Environmental Engineering FOURTH EDITION

228 ENVIRONMENTAL ENGINEERING

Thus

Q2= ” Q1 = -(OS) 16.8 = 3.2m3/h.

2.78

Next, looking at the solids loading (beta), the solids flow rate is

and from Eq. (10.9)

= (4000 - 3950)(4)(1)~(20)(2) = 25,120

& = (3200 - 3150)(8)(1)~(40)(4) = 200,960

2009960 (5) = 40 -, kg

Q2= 25,120 h

corresponding to a flow rate of

-(10-3) 40 = 4 -. m3

0.01 h

The liquid loading thus governs, and the larger machine (machine 2) cannot be fed at

a flow rate greater than 3.2 m’h.

ULTIMATE DISPOSAL

Even after treatment, we are left with a large volume of sludge that needs a final resting

place. The choices for ultimate disposal of sludge are limited to air, water, and land.

Until quite recently, incineration (“air disposal”) was viewed as an effective sludge

reduction method, if not exactly an ultimate sludge disposal method (the residual ash

still required disposal). However, strict controls on air pollution and increasing concern

over global warming are making incineration an increasingly unlikely option. Disposal

of sludges in deep water (such as oceans) is decreasing owing to adverse or unknown

detrimental effects on aquatic ecology. Land disposal, particularly the use of sludge as

fertilizer or soil conditioner, has historically been a favored disposal method, and is

currently growing in popularity as other options become more problematic.

Incineration is actually not a method of disposal at all, but rather a sludge treatment

step in which the organics are converted to H20 and C02 and the inorganics drop out as

a nonputrescible residue. Two types of incinerators have found use in sludge treatment:

multiple hearth and fluid bed. The multiple-hearth incinerator, as the name implies,

has several hearths stacked vertically, with rabble arms pushing the sludge progres-

sively downward through the hottest layers and finally into the ash pit (Fig. 10-16).