Industry Solutions

MBBR vs IFAS for Wastewater Retrofit: Which Fits an Existing Tank?

Compare MBBR and IFAS for wastewater-treatment retrofit projects. Learn how existing activated sludge, clarifiers, aeration, tank volume, ammonia load and media retention affect the choice.

When an existing wastewater-treatment plant needs more biological capacity, building a new aeration tank is not always the first option.

Many retrofit projects begin with a simpler question:

Can we increase biological capacity inside the tanks we already have?

Two common approaches are:

MBBR — Moving Bed Biofilm Reactor

IFAS — Integrated Fixed-Film Activated Sludge

Both use moving biofilm carriers, but they are not the same process.

The right choice depends on the existing plant configuration, not simply on which media has the highest surface area.

Quick Answer

MBBR is generally an attached-growth biological stage in which most of the intended biological capacity is developed on moving carriers. IFAS combines moving biofilm carriers with an existing suspended-growth activated-sludge process.

For an existing activated-sludge plant with functioning secondary clarifiers and return activated sludge, IFAS may be an attractive retrofit option because it adds attached-growth capacity while retaining the suspended biomass system.

Where a project needs a more independent biofilm stage, or where suspended-sludge inventory is not intended to remain the main process mechanism, MBBR may be more suitable.

The decision should be based on the complete process:

Existing Process → Current Limitation → Biological Duty → Tank Conditions → Aeration → Solids Separation → Retrofit Selection

MBBR vs IFAS: Basic Difference

Item	MBBR	IFAS
Main biological concept	Attached-growth biofilm	Attached growth + suspended activated sludge
Moving media	Yes	Yes
Suspended biomass	May exist, but is not normally the main controlled biomass inventory	Intentionally maintained as part of the process
RAS system	Not inherently required for the MBBR stage itself	Normally part of the existing activated-sludge process
Secondary clarifier dependence	Depends on overall process configuration	Usually important because suspended sludge must be settled and returned
Common retrofit role	Add a biofilm stage or increase attached-growth capacity	Increase capacity inside an existing activated-sludge basin
Key retrofit question	Can the tank support media, aeration and retention?	Can the existing AS + clarifier system support the hybrid process?

The important distinction is therefore not the plastic carrier itself.

It is how the carrier is integrated into the biological process.

1. Start With the Existing Process

Before choosing MBBR or IFAS, first document what already exists.

A retrofit assessment should identify:

existing process flow

aeration-tank volume

number of biological stages

activated-sludge concentration

return activated sludge arrangement

secondary clarifier capacity

blower capacity

diffuser arrangement

current dissolved oxygen

current sludge age or operating strategy

existing effluent performance

The same carrier can behave very differently depending on where it is installed.

A retrofit should therefore begin with the existing plant rather than with a media quotation.

2. What Problem Is the Retrofit Trying to Solve?

“Need more biological capacity” is not specific enough.

The plant may actually be experiencing:

high effluent ammonia

insufficient nitrification in winter

increased industrial production

higher BOD/COD load

insufficient aeration-tank volume

peak-load instability

limited sludge age

clarifier overload

excessive solids inventory

poor dissolved oxygen

hydraulic short-circuiting

These problems do not all point to the same retrofit solution.

Example

If ammonia removal is poor because nitrifiers cannot be maintained under the existing operating sludge age, attached biofilm can provide additional habitat for slow-growing nitrifying organisms.

But if the real limitation is insufficient oxygen supply, simply adding media may not solve the problem.

The first design question should therefore be:

What is actually limiting treatment performance today?

3. When MBBR May Fit Better

An MBBR retrofit may be attractive where the project wants to create or strengthen a dedicated attached-growth stage.

Typical situations can include:

limited available tank volume

need for additional biological surface area

new biological stage added inside an existing tank

upgrade from a simpler suspended-growth process

high-load pretreatment

ammonia polishing

industrial wastewater with variable organic load

projects where maintaining a large controlled suspended biomass inventory is not the main objective

An MBBR stage can also be used upstream or downstream of other biological processes.

Main MBBR Retrofit Checks

Before adding media, verify:

effective reactor volume

required biofilm area

allowable media fill

media movement

blower capacity

diffuser layout

dissolved oxygen

retention-screen design

hydraulic conditions

upstream solids load

The related HydroNovus calculation guide explains the basic sizing sequence:

Pollutant Load → Design Surface Loading → Required Biofilm Area → Media Volume → Fill Ratio → Reactor Check

4. When IFAS May Fit Better

IFAS is especially relevant when an existing activated-sludge system already has useful infrastructure that the owner wants to retain.

That can include:

aeration basin

activated sludge

return activated sludge system

secondary clarifiers

sludge-wasting system

Instead of abandoning this process, moving biofilm carriers are added to part of the biological basin.

The reactor then contains two biomass populations:

Suspended Biomass

Activated-sludge flocs remain in suspension and continue to perform biological treatment.

