The SourceCivil Engineering MagazineCalifornia county to benefit from temperature-phased anaerobic digestion facility

California county to benefit from temperature-phased anaerobic digestion facility

By Jay Landers

Looking to improve the marketability of the biosolids generated at its 144 mgd Plant No. 2 in Huntington Beach, Calif., the Orange County Sanitation District is moving forward with the design of a $300 million temperature-phased anaerobic digestion facility. Consisting of six thermophilic digesters and six batch tanks, the new TPAD facility will produce Class A biosolids, which are treated at higher temperatures and capable of being used for land applications, unlike the current Class B biosolids produced by the district. The new facility will also increase the district’s system resiliency in the event of a major earthquake.

In July, the district, which recently rebranded itself as OC San, awarded a $39.3 million design contract to a team comprising Brown and Caldwell and Black & Veatch. The team began its work about a month later. “We are in early preliminary design,” says Dan Bunce, P.E., M.ASCE, the principal in charge for Brown and Caldwell, which is serving as the design lead on the project. The firm is responsible for the overall process design, the design of the thermophilic digesters, and the civil design. Black & Veatch is in charge of designing the digester feed system, the batch tanks, and the overall electrical and mechanical systems for the project.

A provider of wastewater services for 2.6 million people in central and northern Orange County, OC San currently uses mesophilic digestion to treat biosolids at its Plant No. 2. Mesophilic digesters operate between 98 and 100 degrees Fahrenheit for a minimum of 15 days and treat solids to Class B standards, in accordance with the U.S. Environmental Protection Agency’s Standards for the Use or Disposal of Sewage Sludge (40 CFR Part 503). Because the state of California precludes the land application of Class B biosolids, OC San typically hauls its treated solids to composting companies located in California’s San Bernardino and Kern Counties or to a ranch in Yuma County, Arizona.

System upgrade

The new TPAD facility will produce Class A biosolids, which undergo more extensive treatment to achieve greater pathogen reduction as compared with Class B. For this reason, Class A biosolids may be used in more applications, including on edible crops.

Designed to process all solids generated at Plant No. 2 until 2045, the new TPAD facility will heat solids in the six new digesters to a temperature of 131 F. Following an eight-day solids retention process, the solids will be conveyed from the thermophilic digesters and held in the batch tanks for 24 hours. “That is a requirement to achieve Class A status,” Bunce says. Next, the solids will undergo cooling by means of a process that has yet to be determined before then entering the existing mesophilic digesters. “That’s done as a polishing stage to produce high-quality Class A biosolids,” he notes.

Having a diameter of 110 ft, each of the six thermophilic digesters will have a capacity of 2.27 million gal. “This is intended to handle a design capacity that matches the primary and secondary treatment capacity at Plant No. 2,” Bunce says. “It’s equivalent to 1 million gallons per day of primary and thickened waste activated sludge.”

aerial shot of the to-be-built digesters
(Rendering Courtesy Brown and Caldwell)

Having Class A biosolids will “open up markets in California,” says Kathy Millea, P.E., the director of engineering for OC San. “We could do land application in California with Class A.”

The ability to dispose of biosolids closer to home as a result of their higher quality will also be “beneficial in many ways,” Millea says. For example, closer markets mean shorter driving distances to haul biosolids. “That’s big for a few reasons,” Millea says. Along with lower hauling costs and reductions in greenhouse gases from truck trips, OC San will enjoy greater reliability in terms of biosolids disposal. “When we have to drive four hours to haul our solids away, we can run into (transportation) problems,” she notes, particularly as a result of wildfires or flooding.

The addition of the TPAD facility is expected to facilitate increased energy recovery at Plant No. 2, particularly as OC San plans to accept food waste from nearby municipalities after the thermophilic digesters begin operations. Currently, biogas from the mesophilic digesters is captured and used to generate about two-thirds of the energy required by the treatment facility.

Seismic upgrade

The new digesters will also improve the resiliency of the overall system. Constructed between 1959 and 1979, the facility’s 18 existing digesters are aging and vulnerable to seismic damage. While the new digesters will be designed to current seismic codes as part of the project, the existing digesters will be rehabilitated as part of a separate project in the future.

Soils at the project site are vulnerable to liquefaction, an issue the design team will determine how best to mitigate during the construction of the TPAD facility, according to Bunce. The solution for the new digesters could take the form of a pile foundation system or deep soil mixing.

Moving to a Class A product and addressing the seismic risk of the solids handling process exemplify OC San’s ongoing efforts in the area of resiliency planning, Millea says. “It’s something we’re doing in all areas of our facility,” she says. No matter what might happen in terms of climate change, regulatory changes, seismic activity, or other unforeseen events, the district is “making sure that we always have a path forward to continue to treat wastewater,” Millea says.

The team of Brown and Caldwell and Black & Veatch will develop a preliminary and final design for the new TPAD facility by November 2024. Construction is scheduled to begin in 2025 and be completed by 2030.

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