From the nearby island of Oahu, two tower cranes stand as one of the few signs of activity on Sand Island. Host to a variety of industrial facilities and a state park, Sand Island is home and namesake to Hawaii’s largest wastewater treatment plant.

A multiphase effort is underway to add secondary treatment to the Sand Island Wastewater Treatment Plant (SIWWTP), which previously took effluent straight from primary treatment to disinfection and ocean outfall. But completing this upgrade in limited space and on poor soils so close to sea level has required innovative solutions.

The project’s origins date back to 2004, when three environmental groups sued the city and county of Honolulu challenging its wastewater collection and treatment facilities’ Clean Water Act compliance. As part of a settlement reached in August 2010 with these groups, the U.S. Environmental Protection Agency—along with the U.S. Justice Dept., Hawaii Attorney General’s Office and Hawaii Dept. of Health—issued a consent decree requiring secondary treatment upgrades at the Honouliuli plant by 2024 and the Sand Island plant by 2035.

Construction has reached the halfway point on Phase 1 of the Sand Island secondary treatment installation: a $417-million, five-year project that will treat 20 million gallons per day of SIWWTP’s total 90-mgd capacity. Completion is on track for 2026. Phase 2—with an anticipated 2029 start date—will address the remaining capacity and target completion by the consent decree’s 2035 deadline.

Benny Bonilla

Laborer foreman Benny Bonilla and his crew prepare the fine screen structure wall with water stop prior to installing formwork for concrete placement.
Photo courtesy of Hensel Phelps

Membrane Filters

Despite having a full 25 years to complete these upgrades, the city and county of Honolulu was eager to move quickly, says Bev Stinson, executive vice president and water business line leader at AECOM, the project's engineer-of-record. But expanding to improve treatment typically requires a larger footprint, which wasn’t possible here.

“So we came up with a solution: membrane treatment technology that reduces the footprint by more than half of a typical secondary treatment step,” Stinson says. “Whenever you’re trying to build something on a small footprint, oftentimes a smaller, innovative technology keeps costs down. That’s where technologies like membranes are so effective.”

During the city’s procurement process, AECOM performed a competitive evaluation of membrane bioreactor (MBR) technologies to determine which would best meet targets for reduced energy and chemicals usage; reduced footprint and costs; and longevity.

The AECOM team also wanted to have the technology selected so the overall design of the secondary treatment facility would be as streamlined as possible and suit the filtration process precisely. Suez membranes were chosen for meeting all the necessary criteria, Stinson says.

“Membrane treatment technology [...] reduces the footprint by more than half of a typical secondary treatment step.”
—Bev Stinson, Executive Vice President, Wastewater Business Line Leader, AECOM

“What’s special about this project and that technology is it represents the largest number of ZeeWeed 500EV cassettes for a municipal wastewater plant in the country,” says Allie Ackerman, operations manager with contractor Hensel Phelps.

This technology is expected to exceed the EPA requirements for BOD and TSS associated with the 2010 EPA Consent Decree

Although the primary treatment remains unchanged, now primary effluent will travel through two 96-in. lines into a newly built intermediate pump station and blower building. To ensure the primary effluent doesn’t clog the MBR filters, fine screening will remove any particles larger than 2 mm. Then, water travels into a process reactor to reduce the BOD before moving into the MBR filters.

With membrane treatment technology, the effluent quality is superior to older treatment technologies, Stinson says. “So by treating at least a third of that flow to a much superior treatment level, it will have a huge benefit on the environmental benefits of the discharge.”

Following the MBR process, effluent will move through an existing ultraviolet disinfection facility before ocean outfall release. “This is one of the most significant and advanced treatment technologies that they've ever put in Hawaii. And it’s going to make a step change in the environmental benefits,” Stinson says.

SIWWTP’s secondary treatment installation

SIWWTP’s secondary treatment installation will take effluent from the existing primary treatment process into a new intermediate pump station, progressing through the new membrane bioreactor process.
Graphic courtesy of AECOM

Learn From the Past

Notice to proceed on this hard-bid project came on Dec. 13, 2021—just weeks before the EPA’s deadline. “It was, at the time, the largest hard-bid project of the city and county of Honolulu,” Ackerman says.

Scott Jennings, who served as project manager on two projects at SIWWTP more than 20 years ago—an effluent pump station and the world’s first ultraviolet disinfection facility along with the headworks and primary clarifiers 7 and 8 (see ENR, 5/20/2002)—and several others who were on the same jobs are back for the secondary treatment upgrade. Jennings is now construction manager at the city and county’s request on three projects at SIWWTP. Lessons learned on those earlier projects are proving beneficial for the current endeavor.

The two other jobs running concurrently to the secondary treatment project on the same campus include the $200-million In-Vessel Bioconversion Facility Digestion Capacity Upgrades, or Synagro upgrades, along with the $87-million Maintenance Building, Septage and Site Improvements, or maintenance building job. These three ongoing SIWTTP projects, along with the recently completed $536-million Honouliuli WWTP upgrade, account for more than $1 billion in work on Oahu.

