Shaken or Stirred? Testing Biochar Cocktails for Emission Reductions in Composting

By Karie Boone, Center for Sustaining Agriculture and Natural Resources, Washington State University

Washington State is working towards the goal set by HB-1799 to reduce organic waste taken to landfills by 75% by 2030. An important alternative destination for organic wastes—such as manure, yard debris, food waste, wood and garden waste—is composting facilities, which process otherwise landfilled materials that contribute to the potent greenhouse gas, methane, so that they can be reused and ultimately applied to agricultural fields. Existing compost facilities will need to increase the amount of organic waste they can process to achieve this goal. With the addition in some cases of food wastes or other “wet” organics, composting facilities may be at increased risk for emitting emissions and their associated off-site odors (which we discussed in an earlier article) that can upset residents, cause health problems and damage their reputation with the community they serve. That is where biochar comes in: the application of locally available biochar (a form of charcoal made from forest residuals or other dry biomass) to compost piles has the potential to reduce greenhouse gas emissions and odors from composting operations.

Piles of manure with a loader in front, with a conveyor belt above the piles.
A belt transporting compost to maturing piles. Photo: Veronica Crow

A recent Washington State University (WSU) study sought to understand the efficacy of biochar as a tool for reducing emissions and their odors at commercial-scale compost facilities. The project builds on previous work through the Waste to Fuels Technology Partnership, which assessed locally available biochar resources and experimented with their capacity to reduce ammonia (ammonia, when combined with other pollutants can affect human breathing), methane (a greenhouse gas contributing to global warming), and hydrogen sulfide (can cause a wide range of health effects depending on levels of exposure) emissions in the composting process.

An important point underlying this work is that biochars can have very different physical and chemical properties (and therefore different impacts) based on the feedstocks that they are made from, the conditions under which they are made, and any pre- or post-production treatments that are applied. Because the biochar that most effectively adsorbs each gas differs significantly, maximizing emissions reduction requires the development of unique biochar mixtures, or cocktails. Once designed, these cocktails could be deployed in exhaust filters, as toppers on compost rows, as biofilters, or could be co-composted with other organic material.

Sieve with biochar in the middle
A biofilter includes a biochar cocktail held by a sieve in a laboratory-scale composting experiment. Emissions from the composting unit filter up through the biochar-filled sieve. Photo: Veronica Crow

In the work described in the latest WTFT report, WSU researchers Veronica Crow and Manuel Garcia–Perez found that they could effectively use top-layered biochar to reduce ammonia, methane, and hydrogen sulfide emissions in a laboratory setting. Higher pH and ash content biochars were ideal for hydrogen sulfide adsorption, acidic (lower pH) biochars adsorbed ammonia effectively, and a biochar with a high surface area was most successful at methane adsorption. For both ammonia and methane, some locally available biochars tested in this study outperformed adsorption capacities of engineered biochars reported in the literature. Locally available biochars did not adsorb hydrogen sulfide as well as engineered biochars reported in the literature, though the one that performed the best is an end-stage ash product from an existing industrial facility that is readily available in excess and could therefore effectively reduce hydrogen sulfide emissions if properly applied to the composting process. Crow and Garcia-Perez also engineered and tested their own acidic biochar in a laboratory-scale composting design and found that it reduced ammonia emissions by 35%.

The research team also calculated the quantity of biochar needed per gram of compost for targeted emissions reduction, an important question for cost effectiveness and, ultimately, for scaling up the application of biochar to commercial-scale composting processes. The low amounts of biochar required to adsorb emissions (0.01-0.6% by weight) are encouraging in terms of the ultimate feasibility of using biochar to reduce emissions and their odors from commercial-scale composting. Economic feasibility could also help stimulate markets, facilitate new uses of biochar, and build the Pacific Northwest biochar economy.

Additional details on this study, including full methods and results, are available in a technical report Reduction of Odors and Greenhouse Gases from Composting Processes using Biochar from Locally Available Bio-resources.

This work was funded through the Waste to Fuels Technology Partnership between the Center for Sustaining Agriculture and Natural Resources at Washington State University and the Washington Department of Ecology’s Solid Waste Management Program. This Partnership advances targeted applied research and extension on emerging technologies for managing residual organic matter.


Leave a Reply

Your email address will not be published. Required fields are marked *