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San Francisco Plans Billion-Dollar Biosolids Upgrade

Aerial view of the Southeast Treatment Plant, located in San Franciso, California
The Southeast Treatment Plant treats 80 percent of San Francisco’s wastewater. San Francisco Public Utilities Commission

Design work has begun on a $1.6-billion upgrade to the aging solids-handling processes at the Southeast Treatment Plant, the largest wastewater treatment plant owned and operated by the San Francisco Public Utilities Commission.

September 24, 2013—After receiving a notice to proceed in early August, a team comprising several major players in the engineering consulting industry began work on the planning and design of a $1.6-billion upgrade to the aging solids-handling processes at the Southeast Treatment Plant (SEP), the largest wastewater treatment plant owned and operated by the San Francisco Public Utilities Commission (SFPUC). The team consists of the prime consultant, Brown and Caldwell, of Walnut Creek, California, and its major partner, CH2M HILL, of Englewood, Colorado, and also includes Black & Veatch, of Overland Park, Kansas; AMEC, of London; and four firms based in San Francisco: MWA Architects, Geotechnical Consultants, Inc., STRUCTUS, Inc., and SRT Consultants. The Brown and Caldwell team was awarded an $80-million planning and design contract for the project this summer.

Formally known as the Biosolids Digester Facilities Project, the 10-year effort represents a significant component of the SFPUC’s Sewer System Improvement Program (SSIP). To be implemented over a period of 20 years, the SSIP comprises multiple capital improvement projects intended to return San Francisco’s combined wastewater and storm-water system to a state of good repair while achieving certain broad goals, including improved responses to catastrophic events, better management of storm water to minimize flooding, system modifications to adapt to climate change, and fuller adherence to the principles of sustainable development.

Constructed in 1952, the SEP treats on average 59 mgd and produces 57,000 tons (wet) of biosolids per year. During wet-weather events, the facility can treat up to 250 mgd. All told, the SEP treats nearly 80 percent of the city’s total annual flow of wastewater. Along with primary treatment, the facility provides secondary treatment by means of an activated sludge process that uses high-purity oxygen before effluent disinfection. As for solids handling, the SEP employs gravity belt thickening followed by digestion in nine anaerobic digesters, chemical conditioning, and centrifuge dewatering. The digestion process generates roughly 1.2 million cu ft of biogas per day, which can be used to generate electric power or heat the digesters. The gas can also be exhausted through a flare system. Last year the SEP produced 14,000 tons (dry) of biosolids in compliance with the U.S. Environmental Protection Agency’s regulations for class B biosolids in the Code of Federal Regulations (title 40, part 503).

Although portions of the SEP’s solids-handling processes have been updated over the past 60 years, new biosolids facilities are “way past due” at the plant, says Carolyn Chiu, P.E., the SFPUC’s project manager for the Biosolids Digester Facilities Project. For example, the existing digestion process represents “technology from the 1940s,” Chiu says, and the roofs of five of the nine digesters are undergoing emergency repairs so that the digesters can remain in operation until the new facilities are complete. Given their age, the digesters do not comply with current seismic requirements. Odor control also has been a challenge at the SEP, which is situated in the middle of an area that has industrial, commercial, and residential components. In fact, some of the facility’s digesters are located less than 35 ft away from single-family homes.

Along with replacing all of the SEP’s aging solids-handling facilities, the SFPUC has established sustainability goals for the Biosolids Digester Facilities Project, including the beneficial use of all biosolids and biogas generated at the facility, Chiu says. Therefore, the level of treatment of biosolids will be enhanced to ensure that the finished product complies with the Environmental Protection Agency’s stricter requirements for class A biosolids, and cogeneration or other options for beneficially reusing methane will be added at the site.

Because the Biosolids Digester Facilities Project will entail a complete revamping of the solids-handling facilities at the SEP, the design team will be “going from start to finish on the biosolids side,” says Tracy Stigers, P.E., M.ASCE, a vice president and the project manager of the consultant team for Brown and Caldwell. All processes will be evaluated to seek improvements in, for example, the thickening of solids; thermal hydrolysis and other digestion pretreatment methods; digestion processes; the treatment and use of biogas; dewatering methods; facilities for storing and transporting cake solids; and approaches related to nutrient recovery.

Another SSIP goal involves confining odors from the biosolids facilities to the fence line of the SEP, which will mean a state-of-the-art system for collecting and treating odors on-site, says Dave Green, P.E., a vice president of CH2M HILL and the firm’s project engineer for the consulting team. Such a goal is “achievable,” Green says, “but it requires quite a bit of redundancy and quite a bit of engineering to make that happen.” Fortunately, recent decades have witnessed a great increase in the “standard of care for odor control at treatment plants,” Green says. “The bar that we’re all working toward is much higher than it was 20 to 30 years ago.”

Among the many steps that will have to be taken to address odors from the SEP is an effort to reduce potential odors at the source by determining which treatment processes have the effect of minimizing odors, Stigers says. The next steps will entail ensuring that any odors generated are confined within the smallest volume of air possible and selecting and locating the ideal processes for treating that air. To be successful, such processes will have to be seamlessly integrated into the operations of the plant. “Part of maintaining [a successful odor control system] is working all of that into the design of the facility in such a way that it allows the operations and maintenance people to still have appropriate access for control and maintenance of the processes without inhibiting them or creating too many restrictive confined spaces,” Stigers says.

The SSIP is also seeking to ensure that the primary treatment and disinfection facilities at critical wastewater facilities are operational within 72 hours of a major earthquake. To this end, the new features will have to be designed for a magnitude 7.8 earthquake occurring on the San Andreas Fault and a magnitude 7.1 earthquake on the Hayward Fault. However, the geotechnical conditions at the 40-acre site of the SEP range from bedrock to deep basins of bay mud. Such conditions will comprise a “key element” in determining foundation requirements, Stigers says. On the basis of these findings, the design team will “develop site-specific seismic design criteria for different site alternatives,” she notes.

The findings of the geotechnical investigation are expected to influence the project design in key ways. “Getting our arms around the geotechnical issues and how they define the structural and seismic issues will be a big factor in what the new facilities will cost,” Green says. “That will play a big part in site layout,” he says, and may ultimately dictate which processes are selected.

Although the upgrades to the SEP make for a highly complex project, the surrounding community has high expectations, as does the SFPUC, Chiu says. “We’re looking for a complete package,” she says. “We want it to be a very functional, accepted project from the point of view of our ratepayers, regulators, and neighbors—a community asset in the neighborhood and throughout San Francisco.”

Upon receiving the notice to proceed in early August the design team began a 25-month project planning and definition phase. At the end of this first phase the team will have completed planning and predesign, analyzed alternatives, and developed a conceptual engineering report. The SFPUC will choose the delivery method for each project element. The second phase will entail detailed design and procurement, along with engineering construction support. Major construction is expected to begin in 2018.



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