The common coagulants used to treat drinking surface water are
- Aluminum sulfate (Al2(SO4)3-18H2O)
- Iron sulfate (Fe2(SO4)3-9H2O)
- Iron trichloride (FeCl3)
- Sodium aluminate (NaAlO2-12H2O)
These coagulants are all positively charged metal salts. When they are added to water, they react with negatively charged particles in the water, such as dirt, bacteria and viruses. The reaction causes the particles to clump together and form a precipitate called flocculent. The flocs are then removed from the water by sedimentation and filtration.
The choice of coagulant depends on many factors, including the type of water being treated, the level of treatment required, and the cost of the coagulant. Aluminum sulfate is a very common coagulant because it is relatively inexpensive and effective. However, it can also leave residual aluminum in the water, which can be a concern for some people. Ferric sulfate and ferric chloride are more expensive than aluminum sulfate, but they are also more effective at removing certain types of particles. Sodium aluminate is a less common coagulant, but it is often used in combination with other coagulants to improve treatment results.
In addition to the coagulants listed above, there are many other coagulants that can be used to treat drinking surface water. These include
Polymers
Polymers are long, positively charged molecular chains. They work in a similar way to metal salts by reacting with negatively charged particles in the water to form flocs. Natural coagulants are derived from plants or animals. They are often used in combination with other coagulant promoters to improve treatment effectiveness.
There are many common polymeric coagulants used in water treatment. These include:
Polyacrylamide (PAM): PAM is a synthetic polymer that is commonly used as a coagulant in water treatment. It is a long molecular chain with a positive charge. When PAM is added to water, it reacts with the negatively charged particles in the water to form flocs. The flocs are then removed from the water by precipitation and filtration.
Poly Diallyldimethyl Ammonium Chloride (PolyDADMAC): PolyDADMAC is another synthetic polymer that is commonly used as a coagulant in water treatment. It is a long molecular chain with a positive charge. When PolyDADMAC is added to water, it reacts with negatively charged particles in the water to form flocs. The flocs are then removed from the water by sedimentation and filtration.
Natural Coagulants
Chitosan: Chitosan is a natural polymer derived from chitin, a substance found in the exoskeletons of insects and crustaceans. Chitosan is a long chain of positively charged molecules. When chitosan is added to water, it reacts with the negatively charged particles in the water to form flocs. The flocs are then removed from the water by sedimentation and filtration.
The choice of polymeric coagulant depends on many factors, including the type of water being treated, the level of treatment required, and the cost of the coagulant.
The use of coagulants is an essential part of the drinking surface water treatment process. By removing suspended solids and other particles from the water, coagulants help make the water safe to drink.