Coagulation | Activated Sludge Process | Wastewater Treatment Plant

 Coagulation 

Coagulant Chemicals 

Coagulation and flocculation occur in steps designed to overcome the forces that hold suspended particles together, allowing particle collision and flocculation to occur. If step one is not completed, the subsequent step will fail. As a result, a coagulant chemical is added to the water.

The purpose of primary coagulants is to aid in the removal of unsettled solids from water. Used to destabilize particles and cause them to clump together.

In a water treatment plant, a variety of primary coagulants can be used.

Types of Primary Coagulants

Metallic salts 

• Al2(SO4)3 • 14 H2O Aluminum Sulfate (Alum)

1. One of the first coagulants, and still one of the most widely used.
2. It is available in liquid form with an 8.3 percent concentration or in dry form with a 17 percent concentration.
3. When alum is added to water, it reacts with it, producing positively charged ions.
4. A narrow/optimal effective pH range of 5.5–7.5. A pH greater than 9.0 may result in more dissolved aluminum.

• Ferric Sulfate - Fe2(SO4)3 • 9 H2O 
• Ferric Chloride - FeCl3 • 6 H2O 

1. The most common iron salt used to achieve coagulation is ferric chloride (FeCl3).
2. A wider effective pH range of 5.0 to 8.5 is now available.
3. The most serious safety risk associated with the use of ferric chloride is corrosiveness.

Synthetic inorganic polymers 

• Polyaluminum Chloride - Aln(OH)mCl(3n-m) • H2O 

1. Polyaluminium coagulants, in general, use far less alkalinity than alum.
2. Over a pH range of 5.0 to 8.0, it performs admirably.
3. PAC is known for requiring a lower dose, not requiring neutralizing agents such as lime, having a shorter flocculation time, producing less sludge, being easier to backwash, and improving treated water quality.