U.S. builders churn out hundreds of millions of tons of construction and demolition (C&D) debris every year, but there are only so many places to send it; options for fines are most limited.  Increasingly, municipal solid waste (MSW) landfills are rejecting C&D fines because a fair fraction is associated with hydrogen sulfide (H2S) spikes. Even in low concentrations the colorless gas generates toxic emissions, odors, and risk for fire as it’s highly flammable.

Meanwhile, MSW landfill operators are seeing H2S concentrations in the thousands to 30,000 parts per million (ppm) (up from about 20 to 40 ppm about 10 years ago). Typically, this trend is emerging among the few sites that take C&D waste.  

Drywall is the main problem. It leaches into waste, and its sulfate content reacts with water and organics, generating H2S. With increasing precipitation and landfills starting to recirculate leachate, H2S concentrations rise more.

“We are seeing more landfills turn these fines down. There used to be ten or twelve landfills in the midAtlantic that [our company] would send them to. Now we are down to two,” says John Thomas, president of Hainesport Holding, a New Jersey-based waste company that processes C&D fines from its own facility and also transports them for disposal for 13 processors. Most of it goes to a county-owned MSW and C&D site in Pennsylvania and New Jersey. The fines are used for alternative daily cover, road building, and solidification of wet waste.

The 2-inch-plus pieces are causing the issues, but processors like Thomas and some of his clients are finding that if they separate out the smaller pieces – no more than ¾-inches – it makes a huge difference. By this point they have isolated aggregate, or stone, sand, and other tiny particles that are devoid of drywall. Independent labs confirm that ¾- inch particles have about half the sulfate as two-inch fines, Thomas says. But getting down to even two inches is expensive, and catching the smaller particles is pricier yet.  

“Facilities do not want to spend the money to break it down [processing systems range from $4 million to $14 million]. And there is no return because they still pay to landfill it.

You have to buy more screens, more equipment, and spend more time,” says Thomas who would not divulge how much he has invested. But he says his organization has spent “countless hours and money” in processing, researching, testing, and experimentation of “responsible, affordable disposal options” for fines containing sulfate.

Still, money has been a barrier for the industry in general. Two of the largest processing facilities in the MidAtlantic region stopped making screenings altogether.

“They decided it didn’t pay to process it anymore. They just ship it out as bulky C&D waste,” Thomas says.

Dane County’s MSW landfill in Wisconsin takes C&D waste, which is also processed it at an on-site facility. The operation generates about 20,000 tons per year— a combination of multiple C&D materials (dirt, rubble, small pieces of drywall, wood, etc). About 80 to 85% of the fines are beneficially used on site, primarily for alternative daily cover and for building roads inside the landfill footprint.

Some is buried, with methods to circumvent problems—with a focus on placing C&D fines in ways to minimize their interaction with organics and other wet waste.

“We can place it in a thick layer and also near the outer edges of the landfill. This reduces the surface area of fines that are in direct contact with wet MSW. And by placing them on the outer edge they will not be exposed to leachate seeping down from waste above it,” says John Welch, director of Dane County Department of Waste & Renewables, Madison, WI.

Application of intermediate cover soil soon after placement further reduces exposure.

Welch points to an effective gas collection system as the most powerful precautionary tool. Dane County typically installs gas wells in a new area within 12 to 18 months of placing waste, earlier than industry standard practice.  The collection system monitors several gas constituents, including H2S, at multiple locations with automated well heads, involving a computer algorithm to make adjustments several times a day.

“We initially had fairly significant hydrogen sulfide odor problems.  But we began to employ several mitigation strategies, and now have a very low number of odor complaints, equivalent to what we had prior to accepting C&D fines,” Welch says.

Thomas’ focus has been to encourage transfer facilities and processors to get fines below ¾-inch size in order to cut sulfate so debris can be safely landfilled.

It involves a processing line system where incoming material is dumped in a hopper that runs across a sorting line, isolating materials like wood, carboard, and plastics, as well as drywall. The roughly 2-inch C&D fines continue to the end of the line. They are contained in a vibratory screen that shakes out the ¾-inch particles, which drop to a lower screen for further processing and separation. 

The C&D recycling industry is exploring ways to divert fines from landfill and develop products from them. Right now, at least in the U.S., there are few accepted beneficial use options.

Pennsylvania has allowed their use in concrete to backfill coal mines at pilot scale.  In the midWest they have gone into nonstructural building materials that are typically used for applications like decorative pavers and concrete for driveways and sidewalks. But there are no truly commercially sustainable applications, Thomas says.

Canada is a different story. Regulators allow these materials’ use in multiple applications.

SANEXEN Environmental Services, a Quebec-based environmental firm that does design, remediation, and construction work, makes products from C&D fines after removing contaminants.

The company creates compost, aggregate for parking lots and asphalt, as well as sends wood to plants that use it for power.

Mathieu Germain, director of Strategic Development, SANEXEN Environmental Services, describes the company’s technology as a two-step process: mechanical sorting to extract materials, followed by a biotreatment that’s inspired by a traditional treatment of contaminated soil.

SANEXEN’s work in this space began when it converted a facility that used to treat soil to now be able to process C&D fines; and it established a group to analyze them and develop beneficial uses. Its clients are recycling facilities that produce C&D fines.

“Almost 20 percent of inbound material received by C&D facilities results in fines that used to go to landfill (based on our internal numbers). We produce new materials that add value. In the case of compost, we actually use the gypsum from drywall as a nutrient, so it is becoming an asset rather than a problem,” Germain says.

The company can take up to 150,000 metric tons a year and processes all that it receives. So, he says, “technically our solution is zero percent use of landfill.”  

Still, the massive tonnage of C&D fines generated through North America becomes waste with high processing and disposal costs.

Thomas is concerned that this scenario could lead to recyclers choosing to stop making screenings altogether, which would drive demand for more C&D landfills, and hikes in disposal costs as airspace will be at a premium if the material isn’t reduced to fines.

For now, his operation is among a few in the U.S. looking at different technologies to render fines as safely disposable at MSW landfills, such as encapsulating the bacteria contained in them to mitigate odor emissions.

As Dane County continues receiving these materials, Welch has advice for other MSW landfill operators considering doing the same.

“Be prepared for elevated H2S in your landfill gas and be prepared for it to be noticeable sooner after placing waste than you would typically expect to notice landfill gas.  High H2S can lead to significantly increased costs for necessary operational changes, improvements in the gas collection system, and landfill gas processing costs.”