Installing a new head at an existing wastewater treatment plant is a lot like trying to perform a head transplant while keeping the patient alive, says Jignesh Desai, senior project manager in the Office of Project Management of the San Francisco Public Utilities Commission (SFPUC). But that’s exactly what’s happening at the Southeast Treatment Plant (SEP), which was first built in 1952 and is the largest of the city’s three treatment facilities.
A $718 million effort is replacing SEP’s two existing headers: a 150 million gallon per day header and a 100 mgd wet weather header, with a consolidated 250 mgd header facility for the entire time that has improved performance. seismic reliability and operational flexibility, says Desai.
As part of the first stage of wastewater treatment, a header removes debris and grit from the flow, preventing damage to critical equipment downstream.
This project is just one part of a more than $3 billion investment in modernizing the SEP. The new heads will better protect downstream processes and equipment and reduce operation and maintenance requirements. It is designed to withstand an earthquake of up to 7.8 and a projected sea level rise of 36 inches by 2100.
However, the new header facility required some creativity to fit the scope within a limited footprint while ensuring that wastewater treatment operations could continue uninterrupted throughout construction.
The odor control system consists of biological trickling filters and carbon absorbents to remove residual odors from the header building. Clean air is released into the atmosphere at the end of the odor cleaning process.
Photo courtesy of Sundt
Years in the making
A combined sewer system spanning more than 1,000 miles and three treatment plants serve the city and county of San Francisco, treating an average of 40 billion gallons of wastewater and stormwater each year.
In 2012, the SFPUC, California’s third largest public utility with a current portfolio of wastewater capital programs of more than $6.8 billion, approved the target levels of service in its Water Improvement Program sewer system, a 20-year, multimillion-dollar citywide effort. to improve aging sewer infrastructure.
“The main challenges of this job were keeping the plant operational and building such a large facility in a very small footprint.”
—Sam Reidy, President, Water & Wastewater, Sundt Construction
For the SEP header work, “we engaged design consultant Carollo Engineers in 2016 to help SFPUC complete the planning phase, resulting in a conceptual engineering report and support for the subsequent design phase,” says Desai. Under a construction manager/general contractor (CMGC) contract, “we engaged the Sundt/Walsh joint venture in 2017 to provide constructability support during the detailed design phase and to construct the facility of 250 mgd,” he adds.
CMGC allowed the contractor and key business partners to collaborate early with SFPUC and the design team. Pre-construction efforts ranged from constructability reviews and real-time cost estimates, which resulted in significant redesign and cost savings, says Sam Reidy, Sundt’s president of water and wastewater.
“SFPUC has used traditional design-bid-build delivery methods, but in the last decade, we’ve started using alternative delivery methods,” says Desai.
The Southeast Treatment Plant Header is designed to treat stormwater and sanitary flows and provide better screening and sand removal to protect downstream equipment. It was designed and built to fit a 31,500 square meter lot.
Image courtesy of Carollo Engineers
Demo to build
SEP is located in the middle of a mixed industrial, commercial and residential area, with some houses located across the street.
“The main challenges of this work were keeping the plant operational and building such a large facility in a very small footprint and allowing operators access to the surrounding facilities. This required a lot of coordination with engineers and the plant operations staff,” says Reidy. “There’s not a lot of break room, so it was build something to tear something down to build something else.”
To this end, the work is being carried out in three phases. First, the team prepared the site, demolished the existing dry weather headers and installed a temporary 100 mgd bypass in its footprint. This allowed for the demolition of the wet weather headers. Second, crews completed upgrades to the 150 mgd Bruce Flynn Wet Weather Pumping Station located across the street from SEP.
The third phase saw construction begin on the 50 mgd SEP 008 tributary pumping station, which will move wastewater from local sewers to the tributary junction at the new headwater, and the same facility header lation for all time. For SEP 008, crews replaced everything mechanical and electrical inside the station, modifying it from a dry pit pumping station to a pumping station that uses submersible pumps in wet pits, Reidy says.
Meanwhile, a full-scale odor control facility was moved to another space on the site, allowing the team to renovate an existing pump station while dramatically reducing odors. “There had been complaints from the surrounding public, so the client made a significant investment in odor control technologies,” adds Reidy.
