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6.2 CHAPTER SIX

• Mixing is commonly referred to as flash mixing, rapid mixing, or initial mixing. The
purpose of rapid mixing is to provide a uniform dispersion of coagulant chemical
throughout the influent water.
• Enhanced coagulation is a phrase used by the U.S. Environmental Protection Agency
(USEPA) in the Disinfectants and Disinfection By-products Rule. The rule requires that
the coagulation process of some water supplies be operated to remove a specified per-
centage of organic material from the source water, as measured by total organic car-
bon (TOC). Enhanced coagulation (removal of TOC) can be achieved in most cases by
either increasing coagulant chemical dosage or adjusting the pH during the coagulation
reaction.
• Coagulant chemicals are inorganic or organic chemicals that, when added to water at
an optimum dosage, cause particle destabilization. Most coagulants are cationic when
dissolved in water and include chemicals such as alum, ferric salts, lime, and cationic
organic polymers.
• Flocculation is the agglomeration of small particles and colloids to form settleable or
filterable particles (flocs). Flocculation begins immediately after destabilization in the
zone of decaying mixing energy following rapid mixing, or as a result of the turbulence
of transporting flow. In some instances, this incidental flocculation may be an adequate
flocculation process. A separate flocculation process is most often included in the treat-
ment train to enhance contact of destabilized particles and to build floc particles of op-
timum size, density, and strength.
• Flocculation aids are chemicals used to assist in forming larger, denser particles that
can be more easily removed by sedimentation or filtration. Cationic, anionic, or non-
ionic polymers are most often used in dosages of less than 1.0 mgFL.
• Direct filtration is a treatment train that includes coagulation, flocculation, and filtra-
tion, but excludes a separate sedimentation process. With direct filtration, all suspended
solids are removed by filtration. In the process sometimes called in-line filtration, floc-
culation occurs in the conduit between the rapid mixing stage and the filter, in the vol-
ume above the filter media, and within the filter media.
• Solids contact clarifiers are proprietary devices that combine rapid mixing, floccula-
tion, and sedimentation in one unit. These units provide separate coagulation and floc-
culation zones and are designed to cause contact between newly formed floc and set-
tled solids.
• Low-pressure membranes are hollow-fiber membrane systems that provide micro- or
ultrafiltration. These systems have pore sizes that are 10 to 100 times smaller than those
of primary protozoa of concern (i.e., Cryptosporidium and Giardia lamblia). The mem-
brane is a thin layer of polymer capable of separating materials based on size and chem-
ical properties. These membrane systems typically operate in the range of - 12 psi vac-
uum to 40 psi pressure.

THE COAGULATION PROCESS

Coagulation reactions occur rapidly, probably taking less than one second. Principal mech-
anisms that contribute to the removal of particulates when coagulating chemicals such as
alum or ferric chloride are mixed with water include chemical precipitation, reduction of
electrostatic forces that tend to keep particles apart, physical collisions between particles,
and particle bridging.