10. Biochar and contaminants

Contaminants from biochar can either be from the feedstock or formed during production. It is possible that some are present in the ash which can be removed by leaching if necessary.

Some feedstocks are more likely to contain contaminants than others for example using Municipal Solid Waste (MSW) or sewage sludge. Rice husk is likely to contain low levels of contaminants, although residues from chemical fertilisers, pesticides and fungicides sprayed prior to harvesting may be detectable.

PAHs: Results from 6 samples of rice husk in 3 regions of Cambodia showed very low levels of the USEPA (United States Environmental Protection Agency) 16 PAHs (Polycyclic Aromatic Hydrocarbons) of particular concern, including the carcenogenic benzopyrene. Biochars tested in the UK from virgin biomass have contained lower levels than in urban soils.

Dioxins: Likely to be formed if feedstocks contain high chlorine levels.

Heavy metals: This contaminant comes from the feedstock, so MSW for example can be a cause for concern, but virgin biomass is unlikely to have high concentrations.

During application, Silicosis is a risk in particular from feedstocks with high silicon content (for example rice husk) so human health should be considered when handling biochar.

Once the biochar is in the soil it is inert, and will not chemically react to cause the production of any harmful substances.

More work will be done on the safety of the biochar process, looking at airborne emissions, tar and sludge production during this project.

Despite the low risk to human health and the environment, especially looking at the scenarios for agricultural residue biochar in Cambodia, it is essential that the risks are considered, and that regulations and best practice guidelines cover all aspects of production, handling and application. This will be a project outcome.


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