In all biological waste treatment processes some surplus sludge is produced. The objective of residual management is:

• Reduction of water content.
• Stabilization of sludge solids.
• Reduction in sludge solids volume.

In facultative type aerated lagoons and algal waste stabilization ponds, the surplus sludge settles out in the unit itself and is removed only once in a few years after emptying the unit, exposing the wet sludge to natural drying, and carting away the dried sludge for agricultural use or land filling.

In extended aeration process where aerobic digestion of surplus sludge is done, the sludge can be taken directly for dewatering and disposal.

In case of activated sludge and trickling filter plants, the sludge is taken (along with the primary sludge) to a sludge digester for further demineralization and thereafter it is dewatered.

Sludge Dewatering Methods

• Natural: sludge drying beds, sludge lagoons
• Mechanical: sludge thickeners, centrifuges, vacuum filters, filter press
• Physical: heat drying, incineration

Disposal of Sludge

Final disposal of sludge is to land and sometimes to the sea, in one of the following ways:

• Agricultural use of dried or wet sludge.
• Use of dried sludge as landfill in absence of agricultural demand.
• Spreading wet sludge on eroded or waste land, contouring the field, so as to gradually build up a top soil of agricultural value.
• Disposing off wet sludge along with solid wastes for (i) composting, or (ii) sanitary landfill.
• Transporting and dumping into the sea.