Environmental Engineering FOURTH EDITION

140 ENVIRONMENTAL ENGINEERING

The rotational velocity is

2nm

ut = -

60 ’

where r = radius in feet and n = rpm, so that

The velocity differential between paddles and fluid is assumed to be 65% of ut, so that

v = 0.65vt = (0.65)(1.7) = 0.7Oft/s.

Total power input from Eq. (7.1) is

(1.9)( 16)(0.5 ft)(48 ft)( 1.94 lb-s2/ft3)(0.70 ft/~)~

2

P= = 243 ft-lb/S,

and, from Eq. (7.2), the velocity gradient is

ft/s

) = 10.5 ft,

243

= /( (lOO)(5O)(l6)(2.7’3 x

which is a little low. The time of flocculation is

• V (1OO)(50) (16)(7.48)(24)(60)
  t=-= = 28.7 min,
  Q (30)105
  so that the Gf value is 1.8 x 104. This is within the accepted range.

SElTLlNG

When the flocs have been formed they must be separated from the water. This is

invariably done in gravity settling tanks that allow the heavier-than-water particles to

settle to the bottom. Settling tanks are designed to approximate uniform flow and to

minimize turbulence. Hence, the two critical elements of a settling tank are the entrance

and exit configurations. Figure 7-5 shows one type of entrance and exit configuration

used for distributing the flow entering and leaving the water treatment settling tank.

Alum sludge is not very biodegradable and will not decompose at the bottom of

the tank. After some time, usually several weeks, the accumulation of alum sludge

at the bottom of the tank is such that it must be removed. ’Qpically, the sludge exits

through a mud valve at the bottom and is wasted either into a sewer or to a sludge

holding and drying pond. In contrast to water treatment sludges, sludges collected in

wastewater treatment plants can remain in the bottom of the settling tanks only a matter

of hours before starting to produce odoriferous gases and floating some of the solids.

Settling tanks used in wastewater treatment are discussed in Chap. 9. The water leaving

a settling tank is essentially clear. Polishing is performed with a rapid sand filter.