Based on the growth of microorganisms, the wastewater biological treatment systems can be divided into two types: suspended growth (e.g. activated sludge process and MBR) and biofilm growth (e.g. trickling filter, fixed bio-filter and MBBR). Biofilm has advantages as high organic loading and high resistance to environmental changes and toxic chemicals. However, it also has several disadvantages, such as the reaction tank is prone to having blind corners, and red worms and sewage flies like to eat the biofilms, which results in a declined effectiveness of the biological treatment system, as well as an unpleasant feeling having those worms, mosquitoes, and flies around.The main disadvantage of activated sludge process is the management of sludge. When the F/M ratio is too high, the performance of nitrification will be bad; when the F/M ratio is too low, the sludge bulking will occured, so the effluent quality will be effected. So the ASP costs operators more effort to maintain the system.

The MBBR practice is to put carriers with specific geometric shapes in the aeration tank of the wastewater/sewage biological treatment system. The carriers have a high specific surface area that allows microorganisms to attach, grow and form a biofilm. Besides, MBBR carriers have an appropriate size and a density similar to water, so that they can suspend evenly in the water if properly agitated or aerated. Also, since the carriers are in a state of continuous movement, the biofilms on the carriers can have an even contact with the bulk, which contributes to a good mass transfer effect.

MBBR technology has the advantages of high biofilm load and stable treatment effect, while keeping the advantage of good mass transfer in the activated sludge process. Therefore, there are more than 1,000 engineering achievements regarding MBBR in the whole world.