text: Due to early issues with biofilm reactors, like hydraulic instability and uneven biofilm distribution, moving bed biofilm technology was developed. The MBBR system consists of an aeration tank (similar to an activated sludge tank) with special plastic carriers that provide a surface where a biofilm can grow. There is a wide variety of plastic carriers used in these systems. These carriers vary in surface area and in shape, each offering different advantages and disadvantages. Surface area plays a very important role in biofilm formation.  Free-floating carriers allow biofilms to form on the surface, therefore a large internal surface area is crucial for contact with water, air, bacteria, and nutrients. The carriers will be mixed in the tank by the aeration system and thus will have good contact between the substrate in the influent wastewater and the biomass on the carriers. The most preferable material is currently high density polyethylene (HDPE) due to its plasticity, density, and durability. 
  To achieve higher concentration of biomass in the bioreactors, hybrid MBBR systems have been used where suspended and attached biomass co-exist contributing both to biological processes. Additionally, there are anaerobic MBBRs that have been mainly used for industrial wastewater treatment. A 2019 article described a combination of anaerobic (methanogenic) MBBR with aerobic MBBR that was applied in a municipal wastewater treatment laboratory, with simultaneous production of biogas.