Water Treatment - The Drinking Water - page 233

Coagulation Process Operation
The common coagulation unit process operation involves the addition of
coagulant chemicals by rapid mixing—detention time in the rapid mix tank
is typically on the order of minutes (Masters, 1991). During this mixing pro-
cess, polymer (or some other coagulant aid) is added and blended into the
destabilized water prior to during flocculation. The removal of impurities
by coagulation depends on their nature and concentration, the use of both
coagulants and coagulants aids, and characteristics of the water, includ-
ing pH, temperature, and ionic strength. Because of the complex nature of
coagulation reactions, chemical treatment is based on empirical data derived
from jar testing or other laboratory tests and field studies (Viessman and
Hammer, 1998).
Flocculation
The destabilized particles and chemical precipitates resulting from coagula-
tion are designed to enhance their settling qualities and thus their removal
from water; however, even after coagulation has taken place, these particles
and chemical precipitates may still settle very slowly (too slowly). To speed
up the settling process, flocculation is employed.
Note: Flocculation is the clumping together of the fine particles formed
by coagulation. Although the two terms are often used interchangeably,
flocculation and coagulation are actually distinct concepts.
Flocculation is the most important factor affecting particle-removal effi-
ciency. In water treatment operations, flocculation is a slow mixing process
in which the coagulated particles are brought into contact so they will col-
lide, stick together, and grow (agglomerate) to a size that will readily settle.
Enough mixing must be provided (e.g., gentle agitation for approximately
half an hour) to bring the floc particles into contact with each other and
to keep the floc from settling in the flocculation basin. (The heavier the
floc and the higher the suspended solids concentration, the more mixing is
required to keep the floc in suspension.) The most common type of mixer or
flocculator is the paddle type, which uses redwood slats mounted horizon-
tally on motor-driven shafts. Rotating slowly at about one revolution per
minute, the paddles provide gentle agitation that promotes floc growth. The
rate of agglomeration or flocculation depends on the number of particles
present, the relative volume that they occupy, and the velocity gradient in
the basin.