Fixed film or attached growth processes have been in use for biological wastewater treatment since late 1800s. In the attached growth process micro-organisms responsible for organics (BOD, COD) and nutrient (Ammonia, TKN) reduction from wastewater are allowed to grow on specific support medium or carrier material which provides habitat in the form of surface for the growth of microorganisms on it.

Though attached growth processes have same metabolic pathways for pollutant reduction as compared to attached growth activated sludge process, they provides distinct advantages over suspended growth activated sludge process.

Advantages of Attached Growth Processes:

  • Smaller footprint.
  • Minimize needs for secondary settling units.
  • Operational simplicity.
  • Protection against toxic shock loads.
  • Reduced operating and energy costs.

Different type of attached growth processes and type of media used:

One of the first applications of attached growth processes involved use of gravel media as support mechanism in biological filters developed as early as 1890. Over the years due to continuous research and innovation, various types of media and process configurations have been developed. These different media types include simple wood sheets, blast furnace slag, coarse sand, rocks and pebbles, cords, textile materials and asbestos sheets. 

These media have been incorporated into different process configurations like biological filters, contact beds, Trickling Filter, Rotating Biological Contactor (RBC), Biological Aerated Filters (BAF) and Submerged Aerated Fixed Film Filter (SAFF). Most of these processes utilize fixed in place type of media configuration where the attached growth media on which active microorganisms are retained, remain stationary.

Further most of the processes utilized passive aeration system which rely on natural draft for providing required oxygen to the biofilms growing on the attached media. Due to the reliance on this natural draft system compared to mechanical aeration systems, the foot print requirement still remain larger.

Moreover, when the biofilms growing on the media surface become too thick, the diffusion of substrate (BOD,COD, Ammonia) as food for the microorganisms to the biofilms become difficult and also the reduction of metabolic by-products from the biofilms which result in reduced reduction over the period of time.


Disadvantages of traditional fixed in place attached growth processes:

  • Reliance on Natural Draft aeration resulting in still larger foot print compared to mechanical aeration system based suspended growth process.
  • Potential media clogging due to inadequate screening.
  • Excessive biofilm growth resulting in media clogging and development of nuisance organisms like slime, mould, flies.
  • Inadequate mixing or short circuiting which results in inefficient use of entire media surface area for biomass growth.

Moving Bed Biofilm Reactor (MBBR): floating attached growth media-based process Further, with more understanding and knowledge about attached growth processes and importance of media in its operation, there has been a trend towards developing attached growth process utilizing floating media for biological growth in the reactors. This variant termed as MBBR (Moving Bed Bio Reactor) or IFAS (Integrated Fixed Film Activated Sludge) utilizes tiny floating carrier media which are kept in suspension just like the floc particles of suspended growth process.

These tiny media or carriers are made of different materials like Poly Propylene, HDPE, LDPE, PU foam and have different shape and structure which allows growth of biomass responsible for the degradation of target pollutants.

The principal advantages of the Moving Bed Bio reactor (MBBR) process is that it overcomes some of the difficulties of the traditional fixed in place media based attached growth process and also combines few of the advantages of conventional suspended growth based activated sludge process.

How MBBR process works:

Moving Bed Biofilm Reactor (MBBR) utilizes specially designed plastic carrier element having different geometry and surface area for biofilm attachment on it. These tiny carrier elements or media are held in suspension throughout the reactor by providing turbulent energy provided due to aeration, liquid recirculation, or mechanical mixing. The MBBR reactor is usually filled with one third or two third volume with carrier element.

The traditional MBBR process is a once through system and there is no sludge recycle like conventional activated sludge process which results in simplicity of operation and numerous alternatives of clarifiers can be utilized with once through MBBR process.

attached growth process
Fig 1: Traditional Once through MBBR process with a DAF

Advantages of MBBR process:

  • Like other attached growth process, MBBR help to promote development of highly specialized active biofilms based on the reactor conditions. This highly specialized active biomass results in higher reduction efficiencies and higher process stability reducing the foot print of the reactor.
  • MBBR process has much simplified flow process sheet as activated sludge process allowing multiple reactors to configure in flow through series arrangement. This arrangement helps achieving multiple treatment targets like BOD reduction, nitrification.
  • As it is once through flow process, instead of conventional clarifiers, advance clarification process like lamella and Dissolved Air flotation (DAF) can be utilized as solids separation step.

Application of MBBR process:

MBBR process is a versatile biological attached growth process which can be utilized for various applications as below. Over the past three decades, there are hundreds of successful wastewater treatment plants working worldwide based on MBBR process.    

  • Carbonaceous matter (BOD ) reduction
  • COD reduction
  • Nitrification and Denitrification for Ammonia, TKN and Total Nitrogen reduction

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Amit Christian, having MSc degree in Environmental Science from UK university has more than 20 years of experience in the field of water and wastewater treatment. He has expertise in MBBR and IFAS process design, engineering and process start up /commissioning.