Final approval for a major expansion of the city’s Honouliuli Wastewater Treatment Plant in Ewa now rests with the state Land Use Commission.

The main goal of the estimated $340 million project is to develop secondary treatment capability at the plant for the effluent it discharges into the ocean. The city must move to secondary treatment by 2024 under a consent decree with the Environmental Protection Agency and the state Department of Health.

The expansion won approval from the Honolulu Planning Commission last week.

“The ruling by the Planning Commission gives us the green light to finalize the planned expansion of the Honouliuli Wastewater Treatment Plant to full secondary treatment as required under the 2010 consent decree,” city Environmental Services Director Lori Kahikina said in a statement following Wednesday’s decision.

About 27 million gallons of raw sewage flow through the existing plant each day, and currently only about half is treated at a secondary level. Honouliuli services the fast-growing Central and West Oahu regions.

By 2035, average flow is expected to climb to 37 million gallons per day. Six additional clarifier tanks will ensure all of the effluent gets secondary treatment, according to documents the city filed with the Department of Planning and Permitting.

Also proposed are construction of support facilities and relocation of nonprocessing facilities that are now at the Sand Island Wastewater Treatment Plant, including laboratory, administrative support and maintenance facilities.

Nearly 20 new structures of varying sizes are planned, including a warehouse (25,600 square feet), a central shop (23,000 square feet) and an administrative building (22,000 square feet).

That phase of the project would be completed by 2035.

The Honouliuli plant, along Geiger Road near Ka­polei Parkway, began operating in 1984. The city acquired 48.4 acres to the north and east of the existing plant, where a majority of the expansion will happen, in 2011 shortly after the consent decree was signed.

The entire plant, at completion of the expansion, totals just under 100 acres.

The hydrogen sulfide odors emanating from the existing Honouliuli plant have long been a concern to neighbors. But a new odor control system is expected to reduce much of that, said James Niermann, a project coordinator for R.M. Towill, which is designing the expansion.

“The upgrade and expansion will replace the existing granulated activated carbon absorbers with a biological control system to mitigate potential for odor nuisances,” a report by acting Planning and Permitting Director Kathy Sokugawa said. “Grit covers, primary clarifier covers and primary effluent channel covers would be used for odor containment.”

Primary access will be along Geiger Road, just as it is with the existing plant.

A secondary or emergency exit along the northern boundary would spill onto Malio Street and then Renton Road in Varona Village, project officials said. No process-related trucks would go through the Malio gate, Niermann said.

Traffic improvements are planned, but the city says it will also evaluate the situation as the project develops.

The Malio gate crosses over the former Oahu Railway & Land Co. railroad. As a result, the city will need to undergo a National Environmental Policy Act review and obtain federal statutory approvals.

The state Department of Transportation wants the city to sign on to a memorandum of agreement to assess and address airspace easement and wildlife mitigation concerns. Additionally, DOT wants the project to undergo a Federal Aviation Administration construction review to assess the project’s taller structures and solar panels because of the site’s proximity to both the Kalaeloa and Daniel K. Inouye International airports.