Unlocking the Power of MBBR Filters: A Deep Dive into Moving Bed Biological Filter Design

January 10, 2025

MBBR's are industry standard biofilters and it’s easy to see why.

MBBR's can operate under gravity, are aerated to maintain DO levels, are predictable in their performance, and offer biofilm control on the media elements.

Let us cover the fundamentals of MBBR design and operation.

- CO2 degassing

- Oxygenation and Aeration types

- Mixing, sheering and biofilm control

- Media choice

- Dwell Times

- BOD Impacts and Removal

CO2 degassing

MBBR's operate in a constant state of fluidisation from the aeration that circulated the media. This is turn degases CO2 from the water coming out of the tanks, and also from bacterial metabolism as ammonia and nitrite is consumed and nitrate is produced.

Depending on the type of aeration, volume of air, mixing in the reactor and CO2 export systems, Aeration in an MBBR acts much like an aeration basin for CO2 degassing, and given proper attention to design is paid, removal rates of CO2 of between 50-60% per pass can be achieved in high density systems, in low density systems where less CO2 is produced, this may be sufficient to remove almost all of the CO2.

While not the total amount of CO2 is removed, sizing a CO2 degassing unit to do standalone degassing of the entire water stream operating at a 60%-70% removal rate then becomes far more efficient at removing the remaining CO2 post MBBR.

With less CO2 compared to the same water flow and air flow in counter current degassers, CO2 degassing units that could remove 60-70% of the CO2 per pass now can potentially approach complete removal, resulting in very low CO2 levels returning to the tank.

Oxygenation and types of Aeration

Oxygenation in the MBBR itself is something that is overlooked but is incredibly important.

Nitrification is directly rated to Oxygen levels in the MBBR (and BOD levels as well but more on that below), and with only the oxygen exiting from the tanks as the only source of oxygen apart from the aeration in the MBBR, it is important to ensure your aeration system is meeting, and ideally exceeding the amount of oxygen required to not only treat the ammonia and nitrite in the system, but also the BOD.

Drilled pipe is old technology and while robust and cheap to install, the oxygen transfer is very low, the aeration volume is very high to achieve fluidisation, and if it breaks, cracks or develops a leak at a join, the reactor needs to be drained to fix it.

Where does BTA stand on drilled pipe in MBBR's? JUST NO.

Diffused air is the system that BTA utilises to provide mixing and oxygen in MBBR's, and there are two ways to achieve this.

Fixed diffusers such as disk diffusers, these diffusers are resistant to blocking but are fixed in place and hard plumbed to a manifold. They have an acceptable oxygen transfer but have a pressure requirement that is needed to open the membrane for operation which can increase operational dynamic head pressures resulting in higher energy use. If they break off, blow a membrane or get knocked off during routine maintenance, such as scrubbing of outlet screens etc, the reactor needs to be shut down and drained to repair it.
Removable diffusers that are self-weighted solve the issue of removal when maintenance is needed, and depending on reactor design, can be retrieved easily for inspection and cleaning. By using porous diffuser hose, we can achieve incredibly high oxygen transfer rates, minimal back pressure and ease of operation and servicing of the diffusers.

BTA utilise diffused air systems in self-weighted diffuser configurations that are easily retrieved. This removes all fixed pipework from inside the reactor and allows for easy maintenance of the diffusers.

Mixing, shearing and biofilm Control.

First of all: mixing

What good is a biofilter, if the media on which the bacteria is colonised is not utilising the entire volume of biofilter and having contact with the maximum amount of water containing ammonia, nitrite and BOD.

Distribution throughout the biofilter volume is paramount to getting optimum performance.

Height to diameter ratio, media buoyancy, induced flow from aeration and flow patterns all have a significant effect on removal efficiency of biofilters.

Second: shearing

Shearing is a function of localised velocity and ensuring a high velocity in the bubble plume of the diffused aeration system results in effective shearing of biofilm and waste particles from the media.

unlike the outside of the MBBR biocarrier, the inside surfaces do not have direct abrasion of the biofilm.

And this is a good thing.

The outside surface area, while it does contribute somewhat to the overall surface area, is not included in the mass balance calculations of biofilter sizing as the environment is too violent, the protected surface area on the interior of the bio media is where the majority of nutrient reduction occurs.

And because we don't get physical abrasion of the internal structure of the bimedia, we need to remove biofilm and waste by inducing high water flow and air scrubbing to "blast" off excess biofilms thickness and microparticles.

Diffused aeration creates a localised plume of high velocity by design and produces much more induced flow compared to the large bubbles of drilled pipe.

Third: Biofilm Control

Biofilm control is a function of aeration type and the induced shearing from induced velocity.

MBBR's are one of the few biofiltration technologies that have biofilm control.

