In the frame of the joint research project BioKS, the influence of the temperature on the formation of biofilms and furthermore, the corrosion and scaling processes are investigated. The research is accomplished in a bypass system at different full-scale geothermal plants and in laboratory experiments. Furthermore, the influence of the fluid flow as well as traces of oxygen and organic substances on the biofilm formation and structure is investigated. In order to better understand the microbial processes, the microbial community is characterized, quantified and microscopically analyzed. Scales are characterized and interactions between biofilms, construction materials and their surfaces are explored to develop recommendations for an optimized plant operation and to protect the well injectivity as well as the reservoir.