Fixed film reactors are a type of bioreactor used in wastewater treatment and other biological processes. In these systems, microorganisms grow on a solid surface, forming a biofilm. The fixed film allows for efficient mass transfer of nutrients and oxygen, which enhances the degradation of pollutants. These reactors can be designed in various configurations, such as trickling filters or biofilm reactors. They are often preferred for their ability to handle high organic loads and their relatively low energy requirements compared to other reactor types, like suspended growth reactors.