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.

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