In many wastewater treatment systems, when the flow rate or pollution load increases, the existing biological tank may no longer be capable of treatment despite still operating. Building additional tanks is an option, but not every facility has enough space, time, and budget for expansion.
IFAS technology is applied by combining activated sludge with a biofilm growing on media (carriers). By simultaneously maintaining suspended biomass and attached biomass, the system can increase the amount of microorganisms participating in treatment within the same tank volume. However, IFAS is not simply about adding media to a biological tank; it requires synchronized calculation with the pollution load, aeration, mixing, sludge recirculation, and the sludge separation capacity of the entire system.
IFAS stands for Integrated Fixed-film Activated Sludge, which can be understood as activated sludge technology combined with an attached biofilm. The characteristic feature of this technology is that two types of biomass exist simultaneously within the same system.
The first type is suspended biomass in the activated sludge. This amount of microorganisms is maintained through the sludge recirculation process from the settling tank back to the biological tank, similar to a traditional activated sludge system.
The second type is attached biomass on the surface of the media, which develops into a biofilm. The media helps retain an additional amount of microorganisms in the tank without relying entirely on the recirculation and settling capacity of the activated sludge.
In the term "Fixed-film," the concept of "fixed" refers to the biofilm being held on the surface of the material; it does not mean IFAS only uses fixed media. Depending on the design plan, the system can use either fixed media or moving media. Among these, moving media is quite commonly used due to its mobility and flexible contact with wastewater.
Combining these two types of biomass makes IFAS different from conventional activated sludge systems and MBBR. An activated sludge system maintains microorganisms primarily in suspended sludge, while a typical MBBR relies mainly on the biofilm growing on the media and generally does not use sludge recirculation to maintain its primary biomass. IFAS combines both mechanisms in the same system.
Thanks to the biofilm on the media, IFAS can maintain additional groups of slow-growing microorganisms, particularly those participating in ammonia nitrification. This is one of the reasons this technology is often considered when upgrading systems that need to improve their capacity to treat organic matter, ammonia, or Total Nitrogen.
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IFAS technology in wastewater treatment
Wastewater, after pre-treatment and equalization stages, is introduced into the biological treatment zone. Here, the suspended activated sludge and the biofilm on the media simultaneously come into contact with the wastewater and participate in the conversion of pollutants.
The suspended biomass in the activated sludge decomposes organic matter and carries out biological processes depending on the conditions of each treatment zone. Meanwhile, the media provides a surface for a portion of the microorganisms to attach and develop into a biofilm.
In the aerobic tank, the aeration system supplies oxygen for both the suspended and attached biomass. If moving media is used, the airflow also creates movement and helps distribute the media throughout the tank. For the anoxic zone, mixers are used to maintain contact between the wastewater, carbon source, nitrate, and biofilm without excessively increasing dissolved oxygen.
After the biological process, the water and sludge mixture is transferred to a settling tank or sludge separation unit. A portion of the activated sludge is recirculated back to the biological tank to continue maintaining the suspended biomass, while the excess sludge is sent to the sludge treatment stage.
The media and biofilm are retained in the biological tank. For fixed media, the system requires supporting frames and proper flow distribution conditions. For moving media, the tank needs retaining screens to prevent the media from washing out into downstream units, and the aeration or mixing system must ensure the media moves and is distributed relatively evenly.
The biofilm will grow, age, and slough off during operation. The sloughed-off biomass, along with the activated sludge, is sent to the sludge separation stage. Therefore, when upgrading to IFAS, the settling, recirculation, and sludge discharge capacities of downstream units must be checked, rather than focusing solely on the biological tank.
The specific configuration of IFAS can vary depending on wastewater characteristics and treatment goals. The system can be arranged with anoxic zones, aerobic zones, or a combination of multiple stages to simultaneously treat organic matter, ammonia, and nitrogen.
Depending on the design, IFAS can use fixed or moving media. These two groups of media differ in layout, auxiliary equipment, and operating conditions. These differences are analyzed in detail in the article Comparing Fixed and Moving Biological Media in Wastewater Treatment.
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Operating principle of IFAS technology
A notable advantage of IFAS is its ability to maintain additional biomass within the same tank volume. Instead of relying solely on suspended microorganisms in the activated sludge, the system features an added amount of microorganisms growing on the media to jointly participate in the treatment.
This characteristic makes IFAS favorable for consideration in projects requiring upgraded treatment capacity but facing limited expansion space. For an existing system, a portion of the biological tanks and equipment can continue to be utilized if the structures, volumes, and capacities of the auxiliary items meet the upgrade plan.
The biofilm also has the ability to retain slow-growing groups of microorganisms in the tank. When dissolved oxygen, pH, alkalinity, load, and retention time are suitable, the attached biomass can support the nitrification process and improve ammonia treatment capacity.
Retaining a portion of the microorganisms on the media also makes the system less dependent on the settling capability of the entire biomass. However, IFAS still maintains recirculated activated sludge, meaning the quality of the settling sludge and the working capacity of the clarifier remain critical factors.
Alongside its advantages, IFAS has higher design and operational requirements compared to using activated sludge alone. When the total biomass in the tank increases, the oxygen demand, mixing capacity, and volume of generated sludge can also change. Existing blowers and aeration distribution systems must be evaluated before adding media.
If the system uses moving media, the retaining screens must have the appropriate size, structure, and position to keep the media inside without obstructing the flow. Uneven air distribution can cause the media to accumulate in one area, reducing contact capacity and creating sludge accumulation zones.
