If process is carried out at room temperature it is called cold lime soda process.

1) Cold process i) Batch process

ii) Cold Continuous process:-

Coagulant is necessary because at room temperature precipitate is finally divided. But do not settle easily & cannot easily filtered.

$NaAlO_{2}+2H_{2}O--\rightarrow NaOH+Al(OH)_{3}$

Mechanical stirrer is used to increase mixing & rate of precipitate formation

ii) Hot continuous process-

It is carried out at temperature $80- 150^{\circ}c$. It is used when small quantity of water is required. But it takes place at low hardness ie. 15-25 ppm. Due to hot condition-

i) Reactions take place faster.

ii) No need of coagulant

iii) Hardness is low

iv) Dissolved gases are expelled out at high temperature

Advantages:- i) It is more economical

ii) Less amount of coagulant is required.

iii) It increases pH value, there by corrosion is reduced.

iv) Also Fe, Mn & minerals are reduced.

v) Pathogenic bacteria are reduced.

Disadvantages:-

i) The hardness is 50-60 ppm by cold process & about 15-30 ppm by hot process

ii) Careful operation & skilled supervision are required.

iii) Disposal of large quantity of sludge formed during process is a problem.