Attached Biomass

Biofilm develops on the moving carriers and provides additional biological inventory.

This is why IFAS is often described as a hybrid process.

5. IFAS Can Be Valuable for Nitrification Retrofit

Nitrification can become difficult when:

ammonia loading increases

wastewater temperature drops

available sludge age becomes insufficient

the plant receives larger production loads

existing tank volume cannot be increased

Biofilm carriers can provide additional surface for nitrifying organisms.

Because the attached biomass is retained on the carriers, it is not washed out in exactly the same way as suspended biomass leaving with mixed liquor.

This can make IFAS attractive for some nitrification upgrades.

But the carrier alone does not determine nitrification performance.

The retrofit must still check:

ammonia load

temperature

dissolved oxygen

alkalinity

organic loading

carrier surface area

mixing

aeration

hydraulic retention

suspended biomass conditions

6. Existing Secondary Clarifiers Matter Much More in IFAS

This is one of the most important differences in retrofit evaluation.

In IFAS, the suspended activated-sludge population remains part of the treatment process.

That means the secondary clarification system remains important.

Before increasing biological inventory, check:

clarifier surface loading

solids loading

sludge settleability

return sludge capacity

peak hydraulic flow

sludge blanket behaviour

A biological retrofit can create more treatment capacity inside the aeration basin while still being limited by downstream solids separation.

Therefore:

More biomass in the aeration basin does not automatically mean more plant capacity.

The whole biological–clarification system must be checked.

7. MBBR Also Requires Solids Management

MBBR does not eliminate solids production.

Biofilm grows, ages and sloughs from the carrier.

These solids must eventually be separated from the treated water.

Depending on the process arrangement, downstream separation may include:

settling

clarification

flotation

filtration

other solids-removal stages

So an MBBR retrofit should not be designed as if adding carriers removes the need for solids management.

The difference is that the process does not rely on maintaining and returning the same suspended biomass inventory in the way a conventional activated-sludge or IFAS system does.

8. Aeration Can Become the Real Retrofit Limitation

Both MBBR and IFAS depend heavily on aeration in aerobic applications.

Aeration generally has two functions:

Provide oxygen for biological treatment

Maintain adequate carrier movement

Before adding media, check:

current blower airflow

blower pressure

operating water depth

diffuser condition

diffuser layout

dissolved oxygen

oxygen demand

available spare blower capacity

This is especially important in an existing plant.

If the aeration system is already operating close to its capacity, increasing biological inventory may increase oxygen demand beyond what the current system can provide.

Therefore:

Existing tank volume may not be the only constraint. Air capacity can be equally important.

9. Media Movement Must Be Verified

Moving carriers should remain distributed through the intended reactor zone.

Poor movement may lead to:

floating accumulation

media clustering

dead zones

uneven oxygen exposure

reduced usable carrier area

Movement depends on:

carrier density

carrier geometry

fill ratio

tank geometry

diffuser arrangement

airflow

water depth

internal structures

A retrofit therefore needs a hydraulic and aeration check, not just a calculation of media cubic metres.

10. Both Processes Need Media Retention

Moving carriers must remain inside the intended reactor.

Retention screens therefore become part of the retrofit.

Check:

carrier diameter

smallest carrier dimension

screen opening

screen area

maximum hydraulic flow

approach velocity

fouling risk

cleaning access

Adding a retention screen can also introduce hydraulic resistance.

This is especially important where the existing plant has limited hydraulic head.

11. Do Not Choose IFAS Just Because an Activated-Sludge Tank Exists

An existing activated-sludge basin does not automatically mean IFAS is the correct solution.

First check whether the existing suspended-growth process is worth retaining.

Questions include:

Is sludge settleability acceptable?

Are the clarifiers already overloaded?

Is the RAS system reliable?

Is bulking a recurring problem?

Is suspended-solids control stable?

Is the activated-sludge process fundamentally healthy?

If the existing suspended process itself has severe unresolved limitations, simply adding carriers may create a more complicated system without solving the root problem.

12. Do Not Choose MBBR Just Because It Looks Simpler

MBBR is often perceived as a simple retrofit:

Add carriers → add screens → increase capacity.

Actual projects require more engineering than that.

The retrofit may require changes to:

aeration

internal piping

media-retention screens

tank compartmentalization

hydraulic distribution

solids separation

blower capacity

process controls

The carrier is only one component of the biological retrofit.

MBBR vs IFAS Retrofit Decision Table

Project Condition	MBBR May Fit Better	IFAS May Fit Better
Existing activated-sludge infrastructure is strong	Possible	Often attractive
Existing secondary clarifiers are usable	Helpful	Important
Existing suspended biomass should be retained	Not essential	Yes
Need dedicated attached-growth stage	Yes	Possible
Retrofit primarily targets nitrification	Possible	Often worth evaluating
Existing clarifiers are already severely overloaded	Evaluate carefully	Potential limitation
Existing aeration has little spare capacity	Potential limitation	Potential limitation
Tank volume cannot be increased	Possible retrofit driver	Possible retrofit driver
Existing RAS system is unreliable	MBBR may reduce dependence on that process concept	IFAS requires careful review
Need independent biofilm reactor	Strong fit	Usually not the main objective

This table is only a preliminary screening tool.

