Drum filters (microscreens) are the most common primary solids step in modern RAS and semi-flow facilities. A rotating drum with mesh (often about 40–90 µm) removes faeces, feed waste and other suspended particles as water passes outward or inward through the cloth.
When the mesh blinds, backwash starts — usually high-pressure spray nozzles and/or level or pressure control. Backwash water becomes a concentrated sludge stream that continues to settling, thickening or other sludge handling.
Why drum filters are used
- Continuous operation with automatic cleaning.
- Protects biofilters, UV and ozone from solids loading.
- Improves water clarity and oxygen/UV performance.
- Creates a manageable sludge stream instead of letting solids break down in the system.
Mesh size and sizing
Finer mesh clears water more but raises backwash volume, headloss and wash frequency. Coarser mesh lets more fines through to biofilters and UV. Choose micron rating from species, feed, downstream steps and available sludge handling — not “as fine as possible”.
- Hydraulic capacity (m³/h) at the selected mesh and solids load.
- Peak feed and expected TSS — not only averages.
- Backwash volume and where it will go.
- Redundancy: at least two units or a bypass with a planned emergency strategy.
Operation and maintenance
- Check nozzles, pressure and that the mesh is not damaged.
- Replace or repair mesh before tears pass large solids downstream.
- Monitor level/pressure alarms — a stuck filter can empty or flood sumps.
- Log wash frequency: a sudden rise may indicate overfeeding or disease.
Orientation — not a supplier selection. Capacity depends on mesh, load, temperature and water quality.
See also