About the Project
Located at the Sand Island Wastewater Treatment Plant (SIWWTP) in Oahu, Hawaii, the $160 million design-build Synagro In-Vessel Bioconversion Facility Upgrades project is scheduled for completion in 2028. This initiative is part of the broader Sand Island Wastewater Treatment Plant, which serves the City and County of Honolulu.
The project includes the construction of two 2.35-million-gallon (MG) anaerobic digesters, two 0.537 MG sludge storage tanks, a 4,500-square-foot (SF) digester control building and a tunnel extension. This new digester facility ties into the Hensel Phelps’ SIWWTP Secondary Treatment Phase 1 project via the tunnel. Synagro contracted Hensel Phelps to provide both design assistance and construction services for the project.
The project’s primary goal is to increase efficiency and capacity, reduce environmental impact and support the community’s sustainability objectives. Upon completion, the upgraded facility will recycle biosolids that are generated by the SIWWTP Secondary Treatment Phase 1 project. The added digesters use microorganisms in an oxygen-free environment to break down sludge, reducing volume and producing methane-rich biogas, which is used to power various plant equipment. Additionally, this process improves biosolid quality by reducing odor, stabilizing content and lowering pathogens so that it can be utilized for fertilizer. By upgrading the bioconversion facility, the project transforms organic waste into valuable fertilizer for neighboring golf courses and public land, contributing to a cleaner environment and improved public health across the region.
Hensel Phelps’ Largest Concrete Placement in Hawaii
In August 2025, Hensel Phelps achieved a major milestone by self-performing our largest single concrete placement in Hawaii for the Synagro In-Vessel Bioconversion Facility Upgrades project. The foundation consisted of 1,660 cubic yards (CY) of concrete and 672,000 pounds of steel reinforcement (equating to 51 CY of Steel). This section of the foundations took just over eight hours to place, while the masons remained to finish the 9,000 SF slab.
In Hawaii, we typically self-perform 25% to 40% of the total project scope, with concrete as a core specialty. This includes:
• Forming
• Placement and finishing
• Precast and tilt-up concrete
• Related scopes such as earthwork, utilities and masonry
We maintain a skilled local workforce in Hawaii, averaging 300,000 work hours annually. We are also signatory to key craft unions, ensuring access to qualified forepersons, journey workers and apprentices. Self-performing concrete enables Hensel Phelps to leverage specialized resources, including portable batch plants and concrete pumps. Our dedicated engineering team focuses on planning, sequencing, quality assurance, control schedule and cost. Ultimately, this allows us to prioritize maintaining high standards for safety and quality.
Learn more about the work we do in Hawaii and the greater Pacific Region.
