Real-time monitoring of ammonia and greenhouse gas emissions from cattle slurry, digestate and its liquid fraction during storage with mitigation through acidification

Identifying effective ammonia (NH₃) and greenhouse gas (GHG) emission mitigation measures for stored animal slurry or anaerobic digestates is crucial to meet NH₃ emission reduction and net-zero emission targets. In this study, a high-resolution measurement was performed in a pilot-scale (900 L) slurry storage facility equipped with a flow-through system and photoacoustic analyser to; (i) quantify NH₃, CH₄ and N₂O emissions from cattle slurry (CS), anaerobic digestate (DG) and its liquid fraction (LFD), and (ii) evaluate the NH₃, and GHG emission reduction potential through acidification. The acidification began on day-8 using sulfuric acid with target pH of 5.5. During the 98-day slurry storage, DG and LFD emitted 47 % and 112 % higher NH₃ than CS respectively, due to higher pH. LFD emitted 45 % higher NH₃ than DG. CH₄ emissions were reduced by 44 % in DG and 53 % in LFD compared with CS due to a decrease in volatile solids during digestion and separation. Acidification effectively mitigated NH₃ emissions by 59–84 % and CH₄ emissions by 70–86 % compared with their unacidified counterparts. N₂O emissions were related to crust formation, and acidified CS reduced the emissions by 40 % compared to non-acidified CS. This study suggests that anaerobic digestion and separation can reduce CH₄ emissions but exacerbate NH₃ losses if not combined with appropriate mitigation strategies. Acidification proved to be an effective approach for reducing both NH₃ and CH₄ emissions, offering a promising pathway for reducing environmental impacts of slurry management and achieving air quality and climate targets.