Anaerobic co-digestion, the simultaneous digestion of multiple waste materials, leverages existing infrastructure to maximise methane production from organic waste. This thesis integrated well-controlled long-term experiments, metabolic rate evaluation, microbial profiling, and co-digestion capacity assessment to demonstrate how metabolic process kinetics can inform co-substrate selection and define co-digestion capacity. Temperature is a known key determinant for anaerobic process; however, both temperature and feedstock type were identified as key determinants for co-digestion capacity influencing microbial community composition and metabolic functional capacity. At full-scale, food waste co-digestion was demonstrated as an effective strategy to improve energy self-sufficiency at wastewater treatment plants. Temperature is known to significantly impact the biochemical and physio-chemical processes involved in AD. Whilst most digesters are controlled at mesophilic (37°C) or thermophilic (55°C) temperatures, other AD systems - such as covered anaerobic lagoons - are operated at ambient temperature conditions and can be subject to seasonal variations of up to 20°C. Few studies have been conducted at psychrophilic temperatures; understanding the impact of temperature on co-digestion capacity is, therefore, vital to inform seasonal dosage strategies for these ambient AD systems.An understanding of the temperature dependency of co-digestion required the investigation of the impact of operating temperature on long-term anaerobic digestion performance, metabolic activity rates, and microbial composition of different feedstocks. Four bench-scale continuous digesters, of which two treated mixed sewage sludge (SS) and two treated pig manure (PM), were operated at 1-1.5 gVS L-1 d-1 for over 600 days and transitioned through three operating temperature regimes (37°C, 25°C and 15°C, each greater than 100 days). Minimal change in operational performance was observed between 37°C and 25 °C for both the SS and PM systems; however, at 15°C, process stability and performance of both systems was greatly reduced due to slower process kinetics and reduced substrate degradability. The PM system experienced greater process instability than the SS system at 15°C due, in part, to higher degradable OLR and a greater imbalance between upstream and downstream metabolic activity levels.Characterisation of metabolic activity rates were performed using batch activity tests with model substrates and temperature adapted inoculum sourced from the bench scale continuous digesters. The metabolic activity rates measured for the SS and PM systems at 37, 25 and 15°C were hydrolysis of cellulose, gelatin, and oleic acid; fermentation of glucose and glycerol; degradation of propionate and butyrate; and aceticlastic and hydrogenotrophic methanogenesis. The balance between upstream and downstream process rates was impacted by feedstock, with SS and PM systems displaying different activity profiles. Whilst all metabolic process kinetics reduced at lower operating temperature, the relative changes in activity profile measured at 37, 25 and 15°C were different between the SS and PM systems. Propionate and long chain fatty acid (LCFA) degradation were identified as limiting process rates in both feedstock systems at all operating temperatures. Further, acetogenesis was identified as the metabolic step most sensitive to temperature changes due to microbial shifts and the temperature dependency of thermodynamic constraints.Ratios of upstream to downstream metabolic process rates can provide insight into the level of process risk and potential bottlenecks in the AD process. In the SS system, the higher rate of protein degradation relative to the rate of downstream acetogenesis and methanogenesis indicated high process risk for protein-based substrates at all temperatures; similarly, the process risk for carbohydratebased substrates in the SS system increased as operating temperatures decreased. The PM system, comparatively, displayed balanced process rates at 37 and 25°C; however, at 15°C, the balance of the PM system became severely downstream limited, identifying propionate degradation as a potential bottleneck in the AD process. This imbalance in upstream and downstream metabolic rates for the PM system at 15°C correlated with both increased propionate accumulation in the PM continuous digesters and increased imbalance between relative abundance of fermenters and acetogens within the microbial community.Three modifications were made to the ADM1 model to simulate operations at psychrophilic temperatures: (i) temperature was altered to be a function of time; (ii) uptake rates for each metabolic step were altered to be a function of temperature through the inclusion of an additional inhibition factor based on experimental batch activity data; and (iii) biomass decay was altered to be a function of temperature. A comparison of the modified ADM1 model and experimental data