Attached Growth Process for Biological Wastewater Treatment

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.

Advantages of Attached Growth Processes

Though attached growth processes have same metabolic pathways for pollutant reduction, compared to activated sludge process or suspended growth process, Attached Growth Process offer certain advantages such as: 

  • Smaller footprint and Space saving
  • Protection against toxic shock loads
  • Operational Simplicity , Robust and reliable operation
  • Lower sludge production and management 
  • Improved biomass retention 
  • Flexibility for Upgrade and Retrofits

Different Type of Attached Growth Processes

One of the first applications of attached growth processes involved use of gravel media as a support mechanism in biological filters developed as early as 1890s. 

Over the years, various types of  process configuration and different types of media have been utilized.

Several variations of attached growth processes exist, including trickling filters, rotating biological contactors (RBC), submerged aerated filters, and packed bed reactors. Each type employs a different configuration and medium, tailored to specific treatment requirements.

Trickling Filters

trickling filters

Trickling filters are one of the oldest and most common attached growth processes. In this process, wastewater is distributed over a bed of coarse media, typically rocks or plastic materials, called trickling filter media. The microorganisms grow on the surface of the media, forming a biofilm. As the wastewater trickles down through the media, the biofilm degrades organic pollutants. Trickling filters provide good oxygen transfer and are effective in removing organic matter. Common media used in trickling filters include gravel, crushed stone, and plastic materials such as PVC or polyethylene.

Rotating Biological Contactors (RBC)

RBCs consist of cylindrical discs or drums that are partially submerged in wastewater. These discs are covered with media, often plastic or textile material, that provides a surface for microbial growth. As the discs rotate, the media is alternately exposed to wastewater and air, creating favorable conditions for microbial activity. The biofilm on the media removes organic matter from the wastewater. RBCs are efficient in treating high-strength organic wastewater and are commonly used in small to medium-sized treatment plants.

Submerged Aerated Filters (SAF)

submerged aerated filters ( SAF )

Submerged aerated filters utilize a submerged bed of media, typically plastic or other synthetic materials, which provides a large surface area for biofilm development. Wastewater flows upward through the media, and air or oxygen is supplied from below, creating aerobic conditions for microbial growth. The biofilm on the media absorbs and degrades pollutants in the wastewater. SAF systems are effective in removing organic matter, nitrogen, and phosphorus from wastewater and are commonly used in municipal and industrial wastewater treatment.

Moving Bed Biofilm Reactors (MBBR)

moving bed biofilm reactor ( mbbr )

MBBR Technology employ plastic media with high surface area-to-volume ratios. The media is kept in motion within the reactor, either by mechanical agitation or aeration, allowing for continuous contact between the wastewater and the biofilm. Microorganisms attach and grow on the media, forming a biofilm that degrades organic matter and nutrients. MBBRs are flexible, compact, and efficient systems used for a wide range of wastewater treatment applications.

Integrated Fixed Activated Sludge (IFAS):

integrated fixed film activated sludge ( ifas )

Integrated Fixed activated sludge processes incorporate media into traditional activated sludge systems. The media, often plastic or other synthetic materials, enhance the surface area available for microbial growth and provide a more stable environment for the biofilm. The media can be arranged in various configurations, such as in tanks or in the form of suspended carriers. IFAS processes combine the advantages of both attached growth and suspended growth systems, resulting in improved treatment efficiency and reduced sludge production.

Disadvantages of Traditional Fixed in Place Attached Growth Processes

disadvantages of traditional fixed in placed attached growth processes

Most of these processes mentioned above utilize a fixed in place type of media configuration where the attached growth media remain stationery. 

Due to stationary configuration, at times the biomass growing on the media surface becomes too thick resulting in higher diffusion gradient resistance for food and nutrients to the microorganisms growing on it

As the thickness increases, the uptake of organics ( BOD/COD ) reduces which result in lower removal efficiency. 

Further, most of these processes utilize passive aeration system which relay on natural draft system for oxygenation. As the biofilm thickness increases, penetration of oxygen inside the biofilm becomes difficult. Moreover, due to reliance on a natural draft system for aeration, the footprint of such processes is still much larger compared to mechanical aeration system.

  • Reliance on Natural Draft aeration resulting in still larger footprint 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.

Key Components of Attached Growth Process:

For better understanding of attached growth process, it is essential to know its key components.

Media and its role

The choice of media in attached growth systems greatly influences the treatment performance. Common media types include plastic or ceramic materials with high surface area, providing ample attachment sites for microorganisms. The media not only offers physical support but also acts as a substrate for microbial growth.

Microorganisms and their functions

microorganisms and their functions

Microorganisms play a pivotal role in the attached growth process. They form biofilms on the media surface and degrade organic matter through various metabolic pathways. These microorganisms convert organic pollutants into simpler, less harmful substances, promoting the purification of wastewater.

Oxygenation and aeration

Proper oxygenation and aeration are crucial in attached growth systems. Oxygen availability is essential for the survival and growth of aerobic microorganisms responsible for organic matter degradation. Aeration techniques such as diffused aeration or mechanical agitation ensure sufficient oxygen transfer to the biofilm, maximizing treatment efficiency.

Moving Bed Biofilm Reactor (MBBR): floating attached growth media-based process

Further, with more understanding and knowledge about attached growth processes and importance of MBBR 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 MBBR 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 Technology 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.

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

Advantages of MBBR Technology:

advantages of mbbr technology
  • 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 similar to activated sludge process. This simplified arrangement allows multiple reactors to configure in flow through series arrangement which helps to achieve 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 Technology:

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.    

