Environmental Engineering FOURTH EDITION

Sludge Treatment and Disposal 207

j = 0.8 for aerobic,

i = 0.1 for well-operated activated sludge,

i = 0.2 for trickling filters,

Y = 0.5 for activated sludge, and

Y = 0.2 for trickling filters.

If chemical sludges are also produced during the treatment process (e.g., from the pre-

cipitation of phosphorus), these quantities must be added to the total sludge estimated

above.

EXAMPJX 10.1. A wastewater treatment plant has an influent BOD5 of 250 ma, at

a flow rate of 1570m3/h (lomgd), and influent suspended solids of 225 mgL The

raw sludge produced would be equal to kXo, where k is the fraction of suspended

solids removed in the primary clarifier and Xo is the influent suspended solids in

kilogramshour. If k is 0.6 (typical value), Xo can be calculated as

(225 mg/L)(1570m3/h)(1000L/m3)(10-6) mg/kg = 353 kg/h

The raw sludge produced is then

0.6 x 353 = 212 kg/h.

CHARACTERISTICS OF SLUDGES

The important or relevant characteristics of sludges depend on what is to be done to

the sludge. For example, if the sludge is to be thickened by gravity, its settling and

compaction rates are important. On the other hand, if the sludge is to be digested anaero-

bically, the concentrations of volatile compounds, other organic solids, and heavy

metals are important. Variability of the sludges is immensely important in the design

of sludge handling and disposal operations. In fact, this variability may be stated in

terms of three “laws”:

1. No two wastewater sludges are alike in all respects.


2. Sludge characteristics change with time.


3. There is no “average sludge.”
   The first “law” of sludges reflects the fact that no two wastewaters are alike and
   that if the variable of treatment is added, the sludges produced will have significantly
   different characteristics.
   The second “law” is often overlooked. For example, the settling characteristics
   of chemical sludges from the treatment of plating wastes (e.g., Pb(OH)z, Zn(OH)z,
   or Cr(OH)3) vary with time simply because of uncontrolled pH changes. Biological
   sludges are, of course, continually changing, with the greatest change occurring when