Both compost and biochar production are methods to utilize and recycle organic wastes.
Although both biochar and compost use organic wastes as feedstocks, the two operations do not have to be an either/or option; instead, they can be combined for synergistic production and utilization. For example, many materials that make good compost, such as food waste and wet manures, are not easily used for biochar production since a large amount of heat would be needed to dry the materials prior to producing biochar. Ideal feedstocks for composting have from 60 – 70% moisture, high nutrient levels, and low lignin content. Ideal feedstocks for biochar have 10 – 20% moisture and high lignin content, such as field residues or woody biomass.
Based on current findings, the benefits of adding biochar to the composting process may include shorter compost times; reduced rates of GHG emissions (methane, CH4 and nitrous oxide, N2O); reduced ammonia (NH3) losses; the ability to serve as a bulking agent for compost; and reduced odor. For the biochar material itself, undergoing composting helps to charge the biochar with nutrients without breaking down the biochar substance in the process.
A wide range of biochar application rates to compost have been tested, from 5 – 10% to 50% (mass basis) or higher. At adequate doses, biochar has been found to accelerate the composting process—mainly through improving the homogeneity and structure of the mixture and stimulating microbial activity in the composting mix. This increased activity translates to increased temperatures and a shorter overall time requirement for compost development. This may have important economic implications since accelerated composting is a desirable effect.
One challenge to compost operations is the loss of nutrients and the emission of GHGs during the composting process—specifically CH4 and N2O. Adding biochar to compost has been found to reduce these emissions by some studies; however, other studies have shown that biochar has no impact on the overall GHG emissions since these emissions were offset by the enhanced microbial activity on the composting mix containing biochar.
The porous nature of biochar can reduce the bulk density of compost and facilitate aeration in the composting mix. For compost feedstocks that are high in nitrogen (N), such as animal manures, biochar offers the opportunity to reduce the overall N loss over the process, especially that of ammonia.
Although initial publications show measurable benefits on the impact of biochar on composting, the number of studies is still very limited. Many of the traditional indices used for evaluating the quality (‘stability’) of compost (e.g., the carbon/nitrogen, C/N, ratio) are not valid when biochar is included in the mixture, since biochars have very different properties than the rest of the composting material (e.g. high C/N ratio, which will not decrease during the composting process in contrast to the remaining organic material) and may even influence compost quality assays if controlled for biochar (e.g., water-soluble C may adsorb to biochar). Establishing compost quality indices that take the benefits of biochar into account could help increase the commercial use of this activity.
Increasing the use of biochar in compost operations requires education on the benefits of biochar to producers, not only on emissions and odor reductions, but also on the potential economic benefits of accelerated composting time to offset the additional price of producing/purchasing biochar.