High temperature biogas reactors to treat stillage from an industrial bioethanol process: Metabolic and microbial characterization

Stillage derived from bioethanol production process is a side stream conventionally used as feed additive after a cost-intensive dehydration step. From economical and ecological points of view, it also represents an appealing substrate for biogas production. In this work, we examined the biomethanization of thin stillage in a stirred bioreactor under thermophilic conditions (55 degrees C). Different organic loading rates and hydraulic residence times (HRTs) were tested over a long period of operation. Using thin stillage as a mono-substrate, the maximum loading rate reached was 2.1 g(oTS)/L/day (oTS, organic total solid). However, with the addition of a commercially available iron hydroxide additive, a maximum organic loading rate of 5.9 g(oTS)/L/day was achieved. GC-MS and denaturing gradient gel electrophoresis were used to study the metabolites and the microbial population dynamics within the biogas reactor under different process conditions. For all organic loading rates studied, volatile fatty acids were shown to give a clear indication of reactor instability. Products of aromatic amino acid degradation, especially phenyl acetic acid (PAA), were detected earlier in reactors even at very low organic loading rates. PAA concentration above 0.25 g/L indicated an unstable reactor performance and values above 0.5 g/L were found to be inhibitory to the biogas production in batch cultures.