Question

Use of coagulants such as alum

• A. results in increase of pH of the treated water.
• B. results in reduction of pH of the treated water.
• C. may cause the increase and decrease of pH of the treated water.
• D. results in no change in pH of the treated water

Answer 1

The correct option is B results in reduction of pH of the treated water.

The chemistry of alum coagulation:

$\underset{Alum}{A{l}_2(S{O}_4{)}_3}.18H_{2}O+\underset{\left(AlkalinityasCaC{O}_3\right)}{3Ca(HC{O}_3{)}_2}\rightleftharpoons \underset{(ppt\downarrow )}{2Al(OH{)}_3}+\underset{\left(gas\right)}{6C{O}_2}+\underset{\left(permanenthardness\right)}{3CaS{O}_4}+18H_{2}O$

The points to remember from the above equation are:
1. Alum reacts with alkalinity and hence decrease the alkalinity.
2. Precipitate of $Al(OH{)}_3$ is formed.
3. Decrease in pH since alkalinity is reduced
4. With the formation of $CaS{O}_4$, permanent hardness is induced in the water.