Microbial fuel cells for polishing effluents of anaerobic digesters under inhibition, due to organic and nitrogen overloads

Abstract

BACKGROUND Bioelectrochemical systems have been proposed as a possible polishing step for anaerobic digestion (AD). They can also be useful to overcome AD instability in case of AD inhibition while, at the same time recovering ammonia. Continuous assays with a microbial fuel cell (MFC) fed with digested pig slurry were performed to evaluate its operation during malfunction periods of the AD reactor and its feasibility as a strategy to recover ammonia, either by introducing VFA pulses in the MFC or by inducing AD inhibition. A microbial community assessment was performed to study MFC changes over its operation when fed with the digestate. RESULTS The MFC achieved COD removal efficiencies of 50% during AD inhibition, reaching a maximum of ammonium removal of 31% (11.19 gN m−2 d−1). A high throughput 16S rRNA gene based sequencing assessment revealed that the anode biofilm was different from the digestate fed, showing a reduction in microbial population diversity in the anode after a 182-day-operation period with digested pig slurry. The main enriched populations in the anode belonged to Bacteroidetes (Flavobacteriaceae), Chloroflexi (fermentative bacteria Anaerolineaceae), Methanosarcinaceae and hydrogenotrophic methanogens belonging to Methanobacteriaceae. CONCLUSION MFCs have proven to be a reliable technology to complement the operation of AD, improving the quality of the effluent and recovering ammonia, particularly during AD inhibition.