BELT PRESSES

Description

A belt press thickens and dewaters waste sludge continuously. The sludge is deposited on a single moving belt made of fine woven cloth. As the liquid begins to drain through the porous belt, the largest solids coat the belt and trap finer solids. Some systems are no more complex than this and they are known as belt thickeners. Thickeners can achieve a sludge cake of 2-7 % dry solids. Some thickeners turn the sludge over on itself before it is discharged and many arrange the sludge in lines on the gravity filtration bed. Both techniques improve the dewatering efficiency with little additional energy input.

In the more complex belt presses the sludge is first flocculated with polyelectrolytes and thickened and then is pressed between two filter belts forming a wedge that gradually compresses the sludge. The belts pass around large perforated drums and smaller pinch rollers, which apply increasing pressure to the sludge. The sludge is also subjected to shearing forces and friction as one belt moves over the other and these actions release liquid from the sludge. Dewatered sludge is ejected into a container. The sludge cake is about 11-25 % dry solids, though this figure depends greatly upon the nature of the solids to be dewatered. The belts are cleaned with high pressure sprays.

Some belt presses increase their efficiency by incorporating vacuum trays under the belts to enhance the removal of liquid. Other designs use pneumatic compression devices to produce a final sludge cake.

Application

Belt thickeners are used to reduce the volume of sludge for land spreading and to concentrate sludge before dewatering. Belt presses are used for dewatering sludges from water treatment, municipal wastewater, paper mills, the food, metal, and chemical industries and many other sources.

Paper sludges typically produce cakes of high dry solids content (up to 40 %) unlike sewage sludges which tend to be more difficult to dewater.

Sizing

A belt press provides an energy-efficient method of dewatering sludge with a throughput of up to 200 m3/hour. The electrical consumption is about 20 kWh per tonne of dewatered sludge. The drive motor is usually rated between 0ยท4 kW and 4 kW. The weight of belt presses tends to be less than 15 tonnes.