Greenhouse gas emissions from anaerobic and aerobic treatment of biowaste and green waste

This study examines biowaste and green waste treatment in Germany, focusing on climate-relevant emissions from composting and anaerobic dry digestion (AD). Emissions from various sources (Combined heat and power (CHP) unit, leaks, exhaust air treatment, and non-gas-tight digestate storage) were analyzed on-site, paying particular attention to different phases of the composting process. Quantification of the rotting emissions was performed using an Open Dynamic Tunnel measurement system. Inadequate conditions during composting and post-rotting of digestates can lead to significant greenhouse gas (GHG) emissions, including methane (CH₄), nitrous oxide (N₂O), and ammonia (NH₃). Emissions were assessed at five open and two enclosed composting facilities, as well as two batch-operated and three continuously operated AD plants. Total plant-related emissions varied widely, with emission factors (EF) ranging from 22.9 to 574.0 kg CO₂-equivalent (eq) Mg^-1 wet waste (ww), while rotting process emissions ranged from 16.6 to 318.6 kg CO₂-eq Mg^-1 ww across four measurement campaigns. Rotting processes account for the majority of total emissions, with the average emission distribution indicating that CH₄ represents the largest share at 66.4 % (71.2 ± 14.5 kg CO₂-eq Mg^-1 ww), followed by N₂O at 31.6 % (34.5 ± 14.5 kg CO₂-eq Mg^-1 ww), while NH₃ constitutes the smallest fraction at 2.0 % (1.2 ± 0.3 kg CO₂-eq Mg^-1 ww). GHG emissions during composting were only significantly influenced by feedstock materials, with digestate showing higher emissions than biowaste (p < 0.05). Other parameters such as windrow height, structural material mixing ratios, and turning intervals had no significant impact (p > 0.05).