Simulation of the effect of hydraulic and solid residence times on the anaerobic digestion of readily and slowly biodegradable feedstocks for methane and chemicals production

This study uses a kinetic model (Monod and hydrolysis kinetics) of anaerobic digestion (AD) to investigate the effect of the residence time (HRT, hydraulic residence time and SRT, solids residence time) on process performance considering substrate removal, production of intermediate chemicals (modelled as acetic acid and hydrogen) and of the final product methane. Two types of feedstocks were considered, readily or slowly biodegradable substrates, with substrate concentration in the range 1–100 kg/m 3 . Two process configurations were simulated, without (completely mixed reactor, CSTR, without recycle) and with (CSTR with recycle) biomass retention. For the system without biomass retention (SRT=HRT), acetic acid and hydrogen were the main products from readily biodegradable feedstock in a range of intermediate residence times, while for longer residence times methane was the main product. Instead, for a slowly biodegradable feed, no hydrogen production and lower acetic acid production were calculated due to the longer residence times required for hydrolysis. In the system with biomass retention (SRT > HRT), the conversion of slowly biodegradable substrates was impacted not only by the SRT, as usually assumed, but also by the HRT. This can limit the effectiveness of anaerobic digestion in processes with biomass retention. The effect of the kinetic parameters for hydrolysis was investigated, indicating that processes with biomass retention are likely to be less effective than processes without biomass retention for feedstocks characterised by slow hydrolysis rate. Overall, this study provides insight into the effect of operating conditions on AD and on its optimum design to achieve the desired performance. • Choice of process configuration, HRT and SRT greatly affects AD performance. • Hydrogen production is only possible in a narrow range of residence times. • Continuous-flow AD cannot completely remove the acids from the digestate. • For AD with recycle, substrate conversion may be limited by short HRT. • AD without recycle is generally a better choice to achieve high acid yields.