Biological Filters are the heart of a life support system for closed aquatic culture systems.

There are many types of biological filters available that Pure Aquatics have experience in using, deigning, building and implementing.

Pure Aquatics both manufacture and import a variety of biological filters and filter media suitable for a wide variety of biological loads and applications.


Moving bed biological reactors (MBBR’S) have become the standard in biological filtration in recirculating aquaculture systems and for good reason.

They are robust, easy to build, inexpensive to operate, require low pumping heads and they control biofilm thickness, ensuring optimum biological performance. They also simultaneously oxygenate the filter and strip a percentage of CO2 for the system.

Through extensive and ongoing research, experience and development, Pure Aquatics have optimised our MBBR design for high removal, high oxygenation, superior mixing and easy maintenance.


Trickle filters were a very widely used biological filter in the past and still have their place in aquaculture today.

Trickle filters are flexible, they have a wide hydraulic loading range, oxygenate and degass the water as a function of their opperation, and have a very high ammonia and nitrite consumption rate per m2. Just like all filters, the higher the ammonia and nitrite levels the more the biological filter removes (however effluent concentrations go up), however trickle filters really excel at high ammonia concentrations.

The one thing they do not do is self clean, and clogging can be an issue. Ways to negate this is to filter the water to a fine degree, and/or increase the void space of the media, which also decreases surface area. They can also be more expensive to operate due to having to pump to elevated heights for operation. Employed as degassing units under gravity or on low head pumping loops they are very effective, especially for high density culture systems.

Trickle filters are worth considering for biological filters in certain applications and should not be discounted.


Fluid bed filters, also called fluid sand filters, fluidised bed filters or fluidised sand filters, pack a huge surface area into a compact area.

Depending on sand choice, surface areas can range from 8000m2/m3 up to and sometimes exceeding 20,000m2/m3.

They operate well in RAS systems but require the water to be quite clean prior to entering the filter due to their susceptibility to blocking or producing thick bio films that result in excess sand loss.

Fluid bed filters lose sand over time as the sand is colonised by bacteria and becomes lighter and floats out of the system.this can be managed by siphoning the top layer, increasing velocity at the top of the vessel etc but is generally a higher maintenance filter than an MBBR.

They may have a high surface area, but removal rates are generally lower per m2 as a fluid bed is a very abrasive environment, ammonia removal per footprint however, is still at the higher end. Pumping and energy requirements is higher compared to some other biological filter options, but they do have their place in biological filtration.

Moving Bed Biological Filters

Moving bed biological filters, also referred to as moving bed biological reactors or MBBR’s, are ideal biological filters for ammonia and nitrite removal in aquatic systems. Moving bed biological reactors perform biological filtration in a controlled fashion but constantly keeping biological media in suspension with air, which oxygenates the water around the media and transports oxygen to the biofilm. This aeration simultaneously strip a portion of CO2 from the culture water, stabilising pH in the reactor and lowering CO2 levels for the culture species.

MBBR’s (Moving Bed Biological Filters) operate at very low head pressures, and commonly under gravity after a drum filter, reducing pumping costs dramatically. Their main advantage however is biofilm control and self cleaning. Due to the constant aeration, the media exfoliate against one another, removing waste and exfoliating old and inefficient layers of biofilm, keeping the nitrifying bacteria layer thin and young, for maximum ammonia uptake.


Pure Aquatics have optimised our MBBR design and opperation by:

  • Media choice
  • Control of dwell times
  • Optimising pipe sizes and minimising friction loss
  • Careful control of dimensions and mixing requirements
  • Superior biofilm control through advanced mixing
  • Optimised aeration and circulation through diffuser design and material selection
  • Media choice

Our standard media we employ in our MBBR filters is the PAX-1 biological carrier, a standard design that is dimensioned for optimal mixing, water flow, water distribution, waste removal and biofilm surface area growth.

PAX-1 has a minimum surface area of 1000m2/m3.

Also available to utilise in our MBBR’s is the Hel-X Flake biological media, the chip/wafer style media was the first of its kind in the world, and Hel-X continue to lead the way in this style of biological filter media design and manufacture.

The 30mm Hel-X flake is over 5000m2/m3.

Hel-X can be applied at up to 100% of the media volume, but is also applied at between 10-30% as well to augment the biological filter capacity.

People wishing to increase their filtration capacity in their MBBR that utilise a 50% fill rate can add this media at 10% of the media volume to increase the filtration media volume to 60%

Hel X-Flake single
Biological Filters


Dwell time inside any reactor is critical, MBBR’s being no exception. The lower the dwell time (or higher the flux rate) the less ammonia and nitrite the filter will remove per pass, and could result in higher residual TAN levels in the tank.

Generally speaking, dwell times cant be too high, as long as oxygen levels are maintained and sufficient food (ammonia and nitrite) is supplied to the bacteria, some dwell times in wastewater treatment plants exceed 24-48 hours.

However, to maintain an increased dwell time a larger reactor (and potentially more media) is required, increasing construction and operating costs. Therefore, depending on the chosen effluent TAN level exiting the reactor, increasing the required size of the reactor to improve dwell time may or may not be required.

through careful design of the reactor overall, Pure Aquatics can design an MBBR for optimal dwell times and performance for any application.


