How a pump-fed rotary drum filter works

An interactive model of a pump-fed rotary drum filter, built by Play It Koi.

Try the two install-day toggles in particular. Whether a dirty screen ends in a wash cycle or an empty pond comes down to those two, and both get decided the day the filter goes in.

Comparing the two feed styles? The gravity-fed version is here.

Pond pumps push water up into the filter, and the outlet decides where that water sits. Start with a clean screen and the two sides sit close together. As the screen dirties the pump keeps pushing the same water in whether the screen can pass it or not, so the dirty side climbs. That climb is what pushes water through the screen, and it is what the float switches are watching.

Side cutaway. Drawn with z-pipes on the outlets, which is what holds the level inside this filter. Some pump-fed units put the outlet ports higher up the housing instead and arrive at the same place. Either way the inlets and outlets sit on the same plane, and the return runs 4 in the whole way because that side is gravity.
Through the drum. Inside sits higher than outside, and that difference is the push.
Filtering normally
Flow is inside what a pump-fed drum carries and the wash float trips on a reasonable rise.
Running
Pond pump
Auto
Wash control
Idle
Auto-fill
The green band ends where pump-fed capacity ends, at roughly two thirds to three quarters of what the same drum carries gravity fed. Past that, water lands hard on the dirty side of the baffle and the turbulence pushes the level up on its own, with no help from a dirty screen. The check valve is what keeps the crest holding the level once the pump stops.
The longer probe, hanging lower, is the one that calls for a wash. Too low and it sits in the water and washes non-stop. Too high and the safety float gets there first. The band moves with pump flow, because flow lifts the outlet level under it.
Solids collecting on the inside face. The dirtier it gets, the higher the dirty side has to climb to push the same water through. Manual runs the drum on a fixed timer instead of the float, so a screen can load up between washes.
Dirty side, pump to drum Filtered water Captured solids Screen Water lost out the waste line
Drum filters do their best work gravity fed. Two things are working against a pump-fed setup, and both are visible above. Solids reach the pump before they reach the screen, so what arrives at the drum has already been chopped into finer pieces than it started as, and finer pieces are harder for any screen to hold. And the same drum carries only about two thirds to three quarters of its gravity-fed flow, because water arriving under pressure stirs up the dirty side of the baffle instead of settling across the screen. Pump-fed works, and sometimes the site leaves no other option. It is a compromise rather than the better way to run one.

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, outlet height, float positions 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.

Pump-fed drum filter questions

How a pump-fed drum filter works

A pump in the pond pushes water up into the filter. The water enters the inside of the drum and passes out through a fine mesh screen wrapped around it, and the solids stay behind on the inside face. Those are the brown dots that build up on the screen in the drum view above. As the screen loads, the dirty side has to rise higher to push the same water through. When it reaches the wash float, the drum turns and a spray bar hits the screen from the clean side, so the solids drop into the waste tray and go out the waste line. The filtered water leaves through the outlet, which sets the level inside the filter, and carries on to your biological filtration.

What are the advantages of a pump-fed rotary drum filter?

The main one is placement. The pump does the lifting, so the filter can sit on a pad at ground level or even above the waterline. You don't need bottom drains or a filter pit dug below the pond, which makes it the easier way to add a drum to a pond that's already built, and the plumbing is simpler too.

In exchange, the pump chops up solids before they reach the screen, the same drum carries only about two thirds to three quarters of its gravity-fed flow, and the feed pump runs around the clock. Where the site allows it, I'd still go gravity fed. The gravity-fed vs pump-fed comparison goes through the trade-offs in more detail.

Where are pump-fed drum filters used?

You'll see them on ponds built without bottom drains, and on sites where digging a filter pit below the water level isn't practical because of rock or a high water table. They also suit ponds that already run on a pump and skimmer, and setups where the equipment has to sit on a pad away from the pond. Most of the drums we carry come in a pump-fed version, including the SeaSide Aquatics RDFs, the Oase ProfiClear Premium Compact and the Oase ProfiClear Premium XL. You can compare them all on the rotary drum filters page.

What filter media does a pump-fed drum filter use?

The drum's only media is its screen, a fine stainless steel or polyester mesh rated in microns. Most of the drums we sell use screens between 60 and 120 micron, and the micron screen guide explains how to choose.

A screen only catches solids. The ammonia your fish give off is dissolved, so it goes straight through, and you still need biological media after the drum: a moving bed, static media, filter mat, a bead filter or a bakki shower. Combi units put both in one box. The SeaSide Aquatics Combi RDFs include pump-fed models with moving bed and mat chambers, and the Oase ProfiClear Premium Compact takes an add-on moving bed module.

What is the best pump for a pump-fed drum filter?

Pick the pump by the flow it delivers at your actual head, and match that to the drum's pump-fed rating. Two things to check:

  • Size to the drum's pump-fed rating, not its gravity rating. A drum fed by a pump carries roughly two thirds to three quarters of its gravity figure, so use the maker's pump-fed number.
  • Read the pump's performance curve at your real head: the lift from the pond surface up to the filter inlet, plus friction in the pipe and fittings. The big number on the box is the flow at zero head.

Don't overshoot either. Push more than the drum's pump-fed rating and the level rises on flow alone. Drag Pump flow past the green band in the sim and you'll see it. A variable-speed pump lets you set the flow to suit the drum. In my experience external pumps tend to outlive submersible ones, so that's where I'd start: external pond pumps.

Pump-fed RDF installation: the fail-safes

Most of what goes wrong with a pump-fed drum comes down to a few install-day choices. The sim lets you flip the two biggest ones.

  • Wire the pond pump through the filter. If the filter's controls have a pump connection, use it, so the safety float can shut the pump off. A pump on its own outlet keeps going when the screen clogs, and the pond ends up running out the waste line.
  • Put a check valve on the pressure line between the pump and the filter. When the pump stops, it keeps the filter from draining back to the pond through the pump, and it keeps an external pump primed. Evolution Aqua check valves come in 1½ and 2 inch.
  • Plan where the waste water goes. Every wash sends water down the waste line, and an overflow goes the same way. Run it somewhere that can take it, such as a garden bed, a dry well, or a drain where local codes allow.
  • Add an auto-fill valve to top the pond back up after each wash. It keeps up with normal washes but not with water pouring over the tray rim, so it doesn't replace the safety float.
  • Fit unions and ball valves on both sides of the filter, so you can isolate it or take it out without cutting pipe.
  • Set the wash float with some care. Too low and it sits in the water and washes non-stop. Too high and the safety float gets there first. In auto mode the screen washes whenever this float calls for it, so how often depends on feeding, debris and mesh size.

The RDF installation guide covers the plumbing and electrical in full, and the RDF maintenance guide covers priming, winterizing and troubleshooting.

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