This possibility of using sludge as a fertiliser depends on the local legislation and on the sludge composition, which is determined by the wastewater composition and origin and the wastewater treatment system.
Lime stabilisation increases the dry solids content of sludge by raising the temperature and linking each water molecule in the sludge with quick-lime (CaO) in the following reaction:
CaO + H2O Ca(OH)2 + heat
Lime helps to hold heavy metal ions in an insoluble form, to eliminate pathogens and fermentation, and to reduce odours. The sludge must be thoroughly and evenly mixed with lime. Dosing and mixing are critical and require special attention.