Synagro is design-builder on the bioconversion facility upgrade, with Hensel Phelps serving as contractor and R.M. Towill Corp. as designer. The city and county of Honolulu marked the start of construction in early June on two 2.35-million-gal. digesters, two 0.54-million-gal. sludge storage tanks and a digester control building that is connected to the existing digestion and thermal drying facility. Following a notice to proceed in April 2023, crews demolished an abandoned effluent screening and pumping structure to make room for the upgrades. Completion is expected in 2028.

Meanwhile, Hawaiian Dredging Construction Co. is contractor on the MBSSI project—a five-year effort to add a maintenance building, dewatering building, septic receiving station and parking lot equipped with photovoltaics. Completion is set for 2026.

“With several different large projects ongoing and upcoming on the small project site, coordination with both plant operations and other contractors is critical, and Hensel Phelps has been excellent in achieving this,” says Roger Babcock, director of the Dept. of Environmental Services (ENV) with the city and county of Honolulu. “Hensel Phelps has established a very good working relationship with ENV, the design team and the CM by being very communicative, cooperative and responsive.”


The cast-in-place concrete structure for the fine screen well and channels. In the future, this structure will remove particles greater than 2 mm before the water enters the process reactor and MBR system.
Photo courtesy of Hensel Phelps

From the Ground Up

Sand Island was formed in the 19th century as the need for deeper shipping channels required increased dredging efforts. The resulting sediment was brought to this small coral reef island, steadily expanding its size, which now stands at about 500 acres. Its past is a varied one too—it once served as a quarantine area for ships carrying people with contagious diseases and housed a U.S. Army internment camp during World War II.

But being surrounded by ocean water with high winds, storm surge and poor soils has not made for easy building. To mitigate the effects of sea level rise and storm surge, critical, electrical and mechanical facilities were all elevated to resilient locations.

“There are quite a few nature-based types of protection too—even the sand embankments both camouflage the plant and protect against ocean impacts,” Stinson says.

Before construction began, crews demolished an existing drug treatment facility next to the SIWWTP on the footprint where the secondary treatment facility would reside. During excavation, the team encountered a range of unknowns due to the island’s long mixed history, from underground utilities to stray piles and more.

“Hawaii is well known for very challenging soil conditions.”
—Scott Jennings, Construction Manager

The first year of the project required soil testing because of potentially contaminates soil. It also included driving more than 2,500 auger cast-in-place (ACIP) piles for the project’s deep foundation system.

The project required significant measures to mitigate the impacts of natural disasters such as flooding and tsunamis, according to the City and County of Honolulu's Final Environmental Assessment. The design was required to be in accordance with all applicable IBC seismic design standards and CCH standards pertaining to seismic safety. An to mitigate minimize impacts from sea level rise, the proposed facilities were required to be built at a floor elevation of 16‐ft above sea level.

ACIP piles have been trending on the islands of late, Jennings says, noting that deep foundation systems will often go through phases across Hawaii. In the early 2000s, for instance, precast piles were common.

“Hawaii is well known for very challenging soil conditions. This job sits at elevation 8. The ocean is half a mile out at elevation 0, and with the tides, water is usually as high as elevation 2,” Jennings explains. “Sand Island is all built roughly at elevation 8. Because the groundwater table is only 6 feet down at elevation 2, and the work we’re doing now has a finished floor of -10.5, we have a bottom slab at -13.5—so we’re roughly 13.5 feet below elevation 0.”

Past experience is helping inform today’s solutions. On the effluent pump station Jennings managed around 2002, excavation went down to -28. But due to challenging soils and city-mandated use of a jet grout bottom plug, that team couldn’t keep it dry.

On the secondary treatment project, even though excavation is deep and both groundwater and ocean water infiltration are issues, “the team really came together collaboratively to find solutions for that as well as soil stabilization,” Ackerman explains. “So between AECOM, one of our trade partners, Pacific Foundations, and the CM, we came up with these designs that included a secant wall with soldier piles and soil mixing. I think it was the second time that soil mixing had been used on island, and the first time on this scale. That was the key in hitting a lot of our schedule milestones: keeping the water out and going deep enough so we could take the building from the foundation up.”

Jennings calls this “probably the biggest thing that made this job such a success.” The solution—a soil mix bottom plug and perimeter soil mix columns reinforced by soldier piles—saw water infiltration reduced to a trickle, compared with about 2,500 gallons per minute on the jobs during the early 2000s.


Home Stretch

Across the 24-acre site, Hensel Phelps crews have placed about 70,000 cu yd of cast-in-place concrete for foundations and new building structures, channels, wet wells, blend tanks and tunnels and completed about 600,000 work hours as of late May. Hensel Phelps alone has been running about 80-90 craftworkers on site, self-performing more than 50% of the project value.

The project team is targeting substantial completion of the structure by the end of the year.

"We’ve already started the interior build-out and we’ll continue that build-out through 2025. We should have permanent power in fall 2025,” Ackerman says.

In the second quarter of 2026, a seven-month testing and commissioning period will begin, followed by commissioning and hand over during the last 30 days.

“This is a leap in technology—from something that maybe would have been implemented 70 years ago to the most modern types of technology,” Stinson says. “As we went through the process of the design, we had such an engagement with the operators. These are the folk that have to be comfortable with how these technologies operate, they have to maintain and operate them for the next how many years. This plant will be operating for well over the next 100 years, with some mechanical and equipment maintenance and upgrades.”