The disposal of sludge may be carried out by the following methods:
1. Burial or dumping into the sea or other large bodies of water
This is possible only in the case of cities situated on the banks of large rivers or tidal waters providing chance of disposal. Sludge removal in this case is through the process of dilution.
2. Shallow burial into the ground
Wet sludge is run into trenches 0.9 m wide x 0.6 m deep and regularly spaced 0.9 to 1.5 m apart and in parallel rows for disposal. Sludge is then dried to a firm stage, it is covered with a thin layer of soil. After about a month, land is ploughed up with powdered lime and planted with crops. Method of disposal called composting is useful, but the limitation is the area of land required, about 0.84 m square per person.
3. Lagooning (Sludge Lagoons)
This involves making earth tanks or ponds 0.6 to 1.2 m deep, and underdraining them with 100 mm dia. agricultural tile drains, spaced at intervals of 2.7 m. Bottom of the tank is covered with a 15 cm layer of clinker or ashes to prevent unsafe disposal. Sludge is then run in or pumped in and allowed to remain there for a period of 2 to 6 months. When the moisture has been drained or evaporated, contents are dug out to about half of their original volume and used as manure. This method is quite cheap, but its limitation is the nuisance resulting from smell during anaerobic decomposition and files, so that its use is restricted to noninhabited areas.
4. Mechanical dewatering of sludge
The moisture content is reduced to about 50 per cent and the volume to 20 per cent for easy disposal. Sludge dewatering method examples include, filter-pressing, vacuum filtration, centrifuging, heat drying and biological flotation. Sludge cakes may be sold and used for filling low lands or mixed with house-refuse and then burnt up in incinerators.
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