Integrated biogas systems

An integrated biogas system is a synergistic cycle of processes sustainably recovering energy and nutrients by anaerobic digestion systems. It is a value adding sequence managing waste and biomass with a final gaseous by-product biogas or biomethane (a natural gas substitute). This thesis explored its core process, technology and strategies of biogas production and upgrading to biomethane. Various studies of this thesis highlight pathways to conduct and optimise anaerobic digestion at intensified conditions while improving reactor utilisation. An increase in substrate throughput and loading was attained by a pre-treating first stage hydrolysis reactor. The solubilisation of substrate provided upstream carbon dioxide segregation and high quantities of readily available liquid fermentation products. A downstream digester increased methane yields and enriched the methane content to levels of 71% of the biogas composition. Intensified conditions and mono-digestion of a single substrate such as food waste can exhibit deficiencies in essential nutrients and inhibition of methanogenic activity. Supplementation of undersupplied trace elements induced immediate recovery allowing stable digestion at loading rates as high as 5 g VS L-1 d -1 at mesophilic temperatures. An increase in temperature further improved degradation kinetics and stimulated higher biomethane yields at shorter substrate retention in grass digestion. In an integrated biogas system, biogas may be upgraded in conjunction with in-situ and ex-situ biological methanation strategies. The addition of hydrogen revealed positive effects on the methanogenic process. Adverse effects of elevated dissolved hydrogen concentrations on acetogenesis became evident in-situ. A biomethane with methane concentrations in excess of 96% successfully demonstrated the potential for gas grid injection at methane formation rates of 3.7 L per litre reactor volume per day. An approach, supplying gases continuously into a sequential ex-situ reactor system and steadily displacing the upgraded biogas, confirmed similar methane formation yields. A hybrid model, where an in-situ grass digester is followed by an ex-situ reactor suggested an alternative approach to conventional biogas upgrading. The contribution of this thesis is the successful demonstration of optimisation potential in novel and existing digestion systems. The employed biogas upgrading strategies proved to be efficient and suitable for gas grid injection