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MIXING, COAGULATION, AND FLOCCULATION 6.3

Several factors affect the type and amount of coagulating chemicals required, includ-
ing the nature of suspended solids and the chemical characteristics of the influent water.

Coagulant Chemicals
The most commonly used coagulants are
• Alum (aluminum sulfate), A12(SO4)3 • 14H20. The most common coagulant in the
United States, it is often used in conjunction with cationic polymers.
• Polyaluminum chloride, AI(OH)x(C1)y. This is efficient in some waters, requiring less
pH adjustment and producing less sludge.
• Ferric chloride, FeC13. This may be more effective than alum in some applications.
• Ferric sulfate, Fe2(SO4)3. It is effective in some waters and more economical in some
locations.
• Cationic polymers can be used alone as the primary coagulant or in conjunction with
aluminum or iron coagulants.
Although alum is by far the most widely used coagulant chemical, ferric chloride or
ferric sulfate forms a better-settling floc in some waters and may be more consistently ef-
fective in removing natural organic matter as compared to aluminum-based coagulants.
Additionally, polyaluminum chloride often produces a better-settling floc in colder wa-
ters and often results in lower dosages, thereby producing less sludge than alum and fer-
ric coagulants.

Flocculation Aids
Floc formed in many waters with alum is light and fragile and somewhat difficult to set-
tle. Polymers and other additives can often help form a floc that is more efficiently re-
moved by settling and filtration. Typical additives used for flocculation aids are
• High-molecular-weight anionic or nonionic polymers
• Activated silica
• Bentonite

These chemicals are normally added after the application of coagulants, from 5 to
600 s after mixing. If the water to be treated with a flocculent aid is already in the floc-
culation stage, the chemical should be added so that it can be spread across the floccula-
tion basin.

Chemical Selection

The selection of coagulant chemicals and flocculation aids for use in a particular plant is
generally based on economic considerations along with reliability, safety, and chemical
storage considerations. The best method of determining treatability, the most effective co-
agulants, and the required dosages is to conduct bench-scale and, in some cases, pilot
tests. Jar tests can be used to determine treatability and estimate chemical dosages. If pos-
sible, testing should cover all critical seasonal conditions. Pilot plant design and con-
struction are discussed in Chapter 28.