6.2.4 Disposal of the sludge
Several options are available for disposal of the collected sludge (Pickford and Shaw, 2005). It can be put directly onto land and used as a soil conditioner, but this is only possible if it has been left untouched for at least two years (Brikke and Bredero, 2003). Fresh, untreated wet sludge poses high risks for human health and so should not be put on land used to grow crops.
Drying the sludge will kill most pathogens. This can be achieved using drying beds (Figure 6.4), where sludge is put into shallow tanks to a depth of about 300 mm. The base of the tank is sloped and covered with a layer of sand (forming a ‘bed’) to allow liquid to drain out of the sludge. In the warm climate of Ethiopia and without rain, after about a week the sludge will be dry enough to be lifted by a shovel.
The sludge can also be composted by mixing it with vegetable matter, or biogas can be obtained by anaerobic digestion. Whichever method is used, faecal sludge disposal must be carefully managed and operated in order to ensure that the associated risks to health and the environment are avoided.