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US (WA): Turning urban wood waste into biochar

Biochar as a soil amendment has been the subject of much attention in recent years because of its ability to sequester carbon and to improve aggregation, water holding capacity, and organic matter content of soil amended with it (Lehmann, 2007; Marris, 2006). A recent article on the Ag Climate Network blog from Oregon State University discusses what’s needed to economically produce forest to farm biochar. In contrast, researchers at Washington State University are investigating what we could call waste to farm biochar. Waste to farm biochar, if deployed on a larger scale, could offer a two-part benefit – removal of wood from the municipal solid waste stream and creation of a valuable product from this wood. In recent work, researchers are looking at two possible wastes that could be made into biochar: wood-based fractions of municipal solid waste and the large woody material remaining after compost production—referred to as “compost overs.”

Figure 1: Images of the woody biomass sources used to create biochar for this project, including compost overs and wood-based products from municipal solid waste. (source: WTFT 2015-2017 report; photo credit: M. Ayiania)

Figure 2. Michael Ayiania used a spoon pyrolysis reactor to create small samples of biochar from urban wood residuals. (photo credit: R. Esquivel-Garcia)

Wood-based products in municipal solid waste are extremely varied and include materials such as plywood, particle board, and furniture (Figure 1). They also include wood that has been painted or treated, which has the potential to introduce environmental pollutants that can have detrimental effects on crops and soils. Michael Ayiania, a Ph.D. student working with Dr. Manuel Garcia-Pérez in the Department of Biological Systems Engineering, has worked to characterize biochar produced from urban wood waste and compost overs and to examine potential pollutants—heavy metals, polycyclic aromatic hydrocarbons, and volatile organic compounds—in biochar produced from these municipal sources.

Biochar is made through a process called pyrolysis, which is a thermal conversion in the absence of oxygen. For this project, Ayiania prepared samples by grinding and sieving them, and then used a machine called a spoon pyrolysis reactor (Figure 2), set at either 400°C or 600°C, to create small batches of biochar from each of the feedstocks (Figure 3).

Figure 3. Biochar resulting from three forms of urban wood waste examined in this study: compost overs, molded wood pallet, and particle board. (photo credit: M. Ayiania)

Ayiania investigated the fundamental characteristics of the resulting biochar that affect its usefulness as a soil amendment: fixed carbon and ash content, surface area, and micropore volume. Fixed carbon is the fraction of biochar that can persist in the soil for thousands of years. It is desirable for biochar used to amend soil to have a high fixed carbon content. A high ash content can also benefit cropland by raising the pH of acidic soils. Surface area impacts the reactivity of biochar; the greater the surface area, the more chemical interaction with the soil per gram of biochar. Micropores (Figure 4) contribute to the surface area of biochar and affect its ability to adsorb small molecules. For all feedstocks, the biochar with the best characteristics for use as a soil amendment (high fixed carbon and ash content, surface area and micropore volume) was obtained at 600°C.

Figure 4. Micropores in biochar vary based on feedstock type and pyrolysis temperature. Shown are electron microscopy images of biochar made from more typical feedstocks: Douglas fir wood, Douglas fir bark, and hybrid poplar (not included in this study). (photo credit: Waled Suliman)

In this project, concentrations of metals beyond the thresholds established by the EPA for land application of biosolids were found in painted wood (mercury) and treated wood (arsenic). Ayiania also monitored soluble organic compounds (in this case, compounds called polycyclic aromatic hydrocarbons – carcinogenic compounds that can be produced during pyrolysis that have a detrimental effect on soil quality and plant growth), but none were found in biochar from any of the feedstocks.

The presence of metals in soil amendments is particularly problematic because of their tendency to build up in soil with repeated applications. To address this potential problem, Garcia-Pérez’s team experimented with “washing” the biochar to remove the metals and had promising results. Overall metal levels were reduced by 94% and 82% with hydrochloric acid and acetic acid washes, respectively, and levels of all metals were brought under the EPA thresholds.

This study confirms that the wood-based fractions of municipal solid waste and compost overs can be used as feedstocks for producing biochar as a soil amendment—waste to farm biochar—especially if produced at high temperatures. However, the biochars resulting from some treated and painted wood materials will have to be acid washed to remove some of the heavy metals if they are to be land applied and further work will be needed to scale up the acid washing process.

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