The drum rotates slowly partially submerged in a trough of suspended solids. The drum has ends with a hole in the center through which the feed enters. There is no vacuum; a slight difference in level provides only a small driving force. This means that the filtration medium must be quite coarse to get any flux. The screen is fine mesh that captures only fairly large particles such as algae. As the mesh moves to the top, spray dislodges the drained particles.
When a microstrainer is used to collect algae, the original suspension may be faintly green. The liquid in the collection trough is a much more intense green, but the concentration of algae is still low. Small algae or needle-shaped algae that are on end pass the screen and are not collected.