A high-tech upgrade of a century-old wastewater treatment plant in San Luis Obispo, Calif., nears completion after four years of construction that itself followed a decade of planning. Work on the facility that dates to 1920 includes a new membrane bioreactor system and ultraviolet disinfection process, digesters and other infrastructure within the same footprint, which had to operate 24/7 during construction.

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

Dubbed SLO Water Plus, the project includes firms Water Systems Consulting and HDR as program managers, Jacobs providing design and engineering services and Carollo for construction management. 

In extensive digging of test holes, known as “potholing,” use of as-built drawings and consulting with seasoned operators, “we mapped out where we thought stuff would be,” says Jon Merryman, project manager at PCL Construction, which has a contract of about $110 million. Related to 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 plant serves the city, regional airport and California Polytechnic State University, treating 4.5 million gallons of wastewater daily. New infrastructure includes solids-thickening equipment that lowers its energy demand, digesters that increase onsite biogas production for future conversion to electricity and heat, an odor control system, upgraded electrical system with room for future onsite solar power and expanded equalization pond to improve plant capacity to treat storm flows.

The membrane bioreactor system system removes solids from water, improves water quality and reduces chemical demand by about 80%, while the UV treatment system destroys pathogens, also 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. “These plants have redundancy built into them.” 

During the pandemic, another pivot involved bringing in 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 about $20 million worth of equipment sent to the U.S.

The plant also was 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, and 25 bypasses and relocations. The equalization pond was doubled from 4.5 million gal to 9.6 million gal. About 6,000 tons of construction material was recycled.

The plant's membrane bioreactor and ultraviolet disinfection systems have operated 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, generating methane. We recycle that ... into electricity."

Merryman says the project now has completed "the liquids phase, [which is] 85% of the job.” Work now focuses on "retrofit of the old solids facility," which will require eight bypasses to make room for a new digester. "We're getting down, [dealing with shallow] groundwater again." he says. "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.

Facility supervisor Patrick McGrath says the project is preparing the treatment plant for future goals such as capability to produce potable water through such methods as reverse osmosis. 

But for now, he says, "what the community wants from us is not to smell bad."