المحاضرة 3-3

Chemicals in Wastewater Coagulation and Precipitation
Over the years number of different substances have been used as precipitants or coagulants. The most common chemicals are listed in Table N-2.









Table N - 1
Applications of chemical unit processes in wastewater treatment
































Table N-2
Inorganic chemicals used most commonly for coagulation and precipitation processes in wastewater treatment














The reactions involved with ( 1 ) alum, (2) lime, (3) ferrous sulfate (copperas) and lime. (4) ferric chloride. (5) ferric chloride and lime, and (6) ferric sulfate and lime are considered as follow:

Alum:

When alum is added to wastewater containing calcium and magnesium bicarbonate alkalinity, a precipitate of aluminum hydroxide will form. The overall reaction that occurs when alum is added to water may be illustrate as follows:

3 x 100 (as CaCO3) 666.5 2 x 78 3 x 136 6 x 44 18 x 18

3 Ca (HCO3)2 + Al2 (SO4)3.18H2O 2Al(OH)3 + 3CaSO4 + 6CO2 + 18H2O

Calcium Aluminum Aluminum Calcium Carbon
Bicarbonate sulfate hydroxide sulfate dioxide
(soluble) ( soluble) (insoluble) ( soluble) ( soluble)





Because alkalinity is reported in terms of calcium carbonate (CaCO3), the molecular weight of which is 100. The quantity of alkalinity required to reach with 10 mg/L of alum is :
( 10 mg/L)((3 (100 g/mole))/(666.5 g/mole)))=4.5 mg/L