The city’s largest wastewater system upgrade is complete, and allows sewage in Windward Oahu to flow by gravity instead of energy.

The Kaneohe-Kailua Wastewater Conveyance and Treatment Facilities Project features a three-mile-long underground gravity sewer tunnel, new pump station, headworks, generator building, odor control facilities, and tunnel influent facility.

The 13-foot diameter gravity tunnel slopes from a depth of approximately 39 feet below ground level at the Kaneohe Wastewater Pre-Treatment Facility (WWPTF) and ends at the Kailua Regional Wastewater Treatment Plant (WWTP), approximately 77 feet below ground.

“This is an engineering miracle, a real feat, a first of it’s kind in the history of the state of Hawaii,” said Honolulu Mayor Kirk Caldwell. “The way it’s designed where you’re being efficient on energy use, because you’re not pumping under pressure anymore. You’re letting Mother Nature through gravity move sewage.”

The existing force main that is no longer active will be decommissioned.

The $375 million project also increases the system’s storage to accommodate increased flow during heavy rain events, which officials say will greatly reduce the likelihood of spills in Windward Oahu.

The city says the project was completed within budget and ahead of schedule. Officials are now looking into gravity tunnels for Waikiki and Sand Island.

Click here for more information about the project.