Drum filters
Key facts: A drum filter is a mechanical filtration device that uses a rotating cylinder covered in fine mesh screen to strain suspended solids from pond water, automatically backwashing the screen clean with spray jets.[1]
Drum filters, also called rotating microscreens, are mechanical solids-removal devices used in recirculating aquaculture and koi pond systems. A drum shaped structure covered with a fine mesh screen rotates as pond water passes through it, with a typical mesh size of 20 to 60 microns catching settleable and larger suspended solids.[1]In koi keeping, mechanical filters such as drum filters are placed ahead of the biological filter media to trap solids before they can clog biological stages, and are generally recommended to be backwashed or cleaned out often.[2]How drum filters work
A rotating microscreen works by passing water through a fine mesh screen stretched around a drum shaped structure. As solids accumulate on the screen, the drum periodically rotates so that the screen can be counter-flowed with high pressure jets, washing captured material away from the filtering surface and into a waste line.[1]This mesh is described as typically 20 to 60 microns, fine enough to catch settleable solids and a large share of total suspended solids passing through the unit.[1]Backwashing and water loss
Because the screen is cleaned with spray nozzles whenever enough solids build up, microscreens can generate a substantial amount of backwash water, which represents water lost from the system each time the drum cycles.[1]Some recirculating systems address this by using circular tanks with a dual-drain design and swirl separators ahead of the microscreen, reducing the solids load the drum has to process and thereby lengthening the interval between backwash events. Foam fractionators can also be added to remove finer suspended solids and colloids, further reducing how often the screen needs to wash and cutting overall water loss.[1]Capabilities and limitations
Rotating drum microscreens are described as somewhat limited in their ability to capture fine suspended solids compared with some other technologies, but they require very little floor space and do not need daily wash downs, which makes them compact relative to settling basins or other mechanical options.[1]Settling basins, by comparison, rely on gravitational settling and require large surface areas relative to flow, consuming valuable space, and are described as very ineffective at removing fine suspended solids.[1]Place in the treatment train
Mechanical filtration is one of several required unit processes in recirculating systems, alongside waste solids removal, biological filtration, aeration or oxygenation, and carbon dioxide and disinfection steps. Screen filters, including rotating designs, are listed among the common solids-removal components used before biological filtration stages such as fluidized beds, bead filters, moving bed reactors, or trickling filters.[3]In koi pond systems generally, the mechanical filter’s role is to trap solids so they do not clog the biological filter; mechanical filter types mentioned in pond literature include vortex units, brushes, matting, sand and gravel, sieve screens, and settlement chambers in addition to drum-style microscreens.[2]Readers comparing solids-removal options may also consult pages on vortex, sieve and settlement chambers, bead filters, and moving bed filters, which use different mechanisms to capture or process solids and biofilm.[1]Why solids removal matters
About half the feed consumed by fish is excreted as solids, which rapidly break down into finer particles classified as settleable solids, suspended solids, colloids, or dissolved solids. The suspended solids fraction contains a large share of organic material measured as volatile suspended solids.[1]Fine suspended solids below about 30 micrometers can contribute more than half of total suspended solids in a recirculating system, increase oxygen demand, and irritate fish gills, which is part of the rationale for mechanical solids removal ahead of biological treatment stages discussed on the pond filtration overview page.[3]Common questions
What mesh size do drum filters use?
Rotating microscreens typically use a mesh size of 20 to 60 microns, which is fine enough to catch settleable solids and a large portion of suspended solids in the water.[1]
How does a drum filter clean itself?
The drum periodically rotates so the screen can be counter-flowed with high pressure spray jets, washing accumulated solids off the mesh and into a waste stream.[1]
Do drum filters replace biological filtration?
No; drum filters perform mechanical solids removal and are typically placed ahead of a biological filter, such as a moving bed filter, to prevent solids from clogging biological media.[2][3]
What are the main drawbacks of drum filters?
They are described as somewhat limited in capturing very fine suspended particles and can generate substantial backwash water with each cleaning cycle, representing ongoing water loss from the system.[1]
What other mechanical filter types are used in koi ponds?
Koi pond literature lists vortex units, brushes, matting, sand and gravel, sieve screens, and settlement chambers as other mechanical filter types alongside rotating drum microscreens.[2]
Related koi topics
References
- ^ 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.
- ^ Koi pond (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Koi_pond. 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.