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Sustainability is one of the major priorities in our current society. The transition from aerobic to anaerobic wastewater treatment is a crucial issue due to its potential to increase sustainability of wastewater treatment. Anaerobic treatment enables less waste production, less energy consumption and a reduced footprint in contrast to aerobic treatment. Anaerobic wastewater treatment is significantly developing in mesophilic and thermophilic climates while it is hampered in colder climate regions. The cause is a restrained understanding of anaerobic digestion bottlenecks under decreasing temperatures. Consequently, this thesis focusses on the optimization of anaerobic municipal wastewater treatment under psychrophilic conditions. This is done on the one hand by gaining further knowledge on the fundamental process of microbial acclimation to colder temperatures and its consequences on anaerobic digestion efficiency. On the other hand, lipid accumulation is a global issue causing many problems in anaerobic reactors under psychrophilic conditions. Revealing the true bottlenecks of lipolysis is required to enhance lipid degradation into biogas. Therefore, a monitoring tool to quantitively assess lipolysis in sludges is developed to investigate whether lipase activity is limiting. In the first part, anaerobic digestion performances of reactors inoculated with different biomasses, fed with synthetic municipal wastewater and subjected to decreasing temperatures shock are monitored on the long term. It results that sludges acclimate to a temperature shock from 30 to 20°C while no acclimation is revealed after the second temperature shock from 20 to 15°C. Moreover, this experiment confirmed methanogenesis as rate limiting step in presence of an easily degradable feed. In addition, sludges from anaerobic digester are more performant to treat wastewater under psychrophilic conditions compared to cow rumen biomass. The second part addresses the development of a high throughput, easy-to-use and quantitative lipase assay to investigate whether lipase activity is lacking in sludges under psychrophilic conditions. An assay based on an olive oil emulsion stabilized with Arabic gum and visualized through fluorescence changes due to interaction of released long chain fatty acids with Rhodamine B, a fluorescent dye monitored with a microplate reader is a promising solution. Although the efficiency of the assay is confirmed for pure lipase solution, some main obstacles need to be tackled when this assay is adapted to sludge samples. Practically, limited or insignificant lipase activity was recorded in sludges. This highlights the effective lipase activity in sludges or relates reduced activity due to the bottlenecks of the developed assay. To conclude, further research are required to implement anaerobic wastewater treatment plant in cold climate regions.