No abstract

Thermophilic anaerobic digestion (TAD) is characterized by higher biogas production rates as a result of assumedly faster microbial metabolic conversion rates compared to mesophilic AD. It was hypothesized that the thermophilic microbiome with its lower diversity than the mesophilic one is more susceptible to disturbances introduced by alterations in the operating factors, as an example, the supply of nitrogen-rich feedstock such as poultry manure (PM). Laboratory scaled TAD experiments using cattle slurry and increasing amounts of PM were carried out to investigate the (in-) stability of the process performance caused by the accumulation of ammonium and ammonia with special emphasis on the microbial community structure and its dynamic variation. The results revealed that the moderate PM addition, i.e., 25% (vol/vol based on volatile substances) PM, resulted in a reorganization of the microbial community structure which was still working sufficiently. With 50% PM application, the microbial community was further stepwise reorganized and was able to compensate for the high cytotoxic ammonia contents only for a short time resulting in consequent process disturbance and final process failure. This study demonstrated the ability of the acclimated thermophilic microbial community to tolerate a certain amount of nitrogen-rich substrate.

Anaerobic digestion is the process of decomposition of organic matter by a microbial consortium in an oxygen-free environment. The produced biogas from this process is composed of methane, carbon dioxide, nitrogen, oxygen, hydrogen sulfide and traces of other gases.Long-term mesophilic and thermophilic anaerobic digestion experiments were investigated to evaluate the reactor performance and the response of the microbial community under consideration of the structure variations due to an increasing content of NH4 + -N caused by stepwise addition of nitrogen-rich substrates, in this case studies poultry manure (PM).Therefore, laboratory-scale continuously respectively completely stirred tank reactors (CSTR) with a working volume of eight liter and steady organic loading rate (OLR of 3.0 gVS L −1 d −1 ) in mesophilic (37°C) and thermophilic (55°C) conditions were operated.The gradual increasing of NH4 + -N caused by stepwise addition of nitrogen-rich substrates (poultry manure) will lead to an increase in the free ammonia NH3 concentration. Free ammonia is considered a common inhibitor for the anaerobic digestion process due to its cytotoxic effects, resulting from deprotonation of ammonium (NH4 + ). As the free ammonia (NH3) concentration depends on the concentration of NH4 + -N, the pH-value and the reactor temperature, therefore a NH4 + -N and NH3 values of > 3 g kgFM -1 respectively > 0,4 g kgFM -1 which has no impact on the anaerobic digestion process under mesophilic condition caused a serious disturbance and inhibition under thermophilic condition.The anaerobic microbiome acclimated to low PM levels in mesophilic and thermophilic conditions which resulted in a stable anaerobic digestion process. After that, with the consecutive application of medium PM level in mesophilic condition, a process disturbance was induced which was characterized by a shift from a Bacteroidetesdominanted to a Clostridiales-dominated bacterial community accompanied by a change from the acetoclastic to the hydrogenotrophic pathway of methane formation. However, the "new" microbial community in mesophilic condition was functionally redundant as the overall process rates in terms of biogas yield methane content and