for the SS and PM systems revealed that batch kinetic rates using well-adapted inoculum could improve the utility of ADM1 to describe continuous process performance at psychrophilic temperatures.An analysis of the relationship between continuous reactor process performance and microbial community dynamics revealed that process stability could not be correlated with phylogenetic diversity but, rather, the symbiotic activities and balance between different trophic groups (fermenters, acetogens and methanogens). The comparison of SS and PM systems found feedstock to be a greater determinant for microbial community structure than temperature in the range 15-37°C. Differences in feedstock type explained 47% of microbial community variance compared to differences in temperature, which explained 16% of such variance. In both feedstock systems, the reduction in temperature resulted in greater microbial shifts in the bacterial population than the archean population with a single dominant Methanosaeta strain, an obligate acetoclastic methanogen, present in both SS and PM systems at all operating temperatures. This suggests that acetoclastic methanogenesis was the dominant methanogenic pathway regardless of feedstock or operating temperature for systems operating at low organic loading and mesophilic-psychrophilic conditions. Carbon and hydrogen isotope fingerprinting supported this finding, with acetoclastic methanogenesis being indicated as the dominant methanogenic pathway at both mesophilic and psychrophilic operating temperatures.The relative contribution of microbial community dynamics, mass transfer rates, thermodynamics, and chemical equilibrium to the reduction of metabolic kinetics at psychrophilic temperatures was explored. For hydrolytic reactions, feedstock-dependent microbial shifts and digestion pathways exhibited the largest change with decreasing temperature; however, acetogenic reactions are strongly inferred to be more greatly influenced by changes in thermodynamic constraints with temperature. Further, for acetoclastic methanogenesis, the lower methanogenic activity at 15°C is also strongly inferred to be attributed to mass-transfer limitations with decreases in acetate diffusivity and increases in product gas (H2) solubility being the most significant changes with decreasing temperature. Process capacity and the ratio of upstream to downstream metabolic process kinetics were found to influence co-digestion capacity. The highest co-substrate loadings were identified for AcoD systems which co-digested feedstocks with complementary digestion pathways, degradation kinetics that did not exceed the capacity of the rate-limiting metabolic step, and which possessed sufficient alkalinity and nutrients in the combined mixture to promote a balanced microbial community. At all operating temperatures (37, 25 and 15°C), SS had a higher capacity for co-digesting food waste (FW) compared to glycerol (GLY) due to FW having slower digestion kinetics and a greater diversity of degradation pathways not leading to the rate limiting metabolic step of propionate degradation. GLY co-digestion capacity reduced at lower temperatures due to increased imbalance between relative kinetic rates of GLY fermentation and propionate degradation at psychrophilic temperatures.Comparing the GLY co-digestion capacity of SS, PM and cattle slaughterhouse wastewater (SHW) AD systems at 37°C, the greatest amount of GLY (130% additional VS) was able to be co-digested with PM, followed by SS (100% additional VS) and the lowest loading observed for SHW (60% additional VS). GLY degrades primarily into propionate; accordingly, co-digestion capacity was governed by: (i) fraction of base substrate degraded through propionate producing pathways and rate of propionate production from base substrate digestion; (ii) fraction of GLY degrading through propionate producing pathways and rate of propionate production from GLY digestion; (iii) propionate degradation capacity of the inoculum; and (iv) system buffer capacity (alkalinity) to deal with propionate accumulation. The high GLY co-digestion capacity of the PM system can be attributed to the slow digestion kinetics, low biodegradability and high alkalinity of the PM in combination with balanced upstream and downstream microbial capacity of the PM-adapted inoculum.At full-scale, AcoD is an effective strategy to boost biogas production and increase energy selfsufficiency. A case study of Gruneck WWTP revealed that co-digesting food waste at 0.24 kg ¨ VS m-3 d-1 energetically outweighed downstream impacts of reduced dewaterability, increased solids accumulation and nitrogen backload. Food waste co-digestion achieved a net increase in energy production of 4.6 kWh PE-1 d -1 and improved energy self-sufficiency by 16%. Co-digestion, combined with reducing energy consumption through aeration upgrades (3.0 kWh PE-1 a -1, 8% increase in self-sufficiency), enabled Gruneck WWTP to increase energy self-sufficiency from 64% to 88%.