In most cases, a biochar product has a fixed service life. Biochar product disposal (or end-of-life) refers to all handling operations that may occur until the materials are reused in other products (e.g. re-use in soil masses) or disposed in a final application (e.g. landfilling or soil amendement).
The biochar product disposal plays an important role because it is the phase that guarantees the biochar carbon storage over multiple decades to centuries (i.e. biochar is not burnt). The environment in which the biochar is placed also influences its permanence.
A disposal option here refers to a way of managing a product that has reach the end of its service life. Disposal options can be either temporary disposal options such as recycling or re-use, or final disposal options such as incineration, landfilling, or final application to soil.
Recycling & re-use: a biochar-product is re-used in another similar product or re-processed to make a different product. An important notion here is the cascading uses of biochar, which was presented in the work of Wurzer et al (2016). To maximise the environmental and economic value of biochar, multiple uses can be designed sequentially. For instance, Wurzer illustrated the case where biochar can first be used in water or gas filter applications, before being treated and use for animal manure additive, and eventually being spread onto agricultural land. Another example, in urban areas, biochar can first be used in green roof substrates, then re-used in other green roofs, before being used as landscaping soil in parks.
Incineration: if biochar is incierated, all its carbon is returned to the atmosphere and cannot be considered a long-term carbon sink. This said, incineration may still be relevant when biochar has been used for applications that make it unfit for any other disposal option, e.g. treatment of highly toxic substances. In many instances, biochar producers and users who value the carbon removal properties of biochar want to guarantee that their product are not sent to incineration.
Landfilling: biochar can end-up in landfills whenever soil products are disposed off. It is usually assumed that biochar carbon permanence in landfills is similar to soil environments, but not studies are available on the subject. A typical example of biochar ending-up in landfills would be biochar used in construction materials like concrete or facade materials.
Final soil application: application to soil is considered a final disposal option because it is virtually impossible to separate biochar from the soil matrix after application.
Dispersion in the environment: in all cases, during biochar use and its disposal, a fraction of the biochar can get dispersed into the environment. For instance, biochar used in a green roof substrate can move to the water via erosion and runoff. Likewise, biochar used in asphalt can evolve in the surroundings of the roads as the asphalt layers wears off.
Most biochar LCA studies have considered that biochar is used in soil application. In some cases, biochar was assumed to be sent to landfill or incinerated for energy recovery.
Cascading use of biochar in animal husbandry was investigated, but recycling of other biochar-based products was not yet studied.
A key uncertainty is related to the lifetime of the products that is unknown and the novelty of these products: biochar products are still recent, and the first products that have been used are still in-use. Therefore, only assumptions by experts can be made on their service life and the disposal options.
Pyrolysis can also be thought as a disposal option for biomass and biomaterials. For instance, wood products which cannot be recycled can be turned into biochar.
Likewise, any kind of plywood boards, bambo ustensils, plant-based textiles and similar biomaterials can be turned into biochar when they reach end-of-life.
Unlike municipal solid waste incineration, which usually implies mixed waste streams, pyrolysis as a disposal option would usually apply to source-separated waste streams.
To explore the topic further, we recommend the following references:
Azzi, E. S., Karltun, E., & Sundberg, C. (2021). Assessing the diverse environmental effects of biochar systems: An evaluation framework. Journal of Environmental Management, 286, 112154. https://doi.org/10.1016/j.jenvman.2021.112154
Azzi, E. S., Karltun, E., & Sundberg, C. (2022). Life cycle assessment of urban uses of biochar and case study in Uppsala, Sweden. Biochar, 4 (1), 18. https://doi.org/10.1007/s42773-022-00144-3
Thompson, K. A., Shimabuku, K. K., Kearns, J. P., Knappe, D. R. U., Summers, R. S., & Cook, S. M. (2016). Environmental Comparison of Biochar and Activated Carbon for Tertiary Wastewater Treatment. Environmental Science and Technology, 50 (20), 11253–11262. https://doi.org/10.1021/acs.est.6b03239