Monitoring methane and nitrous oxide emissions from digestate storage following manure mono-digestion

Farm-scale anaerobic digestion is increasingly regarded as a greenhouse gas emission reduction measure. To get a full picture of its emission reduction potential, greenhouse gas emissions from digestate storage need to be assessed. This work quantifies methane (CH₄) and nitrous oxide (N₂O) emissions from farm-scale mono-digested dairy manure by continuous monitoring in an on-site digestate storage for three months, in autumn. A dedicated sampling method coupled with an on-line gas phase analyser was developed to enable continuous measurements over longer time periods. The proposed method was a refinement of existing closed chamber approaches and involved repeated sampling cycles, including a gas accumulation phase from which the emission rate was quantified. Daily average methane emissions per stored digestate volume varied from 4.6 to 14 g m⁻³ d⁻¹, equivalent to 3.9 to 8.2% of the methane produced in the digester. Daily average nitrous oxide emissions varied from 0.004 to 0.13 g m⁻³ d⁻¹. The total emission ranged between 170 and 478 g [CO₂,ₑq.] m⁻³ d⁻¹, up to 10% of which was attributed to nitrous oxide. Both methane and nitrous oxide emissions increased for a larger stored digestate volume and a higher temperature. The system under study was simulated through an anaerobic digestion model, taking into account prevailing operating conditions in the digester and digestate storage. Substrate-specific input values were identified by analysing the fresh dairy manure and digestate. The hydrolysis constant and gas–liquid transfer coefficient were estimated at 0.21 d⁻¹ and 0.003 d⁻¹, respectively, for simulation results to match experimentally observed methane emission values.