Environmental Engineering Reference
In-Depth Information
TAble 11.1
Common Coagulant C
hemicals
Common Name
Comments
Aluminum sulfate
Most common coagulant in the United States, often used
with cationic polymers
Ferric chloride
May be more effective than alum in some applications
Ferric sulfate
Often used with lime softening
Ferrous sulfate
Less pH dependent than alum
Aluminum polymers
Synthetic polyelectrolytes; large molecules
Sodium aluminate
Used with alum to improve coagulation
Sodium silicate
Ingredient of activated silica coagulant aids
Source:
Adapted from American Water Works Association and American Society of
Civil Engineers,
Water Treatment Plant Design
, McGraw-Hill, New York, 1990.
the chemical addition for removal of total suspended solids (TSS), biochemi-
cal oxygen demand (BOD), and phosphorus. The equipment required for the
jar test includes a six-position, variable-speed paddle mixer; six 2-quart wide-
mouth jars; an interval timer; and assorted glassware, pipettes, graduates,
and so forth.
Coagulation Chemicals
Several different chemicals can be used for coagulation. Commonly used
metal coagulants are those based on aluminum (aluminum sulfate) and
those based on iron (ferric sulfate). The most common coagulant is alumi-
num sulfate (alum, Al
2
(SO
4
)
3
). Other common coagulation chemicals are pro-
vided in
Table 11.1
.
Coagulant Aids
Coagulation problems often occur because of slow-settling precipitates or
fragile flocs that are easily fragmented under hydraulic forces in basins and
filters (Hammer and Hammer, 1996). A coagulant aid is a chemical added
during coagulation to improve coagulation; to build stronger, more settleable
floc; to overcome the effect of temperature drops that slow coagulation; to
reduce the amount of coagulant needed; and to reduce the amount of sludge
produced (AWWA, 1995). Coagulant aids benefit flocculation by improving
the settling qualities and toughness of flocs. Polymers are the most widely
used materials. Synthetic polymers are water-soluble, high-molecular-weight
organic compounds with multiple electrical charges along a molecular chain
of carbon atoms. In drinking water treatment, polymers are extensively used
as coagulant aids to build large floc prior to sedimentation and filtration. Other
coagulant aids are activated silica, adsorbent-weighting agents, and oxidants.
