Effect of the Addition of Biochar and Coffee Grounds on the Biological Properties and Ecotoxicity of Composts

Propose The aim of this study was to investigate the effect of the composting process, conducted with the addition of various substrates, on the biological and ecotoxic properties of composts. Methods Composts were prepared on the basis of plant material amended with biochar, sewage sludge, coffee grounds, or yeast effluent. The composting process was carried out for 140 days. The biological activity of mixtures was determined in the process, while microorganism abundance and ecotoxicity, in the final product. Results The addition of biochar had no significant effect on the rate of the composting process measured by temperature; however, biochar-amended treatments showed a smaller loss of dry matter and higher C:N ratio. Compared to the control, reduced biological activity measured by the germination capacity of seeds was identified in treatments with maize straw composts and composts enriched with coffee grounds and yeast effluent. The inhibitory effect on the germination capacity was alleviated in treatments with biochar. The addition of biochar to compost with sewage sludge decreased the abundance of Escherichia coli and Salmonella ssp. and increased the total number of bacteria and fungi. In the case of composts with coffee grounds, biochar had an opposite effect to that observed for composts with sewage sludge. Conclusions Biochar to sewage sludge and coffee grounds reduced ecotoxicity of composts to test organisms: Vi brio fischeri, Sianpis alba, Triticum L. and Eisenia fetida.