Reactor dimensions influence mixing inside the reaction chamber, pumping requirements for water flow, and injection depth for air flow.

Utilising the specially designed MBBR diffuser assembly and manipulating biological filter dimensions the Pure Aquatics design approach and MBBR filters achieve optimum results for ammonia removal for the target system requirements.


Unless specified by the customer, all of Pure Aquatics MBBR filters utilise our newly developed MBBR diffuser assembly.

In the past, a plumbed hardline manifold of drilled pipe has been used for aeration of a moving bed, with very inefficient aeration and sub par circulation for the amount of air needed.

A variation of this approach is again a plumbed hard line manifold along the bottom of the reactor, but rather than using drilled pipe, membrane diffusers are used, improving circulation and aeration dramatically through the use of smaller bubbles, concentrated into streams spread throughout the reactor evenly.

This approach, although incredibly effective has 2 downsides, cleaning can sometimes result in technicians unintentionally breaking off diffusers, or when the diffuser finally does fail due to wear, a new diffuser needs to be fitted and possibly plumbing repaired.

This is not only a time consuming process can can also be expensive, requiring either a diver to install the new diffuser, or complete shut down of the filter while repairs are carried out.

Utilising Colorite Aerotube in a specifically designed self weighted configuration we have solved the above issues by making the diffuser assemblies not only removable for maintenance, but also mobile to allow the best positioning of the aeration and circulation.

By utilising the superior aeration and bubble distribution of colorite aerotube, we have also dramatically improved oxygen transfer, mixing and cleaning in our MBBR’s, whilst making set up, operation, maintenance and servicing easier.

Media Volume litres2003006001000200040006000
Dimensions700mmD x 1400mmH700mmD x 1800mmH1100mmD x 1800mmH1300mmD x 2200mmH1850mmD x 2100mmH2700mmD x 2200mmH2700mmD x 2660mmH
Flow rate MIN-MAX40-80lpm60-120lpm120-240lpm200-400lpm400-800lpm750-1500lpm1280-2560lpm
TAN Removal gm’s/day @ 0.25mg/L TAN effluent40gm’s TAN (1kg food)60gm’s TAN (1.45kg food)120gm’s TAN (2.9kg food)200gm’s TAN (4.85kg food)400gm’s TAN (9.7kg’s food)780gm’s TAN (19kg’s food)1200gm’s TAN (29kg’s food)
TAN Removal gm’s/day @ 1mg/L TAN effluent120gm’s TAN (3kg’s food)180gm’s TAN (4.35kg food)360gm’s TAN (8.7kg food)600gm’s TAN (14.5kg’s food)1200gm’s TAN (29.1kg’s food)2360gm’s TAN (57kg’s food)3600gm’s TAN (87kg’s food)


  • Pax1 media
  • 45% protein content
  • Pure aquatics mbbr design
  • 25 degree celsius water temperature

RK2 Fluidised Sand Biofilters

RK2 Fluidised Sand BioFilters utilize continually moving sand in a clear acrylic column to provide large nitrification capacity in a small footprint. These proven bio filters have a distinct advantage over other types of Bio filters as 100% of the media surface area is utilized in the nitrification process. RK2 Fluidised Sand BioFilters are capable of being placed inline in a low pressure system and maintain a constant flow rate with virtually no maintenance.

RK2 Fluidised Sand BioFilters have the ability to pack a huge surface area into a very small footprint. The also provide very high removal rates per pass (especially with smaller sand sizes) and result in very low residual levels of ammonia and nitrite in the system.

As they are not an aerated biofilter, the water exiting them needs to be degassed on order to remove CO2 and reintroduce oxygen to the water before it is processed further and then distributed back to the culture tanks.

RK2 Fluidised Sand BioFilters can be used in either marine or freshwater applications.
Coupled to an Ozone equipped Protein Fractionator, they create a complete Aquatic Life Support System.

Innovation and quality are built into every RK2 Aquatic Life Support product.
Contact Pure Aquatics for specific applications and quotations.


  • Sand media
  • Check valve and flow pumps with union connectors

Pure Aquatics utilises standard sand sizes for predicted nitrification rates. Sand sizes as low as 0.2mm can be utilised for higher loads and require less flow to fluidise them.

Small sands can be used for higher removal rates of ammonia and nitrite per pass, and potentially high nitrification rates, however biosolids accumulation at the top of the fluidised bed periodically need to be maintained to avoid excess sand loss.

Fluid bed filters lose between 25%-100% of their bed volume per year, periodic replacement of sand is recommended for consistent filtration rates.

Upon startup we recommend marking the operating level of sand when the filter is turned off, and periodic temporary shut down to check the sand levels and top up as necessary.

ModelDiameter x Height CMMedia CapacityMedia Height inside filter dryFlow Rate LPMGM’s TAN/day coldwater 15 degreesGM’s TAN/day warmwater 25 degrees
RK10AC-FSF15cmD x 180cmH11L60 CM50 LPM711
RK40AC-FSF20cmD x 230cmH28L90 CM150 LPM1928
RK75AC-FSF25cmD x 230cmH45L90 CM265 LPM3145