Why Wastewater Plants Need Shaftless Screw Conveyors

Municipal and industrial wastewater treatment plants must continuously handle two demanding materials: dewatered sludge and screenings. Dewatered sludge is sticky, high in moisture, and variable in consistency. Screenings contain fibrous rags, plastics, and grit. In a shafted screw conveyor, rags and fibers wrap around the center shaft, while sticky sludge builds up between the shaft and the screw flights. Over time, conveying efficiency drops and the line must be shut down for manual cleaning.

A shaftless screw conveyor removes the center shaft entirely. Material is pushed by a continuous ribbon flight along a smooth U-shaped trough, with no shaft surface for rags to wrap around and no dead zones where sludge can accumulate. For Southeast Asian treatment plants, where influent composition fluctuates strongly between the dry and rainy seasons, this configuration significantly reduces jam-related downtime and manual cleaning.

Key Selection Parameters

Screw Diameter and Conveying Capacity

The WLS series offers six standard sizes with screw diameters ranging from 150mm to 500mm, covering a maximum conveying capacity of 0.5–28 m³/h. For dewatered sludge from belt or plate-frame filter presses, WLS250 to WLS400 are typical choices. Screenings and grit have lower bulk density, so capacity must be recalculated on a volumetric basis, often requiring the next larger diameter.

Conveying Distance and Inclination

A single unit can convey up to 30 meters (WLS400/WLS500 sizes) at an allowable working inclination of 0°–30° (WLS250: 0°–25°). Common layouts include horizontal transfer from the dewatering room to a sludge storage hopper, and inclined feeding to a dryer or incinerator inlet. Actual capacity decreases as inclination increases; layouts above 20° should include a design margin or a larger size.

Trough and Material Selection

Sludge releases moisture and hydrogen sulfide, and coastal plants add salt spray exposure. Stainless steel construction (commonly 304) for both trough and screw resists corrosion and permits direct water washing. The U-shaped trough uses flanged sections joined by bolts, allowing the conveyor length to be extended to match the actual process layout.

Typical Applications in Treatment Plants

• Sludge transfer: dewatered sludge from belt filter press or decanter centrifuge to storage or a drying stage
• Screenings handling: rags and debris from the inlet screens to a collection container
• Grit chamber discharge: dewatered grit to skip bins

Selection Considerations

1. Confirm the moisture content and fiber content of the material first — these define the screw diameter and material of construction.
2. Calculate the design capacity from the plant's dry solids throughput, not from pump flow rates.
3. At inclinations above 20°, derate the capacity or select the next larger size.
4. Specify stainless steel when hydrogen sulfide or salt spray is present, and schedule periodic water rinsing.
5. Reserve maintenance access at the drive end and add overload protection to handle unexpected jams.