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5.4 Thickening

Sludge contains a high concentration of solids, but its water content is still high. Combined primary and secondary sludge from an activated sludge treatment plant contains about 2 % solids and hence 98 % water. One kg of dry sludge is associated with 49 L of water. Thickening to 5 % solids means one kg of dry solids is associated with 19 L of water, thus 30 L of water has to be removed.

Thickening is carried out in a sedimentation tank or in a sedimentation pond (Figure 2.42). The latter is advantageous if land area is available, because the sludge can be allowed to settle over a much longer period and a higher solids content of the thickened sludge is achieved. The water removed from thickening needs treatment. It can be returned to the inlet of an off-site wastewater treatment plant, or in the case of sludge from on-site units by an aerobic treatment process such as lagooning.


Figure 2.42: Sludge thickening pond (Ghana) (Heinss et al., 1999)

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