Screenings, grit and dewatered sludge have to be moved from the machine that produces them to a container, a silo or a truck. In wastewater treatment this is done with screw conveyors or belt conveyors. Both work; which is better depends on the material, the distance and the layout. This guide compares the two and names the data needed for a design.
How the two conveyors work
Screw conveyor
A spiral turns in a U-shaped trough or a tube and pushes the material along. Two designs are used:
- Shaftless spiral — a strong spiral without a centre shaft, running on a wear liner. Nothing can wind around a shaft, so it handles sticky, fibrous and lumpy material such as screenings and sludge cake.
- Shafted screw — a screw on a shaft with bearings at the ends. For free-flowing material such as grit, lime and dry bulk solids, and for steep or vertical conveying.
Belt conveyor
An endless rubber belt runs over a drive drum and a tail drum and is supported by idlers. The material lies on the belt and is carried without being pushed or sheared. Scrapers clean the belt at the discharge end.
The most important differences
| Criterion | Screw conveyor | Belt conveyor |
|---|---|---|
| Enclosure | Closed trough or tube: odour and splashes stay inside | Open, unless covered |
| Distance | Up to about 30 m per conveyor | Up to 100 m and more |
| Inclination | Shaftless up to about 30°, shafted up to 45° and vertical | Up to about 20° with a smooth belt, 30° with cleats |
| Effect on the material | Pushes and mixes; sludge cake can become pasty | Carries gently; cake keeps its structure |
| Feed and discharge points | Several inlets and outlets with slide gates | Usually one feed point and discharge at the head |
| Space | Compact, can pass through walls and floors | Wider, needs access along both sides |
| Cleaning | Little; flushing connections are possible | Scrapers and return belt drop material; needs regular cleaning |
| Wear parts | Liner and spiral | Belt, scrapers, idlers |
| Energy | Higher for a given distance, because the material is pushed | Low |
Which conveyor for which material
- Screenings — shaftless screw conveyor. The material is wet, fibrous and smells; the closed trough and the open spiral suit it.
- Grit — shafted or shaftless screw with wear protection. Grit is abrasive, so the speed is kept low.
- Dewatered sludge over short distances — shaftless screw conveyor, also for distribution into several containers.
- Filter press cake — the cake drops in batches over the whole length of the press. A belt conveyor or a wide shaftless screw under the press takes it away; a belt keeps firm cake in pieces, which makes it easier to stack.
- Long distances and high throughput — belt conveyor.
- Lime, polymer powder and other dry products — shafted screw in a closed tube.
Sizing a screw conveyor
- Throughput — the volume per hour follows from the diameter, the pitch, the speed and the degree of filling. Shaftless spirals for sludge and screenings turn slowly, at 10–30 revolutions per minute.
- Degree of filling — 30–45 % for sticky and abrasive material, more for free-flowing material.
- Inclination — every degree lowers the capacity; above 20–25° a shaftless conveyor needs a closed tube and a larger size.
- Liner — wear-resistant plastic for sludge and screenings, steel wear bars for grit.
The screw conveyor calculator estimates the capacity for a given diameter, speed and inclination.
Sizing a belt conveyor
- Belt width and speed — chosen from the throughput and the lump size; sludge belts run slowly, typically below 1 m/s.
- Troughing — a troughed belt carries more and loses less at the edges than a flat belt.
- Belt cleaning — two scrapers in series and a drip tray under the return belt keep the area clean.
- Covers — hoods reduce odour and keep rain off outdoor conveyors.
- Safety — pull-cord switches along the belt and guards at the drums.
Data we need for a design
- Material, its dry solids content and bulk density.
- Quantity per hour, and whether it arrives continuously or in batches.
- Layout: distance, difference in height, feed points and discharge points.
- Installation indoors or outdoors, and requirements for odour control.
- Operating hours and the machines upstream and downstream.
Our conveyors are built in stainless steel 304 or 316L and matched to the upstream machine, for example a filter press or a fine screen.
Frequently asked questions
Which conveyor is better for sludge cake?
For short distances and where odour matters, a shaftless screw conveyor. For long distances, or where the cake should stay in firm pieces, a belt conveyor.
Can a screw conveyor run uphill?
Yes. Shaftless conveyors work up to about 30°, shafted screws in a tube up to 45° and, as vertical conveyors, at 90°. The capacity falls as the inclination rises.
Why does sludge sometimes become liquid in a screw conveyor?
Some sludges soften when they are sheared. Long screws, high speeds and several transfer points make it worse. A large, slowly turning spiral or a belt conveyor avoids the problem.
How long does a liner last?
With sludge and screenings several years; with grit considerably less. Liners are made in segments and can be replaced without removing the spiral.
Can one conveyor fill several containers?
Yes. A screw conveyor with several outlets and motorised slide gates fills containers one after the other, controlled by level sensors or by time.
Send us your material data and a sketch of the layout via our contact page. We will propose the suitable conveyor type and size.