Environmental Engineering Reference
In-Depth Information
Chapter 5
T
REATMENT OF
S
ECONDARY
S
LUDGE
FOR
E
NERGY
R
ECOVERY
Chunbao (Charles) Xu
*
and Jody Lancaster
Department of Chemical Engineering, Lakehead University, 955 Oliver Road,
Thunder Bay, Ont., Canada P7B 5E1
A
BSTRACT
Primary and secondary sludges are produced as a result of primary and secondary
wastewater treatment in municipal wastewater plant or pulp and paper mills. Sludge
disposal has become a worldwide problem for many reasons including rapidly shrinking
landfill space, increased environmental awareness, more stringent environmental
standards governing the disposal of sludge, and dewatering challenges. Unlike the
primary sludge, the secondary sludge as byproduct of the biological treatment is far more
difficult to dewater and to be disposed. Secondary sludge waste management issues are a
continuing challenge. This together with record high oil prices have contributed to a need
to examine methods of converting secondary sludge waste into energy. In this chapter,
we have overviewed a variety of secondary sludge post treatment methods for energy
recovery, including incineration, gasification, pyrolysis, direct liquefaction, supercritical
water oxidation (SCWO) and anaerobic digestion. A critical comparison between these
methods is presented with respect to their net energy efficiencies. The advantages and
drawbacks of each treatment option are also highlighted in this chapter.
1.
I
NTRODUCTION
Primary and secondary sludges are produced as a result of primary and secondary
wastewater treatment in municipal or industrial wastewater plants, e.g., pulp and paper mills.
For instance, about 40-50 kg of sludge (dry)is generated in the production of 1 tonne of paper
at a paper mill in North America (Joyce et al., 1979), and of that approximately 70%is
primary sludge and 30% secondary sludge (Elliot and Mahmood, 2007). The primary sludge
