The MBBR system is based on the use of a biofilm that grows attached to the surface of carriers. The carriers are made of polyethylene or polypropylene with a density close to that of water. They are designed to provide a large protected surface for bacterial growth.
The reactor volume is filled with carriers up to a maximum value of 67%. Due to their density being close to that of water and the fact that only part of the reactor volume is filled with carriers, the packed bed is allowed to move freely in the reactor as can be observed in Figure below, hence the name moving bed biofilm reactor. Medium bubble aeration or mixing by slow-speed mixers is applied to keep the carriers in suspension and also in order to control the biofilm thickness.
The waste water treatment using MBBR process involve the below processes
i. The Flow equalization chamber
This chamber act as waste water holding facility for the MBBR systems before being pumped in to the plant by the help of the side hill screens. The chamber is continually aerated with the help of the blowers to help remove bad odor and break down the fine suspended solids. This chamber makes the MBBR method flexible in that in the case of plant technicalities, the plant stores waste water temporarily as the plant undergoes repair and maintenance.
ii. Aeration chambers
The aeration chambers are aerated entirely with the help of diffuser channels. These chambers contain floating plastic carriers called media balls that are suspended and move freely in the continuously aerated mixed liquor. They are perforated circular like balls made up of non-degradable plastic with a biofilm made of propylene and filling carrier that provide a surface area for the attachment and growth of the microorganisms as they rotate. The perforations vary with the type of the media balls ranging from 350m2/m3 – 800m2/m3. The small perforations increase the surface area reducing the foot print of the tanks that would rather be used to treat the waste water hence increasing the rate of degradation of the suspended solids.
The media balls are preferred rather than other biofilms because;
• They provide a large surface area for attachment of microorganisms and they
are evenly distributed as the mixed liquor is being aerated, they move randomly
in the chambers.
• They do not block easily as the perforations are of suitable size for bacteria
interactions with the suspended solids.
• They are easy to maintain since they do not get worn out and if the biofilms
are worn out, they can easily be replaced.
iii. The Clarifiers
Most of the MBBR designs have clarifiers for sludge removal, skimming of floating flocs, fats, grease and oils and they also have weirs that are designed to regulate the speed of the moving mixed liquor to help ambient settling of the sludge particles at the bottom of the hopper. They use the pressure mechanisms for all these processes. These chambers have a hopper that is in a V shape to increase sludge settlement at the clarifiers and easy removal by the desludging pipes. The skimmers are efficient in the removal of the fats, grease and oils making this method suitable for commercial industries dealing with food products. The clarifiers have a higher sludge holding retention time hence giving it time for decomposition and breakdown of the nitrogenous waste present in the sludge. Since these chambers are not aerated the nutrients present in the sludge are broken down and removed through nitrification process improving the quality of the effluent. In this chamber most of the nitrogenous waste are removed.
Advantages of MBBR
a. Compared to Extended Aeration waste water treatment systems they cover a lesser space.
b. It is suitable for handling large volume of generated waste hence good for industrial as well as large residential estates
c. Has a higher sludge retention time that necessitates the removal of nutrients from the sludge through nitrification of the nitrogenous compounds in the mixed liquor.
d. It has a low sludge production since some of the sludge is recycled back for seeding the aeration chambers.
e. It is suitable to handle chemicals and can perfectly deal with the fats, grease and oils.