Water Treatment Processes - Mathematics and Statistics for the Environment - page 613

23.4 COAGULATION, MIXING, AND FLOCCULATION

23.4.1 C oagulation
Following screening and the other pretreatment processes, the next unit process in a conventional
water treatment system is a mixer where the first chemicals are added in what is known as coagula-
tion . The exception to this situation occurs in small systems using groundwater, when chlorine or
other taste and odor control measures are introduced at the intake and are the extent of treatment.
The coagulation process involves a series of chemical and mechanical operations by which coagu-
lants are applied and made effective. These operations are comprised of two distinct phases: (1)
rapid mixing to disperse coagulant chemicals by violent agitation into the water being treated, and
(2) flocculation to agglomerate small particles into well-defined floc by gentle agitation for a much
longer time. The coagulant must be added to the raw water and perfectly distributed into the liquid;
such uniformity of chemical treatment is reached through rapid agitation or mixing.
Coagulation is a reaction caused by adding polymers or metallic salts such as iron or aluminum
compounds to the water. Common coagulants include

• Alum (aluminum sulfate)
• Sodium aluminate
• Ferric sulfate
• Ferrous sulfate
• Ferric chloride
• Polymers

23.4.2 m ixing
To ensure maximum contact between the reagent and suspended particles, coagulants and coagulant
aids must be rapidly dispersed (mixed) throughout the water; otherwise, the coagulant will react
with water and dissipate some of its coagulating power. To ensure complete mixing and optimum
plug flow reactor operation, proper detention time in the basin is required. Detention time can be
calculated using the following procedures.

For complete mixing:

V
QK

1 CC

C

−

==

i e

t (23.20)

e

For plug flow:

V
Q

L
v

1 C

C

===

i
e

t ln (23.21)

K

where
t = Detention time of the basin (min).
V = Volume of basin (m 3 or ft 3 ).
Q = Flow rate (m 3 /s or cfs).
K = Rate constant.
C i = Influent reactant concentration (mg/L).
C e = Effluent reactant concentration (mg/L).
L = Length of rectangular basin (m or ft).
v = Horizontal velocity of flow (m/s or ft/s).