At just 31,500 square feet, the site presented multiple logistical challenges. It is also bordered by transport corridors to the east and north, with other process buildings to both the south and west.
This made coordination critical; stakeholders, from neighboring businesses to plant operations teams, subcontractors, and the Union Pacific Railroad, were deeply involved and knowledgeable about the construction activities. The team prepared maintenance plan operations with hour-by-hour schedules if construction was expected to affect plant operations and used daily subcontractor meetings and a state-of-the-art planning system to provide anticipation of a at six weeks
Because work was done along the property line and adjacent railroad tracks, the project team had to obtain a special permit, use third-party signage, and plan work around scheduled train times, Reidy says.
The sand gallery is where the sand pumps and accessories collect the sand and pump it downstream for disposal. This equipment is currently undergoing functional testing.
Photo courtesy of Sundt
Building up
SEP not only treats about 80 percent of the city’s and surrounding wastewater, but also manages stormwater and sanitary flows, explains Jim Hagstrom, executive vice president and managing director of technical practices for Carollo Engineers. This combined system means there are large variations in flow: on average, the plant treats about 80 mgd, but this flow can reach 250 mgd during storms.
One way to mitigate such a wide range of potential flows is to have a large storage capacity in the collection system, “but that allows a lot of sand to be retained,” Hagstrom says.
Peak grain loadings at SEP can exceed 6,000 lbs/mg, nearly twice the industry standard, Desai adds.
Meanwhile, biosolids digester facilities are also being built downstream of the headwaters, scheduled for completion in 2028, with very strict limits on sand content.
“Instead of adding more screens, we actually changed the water level.”
—Jim Hagstrom, Executive Vice President, Managing Director, Technical Practices, Carollo Engineers
“The challenge is that you have these high flow variations, very high sand loads because it’s a combined system, and the performance has to be higher than most wastewater treatment plants because of the biosolids system that they’re putting downstream,” says Hagstrom. “So you have to have very good sand removal or high-efficiency sand removal from the header component.”
To find out which technologies would be best for headwater jobs, SFPUC conducted a large-scale 5 mgd side-by-side pilot to test different sand washing and separation technologies.
“We selected the main cell for sand removal that provides a much higher level of sand removal than the typical industry standard,” says Hagstrom. “Then we take that sand and wash it to reduce both the odors and the total weight it takes away.”
Another challenge was finding a way to install an adequate amount of screening, given the limitations of the site.
“Instead of adding more screens, we actually changed the water level. So in high-flow conditions, we increase the hydraulic grade line through the screen so that we don’t get too high a velocity through of them,” says Hagstrom. “What limits effective screening is speed across the screen. As speed increases, you have to add more screens. But we didn’t have that luxury, given the footprint, so we added depth.”
Instead of the eight screen channels specified in the original concept, the team installed just four screens that complete the same job through the use of hydraulics. “I think it’s probably one of the most unique things we did on the header to try to fit it into place,” he says.
Vertical stacking and construction processes also proved a vital solution in other areas, from odor control to sand washing and handling and electrical support facilities, ensuring the installation of these vital elements while they kept within the site lines.
Top view of grease tanks under construction. The gates will control flow into the sand tank chambers when the plant is operational. Crews also applied coatings to protect the concrete inside the tanks and chambers.
Photo courtesy of Sundt
Critical impacts
The aesthetic improvement at SEP is helping the facility be a better neighbor during construction and beyond. Since 2020, four temporary public art installations have been featured on the construction’s security fences, while a permanent 335-foot-long by 35-foot-high mural was recently installed, which refers to the processing of headers.
Because LEED certification was not appropriate for this type of facility, the design team suggested to SFPUC that Envision, designated by the Institute for Sustainable Infrastructure, would be better, Hagstrom adds. In addition to being the first project the City of San Francisco has implemented with Envision, the headworks also recently won an Envision Gold Award.
Construction is nearing completion and commissioning began at the end of August. The project is on budget and substantial completion is on track for December, with final completion scheduled for May 2025.
“The Headworks project will be a critical element in achieving SFPUC’s commitment to protecting public health in our community,” says Desai.
