a. ALUMINIUM SULPHATE or ALUM
A
ALUM reacts with natural alkalinity in water to form aluminuium hydroxide floc..
If water doesnot contain sufficient alkalinity to react with the alum, lime or soda ash is fed to provide the necessary alkalinity.
2 4 3 2 2 3 4 2
2 4 3 2 2 3 2 3 2 4 2 2
) .18 3 2 3 18
) .18 3 3 2 3 3 18
i AL SO H O Ca OH AL OH CaSo H O
ii AL SO H O Na Co H O AL OH Na So Co H O
The effective PH range for alum coagulant is 5.5 – 8.0 An advantage of using soda ash is that unlike lime it does not increase water hardness, only corrosiveness.
SJB Institute of Technology, Department of Civil Engineering Page 89 b. FERROUS SULPHATE (COPPERAS)
Ferrous sulphate reacts with natural alkalinity but the response is much slower than that between alum and natural alkalinity. Lime is generally added to raise the pH to the point where ferrous lons are precipitated as ferric hydroxide by the caustic alkalinity
4 2 3 2 3 2 4 2
3 2 2 2 3 2
2 2
2 3
.7 ( ) 7
2 ( ) 2 2
4 2 4
FeSo H O Ca HCu Fe HCo CaSo H O Fe HCo Ca OH Fe OH CaCo H O
Fe OH O H O Fe OH
This treatment adds some hardness but no corrosiveness but dosing operation with two chemicals is more difficult.
c. CHLORINATED COPPERAS
Chlorinated copperas treatment is a second method of using ferrous sulphate. In this process chlorine is used to oxidize the ferrous sulphate to ferric sulphate.
4 2 2 2 4 3 3 2
3Feso .7H O 1.5cl Fe So Fecl H O
Ferric sulphate and ferric chloride react with natural alkalinity or lime, as illustrated by the following reactions with ferric chloride
3 3 2 3 2 2
3 2 3 2
2 3 2 3 6
2 3 2 3 .
FeCl Ca HCO Fe OH Cacl CO
FeCl Ca OH Fe OH Cacl
Colour in water is generally not affected by copperas and lime treatment where as chlorinated copperas is effective in the removal of colour.
d. OTHER COAGULANTS (SODIUM ALUMINATE).
2 2 4 3 2 3 2 4 2
6nALo AL So .18H O 8AL OH 3Na So 6H O
PROBLEMS
1. Determine the quantity of alum required to treat 10MLD of water at treatment plant the dosage of alum is 12ppm. Also determine the amount of carbon dioxide released per litre of water.
1000mg = 1gm Solution:-
Qty of water to be treated = 10MLD = 10 106Lit day/ Alum dosage = 12ppm = 12mgll (Given)
SJB Institute of Technology, Department of Civil Engineering Page 90
The Chemical reactions involved in treating water with alum are given by:
2 4 3 2 3 2 3 4 2 2
The molecular weight of Co 1 12.01 2 16 44
Since, 12 mg of alum is required for treating on litre of water, the Qty of Co2 released per litre of water = 4.756mg.
2. 10mg of copperas is consumed with lime at a coagulation basin per litre of water.
Determine the qty of copperas and the quick lime required to treat 10MLD of water, for one year. Molecular weight of copperas = 278, mol.wgt of quicklime = 56
Solution:-
Qty of water treated per day = 10 10 Litres 6 Dosage of copper as given = 10mg/lit.
6
6
Qty of copperas required / day 10 10 10 100 10
10
mg kg Total Qty of copperas required per year
4 2 2 2 4 2 2 2
Given molecular wgt of copperas = 278 and
SJB Institute of Technology, Department of Civil Engineering Page 91 Given molecular wgt quick lime = 56
From the above equations, one molecular of copperas require one molecule of lime 278 mg of copperas require = 56mg of lime
10kg of copperas 56
100 20.143 .
278 kg of lime Qty of Lime required per year =
20.143 / 365
7352.19 7.352
kg day kg tonnes
Hence 36.5 tonnes of copperas and 7.352 tonnes of lime are required for one year to treat 10MLD of water.
3. A water treatment plant treating 50MLD of water requires 20mg/l of filter alum. If the water has 6mg/l of alkalinity as CaCo3 determine the Qty of filter alum and quick litres required per year. The alum is 80% pure and the quick lime contains 85% Cao.
(Given mol.wt of Al=27, S = 32, 0=16, h=1, Ca = 40 and C=12) Solution:
Total qty of water treated/day = 50 10 Litres given dosage of alum = 20mg/lit 6 Qty of filter alum required
20 50 106 / 1000 /
But alum is 805 Pure met Qty of alum required = 1000
80100 1250 / For oneyear, alum requirement
=1250 365
=456250kg
=456.25tonnes.
mg day kg day
kg day
When filter alum is added, the following reactions are followed.
SJB Institute of Technology, Department of Civil Engineering Page 92
Now from the above equations it is clear that alum requires (3 162) parts of natural alkalinity as ( 3 2)
Ca HCo for every 666 parts of alum. (3 162) parts of alkalinity as Ca HCo( 3 2) is equivalent to
(3 100) parts of alkalinity required as CaCo3.
Hence the alkalinity required as CaCo3 for the water containing alum as 20mg/lit 3 100
666 20
9.009mg lit/
But the natural alkalinity present in water is 6mg/lit.
Additional alkalinity required = 9.09-6
= 3.009mg/lit as CaCo3
From the equation, it is clear that 10parts of CaCo3 is produced by 56parts of Cao Qty of Cao required =
56 3.009 100
1.685mg lit/ .
Since the quick lime contains 85% of Cao, the Qty of quick lime required 1.685
85 100
1.9824mg lit/ .
Qty of quick lime required per day.
6
Yearly consumption of quick lime
SJB Institute of Technology, Department of Civil Engineering Page 93
4) The water works of a city treats 50MLd in a coagulation ssedimentation tank. The qty of alum consumed is 15mg/l. If the alkalinity of raw water is equivalent to 1mg/l as CaCo3, Calculate the quantity of alum and quick lime (Containing 80% Cao) required per month by water works.
(Atomic weights: Al = 27, S = 32, O = 16, H = 1, Ca = 4, C = 12).
Solution:
Qty of water to be treated = 50 106lit day/ Qty of filter alum required perday at 15mg/lit
15 50 106
750
mg kg
Qty of filter alum require per month 750 30
22500
22.5 .
kg tonnes
The chemical reactions that take place are
2 4 3 2 3 2 3 4 2 2
Now from the above equations. It is clear that alum requires 3 162 Parts of natural alkalinity as Ca(Hco3)2 for every 666 parts of alum. 3 162 parts of alkalinity as Ca(Hco3)2 is equivalent to 3 100 parts of alkalinity required as CaCo3.
Hence the alkalinity required as CaCo3 for containing alum of 15mg/l 3 100
666 15 6.757mg l/
Natural alkalinity available as CaCo3 = 1mg/L