Encyclopedia of Environmental Science and Engineering, Volume I and II

WATER TREATMENT 1315

on which one or more propeller blades are mounted. Baffles

are frequently used to reduce the vortexing about the propel-

ler shaft. Vortexing hinders the mixing operation. Detention

periods are usually of the order of one to five minutes, usu-

ally at the lower end of this range. Considerable study has

been done on the baffling arrangement in a flash mixing

unit, and a variety of arrangements have been shown to be

successful.

Sedimentation

Sedimentation or settling may be accomplished by a variety

of means and mechanisms, depending on the material which

is to be settled from the liquid.

Discrete settling This type of sedimentation is primarily

concerned with the settling out of non-flocculent discrete

particles in a fairly dilute system. The primary feature of

this type of settling is that the particles do not flocculate and

therefore their settling velocity and particle size remain the

same throughout the period of settling. It will be seen later

that this is quite different from other forms of settling.

The particles in discrete settling will accelerate until the

fluid/drag reaches equilibrium with the driving force acting

on the particle. In other words, the resistance of the water

is equal to the accelerating force of gravity of the particle.

When this velocity is reached, it will not increase. This is

known as the terminal settling velocity, and it is normally

achieved quite rapidly. The loading rate which is used fre-

quently for the design of a settling tank is known as the over-

flow rate and may be expressed in cubic feet per square foot

per day based on the area. It can be seen that cubic feet per

square foot per day is in fact the same as feet per day, or in

fact a simple velocity. This velocity is defined as the set-

tling velocity of the particles which are removed in this ideal

basin if they enter at the surface.

Overflow rates or surface loadings of 150 gallons per

day per square foot of tank surface are not unusual where

the settling and sand, silt or clay are being accomplished by

plain sedimentation.

Flocculent settling The primary difference between this

type of settling and the previous one is that in a flocculent

system the larger particles subsiding at a slightly higher rate

CHEMICAL

FEED

HIGH SPEED

PROPELLER

PADDLE

R OTAT I O N

FLASH MIXING UNIT FLOCCULATOR

FIGURE 3

OUTLET

WEIR OUTLET

IN LET

SLUDGE

HOPPER

SLUDGE

COLLECTOR

CHAIN SLUDGE

MIXING ZONE INLETRAW

WATER

(Accelator by Infilco)

CIRCULAR COMBINATION

SETTLING FLOCCULATOR

LONGITUDINAL SETTLING

TA N K

FIGURE 4

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