The recently completed $375 million Kaneohe-Kailua Wastewater Conveyance and Treatment Facilities project, which features the state’s first gravity-fed underground sewage tunnel, is expected to lower energy costs, reduce the risk for overflows during storms, offer odor relief for Aikahi area residents and possibly provide more sewage capacity for the area.

Just as important for city officials, the project — believed to be the largest sewer project ever undertaken in Hawaii — was completed in time to meet a June 30 deadline mandated under a global consent decree reached between the city and the Environmental Protection Agency in 2010.

Officials with the city and the various contractors involved in the project announced its completion Thursday, although the facilities actually began operating last month.

Mayor Kirk Caldwell said the city has now completed more than half of the 424 projects, originally envisioned to reach through 2030, that have been deemed necessary to fulfill its consent decree obligations.

The most intriguing part of the project is the 3-mile long, 13-foot diameter tunnel that feeds raw sewage from the Kaneohe Wastewater Pre-Treatment Plant to the Kailua Regional Wastewater Treatment Plant by means of gravity, largely through Oneawa Hills. The tunnel is the first in the state to use gravity flow, eliminating the need for an electric-powered force main to pump the effluent to the Kailua facility, where it undergoes treatment.

“This is an engineering miracle, a real feat, a first of its kind in the history of the state of Hawaii,” Caldwell said.

The new technique offers enough capacity to handle additional sewage or stormwater in the event of heavy rain. “So we’re not going to have the type of spills that we’ve had in the past,” the mayor said.

The technique also eliminates the need for surface- level wastewater storage, reduces overall costs and helps preserve Kaneohe Bay, officials said. The original plan was to replace an existing 42-inch force main with a new one in the bay, an idea that officials concluded posed a risk of contamination in the event of a rupture.

Environmental Services Director Lori Kahikina said the city successfully lobbied the EPA to switch to the tunnel plan and provide extra time to finish the project.

Councilman Ikaika Anderson said that the improvements may add sewage capacity for an area where homeowers have been denied permits for accessory dwelling units due to the lack of available sewer hookups.

“I’m hopeful that with this project coming online, that we will now be able to fulfill that responsibility and allow these accessory dwelling units … for long-term rentals for our families,” Anderson said.

Gary Weller, who represents the Aikahi area on the Kailua Neighborhood Board, said the community has been plagued by the odors.

“This plant, modified the way it is today, is supposed to eliminate that odor and if it does that, at least my neighborhood would be pleased with the results,” Weller said.

He added that he’s also hopeful the improvements will eliminate much of the noise neighbors have complained about.

It’s the first tunnel locally to use a tunnel-boring machine as the chief drilling tool. Drilling began on the Kailua side in April 2015 and broke through to the Kaneohe side in June 2016. The tunnel slopes from a depth of approximately 39 feet below ground on the Kaneohe end to about 77 feet below ground on the Kailua side.

Other components of the three-phase project include a new pump station, generator building and odor- control facilities.

For more information on the project, go to kktunnel.org.

KK Tunnel Fact Sheet by Honolulu Star-Advertiser on Scribd

KK Tunnel Route Map by Honolulu Star-Advertiser on Scribd