Anaerobic degradation of volatile fatty acids in continuous biogas reactors at very short hydraulic retention times.

Sammanfattning: There is an increasing demand for safe, affordable and above allsustainable energy, due to the exhaustion of fossil fuels and increasing greenhousegas emissions. In this context, anaerobic digestion is of growing interest, as it enablesthe production of the versatile renewable energy source biogas from various types oforganic wastes. Though, in order to establish more efficient process policies, a deepunderstanding of the microbiology underlying this process is necessary.This master thesis project focused on the rate limiting steps of anaerobic digestion,acetogenesis and methanogenesis. Therefore, three lab-scale, continuous stirred tankreactors R1 (control) R2, R3 were fed with a synthetic medium, which contained thevolatile fatty acids, acetic acid (45 % COD), butyric acid (45 % COD) and propionicacid (10 % COD) as the sole carbon source. The effect of reduced hydraulic retentiontimes (HRT) and increased organic loading rates (OLR) of volatile fatty acids (VFA)on the dynamics and composition of the microbiological community was studied byhigh-throughput MiSeqR©Illumina conserved marker gene amplicon sequencing ofthe 16S rRNA gene and themcrAgene. Thereby, the aim was to investigate thelink between operational parameters, microbial community present and the processperformance.The HRT of the reactors could be reduced to 1.9 days of HRT (R2) and2.7 days of HRT (R3) at which process breakdown occurred. Increased processefficiency and stable process performances were observed up to a HRT of 3.9 days(OLR of 7 g VFA L-1d-1). A surprisingly stable biomass concentration and microbialcommunity were observed over time, whereat only R2 showed an increased relativeabundance of the archaeal genusMethanothrixat reduced HRTs and increased OLRs.Unintended biofilm formation was found in all three reactors and is assumed to beinterrelated with this unexpected findings.