Page 225 - Water and wastewater engineering

6-2 WATER AND WASTEWATER ENGINEERING

6-1 INTRODUCTION

Coagulation and flocculation are essential components of conventional water treatment systems
that are designed to

• Remove infectious agents,
• Remove toxic compounds that have adsorbed to the surface of particles,
• Remove precursors to the formation of disinfection byproducts, and

• Make the water palatable.
Surface water supplies contain organic and inorganic particles. Organic particles may include
algae, bacteria, cysts of protozoa, oocysts, and detritus from vegetation that has fallen into the
water. Erosion produces inorganic particles of clay, silt, and mineral oxides. Surface water will
also include particulate and dissolved organic matter, collectively referred to as natural organic
matter (NOM), that is a product of decay and leaching of organic detritus. NOM is important
because it is a precursor to the formation of disinfection byproducts.
Groundwater treated to remove hardness, or iron or manganese, by precipitation contains
finely divided particles.
Both the precipitates and the surface water particles may, for practical purposes, be classi-
fied as suspended and colloidal. Suspended particles range in size from about 0.1
m up to about
100
m in diameter ( Figure 6-1 ). Colloidal particles are in the size range between dissolved sub-
stances and suspended particles. They are in a solid state and can be removed from the liquid by
physical means such as very high-force centrifugation or by passage of the liquid through filters
with very small pore spaces. Colloidal particles are too small to be removed by sedimentation or
by sand filtration processes.
The object of coagulation (and subsequently flocculation) is to turn the small particles into
larger particles called flocs, either as precipitates or suspended particles. The flocs are readily
removed in subsequent processes such as settling, dissolved air flotation (DAF), or filtration.
For the purpose of this discussion coagulation means the addition of one or more chemicals to

Giardia cysts
Viruses
Cryptospordium oocysts
Algae
Bacteria
Fog Mist Rain
Pollens
Human hair
Visible to eye
Screen mesh

0.01 0.1 1 10 100 1000
Particle size,
m
FIGURE 6-1
Particulates in water and miscellaneous other reference sizes.