Moving bed filters
Key facts: A moving bed filter is an aerated biological filter in which free-floating plastic media are kept in constant motion by air or water flow, providing surface area for nitrifying bacteria biofilm.[1][2]
Moving bed filters are listed among the aerated types of biological filter used on koi ponds, alongside Bakki showers and trickle filters, as distinct from non-aerated sand or cross-flow filters.[1] The underlying technology, the moving-bed biofilm reactor (MBBR), was originally developed for municipal and industrial wastewater treatment before being adapted to recirculating aquaculture systems (RAS).[3][2]How moving bed filters work
In an MBBR, wastewater or pond water passes through an aeration tank containing plastic carriers on which a biofilm of bacteria grows.[3] The carriers are kept in motion by the aeration system, which also ensures contact between the substrate in the water and the biomass on the carriers.[3] A sieve retains the carriers inside the tank while allowing treated water to pass.[3] In aquaculture-specific designs, large plastic media of about 6 to 13 mm (1/4 to 1/2 inch) are manufactured to provide protected surfaces for fixed-film bacteria, and the media bed is constantly aerated to keep it moving.[2]Because a significant volume of air must be injected to keep the bed moving, moving bed reactors can make a meaningful contribution to a system’s aeration and degasification needs in addition to biofiltration.[2] See also aeration and air pumps and biological filter media for related components.Media and materials (K1 and similar carriers)
MBBR systems use a wide variety of plastic carriers that vary in surface area and shape, each with different advantages and disadvantages; surface area is considered crucial because it determines contact between the biofilm and the water, air, bacteria and nutrients.[3] High-density polyethylene (HDPE) is described as the most preferable carrier material due to its plasticity, density and durability.[3] Media can occupy as much as 70 percent of the tank volume in these designs.[3]Some polystyrene bead materials have shown initial resistance to bacterial colonization, possibly due to surface additives such as flame retardants, though this resistance is reported to be short-lived.[5] In moving bed or mixed bed biofilters specifically, bacteria can also be slow to colonize because of excessive agitation or aeration abrading the media surface and scraping off cells trying to adhere; reducing aeration and water flow during start-up, then gradually increasing it as ammonia and nitrite readings fall, is one method operators use to address this.[5]Capacity and performance
According to the Southern Regional Aquaculture Center, moving bed reactors have a robust design and are relatively easy to design and operate.[2] Their nitrification capacity tends to be only one-quarter to one-half that of fluidized sand beds, with filters typically sized to support about 0.5 to 1 pound of feed ration per cubic foot of media (8 to 16 kg/m3/day).[2] Low cost of construction and simplicity of operation are cited as offsetting this lower conversion rate, which is said to make the design popular across all sizes of operation.[2] By comparison, a well-designed fluidized bed is described as capable of achieving the highest volumetric conversion rates among the biofilter types reviewed, in the range of 1 to 2 pounds of daily feed ration per cubic foot of sand (16 to 32 kg/m3/day).[2]Advantages and disadvantages
Advantages reported for MBBR technology in general wastewater applications include increased performance and volumetric treatment capacity, higher effective sludge retention time favorable for nitrification, automatic response to load fluctuations, lower sludge production, less required area, and resilience to toxic shock.[3] Because there is no sludge return line, process performance is said to be independent of a secondary clarifier.[3] MBBRs do not require recycling of sludge, unlike activated-sludge systems, and can often be retrofitted into existing tanks to increase treatment capacity without expanding the footprint.[3]Reported disadvantages include bioclogging and build-up of headloss compared to other biofilm processes, along with feed pipe or effluent sieve blocking, nonhomogeneous mixing, carrier voids blocking, destroyed carriers, carriers accumulating at effluent sieves, and carrier overflow.[3]Place within a pond filtration system
In recirculating aquaculture tank system designs reviewed by the Southern Regional Aquaculture Center, a moving bed reactor is commonly paired with a mechanical solids-removal stage, such as a bead filter or microscreen, placed upstream to reduce the organic load reaching the biofilter.[2][4] This mirrors general practice in pond filtration, where settlement or mechanical stages such as bead filters, drum filters and vortex and sieve chambers typically precede biological stages.Fixed-film biological filtration in general relies on a substrate with large surface area where nitrifying bacteria attach and grow, converting ammonia and nitrite to the less toxic nitrate; common substrates listed include gravel, sand, plastic beads, plastic rings, plastic tubes and plastic plates.[4] This process is part of the broader nitrogen cycle that koi keepers must manage, alongside monitoring of ammonia and nitrite.Related biofilter types
- Fluidized bed filters suspend a bed of sand in an upflowing current of water, offering very high surface area and, when well designed, the highest volumetric conversion rates among the biofilter types compared.[2]Floating bead filters use a bed of static or continually moved plastic beads and can double as clarifiers, supporting both solids capture and biofiltration.[2]In home aquarium use, a fluidized bed filter directs water through a sand or similar media bed from below so the media behaves like a fluid, giving a very large surface area for denitrification bacteria in a comparatively modest-sized filter.[6]Biofiltration as a general pollution-control technique relies on a fixed biofilm formed by bacteria, fungi, and other microorganisms growing on a media bed, whether for water or air treatment.[7]Constructed wetlands rely on a related principle, using biofilm on coarse sand or gravel media for aerobic or facultative bacterial filtration as a secondary treatment stage.[8]
Common questions
What is a moving bed filter on a koi pond?
It is an aerated type of biological filter in which plastic media are kept moving by aeration, providing surface area for nitrifying bacteria biofilm; it is listed as one of the aerated biofilter types used on koi ponds, alongside Bakki showers and trickle filters.[1]
What media is used in moving bed filters?
How does a moving bed filter compare to a fluidized sand bed?
A moving bed reactor’s nitrification capacity is typically only one-quarter to one-half that of a well-designed fluidized sand bed, but it offers robust, simple construction and operation that offsets the lower conversion rate.[2]
Why does moving bed media sometimes take longer to grow bacteria?
Excessive agitation or aeration can abrade the media surface and scrape off colonizing bacteria cells; reducing aeration and water flow during start-up, then gradually increasing it as ammonia and nitrite decrease, can help establish the biofilm.[5]
Can a moving bed filter also help with aeration?
Yes; because a significant amount of air must be injected to keep the media bed moving, moving bed reactors can make a significant contribution to a recirculating system’s aeration and degasification needs.[2]
Related koi topics
References
- ^ Koi pond (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Koi_pond. Retrieved November 2026.
- ^ SRAC Publication No. 453: Recirculating Aquaculture Tank Production Systems: A Review of Current Design Practice. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/453%20Recirculating%20Aquacultu…. Retrieved November 2026.
- ^ Moving-bed biofilm reactor (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Moving_bed_biofilm_reactor. Retrieved November 2026.
- ^ SRAC Publication No. 451: Recirculating Aquaculture Tank Production Systems: An Overview. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/451%20Recirculating%20Aquacultu…. Retrieved November 2026.
- ^ SRAC Publication No. 4502: How to Start a Biofilter. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/4502%20How%20to%20Start%20a%20B…. Retrieved November 2026.
- ^ Aquarium filter (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Aquarium_filter. Retrieved November 2026.
- ^ Biofilter (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Biofilter. Retrieved November 2026.
- ^ Constructed wetland (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Constructed_wetland. Retrieved November 2026.