A gravity sewer tunnel that is being hailed as the largest wastewater system upgrade in Hawaii’s history has been completed in Honolulu. Plans for the $375-million Kaneohe-Kailua Wastewater Conveyance and Treatment Facilities Project were accelerated by environmental engineering and construction firm Brown and Caldwell following a U.S. Environmental Protection Agency Consent Decree to improve Windward Oahu’s sewage collection and treatment system by June 2018 to replace the area’s force main pipeline.
“Our goal was to help the city construct and deliver an important asset to protect community health and wellbeing while not adversely affecting the environment,” said Ray Matasci, Brown and Caldwell’s Pacific Area leader.
The team’s solution involved connecting the Kaneohe Wastewater Pretreatment Facility (KWWPTF) to the Kailua Regional Wastewater Treatment Plant (KRWWTP) via a 4.8-km (3-mi) long, 3-m (10-ft) diameter gravity sewer tunnel, which would increase the region’s wastewater conveyance and storage capacity while reducing overflows. The tunnel conveys wastewater by gravity flow, sloping from a depth of 11.9 m (39 ft) below ground level at the KWWPTF down toward the KRWWTP, before ending at 23.5 m (77 ft) below ground level.
To lift the wastewater to the surface for treatment, Brown and Caldwell designed a 170.3-million liter per day (45-million gallon per day) influent pump station and constructed it at the KRWWTP. The project also includes generator and headworks buildings, as well as odor control facilities.
The tunnel was put under Oneawa Hills, rather than through Kaneohe Bay, to avoid potential sewage overflows into the protected area.
“This first-of-its-kind project in Hawaii is one the entire community can be proud of,” said Kirk Caldwell, mayor of Honolulu. “Our team has created a world-class sanitation facility that will last for generations, while also protecting the environment that’s so important to our island lifestyle.”
To learn more about the project, check out the video below, courtesy of Brown and Caldwell.