Numerous chemicals are used in coagulation and flocculation processes. There are advantages and disadvantages associated with each chemical. Following factors should be considered in selecting these chemicals:
- Reliability of supply.
- Sludge considerations.
- Compatibility with other treatment processes.
- Secondary pollution.
- Capital and operational costs for storage, feeding, and handling.
Coagulants and coagulant aids commonly used are generally classified as inorganic coagulants and polyelectrolytes. Polyelectrolytes are further classified as either synthetic-organic polymers or natural-organic polymers. The best choice is usually determined only after jar test is done in the laboratory.
Following table lists several common inorganic coagulants along with associated advantages and disadvantages.
Table 3.6.1: Advantages and disadvantages of alternative inorganic coagulants
Polyelectrolytes are water-soluble polymers carrying ionic charge along the polymer chain and may be divided into natural and synthetic polyelectrolytes. Important natural polyelectrolytes include polymers of biological origin and those derived from starch products, cellulose derivatives and alginates. Depending on the type of charge, when placed in water, the polyelectrolytes are classified as anionic, cationic or nonionic.
- Anionic—ionize in solution to form negative sites along the polymer molecule.
- Cationic—ionize to form positive sites.
- Non-ionic—very slight ionization.
Common organic polyelectrolytes are shown in following table.
Table 3.6.2 : Common organic polyelectrolytes 
Polyelectrolytes versus Inorganic Coagulants
Although they cannot be used exclusively, polyelectrolytes do possess several advantages over inorganic coagulants. These are as follows.
- During clarification, the volume of sludge produced can be reduced by 50 to 90%.
- The resulting sludge is more easily dewatered and contains less water.
- Polymeric coagulants do not affect pH. Therefore, the need for an alkaline chemical such as lime, caustic, or soda ash is reduced or eliminated. · Polymeric coagulants do not add to the total dissolved solids concentration.
- Soluble iron or aluminum carryover in the clarifier effluent can result from inorganic coagulant use. By using polymeric coagulants, this problem can be reduced or eliminated .
- In some waters, an even large dose of primary coagulant does not produce a satisfactory floc size and hence good settling rate. In these cases, a polymeric coagulant aid is added after the coagulant, to hasten reactions, to produce a denser floc, and thereby reducing the amount of primary coagulant required.
- Because of polymer bridging, small floc particles agglomerate rapidly into larger more cohesive floc, which settles rapidly.
- Coagulant aids also help to create satisfactory coagulation over a broader pH range.
- Generally, the most effective types of coagulant aids are slightly anionic polyacrylamides with very high-molecular weights.
- In some clarification systems, non-ionic or cationic types have proven effective.