At the Sand Island Wastewater Treatment Plant in Honolulu, construction continues to advance on one of Hawaii’s most significant wastewater infrastructure projects.
Hensel Phelps is delivering the $163 million Synagro In-Vessel Bioconversion Facility Upgrades project, a design-build effort that will enhance biosolids processing capacity through the construction of two 2.35-million-gallon anaerobic digesters, sludge storage tanks, a control building and associated tunnel improvements.
Crews have already completed a major milestone with a record-breaking concrete placement of the Digester Three hopper foundation. This milestone moves the project closer to delivering critical infrastructure that will support long-term sustainability and resiliency for the City and County of Honolulu.
What Is an Anaerobic Digester?
Anaerobic digesters are a critical component of modern wastewater treatment facilities. These large structures support biological processes that break down organic material in the absence of oxygen, reducing waste volume while producing renewable biogas that can be used as an energy resource.
The Synagro project will add two new 2.35-million-gallon digesters designed to improve treatment efficiency, support sustainability goals and increase long-term resiliency for Oahu’s wastewater infrastructure.
Overcoming Complex Digester Construction Challenges
Constructing large-scale anaerobic digesters requires extensive planning, technical precision and close coordination between structural, mechanical and process systems. For the Digester Three hopper foundation, crews navigated an inverted cone design, specialized formwork and multiple system connections that had to be integrated into the structure with accuracy.
The hopper placement marked the first of its kind on the project and required a continuous, 24-hour concrete pour to achieve a monolithic, watertight structure forming the base of the 75-ft-tall digester. Complex geometry, specialized formwork and tight coordination among Hensel Phelps, DN Tanks and trade partners were critical to safely executing the large-scale concrete placement.
Success on work like this starts long before concrete placement. With complex geometry like the inverted cone hopper, with complex mechanical systems entering and exiting the cone, there’s no margin for error—executing a continuous 24-hour pour requires detailed planning, early coordination with trade partners and a disciplined approach to risk management to deliver it right the first time.”
— Rick Crago, National Water/Wastewater Enterprise Lead and Pacific Region Operations Manager, Hensel Phelps
As work advances, the project team is building on this milestone to deliver the critical structures and systems needed to support biosolids processing, renewable resource recovery and long-term wastewater treatment operations.
Supporting Sustainable Wastewater Infrastructure in Honolulu
This project represents a significant investment in resilient wastewater treatment infrastructure for the City and County of Honolulu. Once complete in 2028, the facility will help convert organic waste into renewable resources while enhancing environmental stewardship, public health and long-term wastewater system resiliency across Oahu.
As a national leader in integrated construction delivery, Hensel Phelps continues to support complex water and wastewater projects that strengthen essential community systems and advance sustainable infrastructure solutions.
Interested in learning more about Hensel Phelps’ water and wastewater expertise across Hawaii and the Pacific? Explore our Pacific Region projects to see how teams are delivering critical infrastructure that supports communities, sustainability and long-term resilience.
Synagro Wastewater project arial view of construction site
