Anaerobic digestion of sewage and domestic waste: how small can it be?

Anaerobic digestion (AD) is a well-established technology, particularly suited to handle organic material. It is widely used at large scales as a renewable energy source as it produces biogas, while the remaining digestate is used as fertilizer. However, as energy and transportation prices rise, small-scale applications and decentralized local energy production will become more attractive. The aim of this work is to evaluate the sustainability of small-scale urban AD systems and investigate the scale down to which they remain techno-economically viable.
Data on small-scale domestic AD applications was gathered from literature and analyzed to provide an overview of the current situation and identify gaps, potential correlations and trends. A steady-state mathematical model of the AD process was developed for scenario evaluation, based on feedstock analysis, mass balances and kinetics. Literature data from AD applications were simulated, and a sensitivity analysis was also carried out to study the effect of several model parameters on the process.
The data analysis confirmed the challenges of small-scale systems, as larger reactor volumes are required to treat more dilute feedstocks, such as sewage. The analysis showed that the clear majority of small AD systems are installed on farms and in rural areas, and conversely, very few small-scale applications treat municipal wastes or sewage, indicating a technical gap in these systems. Finally, the model can successfully predict the efficiency of AD systems based on feedstock characterization and it will be used to evaluate the feasibility of such systems.