The final decision should be based on process calculations.

A Practical Retrofit Decision Sequence

A useful engineering workflow is:

Step 1 — Define the Performance Gap

Determine:

current influent

current effluent

future load

required effluent

Step 2 — Calculate Biological Loading

Review:

flow

BOD/COD

ammonia

nitrogen target

temperature

peak conditions

Step 3 — Check Existing Tank Capacity

Document:

effective volume

water depth

number of basins

available retrofit zones

Step 4 — Check Aeration

Determine whether the current blower and diffuser system can support additional biological activity and carrier movement.

Step 5 — Check Existing Activated Sludge

If considering IFAS, evaluate:

MLSS

sludge age

settling

clarifier loading

Step 6 — Estimate Required Biofilm Area

Only now should the required carrier surface area and media quantity be evaluated.

Step 7 — Check Media Retention and Hydraulics

Verify screens, head loss and peak flow.

Step 8 — Compare MBBR and IFAS

Select the configuration that best fits the existing plant, rather than forcing the plant to fit a particular media product.

Data Needed Before Evaluating an MBBR or IFAS Retrofit

Wastewater Data

average flow

peak flow

ammonia / NH₄-N

total nitrogen where relevant

suspended solids

alkalinity

temperature

Treatment Target

required BOD/COD

ammonia target

TN target where applicable

discharge or reuse requirement

Existing Biological System

process type

tank dimensions

effective volume

MLSS

current sludge age if available

dissolved oxygen

operating problems

Aeration System

blower quantity

blower airflow

operating pressure

diffuser type

diffuser layout

water depth

Clarification System

For activated-sludge / IFAS retrofit:

clarifier dimensions

peak hydraulic load

solids loading

sludge settleability

RAS flow

Existing Media

If carriers are already installed:

carrier type

media volume

fill ratio

carrier dimensions

current movement

Common Retrofit Mistakes

Mistake	Better Approach
Selecting MBBR or IFAS before identifying the real bottleneck	Diagnose the existing process first
Calculating media from tank volume alone	Calculate biological load first
Ignoring aeration capacity	Check oxygen demand and mixing
Assuming IFAS solves clarifier limitations	Verify solids-separation capacity
Treating IFAS as simply “MBBR inside activated sludge”	Evaluate both attached and suspended biomass systems
Ignoring retention screens	Check carrier size, screen area and hydraulics
Selecting the highest-surface-area carrier	Compare usable area and actual reactor conditions
Assuming adding media always increases plant capacity	Check the complete process train

Frequently Asked Questions

Is IFAS the same as MBBR?

No. Both use moving biofilm carriers, but IFAS intentionally combines attached biofilm with a suspended activated-sludge population. MBBR is primarily designed around attached-growth treatment.

Can MBBR media be added directly into an existing activated-sludge tank?

Physically adding media is only one part of the retrofit. The project must also evaluate media retention, aeration, mixing, biological loading, hydraulics and downstream solids separation.

Is IFAS better for ammonia removal?

IFAS can be attractive for nitrification retrofit because attached biofilm provides additional biomass retention, including habitat for nitrifying organisms. Actual performance still depends on temperature, oxygen, alkalinity, ammonia load and other operating conditions.

Can MBBR increase capacity without building a new tank?

Potentially, yes. Adding attached-growth surface can increase biological inventory within an existing volume. But aeration, hydraulics, media movement, retention and downstream treatment may become the limiting factors.

Does IFAS require a secondary clarifier?

Typical IFAS systems retain the activated-sludge portion of the process, so solids separation and return sludge are normally important parts of the overall system.

How much MBBR media should be added during a retrofit?

Media quantity should be calculated from biological load, design surface loading, usable carrier surface area and reactor conditions. It should not be determined from tank volume alone.

Related Technical Resources

How to Calculate MBBR Media Volume: Load, SALR, Fill Ratio and Reactor Volume

Why Specific Surface Area Alone Is Not Enough to Select MBBR Media

HDPE MBBR Biofilter Media for Wastewater & RAS

Fine Bubble Disc Diffuser

Fine Bubble Tube Diffuser

Planning a Biological Retrofit?

If an existing wastewater-treatment plant is approaching its biological capacity, the first decision should not be:

How much media should we buy?

Start with:

What is limiting the existing process, and what biological capacity must be added?

Send HydroNovus the flow, influent and effluent data, biological tank dimensions, ammonia/BOD load, temperature, aeration information and existing sludge-process data.

We can then evaluate whether MBBR, IFAS or another biological retrofit approach is more appropriate before calculating media quantity.

Discuss Your Retrofit Project → /contact