Gaseous Emissions During Composting of Biowaste Digestate with Different Biochar Ratios at Laboratory Scale

Abstract Digestate from organic source-selected municipal solid waste is a material that has experienced an exponential growth in its production, due to the current boost of anaerobic digestion. Composting is an environmentally-friendly technology required by EU legislation for the proper management and valorisation of this waste prior to its application to soil as organic amendment. Although some studies have examined the impact of biochar on the composting of raw organic waste and its resulting emissions, the effects of biochar on the composting of low-biodegradability organic waste, such as digestate, have rarely been investigated. In this work, biochar from forestry biomass was tested at different application ratios as a material to minimize the gaseous emissions from biowaste digestate composting, including: ammonia, methane, volatile organic compounds (VOC) and odours. Biochar composting ratios were 5%, 10% and 15% (based on digestate dry matter) together with a control treatment without biochar at bench scale (10 L) during the typical times used in industrial scale digestate composting (12 days). The addition of biochar resulted in a reduction of up to 95% of methane, 60% of total VOC and 70% of odour units emissions, being these numbers obtained with the highest application ratio of biochar (15%). On the contrary, ammonia emissions increased significantly threefold with 5% biochar, according with the high process temperature reached. In conclusion, biochar has the potential to reduce the emission of certain pollutant gases during digestate composting. However, further large-scale experiments are needed to investigate the effect of composting mass on temperature and, consequently, the quantity and composition of gaseous emissions. Graphical Abstract