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CHAPTER 6

MIXING, COAGULATION,

AND FLOCCULATION

Paul J. Delphos
HDR Engineering, Inc.
Norfolk, Virginia
George M. Wesner
Consulting Engineer
San Clemente, California

Coagulation and flocculation may be broadly described as chemical and physical pro-
cesses that mix coagulating chemicals and flocculation aids with water. The overall pur-
pose is to form particles large enough to be removed by the subsequent settling or filtra-
tion processes. Particles in source water that can be removed by coagulation, flocculation,
sedimentation, and filtration include colloids, suspended material, bacteria, and other or-
ganisms. The size of these particles may vary by several orders of magnitude. Some dis-
solved material can also be removed through the formation of particles in the coagulation
and flocculation processes. The importance of dissolved material removal has become
much more critical in recent years with increased regulatory emphasis on disinfection by-
products and total organic carbon removal.
There are several excellent discussions on the theory of coagulation and flocculation
in Water Quality and Treatment and other AWWA publications listed at the end of this
chapter.

DEFINITIONS

Terms used in this chapter are defined as follows:
• Coagulation is the process in which chemicals are added to water, causing a reduction
of the forces tending to keep particles apart. Particles in source water are in a stable
condition. The purpose of coagulation is to destabilize particles and enable them to be-
come attached to other particles so that they may be removed in subsequent processes.
Particulates in source waters that contribute to color and turbidity are mainly clays,
silts, viruses, bacteria, fulvic and humic acids, minerals (including asbestos, silicates,
silica, and radioactive particles), and organic particulates. At pH levels above 4.0, par-
ticles or molecules are generally negatively charged. The coagulation process physi-
cally occurs in a rapid mixing process.

6.1