recycled construction material, and doubling the equalization pond from 4.5 million gal to 9.6 million gal.

The MBR and UV systems have been operating since May, says Bercenas. "The new process breaks ammonia down into nitrogen and oxygen through biological reactions. Waste goes into another tank, a solids handling facility. That waste decays, generates methane. We recycle that ... into electricity."

“We made it through the liquids phase, [which is] 85% of the job,” says Merryman. “Now we’re focusing on ... retrofit of the old solids facility." That will require eight bypasses to make room for a new digester. "We're getting down, [dealing with shallow] groundwater again. We'll get the structure built out, then enter the mechanical phase." After testing the new systems, crews will decommission existing digesters in sequence and use the sludge to seed the new digester. Completion is expected early next year.

WRFF facility supervisor Patrick McGrath says the project is preparing the plant for future goals such as the capability to produce potable water through such methods as reverse osmosis. But for now, "what the community wants from us is not to smell bad."

Aileen Cho, ENR's senior transportation editor, is a native of Los Angeles and recovering New Yorker. She studied English and theater at Occidental College, where a reporter teaching the one existing journalism course encouraged her to apply for the LA Times Minority Editing Training Program. Her journalism training led to her first stories about transportation, working as a cub reporter with the Greenwich Time. Her work has appeared in the Los Angeles Times and New York Times. Many of her experiences with engineers and contractors have inspired material for her alternative theater productions way, way off Broadway. For ENR, Aileen has traveled the world, clambering over bridges in China, touring an airport in Abu Dhabi and descending into dark subway tunnels in New York City. She is a regular at transportation conferences, where she finds that airport and mass transit engineers really know how to have fun. Aileen is always eager to hop on another flight because there are so many interesting projects and people, and she gets tired of throwing her cats off her computer in her home office in Long Beach, California. She is a very conflicted Mets/Dodgers fan.