A biofilm reactor is a type of wastewater treatment system that uses microorganisms to break down organic matter. In these reactors, bacteria and other microbes form a thin layer, or biofilm, on surfaces within the reactor. This biofilm helps to efficiently degrade pollutants as water flows through the system. These reactors can be designed in various configurations, such as fixed-bed or moving-bed, to optimize the growth of the biofilm. They are commonly used in industries and municipal wastewater treatment plants to improve water quality before it is released into the environment.