Digestate storage at anaerobic digestion (AD) sites results in methane release from entrapped biogas and continued methanogenesis. While emissions are expected to change with flowsheet configuration (e.g. pretreatments, AD in series or parallel) and asset operating conditions like hydraulic retention time, organic loading rate  and temperature, their impact on post-AD emissions remain understudied. There is also limited knowledge on the efficiency of emerging methane abatement strategies, discerning between methane-recovery technologies (e.g. vacuum degassing), innovations that reduce methanogenic activity (e.g. aeration, acidification, dewatering) or those that reduce available carbon (e.g. longer HRT, reactors in series). This study addresses this gap, linking site practices to digestate emissions potential and comparing efficiency of abatement strategies. Digestate handling (storage time, temperature) and aeration were amongst the first to be tested. 
A 3-month trial assessed residual biogas potential of digestate stored at 4, 18 and 38°C. Two-way ANOVA showed significant effects of time, temperature and their interaction (p0.05) on total volatile fatty acids (VFA) and free ammonia. VFA levels were higher at 4°C (450 ±47 mg/L) and decreased at 18°C and 38°C (272 ±20 and 156±13 mg/L); while ammonia shifts towards its free form at elevated temperatures (319 ±50mg/L at 38°C). Findings indicate slowed methanogenic activity at cold temperatures, shedding light on the link between storage time and emissions kinetics. Warmer storage can elevate free ammonia beyond toxicity values for acetoclastic methanogenesis (ca. 150mg/L free ammonia). Trials to quantify the impact of aeration intensity and duration on residual biogas potential are ongoing and will be presented at the conference. The project has also developed an RNA-based method to assess activity of methanogenic communities (rather than presence), so as to study reactivation patterns of different communities exposed to abatement strategies. 

To be presented in the 'Methane Emissions' session.