The Sand Island Wastewater Treatment Plant, located in metropolitan Honolulu, is the largest such facility in Hawaii. It began operations in 1978 and was originally designed to provide 82 million gallons per day (MGD) of primary treatment. Today, it serves a wide region extending from Kuliouou Valley in East Honolulu to Aliamanu in the west.

The fabricator supplied structural steel and miscellaneous metals for the Sand Island Wastewater Treatment Plant in Honolulu, Hawaii. One of the most significant fabrication and coating challenges involved the truss web members for the Membrane Bioreactor (MBR) Canopy. These trusses were designed with back-to-back angles that created a narrow interior gap, an area that could not be accessed for traditional blasting and painting. Recognizing this issue early in the process, the team collaborated closely with the design engineers to develop a coating strategy capable of fully wrapping and protecting the tight, enclosed space. The final solution utilized full hot-dip galvanizing, followed by a primer and topcoat system, ensuring long-term corrosion protection despite the geometry’s inherent inaccessibility. A rendering of the upgraded Sand Island Wastewater Treatment Plant illustrates the significant improvements underway as part of a consent decree aimed at elevating wastewater treatment performance. The project includes converting the existing primary treatment plant into what will become the largest secondary treatment facility in the state. New infrastructure—such as anaerobic digesters, sludge storage tanks, and a digester control building—was added as construction advanced in early 2024. Once complete, the upgraded system will be capable of producing Class A biosolids from both primary and secondary sludges, suitable for agricultural and landscaping use. These efforts are part of a broader initiative to modernize the facility and meet expanding environmental and operational requirements. Current upgrades include increasing treatment capacity from 82 million gallons per day (MGD) to 90 MGD, along with significant modifications to enhance chemically enhanced primary treatment. These improvements help optimize performance ahead of the new secondary treatment systems being implemented in compliance with the consent decree. The plant currently processes an average flow of 65 MGD, a maximum monthly flow of 90 MGD, and peak wet-weather flows reaching 240 MGD, with treatment consisting of chemically enhanced primary clarification and UV disinfection prior to ocean discharge. The modernization effort is unfolding in phases. Phase 1 includes constructing a 20 MGD Membrane Bioreactor system, establishing the foundation for full secondary treatment. Phase 2 expands on this with a dual secondary treatment system that combines Aerobic Granular Sludge (AGS) and MBR processes. Additional upgrades include peak-flow equalization, enhancements to preliminary and primary treatment, and increased solids-handling capacity to support the expanded biological processes. The facility’s Organic Waste Sustainability Plan further contributes to long-term environmental goals by integrating organic feedstocks such as fats, oils, grease, and food waste to enhance anaerobic digestion, reduce greenhouse gas emissions, and generate renewable energy through cogeneration. The complexity of this project required extensive coordination across steel fabrication, coating applications, marine shipping logistics, and multi-phase construction activities—all while maintaining continuous operation of Hawaii’s largest wastewater treatment facility. The coastal location exposes structural steel to high humidity, salt air, and accelerated corrosion, making advanced protection essential. The multilayer corrosion-control strategy, consisting of hot-dip galvanizing followed by primer and topcoat, was selected to withstand these harsh environmental conditions and ensure complete coverage, even in areas that could not be prepared through conventional means. This approach also contributes to long-term cost effectiveness by minimizing maintenance requirements and extending the service life of the structural components. Overall, the Sand Island WWTP modernization represents a major investment in Hawaii’s critical wastewater infrastructure. The fabricators contributions—including innovative coating solutions, early engineering coordination, and the supply of nearly half a million pounds of structural steel—play an essential role in supporting the facility’s durability, performance, and readiness for the next generation of wastewater treatment needs.