Determination of Methane and Carbon Dioxide Formation Rate Constants for Semi-Continuously Fed Anaerobic Digesters

To optimize commercial-scale biogas production, it is important to evaluate the performance of each microbial step in the anaerobic process. Hydrolysis and methanogenesis are usually the rate-limiting steps during digestion of organic waste and by-products. By measuring biogas production and methane concentrations on-line in a semi-continuously fed reactor, gas kinetics can be evaluated. In this study, the rate constants of the fermentative hydrolysis step (<i>k</i>_c) and the methanogenesis step (<i>k</i>_m) were determined and evaluated in a continuously stirred tank laboratory-scale reactor treating food and slaughterhouse waste and glycerin. A process additive containing Fe²⁺, Co²⁺ and Ni²⁺ was supplied until day 89, after which Ni²⁺ was omitted. The omission resulted in a rapid decline in the methanogenesis rate constant (<i>k</i>_m) to 70% of the level observed when Ni²⁺ was present, while <i>k</i>_c remained unaffected. This suggests that Ni²⁺ mainly affects the methanogenic rather than the hydrolytic microorganisms in the system. However, no effect was initially observed when using conventional process monitoring parameters such as biogas yield and volatile fatty acid concentration. Hence, formation rate constants can reveal additional information on process performance and <i>k</i>_m can be used as a complement to conventional process monitoring tools for semi-continuously fed anaerobic digesters.