Chemical precipitation or coagulation and flocculation with various salts of aluminum (e.g., alum), iron, lime and other inorganic or organic chemicals are widely used processes to treat water for the removal of colloidal particles (turbidity) and microbes. Treatment of water by the addition of chemical coagulants and precipitants has been practiced since ancient times, even though the principles and physico-chemical mechanisms may not have been understood. Sanskrit writings refer to the use of vegetable substances, such as the seed contents of Strychnos potatorum and Moringa oleifera, which are still in use today for household water treatment (Gupta and Chaudhuri, 1992). Judeo-Christian, Greek and Roman records document adding “salt”, lime, “aluminous earth”, pulverized barley, polenta as precipitants to purify water. Although alum and iron salts are the most widely used chemical coagulants for community drinking water treatment, other coagulants have been and are being used to coagulate household water at point of use, including alum potash, crushed almonds or beans and the contents of Moringa and Strychnos seeds.
Pliney (c.77 A.D.) said that polenta, a kind of food, added to nitrous or bitter water would render it potable in two hours, and that a similar property is possessed by chalk of Rhodes and the argilla of Italy. This is the first mention found of lime and aluminous earth as precipitants.
References:
- http://www.who.int/water_sanitation_health/dwq/wsh0207/en/index6.html
- Engelhardt, Terry L. 2014. COAGULATION, FLOCCULATION AND CLARIFICATION OF DRINKING WATER. l. : HACH, 2014.