Modeling the Flow
Gresham Smith first conducted a conducting a wastewater characterization study of the influent waste stream, with a focus on the plant’s liquid stream. The goal of the study was to determine the constituent concentrations in the influent wastewater, which would be used in process modeling, process selection and equipment sizing. Our team also collected and analyzed plant operating data, which, together with the study data, was used to build a dynamic process model simulation to evaluate treatment alternatives for technical feasibility and cost effectiveness.
A Combination Approach
To save time and budget, as well as reduce the need for labor since workforce was in short supply, Gresham Smith took a combination approach, repurposing existing structures alongside new construction. Engineers used qualitative factors—benefits to overall treatment process, benefits to construction cost or schedule, ability to conform to the site’s limited footprint, and potential future benefits—to evaluate the plant’s existing structures to determine viability for re-use.
Project Components
- • Renovated Southern Link influent pump station and force main
- • Repurposed existing headworks for new pump station to new headworks
- • Repurposed chlorine contact basin to serve as UV disinfection chamber
- • Repurposed primary clarifier and sludge pumping building to house blowers and store WAS
- • New influent pump station and force main
- • New headworks with influent flow measuring, fine screens and vortex grit removal
- • Three new pre-treatment trains that operate individually to create redundancy
- • New passive weir gate and manual screen for flood protection
- • New two-stage odor control process
- • New oxidation ditch
- • Three new companion secondary clarifiers
- • Hydraulics for future chemical phosphorus removal with coagulation and tertiary filtration
Benefits of Biological Treatment
After evaluating three biological treatment processes—conventional activated sludge, oxidation ditch and sequencing batch reactor—the team determined the oxidation ditch was the preferred solution to meet performance and operations needs. To further refine the treatment process, engineers used BioWin process models to analyze a variety of influent loading scenarios and confirmed that the oxidation ditch will produce effluent consistent with the requirements of the plant’s NDPES discharge permit.
A Complex Site
Once a design solution was selected, the project team evaluated the dedicated project site—a 17-acre greenfield site 300 feet above the Philippine Sea—and prepared a site layout that would better accommodate the necessary treatment processes. To work with the site’s slope, Gresham Smith selected a layout that accommodated locating the headworks in the southern portion of the expansion site, where the existing elevation was lowest and sloped upwards toward the north end. This allowed the headworks to be constructed at grade with the screening system located above the first floor, keeping the hydraulic profile low, eliminating the need for screen conveyor systems and accommodating a gravity drop to dumpsters below. This solution also significantly reduced excavation requirements, which was important since the site was comprised of volcanic deposits.
Setting a New Standard
As only the third wastewater treatment plant with a secondary treatment system on the island of Guam, this project sets the standard for overcoming climate and site constraints and adhering to stringent design requirements set by the U.S. government to deliver an operationally efficient design solution that benefits residents of Guam. The upgraded treatment processes, particularly the biological secondary treatment and UV disinfection system, will improve the quality of the effluent discharged to the Philippine Sea, reducing the impacts to coral reefs and protecting the natural resources vital to Guam’s economy.