The coagulant widely used for sewage treatment is 

Concept:

Coagulation:

• Coagulation is a process used to neutralize charges and form a gelatinous mass to trap or bridge particles thus forming a mass large enough to settle or be trapped in the filter.
• Coagulation destabilizes the charges of the particles. Coagulants with charges opposite to those of the suspended solids are added to the water to neutralize the negative charges on dispersed non-settable solids such as clay and organic substances.
• Coagulant chemicals are substances that promote the coagulation of liquids into solids.

Coagulants used for wastewater treatment:

• Aluminium chloride
• Ferric chloride
• Ferric and ferrous sulphate
• Aluminium chlorohydrate

Coagulants used for water treatment:

• Alum
• Ferrous sulphate
• chlorinated ferrous sulphate
• Sodium aluminate

Ferric chloride is widely used for sewage treatment and Alum is widely used for water treatment.