Biofilm thickness directly impacts nitrification capacity, lower velocity and shearing encourage the formation of thicker biofilms; thicker biofilms have a higher percentage of heterotrophs which have a much higher requirement for oxygen.

This means the base biofilm of nitrifiers that the heterotrophs adhere to get starved for oxygen and are either significantly reduced in their capacity or die and are sloughed off.

The objective is to remove heterotrophs and waste particles, this allows the autotrophic nitrifying bacteria to dominate the available surface area. This means that the nitrifying bacteria have almost exclusive access to the available oxygen, ammonia, nitrite and BOD in the water and have minimal competition.

To put it in mathematical terms, if a biofilm consisting of a 300-micron thick layer is 150microns thick of heterotrophs and 150 microns thick of autotrophs, by removing the heterotrophic layer, you now allow the autotrophic layer access to 100% of the oxygen, ammonia, nitrite and BOD.

As heterotrophic bacteria are about 20 times less efficient at nitrification than autotrophic bacteria, by removing this layer through adequate shearing and biofilm control, we can increase nitrification substantially.

Media Choice

Media choice in MBBR's is important, lets not dispute that.

But just as important is the mixing, aeration and overall design of the reactor.

Without the core concepts of MBBR design being taken into account, and without the process being optimised, it doesn't matter how high the surface area is on your media, if it’s not operating at optimum performance then it doesn’t matter how good the media design is.

On the slip side of this, even a media carrier with a modest surface area can achieve higher than expect nitrification rates if the reactor is designed properly.

Knowing what media to use, in which circumstance, based on the system design flow rate, system water quality, TSS, BOD, feed rate and reactor design is the most important part of media selection.

For most RAS applications, the 13KLL+ media https://www.btaqua.com.au/biologicalmedia is an effective choice for nitrification and BOD removal, as long as the reactor is designed correctly.

RAS systems generally do not have high TSS and solids in general that would result in the biofilter elements clogging.

Compared to say a wastewater treatment plant, which have very high TSS and solids that would require specific biocarrier design to ensure they don't clog, become weighed down with excess biofilm, or don't self-clean effectively, which not only effects nitrification, but can also impact the performance of the activated sludge process.

The HEL-X flake Pro, we then generally use as a booster to existing RAS biofilters, or to maximise nitrification in a reactor when designing them. even at the highest of densities and feed rates it is highly unlikely that a MBBR with only HEL-X Flake Pro in it would be able to achieve a sufficient dwell time, which is why we mix it in only as a portion of the overall media volume.

Dwell Times

Dwell time is incredibly important in an MBBR and is right up there with oxygen availability and mixing/shearing.

The longer water spends in the MBBR, the better the removal rates will be however this is also dependent on the other design factors of the MBBR itself.

High dwell time won’t make up for everything, and conversely, optimising other design considerations can make up for sub optimal dwell times.

Generally speaking, ideally the dwell time in an MBBR should range between the 5–7-minute mark, with 7-10 minutes being ideal.

In cold water systems, higher dwell times are required, and are usually easily achieved due to the low temperature requiring more media in the first place.

BOD (biological oxygen demand) its impacts and removal.

BOD is the biggest influence on nitrification.

You need oxygen to feed the microbes, microbes to break down the BOD, and then additional surface area for nitrifiers.

BOD fuels biofilm growth, it’s an excellent source of organic carbon to fuel heterotrophic bacterial growth, and this results in less surface area for nitrifying bacteria to colonise, as well as impacts the thickness of biofilms which effects oxygen diffusion to the bacteria.

There are two ways to compensate for BOD impact on Nitrification:

Size the biofilter accordingly - It sounds simple but not many people do it.

NOTE: this also requires careful attention to biofilter design to properly control biofilm, are you sensing a trend here?

2. Ozone - Ozone directly degrades BOD

Ozone directly degrades BOD, it helps with mechanical waste removal, and because of the removal of BOD, it leaves most of the oxygen to the biofilter and nitrifiers.

It could also be argued that it helps somewhat with biofilm control too.

This of course is assuming that the ozone is sized correctly.

This is why people see a reduction in bacteria ammonia and nitrite and an increase in biofilter efficiency when applying ozone.

The biofilter is the heart of any recirculating aquatics system. It NEEDS to be designed right.

A relatively small investment into the correct design of biofilter reactor to begin with can see improvements in growth and production performance that far outweigh capital expenditure.

And apples to apples, MBBR's cost the same to run generally speaking. So, we prefer to do it the best we can the first time to ensure continual optimised performance throughout the life of the MBBR and the system as a whole.

BTA's MBBR reactor designs aims to optimise MBBR efficiency, streamline operator maintenance and maximise waste removal.

Back to blog