4.12 Sludge treatment and disposal
The sludge collected in any sedimentation tank in the water treatment process has to be disposed of. In some instances the wet sludge is transported to the nearest sewage works where it is discharged into the raw sewage inlet channel. The presence of the added chemicals can help in the primary sedimentation of the crude sewage. Alternatively, the sludge can be sent to a landfill site after it has been concentrated into a cake by dewatering.
The dewatering is carried out by pressure filtration, vacuum filtration or centrifugation. In pressure filtration, the sludge is pressed at high pressure between filter cloths. Vacuum filters are popular in the USA but few treatment works in the UK employ them. A slowly revolving drum, partly immersed in the sludge, carries a filter cloth through which water is sucked from the sludge under vacuum. In centrifuging, chemically or biologically conditioned sludge falls onto the centre of a rapidly rotating bowl. The solids are thrown to the outer edge of the bowl where they are removed by a scraper. With chemical-conditioned sludge, 80–98% of the solids can be separated this way. These solids would contain typically 10–35% water.