Environmental Engineering Reference
In-Depth Information
4.
C
ONCLUSIONS
Unlike primary sludge, secondary sludge as a byproduct of biological treatment is far
more difficult to dewater and to be disposed. Secondary sludge waste management issues are
a big challenge especially with the implementation of more stringent environmental
legislation. Typical post-treatment methods for secondary sludges include incineration,
pyrolysis, gasification, direct liquefaction, super critical water oxidation and anaerobic
digestion. The operating conditions (temperature, pressure, atmosphere and products, etc.)
vary among the methods. For example, incineration, gasification and SCWO methods utilize
air or oxygen while the remaining methods are conducted under oxygen depleted or anaerobic
conditions. Incineration, pyrolysis and gasification operate at high temperatures, while these
methods differ in the objective products. Incineration aims to produce heat and
steam/electricity, pyrolysis targets a high yield of oil, and gasification favors production of
gas. The greatest sludge volume reduction (over 90%) can be achieved with the high-
temperature methods including incineration, pyrolysis and gasification, which is
advantageous as it effectively reduces the physical amount of sludge for disposal. The major
disadvantage for these high-temperature processes is their lower net energy efficiency for the
treatment of secondary sludge containing very high content of water (98-99%), resulting from
the need of the energy intensive operations of dewatering/thickening and complete
evaporation of the water in the sludge. In contrast, the other three treatment methods, i.e.,
direct liquefaction, SCWO and anaerobic digestion, operate at a relatively lower temperature
and more importantly without the need of dewatering /thickening and complete evaporation
of the water in the sludge. Accordingly, these methods are more promising for the treatment
of secondary sludge from the standpoint of energy recovery.
A
CKNOWLEDGMENTS
Part of this work was financially supported by the Natural Science and Engineering
Research Council of Canada (NSERC) through the Discovery Grant awarded to Dr. Charles
XU.
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