Aluminum Salts:
Aluminum Sulphate (Al2(SO4)3 . 18H2O)
This is commonly known as filter alum or alum. When it is added to raw water, it reacts with bio-carbonate alkalinity present in raw water and forms gelatinous precipitate of aluminum hydroxide (floc). The floc attracts under colloidal particles, grows in size and finally settles down. The chemical reaction involved is:
Al2 (SO4)3 + 3Ca(HCO3)2 → 2Al(OH)3 (ppt) + 3CaSO4 + 6CO2
Likely Dosage
10 - 100 mg/1 commercial product in water treatment is according to the quality of raw water; with lime added:
Al2 (SO4)3 + 3Ca (HCO3)2 → 2Al (OH)3 (ppt) + 3CaSO4
In water treatment, 1 part of 3Ca(OH)2, 3 part of Al2 (SO4)3 . 18H2O
(commercial aluminum sulphate) with soda ash added :
Al2 (SO4)3 + 3Na2CO3 + 3H2O → 2Al(OH)3 + 3Na2SO4(ppt) + 3O2
1 part of anhydrous sodium carbonate to 1 or 2 parts of Al2 (SO4)3 . 18H2O
(commercial aluminum sulphate).
Al2 (SO4)3 + 6NaOH → 2Al(OH)3 + 3Na2SO4
In water treatment, the requirement of NaOH is 36% of the dose of
Al2(SO4)3 . 18H2O (commercial aluminum sulphate).