Exploring different options

Photo courtesy of Sanexen

With any kind of recycling process, the byproduct left behind has historically puzzled processors wondering about what to do with it. This is especially true with the recycling of construction and demolition (C&D) materials, where C&D fines have continued to pose a challenge because of limited end markets.

Currently, C&D fines are most commonly used as alternative daily cover in landfills or as structural fill; however, more opportunities for the material have emerged as processors look to push the boundaries of a once inconvenient waste product.

At this year’s C&D World in Atlanta, organized by the Construction & Demolition Recycling Association (CDRA), conference programming highlighted some of the new opportunities for C&D materials during a session titled Continued Exploration: Markets of C&D Debris.

Led by Timothy Townsend, Ph.D., a professor in the department of environmental engineering sciences at the University of Florida, the session covered new advancements and market changes for C&D fines.

“It feels like we’ve been talking about [C&D fines] for years and years, and in a lot of cases, it’s [processors] wondering what they can do with the leftover material at the end of the [recycling] process,” Townsend said.

Thus far, slightly altering some aspects of the screening process has proven to yield the greatest success in creating new end markets for fines.

“We know that if you take C&D fines, and if you screen them a certain way, you can make the outcome a little bit different,” Townsend said, noting that a reduction in lead content was possible, for example. “So, there are opportunities, potentially, if you know your product [well enough] to be able to process that material in a way that you now have a material that you can go out and have an end market for.”

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Mixing it up

Given the variety of materials found in C&D fines, including soil, wood, concrete, drywall and rock, it can be difficult to create a consistent product that is suitable for use on large-scale projects. Many of these material streams are sourced from diverse job sites, which can leave recyclers with varying fines compositions.

To tackle this dilemma, equipment vendors have launched “all sorts of different [technology] and technical approaches for things like screening, washing, pulverizing and composting” to provide different options for hard-to-recycle materials such as fines, Townsend said. While these advancements have given rise to more standardized fine byproducts, material blending is quickly gaining traction as a potential recycling method.

“I’m seeing this more and more with all the other waste products that we deal with and try to find markets [for],” Townsend explained. “One [question] is ‘How do you combine one waste with another waste product?’ The other is ‘How do you combine a waste product with a virgin product to be able to create something that’s ultimately going to [be] a future desired end product?’

“Whether you’re talking about the different soils we get from C&D, aggregates, organic mulch products or things like crushed shingles, we’re looking to create a product that is going to provide a reliable … and economic end [market],” he added.

In some cases, waste management firms have used blending methods to reduce the leachability of certain materials in landfills. The most prominent example of this is the production of hydrogen sulfide (H2S), which is emitted from the gypsum found in fines. The colorless gas can be toxic, produce odors and present a risk for fires even in low concentrations.

Finding solutions

At the C&D World 2021, Townsend first discussed his research surrounding the benefits of using biochar-blended fines in landfill applications to neutralize not only H2S but per- and polyfluoroalkyl substances, or PFAS, as well.

His research has shown that, in theory, the more biochar that is added to the fines, the more H2S that is sequestered. However, he conceded that given the propensity for landfill material composition to vary— especially at landfills that accept C&D debris—this is not always the case.

Townsend said a mixture that contains 15 percent biochar has been found to reduce H2S production by one to two orders of magnitude in lab testing. At 30 percent biochar, H2S production is reduced by two to three orders of magnitude or more.

“Hydrogen sulfide generation is a very complicated process, and it’s very finicky,” Townsend said at this year’s C&D World. The sweet spot for economical and effective use of biochar lies in blends that range from 10 percent to 20 percent, he added.

To better assess the ability of biochar-blended fines to reduce H2S emissions, CDRA-sponsored research has included the construction of small, simulated landfills that are filled with different amounts of biochar and C&D fines.

“When we analyzed the results, the [simulated landfill] that produced the most hydrogen sulfide actually had the most biochar in it,” Townsend said of the research. “There was a certain point where a little bit of biochar seemed to stimulate the activity and make the environment better for the hydrogen sulfide to be produced in the first place.

“It wasn’t until you reached a certain threshold amount of biochar that it actually got lower. So, it’s a much more complicated story than being able to say, ‘Yes, you add the biochar to the landfill … and you’re not going to have hydrogen sulfide production anymore.’”

Townsend said a follow-up study is underway at a landfill in Sarasota County, Florida. Researchers have added biochar to the surface of the landfill to determine how much of a reduction in H2S is effectively achieved.

“On a small scale, it works,” he said of mixing biochar with C&D fines. “But, in practice, with the complications and heterogeneity, it’s been very difficult to show a large-scale impact [where] this is going to be a solution to the hydrogen sulfide problems from C&D fines in a landfill.”

The author is associate editor for Construction & Demolition Recycling and can be reached at hrischar@gie.net.