How a gravity-fed rotary drum filter works
An interactive model of a gravity-fed rotary drum filter, built by Play It Koi.
The failure modes are in here too, including the one where a filter set too low for its pond keeps running water out the waste line while the auto-fill valve pays for it. Move the controls and watch what the water does.
Comparing the two feed styles? The pump-fed version is here.
With the pump off, the water inside the filter sits level with the pond. Start the pump and it draws from the clean side, pulling that level down. The gap it opens is what pushes water through the drum screen, carrying it from the dirty side into the clean side. Everything else is bolted to the housing, so where the pond sits decides how the whole thing behaves.
About this model. Proportions are generic rather than any one manufacturer's unit, and time runs faster than real life so a wash cycle and an overflow are both watchable. Drum size, drawdown and wash water use vary widely from one unit and one pond to the next, so this shows how the parts behave rather than what any particular filter will do.
Gravity-fed rotary drum filter questions
What are the main parts of a gravity-fed rotary drum filter?
Dirty water comes in from the bottom drains through gravity lines to the filter's inlet chamber. Inside the housing is the drum, a cylinder wrapped in stainless mesh, usually 60 to 120 microns. Water passes through the mesh from the inside out into the clean chamber, where a pump draws it off and sends it on to the biological filter and back to the pond.
The cleaning gear sits around the drum. A wash pump feeds a spray bar outside the screen, and a waste tray inside the drum catches what the spray knocks loose and carries it to a drain. A float switch or optical sensor watches the water level, and a control box turns the drive motor and the spray on when the screen needs a wash. You can watch all of these work in the simulator above.
Gravity drum filter vs pump fed: what is the difference?
In a gravity-fed system the filter is set so its water level matches the pond, and water runs in from the bottom drains by itself. The only pump comes after the drum, so it moves water that has already been screened. In a pump-fed system a pump pushes pond water into the filter, which can then sit on a pad at or above the water line. That is much easier to add to a pond without bottom drains. The trade-off is that the pump's impeller breaks fish waste into smaller pieces before the screen sees it, and the same drum is rated for less flow. FREE Drum, for example, says to use 65% of the gravity rating when pump-fed. The gravity-fed vs pump-fed RDF guide covers the plumbing for both, and the pump-fed simulator shows the other layout running.
What solids does it remove?
Anything the mesh is too fine to let through: fish waste, uneaten food, string algae, leaves, bits of plant and fine suspended debris. Most of the drums we carry use a 70 to 75 micron screen. A finer screen catches smaller particles but washes more often and sends more water to drain. The drum does nothing about dissolved waste such as ammonia and nitrite, so a gravity-fed system still needs a biological filter after it.
Where are gravity-fed drum filters used?
Mostly on koi ponds built with bottom drains, and above all on new builds, where the filter pit can be planned before anything is poured. They earn their keep on larger or heavily stocked ponds, because the drum keeps clearing waste through the day without anyone rinsing a sieve or pads. The same filters are used in commercial aquaculture and aquaponics, where solids have to come out of recirculating water before it reaches the biofilter or grow beds.
How do I set up a gravity-fed drum filter?
Start with the height. Water inside the filter settles at the same level as the pond, a little lower while it is flowing. So the housing goes into a pit or vault deep enough that its working water level lines up with the pond, with its rim above the pond's waterline. Your filter's manual gives the exact range. Set it too high and the drains can't feed it. Set it too low and the pond level sits above the waste tray rim, so pond water runs straight out the waste line, and an auto-fill valve will keep replacing it. Raise the pond level slider in the simulator to see that happen.
Gravity lines are usually 4 inch, one per bottom drain, kept short and run with long sweep bends rather than standard elbows. A 4 inch line carries about 3,600 GPH by gravity, so the drain lines together have to carry at least what your pump moves. If the pump pulls more than the drains can deliver, it empties the clean chamber. A valve on each drain line where it meets the filter lets you shut one off for service.
A pond overflow keeps heavy rain from lifting the pond level toward the waste tray rim. Some drums carry their own protection too. The FREE Drum Filter, for one, has bypass ports you can open to keep water moving during a repair, and its control box can cut power to the pump when the water gets low.
What happens during a gravity fed RDF wash cycle?
As solids build up on the screen, less water gets through it, so the level on the clean side drops while the pump keeps drawing. When it falls to the sensor, the controller turns the drum and starts the spray. The dirty part of the screen rotates up out of the water, the spray knocks the solids into the waste tray, and they flush to drain. A wash takes seconds and uses a few gallons, which the pond makes up from rain or the auto-fill. In peak feeding season a drum can wash dozens of times a day. In the simulator the brown dots are the solids. They stop on the inside of the screen and ride up to the spray when the drum turns.
How often does it need maintenance, and what about winter?
The drum cleans its own screen, but give it a check every few months in the active season. Soak the spray nozzles in vinegar if hard water has scaled them, make sure the waste line runs clear, look the mesh over for tears or algae, and confirm the sensor still starts a wash. Grease bearings and seals on the schedule in your manual.
In cold climates koi stop eating over winter and make far less waste, so many keepers shut the drum down or run it less. Either way, keep water from freezing in the housing, the spray lines or the waste line. The rotary drum filter guide has more on care through the year.
What is the best gravity fed drum filter for koi pond builds?
Size comes first. Pick a drum whose gravity flow rating covers the pump flow you plan to run, and go a size up if you can, since a bigger drum washes less often and wears less. The drum filter sizing guide walks through the numbers. After that, compare screen sizes and check how deep a pit each one needs.
Gravity-fed models we carry include the NishikigoiRDF, which is built for gravity feed only, the FREE Drum Filter, whose top sits just 4 inches above pond level, the Sea Side Aquatics RDFs in their Gravity Fed versions and the Oase ProfiClear Premium XL in its Gravity configuration. You can compare them in our rotary drum filter collection, and the gravity-fed system design guide shows how the drum fits between the drains and the biofilter.