Converting fines and less desirable parts of the wood stream into marketable products is a constant challenge for C&D recyclers. However, new research from the University of Florida shows the potential benefits of using biochar and biochar fines in landfills to neutralize both odorous hydrogen sulfide (H2S) and dangerous per- and polyfluoroalkyl (PFAS) substances.
Timothy Townsend, professor in the Department of Environmental Engineering Sciences at the University of Florida, gave a talk on March 22nd at the Construction & Demolition Recycling Association (CDRA) C&D World Convention entitled “Opportunities with C&D Fines and Biochar obtained from wood ”.
Townsend, whose research focuses on identifying the beneficial uses of waste materials, has partnered with the CDRA to specifically identify opportunities for C&D wood and fines.
C&D wood is usually obtained from lumber, pallets, clearing material as well as wood-based materials and treated wood. According to Townsend, sizing lumber, or what recyclers refer to as A-line lumber, can often be sold without much difficulty to those who make landscape products or other markets. However, engineered wood and treated wood can be more challenging due to the possible presence of adhesives, paints, and chemicals.
Fines, which are the shaky by-product of screening and sorting processes, typically consist of a variety of materials. Common markets for fines include use as landfill cover, filler material, and as soil and agricultural alterations. Similar to C&D wood, however, fines can be difficult to market because the composition and quality varies depending on a recycling company’s waste stream and screening and sorting processes.
“If you want to market fines as landfill cover and there is gypsum in the fine composition, you can produce hydrogen sulfide, which limits the number of areas across the country where this material can be used,” says Townsend.
“Regarding the use of fines as filler material, I’ve spent a lot of time with CDRA members trying to see if there is a way to make those fines a better product,” he continues. “I want to encourage recyclers to look for ways to process and manufacture better products. I think there is an opportunity on the horizon to create fines that are a stronger material that could be suitable for a base coat or a more structural material, or even material that is more suitable for agriculture. But the big challenge, if, like all waste products, you want to use fines in the environment, there can be small amounts of chemicals like arsenic, lead and sulfate that can be a cause for concern. “
Because of these environmental issues, Townsend says recyclers can run into problems when trying to market fines “as is.” However, once recyclers understand what concerns they may have, investing in new technology can help them create a better product. He says this will require recyclers to change their mindset from “managing by-products to developing products”.
Townsend says air separation, screening and washing processes can be particularly useful in separating the concentrations of aggregates, wood, paper, plastic and drywall in the fines. By better sorting this material, recyclers can market fines in a way that can help address regulatory challenges.
Once recyclers fully understand the makeup of their fines, they can make strategic decisions and investments to optimize that mix and target different end markets.
By being more selective about the waste they process, the size fractions produced, and the use of washing and blending solutions, operators can overcome some of the limitations often found in finding markets for C&D fines, Townsend said.
Benefits of biochar
Townsend says recyclers have started converting less salable wood from their B and C lines into biochar. Biochar, a char made from the pyrolysis of wood in a low-oxygen environment, can be used alone or with fines in landfill cover applications to reduce the H2S production that occurs naturally in landfills and in some gypsum-containing fines.
According to Townsend, his research has shown that the more biochar you add to fines, the more H2S you bind. However, he admits that a mixture of 90 percent biochar and 10 percent fines, for example, defeats the purpose of trying to find economic use for fines.
He says a blend of 15 percent biochar reduces H2S by 1 to 2 orders of magnitude in laboratory tests. At 30 percent biochar, H2S is reduced by 2 to 3 orders of magnitude or more. The sweet spot for economical and effective use is likely to be 10 to 20 percent biochar blends, he says.
Townsend said fines used as cover material in humid climates can tend to produce more hydrogen sulfide than in drier areas because moisture levels can worsen H2S production in drywall fines.
In initial field tests, in which fines were used as an alternative daily cover in landfills, Townsend found measurable H2S concentrations on a regular basis, albeit in small amounts. However, when the fines were mixed with biochar, the H2S values in the field tests were evenly below the detection. Townsend admitted that these studies, which were delayed due to COVID-related shutdowns at the university, were still ongoing.
A word about PFAS
According to Townsend, biochar is not only showing great promise for slowing down H2S production in landfills, but also showing promise for the capture of PFAS chemicals. These chemicals have been scrutinized by the EPA and other regulatory agencies in recent years for their health risks, which include liver damage, thyroid disease, and cancer, among others.
“It’s not much fun talking about PFAS because it doesn’t have a lot of good implications because it’s a new chemical. [It’s like] If only we discovered lead-based paints or asbestos or something like that, ”says Townsend. “For the C&D recycling industry, PFAS isn’t going to be as big a role as these issues, but the idea is that it’s better to do some further training every now and then and think a little about how the industry can position itself with some of its markets to perhaps be able to address the problem. There are some in the industry who are already working on it. “
According to Townsend, these PFAS chemicals are “forever,” used to make a range of commercial and consumer goods such as non-stick cookware, waterproof clothing, stain-resistant fabric and carpet, and packaging and fire-fighting foam, in leachate found in every landfill in the world .
“If you look at C&D landfills, you’ll find PFAS in leachate – it’s not that different from normal MSW landfills. … In terms of bulkier C&D items where PFAS may be mattresses, furniture, clothing, tents, umbrellas, rugs, and carpet upholstery. The good news is that for concrete, drywall, and wood, most of the products that C&D recyclers deal with should have overall PFAS levels that are pretty low, ”he says.
Townsend says that while PFAS is not found at the same concentrations as in MSW streams, the EPA’s focus on identifying and mitigating the substance requires that C&D recyclers be at least informed about possible materials containing these chemicals can.
Townsend selects carpets, fines, and asphalt shingles as the main sources of PFAS that C&D generators should look out for.
When discussing how PFAS could affect the shingle recycling markets, Townsend said that recycled shingles in asphalt concentrations with hot mix would likely not cause a problem, whereas floor shingles used in pavement could pose the problem of pollution due to their potential to wash out the groundwater.
Despite PFAS’s environmental concerns, Townsend says there are some potential opportunities for recyclers.
“We know that biochar can help [sequester] H2S, but can PFAS take on it? Absolutely possible as many PFAS remediation sites across the country use activated carbon [that is similar to biochar] to remove that, ”he says.
During his presentation, Townsend shared laboratory results showing how biochar mixed with fines helped greatly neutralize the PFAS concentrations found in test samples. According to tests by the University of Florida, including biochar in a sample was equivalent to a reduction in PFAS found in leachate by more than 80 percent.
“Just like using biochar to neutralize H2S, C&D recyclers would have to invest in technology and equipment to make the biochar from a fraction of their wood stream [for PFAS mitigation], but then there might be an opportunity to market that as a product, ”says Townsend.
“Landfills need soil products. [MSW landfills] I need this because it’s part of Subtitle D. And in many cases they have to pay to get land because they don’t always have large loan pits, ”he says. “So imagine that the soils that recyclers could give them are made to handle the H2S problem, but also to absorb and trap materials like PFAS. … If you have integrated some of these fines into biochar or only used the biochar itself from C&D sources … you have the option of binding these chemicals. This may be more applicable in some markets than others, but for C&D recyclers it is something to think about. Biochar from C&D continues to show promise. “