Chapter 10
Sludge Treatment and
Disposal
The field of wastewater treatment and engineering is littered with unique and imagi-
native processes for achieving high degrees of waste stabilization at attractive costs.
In practice, few of these “wonder plants” have met expectations, often because they
fail to pay sufficient attention to sludge treatment and disposal problems. Currently
sludge treatment and disposal accounts for over 50% of the treatment costs in a typical
secondary plant, prompting renewed interest in this none-too-glamorous, but essential
aspect, of wastewater treatment.
This chapter is devoted to the problem of sludge treatment and disposal. The
sources and quantities of sludge from various types of wastewater treatment systems
are examined, followed by a definition of sludge characteristics. Such solids concen-
tration techniques as thickening and dewatering are discussed next, concluding with
considerations for ultimate disposal.
SOURCES OF SLUDGESludges are generated from nearly all phases of wastewater treatment. The first source
of sludge in a wastewater treatment facility is the suspended solids from the primary
settling tank or clarifier. Ordinarily about 60% of the suspended solids entering the
treatment facility become raw primary sludge, which is highly putrescible and very
wet (about 96% water).
The removal of BOD is basically a method of wasting energy, and secondary
wastewater treatment plants are designed to reduce the high-energy organic material
that enters the treatment plant to low-energy chemicals. This process is typically accom-
plished by biological means, using microorganisms (the “decomposers” in ecological
terms) that use the energy for their own life and procreation. Secondary treatment pro-
cesses such as the popular activated sludge system are almost perfect systems. Their
major fault is that the microorganisms convert too little of the high-energy organics to
C02 and H2O and too much of it to new organisms. Thus the system operates with an
excess of these microorganisms, or waste activated sludge. As defined in the previous
chapter, the mass of waste activated sludge per mass of BOD removed in secondary
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