Lime:
Lime is widely used with other coagulation in water treatment for raising pH and increasing alkalinity as indicated above. Lime is used alone as a precipitant to aid coagulant by iron salts. The reactions taking place is:
Ca(OH)2 + H2CO3 ↔ Ca CO3 + 2H2O
Calcium Hydroxide Carbonic Acid calcium Carbonate
(Lime) (ppt)
Ca (OH)2 + Ca(H2CO3)2 → Ca CO3 (OH)2 + H2CO3
(Calcium Carbonate) (ppt)
A sufficient quantity of lime must be added to combine with all the free carbonic acid and Calcium Bicarbonate, which act as a coagulant. Like all the other coagulants, the quantity of lime required for precipitation or coagulation need to be determined by the JAR TEST. Lime can be brought as CaO (quick lime) or Ca(OH)2 (slaked or hydrated lime). The major problem in using lime as coagulant is the disposal of the large quantity of sludge produced. Lime is a unique coagulant in cases where phosphorous removal is desired and also in instances where it may be desired to remove ammonia by air stripping. For phosphorous removal, the operating equation is:
(pH above 9.5)
10Ca++ + 6PO34- + 2OH- ↔ Ca10(PO4)5 (OH)2 (ppt)