  • BOD and COD removal from municipal and industrial wastewater
  • Ammonia, TKN removal from municipal and industrial wastewater
  • Upgradation of existing Sewage Treatment Plant (STP) and Effluent Treatment Plant (ETP) based on activated sludge process

MBBR for Municipal Wastewater Treatment

MBBR and IFAS have proven effective in municipal wastewater treatment, providing reliable and efficient removal of organic matter, suspended solids, and nutrients. These processes help meet stringent effluent standards and contribute to sustainable water management practices in urban areas

Case study: Ammonia Nitrification is cold weather

MBBR for Industrial Wastewater Treatment

Industries generate complex wastewater containing diverse pollutants. Attached growth processes offer a reliable solution for treating industrial effluents, tackling contaminants such as organic compounds, heavy metals, and toxic substances. The flexibility of the attached growth process makes it suitable for a wide range of industries, including food processing, pharmaceuticals, and petrochemicals.

MBBR for Upgrading Existing Wastewater Treatment Plants

MBBR and IFAS present viable options for upgrading existing treatment plants. These processes can augment the capacity and performance of conventional treatment systems without significant infrastructure modifications. Retrofitting existing plants with attached growth processes allows for improved treatment efficiency and increased resilience to changing wastewater conditions.


attached growth and suspended growth processIn the attached growth process, Microorganisms attach to a medium and form a biofilm, which breaks down organic matter in the wastewater. The medium, made of materials like rocks, plastic, or synthetic substances, provides a large surface area for the microbes to grow.

In the suspended growth process, microorganisms are floating freely in the wastewater. These microbes are kept suspended by aeration and mixing. Where it allows microorganisms to grow and consume the organic pollutants.

Attached growth processes are wastewater treatment processes where the microorganisms are attached to a solid surface. The different types of attached growth processes include:

  • Trickling filters
  • Rotating biological contactors (RBCs)
  • Moving bed biofilm reactors (MBBRs)
  • Fluidized bed reactors
  • Biofilm aerated reactors (BARs)
  • Membrane bioreactors (MBRs)

Difference between suspended and attached growth

Comparison Factors Attached Growth Process Suspended Growth Process
Growth Mechanism Microorganisms grow on a fixed medium, forming a biofilm. Microorganisms are suspended in the wastewater as free-floating cells.
Treatment Efficiency High surface area for microbial activity due to the biofilm, suitable for high organic loads, better resistance to shock loading and toxic substances. Treatment efficiency depends on contact time between microorganisms and wastewater, effective for lower organic loads, may be less efficient for high-strength wastewaters.
System Complexity Requires structured medium (trickling filters, RBCs, etc.), control of flow rates, media maintenance required. Implemented in activated sludge systems (aeration tanks, secondary clarifiers), less complex in terms of equipment and maintenance.
Sludge Production Less sludge production as microorganisms remain attached to the medium and not removed. More sludge production due to continuous growth, excess biomass needs to be removed as waste sludge.
Footprint and Reactor Size Smaller reactor volumes required, resulting in a smaller footprint. Larger reactor volumes needed to provide sufficient contact time between microorganisms and wastewater.

advantages of attached growth process over suspended growth process

Advantages of attached growth over suspended growth:

  • Higher efficiency: Attached growth processes are more efficient in removing pollutants from wastewater. This is because the microorganisms have a large surface area to grow on, which allows them to come into contact with more wastewater.
  • Wider range of applications: Attached growth processes can be used to treat a wider range of wastewaters, including domestic sewage, industrial wastewater, and agricultural runoff.
  • More resistant to shock loads: Attached growth processes are more resistant to shock loads than suspended growth processes. This means that they can still function effectively even if the wastewater contains a sudden increase in pollutants.
  • Less prone to bulking: Attached growth processes are less prone to bulking than suspended growth processes. Bulking is a condition in which the microorganisms in a wastewater treatment process grow out of control and form a thick layer on the surface of the wastewater. This can lead to problems with the treatment process, such as clogging and reduced efficiency.
  • Simpler operation and maintenance: Attached growth processes are relatively simpler to operate and maintain than suspended growth processes. This is because they do not require the use of chemicals or other harsh treatment methods.

Overall, attached growth processes are a more efficient and versatile wastewater treatment process than suspended growth processes. They are a good option for wastewater treatment plants that need to remove a wide range of pollutants from wastewater

The lifespan of the media depends on various factors such as the type of media, operating conditions, and maintenance practices. In well-maintained systems, the media can last for several years before requiring replacement.

Absolutely! Attached growth processes can be utilized for advanced treatment, including the removal of nutrients such as nitrogen and phosphorus. These processes can be integrated into existing wastewater treatment plants to achieve higher effluent quality standards.

 Attached growth processes can be tailored to treat a wide range of wastewater types. However, certain wastewater characteristics, such as extreme pH levels or high levels of toxic substances, may require additional treatment steps or pretreatment before employing attached growth processes.

Maintenance requirements for attached growth systems include periodic cleaning of media, monitoring of microbial activity, and regular inspection of mechanical components. Proper maintenance ensures the long-term performance and reliability of the system.

Author Bio

Amit Christian is a MSc graduate in Environment Science from Middlesex University, London, UK. He has been active in the field of water and wastewater treatment since 1998. He specializes in design, engineering, and management of various biological wastewater treatments such as Activated Sludge Process (ASP), Sequencing Batch Reactor (SBR), Moving Bed Bio Reactor (MBBR), Integrated Fixed Film Activated Sludge (IFAS). He has helped various Industrial and Municipal clients in troubleshooting , optimizing their biological wastewater treatment processes to achieve latest Stringent norms for Ammonia Removal.