Challenges associated with high-solid anaerobic digestion: down-scaling, plug-flow behaviour and ammonia inhibition

Biogas production through anaerobic digestion (AD) enables use of resources contained in organic waste streams to produce renewable energy and biofertiliser. For treatment of organic material with high total solids content, high-solid anaerobic digestion (HSD) reduces the need for dilution, but has not been as extensively studied as more conventional wet AD processes. The low dilution rate in HSD involves a high risk of accumulation of toxic substances, such as ammonia, a well-known inhibitor of AD processes. Monitoring of thermophilic HSD systems in this thesis indicated disturbances, with associated accumulation of propionate and changes in microbial population at ammonium-nitrogen concentrations >4 g/L. Continuous HSD processes are typically run in reactors of plug-flow type (PFR), characterised by a concentration gradient of organic material and degradation products from inlet to outlet. This thesis demonstrated challenges in obtaining plug-flow dynamics and phase-separated microbial communities in PFRs both at laboratory and industrial scale. Serial digestion systems, which have characteristics similar to plug-flow behaviour, improved methane yield and protein degradation significantly when food waste was digested in wet conditions. A down-scaling study of an industrial-scale HSD process showed that similar process performance and yield could be obtained in laboratory-scale PFRs, but also that HSD experiments can be carried out in completely stirred-tank reactors (CSTR) usually used for wet materials. Thus the work in this thesis addressed some of the challenges associated with HSD and demonstrated that laboratory-scale studies can be used for future process optimisation trials.