The amount of lime required to stabilize sludge is determined by the type of sludge, its chemical composition, and the solids concentration. In liquid lime stabilization, dosages of lime [Ca(OH)2] required to raise the pH to 12.5 or above and to maintain it at minimum pH 12 for 30 minutes for various types of sludge vary from 16 to 40% of sludge dry solids.
Chemical composition also determines the lime dosage required. This composition is a combination of the type of sludge as well as the type of treatment process from which it is produced, such as whether chemical coagulation treatment is used. The third factor affecting lime dosage is solids concentration. For liquid lime stabilization, lime dose per unit mass of sludge solids required to attain a particular pH level is relatively constant. That is, the lime requirement, rather than the sludge volume, is more closely related to the total mass of sludge solids. However, the lime dose required per unit mass of solids tends to be somewhat higher for very dilute sludge because more lime is required to raise the pH of water.
For dry lime stabilization, the dosage of lime required depends greatly on the type of final product required. Figure 6.4 shows the theoretical dry lime stabilization dose for both class B and class A stabilization (WEF, 1998). The
Required for 70°C
'('Class A temperature)'
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