The accumulated precipitate is mechanically collected from the bottom of the basin and pumped away. Sludge removal is an important step in effective sedimentation. The University plant has three softening units which operate in parallel. A slurry of slaked lime and ferric sulfate enters the softener riser and reaction zones , where a turbine mixes it with water from the settling basin. Ferric sulfate again acts as a coagulant for turbidity removal.
Slaked lime acts as a softening agent.
The chemical reactions for softening are relatively simple. The anions present with calcium and magnesium are usually bicarbonate and sulfate. Hardness is removed by introducing carbonate ions in the form of hydrated lime. Since the solubility of magnesium is low at pH above 10.8. Because the pH is relatively high in the softeners , a considerable amount of hydrated lime and calcium carbonate remains in the solution.