Full-scale co-composting of low-biodegradable digestate from the source-selected organic fraction of municipal solid waste with wood-derived biochar: Insights on process performance, gaseous emissions, and engineering implications

A full-scale trial was performed to examine the effect of wood-derived biochar on the composting of digestate from the anaerobic digestion of source-selected organic fraction of municipal solid waste (OFMSW), compared with that of only OFMSW digestate. Both materials were separately composted in forced-aerated tunnels for 10 days during which process performance and gaseous emissions were evaluated. Despite the anticipated limited biodegradability of digestate, composting with biochar allowed the pile to reach a sustained temperature close to 70 °C (for 4 days), whereas this was not achieved in the control pile in which temperature rapidly dropped (after day 2). This resulted in the fulfilment of sanitation regulatory requirements of the composting process when using biochar, yet at the cost of an 86 % increase in ammonia emissions due to the higher temperature and the associated increase in aeration based on the SCADA temperature control system. Cumulative methane (0.45 vs. 0.32 kg CH 4 Mg -1 OFMSW digestate) and volatile organic compounds (VOC) (2.87 vs. 0.77 kg C-VOC Mg -1 OFMSW digestate) emissions also increased from the biochar-amended pile with respect to the control. Regarding the final products, both composts appeared suitable as soil amendments based on Spanish compost legislation. The use of biochar in the composting of low-biodegradable waste, such as OFMSW digestate, at commercial scale contributed to heat retention, an effect that is often neglected in small-scale studies. These novel results are key for ensuring the sanitation of the final compost obtained from low-biodegradable substrates. Moreover, the necessity of conducting full-scale experiments has been elucidated to comprehend the mechanisms ruling the change in emissions resulting from biochar co-composting and to assess the sustainability of the process.