Investigations of the Effects of Lowering the Temperature in Full Scale Mesophilic Biogas Digesters at a Wastewater Treatment Plant

This thesis has investigated the effects of running the two full scale biogas digesters at Slottshagen wastewater treatment plant at 34 °C compared to 37 °C, in terms of process stability, biogas production and energy savings with the aim of saving energy and money by not heating the digesters as much. The main objective was to investigate whether it is at all possible to operate the biogas process at 34 °C or if the process becomes inhibited or otherwise unstable. If the process could be operated at 34 °C it might mean savings of both energy and money, provided that there is still a sufficient production of biogas.The experiment lasted for three months and investigated the short-term effects of the reduction of temperature. The process was monitored closely, and samples from the reactors were collected and analysed twice a week to ensure the stability of the biogas process. Several parameters were monitored online, the biogas production and methane content amongst others. Other parameters were calculated, such as the degree of degradation and specific methane production. This was done to ensure process stability and a sufficient production of biogas. The energy balance was calculated to evaluate if energy was saved by lowering the temperature in the digesters.The results show that the biogas process does remain stable at 34 °C while still producing a satisfactory amount of biogas during the short time of the experiment. Calculations show that both energy and money has been saved during the experiment. However, the system is largely dependent on seasonal variations, therefore further studies over a longer time period would be desirable. During the course of the thesis it has also become evident that the biogas process at Slottshagen is irregular in several aspects, and that it would be beneficial to even the process out, especially with regards to the hydraulic retention time. Making the process more even would enable further improvements to be made and simplify interpretations and comparisons of processstability data