On the island

A combination of site-specific factors makes for a unique situation at the integrated solid waste management facility run by PVT Land Company Ltd. in Waianae, Hawaii. The Honolulu County-based company operates the only construction and demolition (C&D) landfill on the entire island of Oahu, the most populated island in Hawaii. Most of the material on the 179-acre landfill consists of wood, concrete and unrecyclable construction debris.

The relatively clean waste materials make for an unusual condition at the landfill: The leachate there contains fewer contaminants than the groundwater swelling below the site, says Steve Joseph, PVT’s vice president of operations and director of planning and permitting.

“What’s kind of unusual about our leachate is we’re over brackish water, so the salt content and a lot of the cations and anions in our underlying groundwater are higher than they are in the leachate,” Joseph says. “So unlike other sites, the way I would know that we’ve got a leak is if the groundwater actually improved.”

Atypical situations call for atypical operations. Between seven landfill liner layers to prevent leakage—aptly called the “seven layers of environmental protection”—and a solar-powered water sump that collects leachate for dust control, PVT’s innovative leachate management system helped earn it Solid Waste Association of North America’s (SWANA’s) 2018 Landfill Management Excellence Gold Award (in tandem with OC Waste and Recycling’s Prima Deshecha Landfill in California.)

“I awarded PVT Land Company top points in environmental controls and monitoring for their ‘seven layers of environmental protection’ when it came to leachate management,” says Bob Watts, SWANA’s landfill management technical division awards chairman. “Every landfill site is unique, but their use of solar-powered pumps and leachate for dust control is also a sustainable solution to leachate management.”

Liner layers

In addition to accepting C&D debris, the PVT landfill is also Oahu’s only approved site for waste classified under the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA), the emergency management site for natural disasters and the disposal site for debris from the North Pacific.

The company is Hawaii’s largest recycler, accepting nearly 2,000 tons of C&D waste each day. About 1,200 tons of feedstock are processed daily in the facility’s “Big Green Machine,” a massive recycling system designed and installed by CP Group of San Diego. Whatever can’t be recycled—about 20 percent of the facility’s total debris—is stored in PVT’s landfill.

"Every landfill site is unique, but their use of solar-powered pumps and leachate for dust control is also a sustainable solution to leachate management.” -Bob Watts, SWANA’s landfill management technical division awards chairman

There, multiple layers of protection are rolled out to prevent leachate leakage. The first is a geosynthetic clay liner composed of a thin layer of high-density, processed bentonite clay sandwiched between pieces of synthetic fabric. Flexible, high-density polyethylene is placed on top of that in wide sheets hard as roofing shingles to prevent drainage. Next is a layer of geotextile fabric to block particles that would scratch or wear on the plastic.

About a foot of gravel covers those layers to allow liquid to drain out through a network of perforated HDPE pipes aligned in trenches beneath it. Gravity carries the leachate through the pipes to a sump, where it is then drained.

That layer is covered by another piece of geotextile fabric, followed by two more layers of dirt and soil. Together, the seven layers work to keep contaminants from sinking further into the ground while diverting leachate to one central location.

Joseph says that over the years, PVT has discovered one key element it adds before its dirt layer: coal ash. “The beauty of the ash ... is that once you put it down, if you don’t bust it down or drive over it, it has a slight cementitious quality to it, so it becomes nice and firm,” Joseph says. “But the great thing about it is it has about the permeability of beach sand, so the leachate goes right through it.”

The ash layer acts as a drain for the leachate to the impenetrable layers underneath, but it also assumes double-duty as a firebreak. Joseph says C&D facilities like his are especially prone to fires because of the flammable materials they accept, so he regularly injects carbon dioxide into the cells of his landfill, which penetrates straight through the coal ash and cuts off the high levels of oxygen that can contribute to fires. If a fire were to start, Joseph says the coal ash works to contain it to a localized pocket within the cell rather than allowing it to spread throughout the site.

Drainage

Joseph says PVT’s storm management system contributes to its leachate management as well. Controlled grading on the landfill cells, along with an engineered system of drainage ditches, channels, pipes and six sedimentation and retention basins, is designed to contain up to a 100-year flood and minimize the amount of leachate generated. With that system in place, Joseph says the landfill pumps only about 1,000 gallons of leachate a month.

The water that does seep through the debris is carried by the HDPE pipes, which are designed to store and pump up to 75,000 gallons of liquid in a 10-day period, to a sump in the west central part of the landfill. The sump is too far from power lines to run on electricity, so PVT installed a more sustainable solution about four years ago: solar panel-powered pumps manufactured by Grundfos Pumps Corporation, a Denmark company with U.S. headquarters in Brookshire, Texas.

“The panels work out perfect. It’d really be a challenge to get power out to [the sump],” Joseph says.

The solar panels’ practicality has proven to be just one of their many benefits. With PVT’s hurricane-prone location, Joseph says the panels are more reliable during rushing storms than electricity—even if some of them fly off in the wind, as 15 of them did during Hurricane Lane in August 2018. If the working pumps do happen to fail, PVT maintains a backup system that can replace the broken pumps while they’re out for repair.

Joseph says the solar power has already provided economic benefits too, as PVT also uses them for groundwater monitoring and to power office operations. Having no generators, compressors or extra infrastructure results in minimal service requirements. “All together, the use of solar systems has led to a significant reduction in overall utility costs—important in a state where the kilowatt price for businesses is more than double other states in the nation,” PVT noted in its SWANA award application.

Dust control

Though a small quantity, the leachate PVT produces doesn’t go to waste. It’s used as a solution to one of the landfill’s biggest problems—dust. The island’s trade winds and moderate aridity constantly kick up dust that PVT controls, in part, by spraying it with leachate.

“Our leachate is so clean, it allows us to use it for dust control. It would improve the groundwater,” Joseph says.

The small amount of leachate PVT produces only comprises a marginal portion of the 50,000 to 100,000 gallons of water PVT uses a day for dust control. But when using that much water, every drop counts. Joseph says the leachate supplement allows him to use a smaller holding tank that can be emptied at more convenient times. The use of leachate for dust control has been “much more cost-efficient,” Joseph says.

Gaining permission to use even that small amount didn’t come easy, though. Joseph says he spent years trying to convince the state department of sanitation that the water was clean enough to reuse. It wasn’t until Joseph brought in independent analyses of the water that the department finally consented.

“Because it’s leachate, they had this preconceived notion instead of looking at the numbers and looking at the risk. It was an uphill push to be able to do it,” Joseph says. “To do this, you really need to do a risk assessment on it. That’s the best way, because then you have science working for you.”

PVT now maintains a staff of two geologists, one environmental engineer and one environmental technician for monitoring and maintenance purposes of all the landfill systems, including leachate. In addition, an independent third-party hydrologist—required by the state to ensure impartiality—takes samples of groundwater once a quarter. PVT’s SWANA application says groundwater monitoring conducted from 1998 to present has indicated no impact to groundwater from the landfill dust control operations.

“Leachate was the perfect choice to use for dust control, especially since my pumped water was worse than my leachate,” Joseph says. “It didn’t make sense to do it any other way.”

The author is the assistant editor of Waste Today magazine and can be contacted at tcottom@gie.net.