The city is expected to begin work on an $85.7 million sewer rehabilitation project in Nuuanu Valley next summer.

According to a draft environmental assessment released in July, the two mains that transport sewage from the area — the Waolani Stream Sewer and the Nuuanu Stream Sewer — no longer have the capacity to support homes in Nuuanu Valley.

The project, known as the Dowsett Highlands Relief Sewer, will add a larger pipe down the Pali Highway and Nuuanu Avenue to relieve those two sewer mains, Markus Owens, a spokesman for the city department of environmental services, wrote in an email.

“The main reason for this project is to reduce the risk of sanitary sewer overflows in this area,” Owens wrote in an email.

A total of 12.1 million gallons of sewage a day could be diverted from the two sewer lines to the new relief line. There are also plans for smaller relief lines on Laimi Road, Ahi Place, Jack Lane, Wyllie Street and Pelekane Drive according to the draft assessment.

Blaze Lovell/Civil Beat

Final designs for the sewer project should be completed later this year, with bids going out in February of next year. Construction is expected to start in April 2018, according to the draft environmental assessment.

The project must be completed before June 30, 2020 to comply with a court-approved decree from the Environmental Protection Agency.

A small section of the project area between Puiwa Road and Coelho Way could utilize a construction technique called “pilot tube microtunneling” that involves a boring a hole for the pipes without opening a trench on the surface.

The rest of the project will require trenching, and the placement of barricades and lane closures.

The city first identified sections of the city’s sewer system that needed repairs in a 1999 study after a joint suit brought by the EPA and the state Department of Health against Honolulu.

The city was sued again in 2007 after heavy rains ruptured sewer lines, dumping 48 million gallons of effluent into the Ala Wai Canal.

In response, the city was required to inspect more than 600 miles of its aging sewer and pay a $1.6 million settlement to the EPA and the state.

Screenshot from Department of Environmental Services

The sewer work in Nuuanu will be done in conjunction with several transportation department projects. Those include replacing street lights, widening the shoulder lanes and resurfacing.

HDOT will be reducing the medians to add another lane. It will also replacing 266 lights along with adding 23 new ones, said Tim Sakahara, a department spokesman.

Crews will resurface the strip of Pali Highway between Waokanaka Street in Nuuanu to Kamehameha Highway near the Castle Junction in Maunawili. In the meantime, the city will begin its work on the sewer while HDOT replaces the streetlights along the road.

After the city finishes its sewer project along Nuuanu Avenue down to School Street, the transportation department will resurface the Pali Highway to Vineyard Boulevard.

The HDOT projects are expected to cost about $99 million, according to a department presentation.

The Dowsett Highlands sewer project is just one of more than 400 sewer projects needed to complete the upgrade of the Honolulu’s sewer system.

So far, the city has completed 364 projects and has been in compliance with the EPA since the decree was filed in 2010.