Bioelectrochemically improved anaerobic digestion (AD BES) of agricultural digestates: are digestate pre treatments an option for improving biogas production?

Mixture of agricultural, agrofood industry and collectivities digestates represent a significant source of renewable biomethane. ENGIE biomethane plants in France have the potentiality to inject it in the natural gas distribution and transport grids. Their biogas production rate can be increased by bioelectrochemically improved anaerobic digestion (AD-BES), compared to a sole anaerobic digestion (AD) process. Four pretreatments of digestate (ultrasonic, thermal, sieving and grinding) were tested to further improve the methane potential of the feedstock, while assessing their effect on the bioelectrochemical system and their convenience in terms of global energy balance. Trials at different applied voltage for AD-BES reactors demonstrated that higher biogas production rate and/or CH4 content can be achieved. Powering them at voltage C caused a 142% increase of biogas production vs a control AD (Figure 1), though not leading to a proportionally higher CH4 production rate, demonstrating that electromethanogenic biocathode was the limiting step of the process. Pretreatments caused a significant improvement on organic matter biodegradability (+65%) and a reduction of viscosity of the digestate (-82%). In particular, the ultrasonic pretreatment had a clear effect on the internal resistance and bioelectrochemical performance of AD-BES, in terms of chronoamperometry and cyclic voltammetry of both bioelectrodes. A positive energy balance could not be demonstrated. AD-BES is a potential technology for increasing biomethane production in existing biomethane plants operating with a mixture of agricultural, agrofood and collectivities feedstocks. Its increase in biogas/biomethane production with respect to sole AD plants could lead to a significant decrease of biomethane production cost (estimated, 13%). However, its optimization is complex depending on a number of interconnected factors, including inoculum, electrodes materials and applied voltage. Trials at pilot scale are required for a technical-economical balance and will be run in the course of Biomethaverse project.