In attached growth process, the microorganisms grow on the media such as plastic, stone, and etc. The media in the aeration tank can be as flotation bed or fixed bed.

The microorganisms attached to the solid surfaces are basically from the heterotroph group of activated sludge, which use from organic carbon as a food source for the biomass production. For the formation of biofilm on the media, the components on the media are bed, biofilm, film liquid, and bulk liquid, respectively. On the biofilm surface, a resident layer separates the biofilm and mixed liquid. The nutrients and oxygen penetrates through this resident layer into the biofilm layer. On the other hand, the products of biological degradation diffuse from biofilm layer to bulk liquid. These processes are performed continuously. Attached Microorganisms cause the growth of dense, viscous or gel-like layers. By growing the microorganisms, the biomass on the media increases and thickens. Thickening of this layer affects the access of oxygen and nutrients for microorganisms.

The treatment of wastewater by Moving Bed Biofilm Reactor (MBBR) method is actually the same as the activated sludge method, which the media is added to aeration tank for increasing the contact area and consequently the increasing of chemical oxygen demand (COD) removal efficiency. Also, the aeration tank volume is reduced due to growing the biofilm on the media. The required air for moving the media and requiring the microorganisms was supplemented by air blower. After contacting air with wastewater, some of the oxygen in the air was dissolved in the wastewater, thus the required microorganisms to oxygen were supplemented

The results of this study showed that

The treatment of the wastewater of petrochemicals is possible by biological method and reduction of COD below about 60 mg/L.

The rate of COD removal from the MEG unit wastewater using MBBR process was over 82%.