Page 246 - Water and wastewater engineering

COAGULATION AND FLOCCULATION 6-23

Polymer. In rare instances, usually when the turbidity and alkalinity are high, cationic polymers
(poly-DADMAC and epi-DMA) have been used as primary coagulants, but their use typically
has been in conjunction with a metal salt. The main advantage of using polymers in conjunction
with metal salts is the ability to reduce the metal salt concentration and resulting sludge produc-
tion by 40 to 80 percent.
The epi-DMA dose generally decreases as the pH increases. The dose for poly-DADMAC is
only slightly affected by pH. Typical dosages are on the order of 1 to 10 mg/L.
Polymers are not effective in removing NOM.

Coagulant Aids
Insoluble particulate materials such as clay, sodium silicate, pure precipitated calcium carbonate,
diatomite, and activated carbon have been used as coagulant aids. They are used in waters that have
low concentrations of particles and, thus, have few nucleating sites to form larger floc. Because
their density is higher than most floc particles, floc settling velocity is increased by the addition of
coagulant aids. The dosage must be carefully controlled to avoid lowering the water quality.

Flocculant Aids
Uncharged and negatively charged polymers are used as flocculant aids. Their purpose is to
build a stronger floc. They are added after the coagulants are added and the particles are already
destabilized.
Activated silica and sodium silicate are common flocculant aids. In processes where these are

added, called ballasted flocculation, micro-sand is added after chemical coagulation but before
flocculation to act as a nucleus for floc formation. The sand has a higher density than the floc and
increases its settling velocity (Willis, 2005).

6-5 FLOCCULATION THEORY
Smoluchowski (1917) observed that small particles undergo random Brownian motion due to
collisions with fluid molecules and that these motions result in particle to particle collisions.
Langelier (1921) observed that stirring water containing particles created velocity gradients that
brought about particle collisions. These observations provide the basis for describing the mecha-
nisms of flocculation.

Microscale Flocculation
The flocculation of small particles (less than 0.1
m in diameter) is caused by diffusion. The rate
of flocculation is relative to the rate at which the particles diffuse. Thus, the primary mechanism
of aggregation is through Brownian motion. This aggregation is called microscale flocculation or
perikinetic flocculation. After a period of seconds, the microflocs range in size from 1 to about
100
m in diameter.

Macroscale Flocculation
Mixing is the major flocculation mechanism for particles greater than 1
m in diameter. This
mechanism is known as macroscale flocculation or orthokinetic flocculation. Mechanical mixing
is employed to achieve orthokinetic flocculation.