Methane Emissions During Sludge Storage in Relation to the Microbial Community Composition of Sludge - A Benchmark Study and Metagenomic Analysis Concerning Different Sludge Treatment Processes

Sammanfattning

Wastewater treatment plants (WWTPs) are one of numerous actors influencing the development towards a sustainable society. In the process of wastewater treatment, sludge is formed as a by-product. During storage of finished treated sludge, it is desirable to minimise methane emissions due to the contribution of greenhouse gas emissions to climate change. In this project, the methane emissions of finished treated sewage sludge from four WWTPs employing different sludge treatment processes (mesophilic and thermophilic digestion, as well as liming) was investigated. The goal was to generate benchmark values of methane emission during sludge storage to be used in tools for estimating the climate impact of WWTPs in Sweden. Additionally, a metagenomic analysis was performed to see if any relation between the amount of emitted methane and the microbial community composition of the sludge could be found. Benchmark values for methane emission during storage of sludge treated by mesophilic and thermophilic digestion as well as liming was successfully obtained. Furthermore, the results indicated that the implementation of thermophilic digestion conditions could potentially decrease methane emission from sludge storage. The results from the metagenomic analysis showed that a higher relative abundance of methanogens resulted in increased methane emissions and that sludge containing more diverse microbial communities were able to emit larger amounts of methane at lower temperatures. Since targeting these areas to decrease methane emission during sludge storage would negatively affect the biogas production efficiency, other alternatives to minimise methane emission from sludge storage should be explored.