For fixed media, the design needs to control the layout density, flow direction, and accessibility for inspection or cleaning. If the wastewater contains high TSS or grease, solids can accumulate in the gaps of the media and reduce the actual active surface area.
Furthermore, IFAS cannot fix all underlying causes of poor system performance. If the issue stems from an inadequate equalization tank, lack of oxygen, degraded equipment, inhibitory substances in the wastewater, or an overloaded settling tank, these factors must be resolved prior to or concurrently with the technology upgrade.
Activated sludge, MBBR, and IFAS all use microorganisms to treat pollutants but differ primarily in how biomass is maintained within the system.
| Criteria | Activated Sludge | MBBR | IFAS |
| Suspended biomass | Is the main treatment component | Not actively maintained like the activated sludge system | Maintained alongside attached biomass |
| Biofilm on media | None | Is the main treatment component | Present and jointly participates in treatment |
| Use of media | No | Yes | Yes |
| Activated sludge recirculation | Yes | Yes | Usually not used to maintain the primary biomass |
| How biomass is maintained | Mainly in suspended sludge | Mainly on the media surface | Combines suspended and attached biomass |
| Sludge separation requirement | Separate and recirculate activated sludge | Separate sloughed biomass and solids | Separate sludge, recirculate activated sludge, and receive sloughed biofilm |
| Ability to upgrade existing tanks | Depends on the ability to increase MLSS, aeration, and clarifier | Can be applied if the tank and auxiliary equipment are suitable | Advantageous when needing to maintain extra biomass while still using the activated sludge system |
| Operational requirements | Control sludge, oxygen, and recirculation | Control biofilm, media, and aeration or mixing | Simultaneously control activated sludge, biofilm, media, and sludge separation |
To put it simply, the activated sludge system maintains microorganisms mainly in suspended sludge, MBBR maintains the majority of the necessary biomass on the media, and IFAS combines both. Therefore, IFAS is neither just another name for MBBR nor an activated sludge system where media is added arbitrarily.
The selection of technology should not be based on which is newer, but rather on wastewater characteristics, loading rates, treatment goals, available construction space, and the capabilities of existing structures.
IFAS is suitable for consideration when an existing biological system can no longer handle the pollution load, but building additional tanks faces space constraints. This scenario can occur when a factory increases production capacity, a facility expands operations, or the actual wastewater load is higher than the initial design.
This technology can also be considered when a system needs to enhance the retention of nitrifying bacteria to support ammonia treatment. However, effectiveness is only achieved when the tank has sufficient oxygen, alkalinity, retention time, and proper environmental conditions for the nitrification process.
For Total Nitrogen treatment goals, adding media must be considered alongside the configuration of the anoxic zone, carbon source, nitrate recirculation ratio, and dissolved oxygen. Increasing biomass cannot replace the necessary conditions for the denitrification process.
Before selecting IFAS, it is necessary to determine the inlet concentration, wastewater flow rate, and the kg/day loads of BOD, COD, ammonia, and nitrogen. This data serves as the basis for calculating the amount of biomass to maintain, the required media surface area, and the capacity of the existing tank volume.
Aeration capacity is one of the first factors to check. Air blowers, piping, and air distribution devices must simultaneously meet the oxygen demand of the microorganisms and the mixing requirements in the tank. If moving media is used, air distribution also directly affects the movement and contact capability of the media.
Activated sludge concentration, sludge age (SRT), and the F/M loading rate must be maintained suitably for the actual load and settling capacity. IFAS does not eliminate the requirement to control activated sludge but simply adds the biofilm as an additional component to monitor.
For the ammonia treatment process, pH and alkalinity must be checked alongside dissolved oxygen. When alkalinity is insufficient, nitrification efficiency can drop even if the system has maintained a relatively large amount of microorganisms.
The recirculated sludge flow and excess sludge volume must be adjusted to maintain suspended biomass without causing excessive sludge accumulation. Simultaneously, it is necessary to monitor settling capability, the amount of solids entering the clarifier, and effluent TSS.
The biofilm on the media also needs periodic observation. A film that is too thin may indicate unstable attachment conditions, while one that is too thick can reduce the transfer of oxygen and substrates to the inside. Biofilm sloughing is a natural phenomenon, but the sloughed-off amount must be within the handling capacity of the sludge separation stage.
With moving media, it is necessary to check the fill fraction, movement capability, distribution in the tank, and the condition of retaining screens. With fixed media, checks should include the supporting frames, flow distribution, solids accumulation, and access conditions for maintenance.
When treatment efficiency drops, the entire chain of structures should be evaluated instead of just increasing the amount of media or altering a single parameter. IFAS is only effective when the biological tank, aeration system, recirculation, sludge separation, and operating conditions are synchronously calculated.
IFAS is a technology that combines activated sludge with a biofilm growing on media, helping the system maintain both suspended and attached biomass simultaneously. This characteristic provides an advantage in certain cases where treatment capacity needs to be enhanced or systems need to be upgraded while expansion space is limited.
However, IFAS is not simply about adding media to an Aerotank. The technology's efficiency depends on the pollution load, the type and amount of media, aeration capacity, mixing conditions, sludge recirculation, and the capability of the downstream sludge separation unit.
For systems needing capacity upgrades, improved ammonia treatment, or utilization of existing biological tanks, Dai Nam conducts load surveys, checks aeration capacities, clarifiers, and recirculation systems before proposing IFAS. The upgrade plan is synchronously calculated across technology, media, auxiliary equipment, and actual operating conditions, avoiding isolated media additions that fail to resolve the system's root causes.
