Wastewater is in many ways a valuable resource, which must not only be "cleaned". The biological purification plants of the future use the ability of microorganisms to transform different organic substances into energy (e.g. methane) and valuable products such as bioplastic, as well as their ability to remove limited resources, such as phosphate in a way that It can be easily reused. The lecture gives an overview of how microbial communities can be used for these important processes, as well as how to study such complex bacterial communities at a hitherto unknown level of detail with "metagenomics". New technologies have made it possible to make sequence analysis of the whole bacterial community's genetic heritage (metagenomics), and it can be used for analysis of bacterial composition, functions and interactions, a knowledge that provides completely new and exciting perspectives to Develop and optimise the future biological "cleansing processes".