Page 385 - Water and wastewater engineering

10-2 WATER AND WASTEWATER ENGINEERING

10-1 INTRODUCTION

One of the objectives of the coagulation and flocculation processes is to enhance the size of
particles so that they will settle in a reasonable period of time. The lime-soda softening process
objective is to remove hardness by forming an insoluble precipitate. Once the particles and pre-
cipitate are formed, the most common means of removing them from the water is by gravitational
settling in a sedimentation basin (also called a clarifier or settling tank ). Other means, such as
direct filtration or flotation, may also be employed. Gravitational settling is the subject of this
chapter. The other processes are described in Chapters 11, 15, and 27.

10-2 SEDIMENTATION THEORY
In the design of an ideal sedimentation tank, one of the controlling parameters is the settling
velocity ( v s ) of the particle to be removed. For the purpose of discussion and illustration, the set-
tling properties of particles are categorized into four classes: (1) discrete particle settling, (2) floc-
culant settling, (3) hindered settling, and (4) compression settling. By convention these categories
have been labeled Type I, Type II, Type III, and Type IV settling, respectively. In actual settling
tanks, it is not uncommon to see all of these types of settling. The value of separating the discus-
sion into these categories is that it provides a means of understanding the relationship between
variables in the design of the sedimentation basin.

Type I Sedimentation
Type I sedimentation is characterized by particles that settle discretely at a constant settling
velocity. They settle as individual particles and do not flocculate during settling. Examples of
these particles are sand and grit (a mixture of abrasive particles that may include sand, broken
glass, etc.). Generally speaking, the only applications of Type I settling are during presedimenta-
tion for sand removal prior to coagulation in a potable water plant, in settling of sand particles
during cleaning of rapid sand filters, and in grit chambers.

Stokes’ Law. When particles settle discretely, the particle settling velocity can be calculated,
and the basin can be designed to remove a specific size particle. In 1687, Sir Isaac Newton showed
that a particle falling in a quiescent fluid accelerates until the frictional resistance, or drag, on the
particle is equal to the gravitational force of the particle ( Figure 10-1 ) (Newton, 1687). The three
forces are defined as follows:
F () g V (10-1)
G s p
F B () g V p (10-2)

v 2

F D C A p () (10-3)
D
2

where F G gravitational force
F B buoyancy force
F D drag force
3
s density of particle, kg/m
3
density of fluid, kg/m