Environmental Engineering Reference
In-Depth Information
Other main problems concerning these high-temperature thermal processes include
excessive energy to reach high temperatures, need for extensive air pollution equipment and,
therefore, high capital costs (Monte et al., 2008). 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. As a matter of fact, for SCWO and direct liquefaction
methods, the water in the sludge is the reaction medium and participates directly in the
reaction through the formation of free radicals (Griffith and Raymond, 2002). Accordingly,
these methods are more promising for the treatment of secondary sludge from the standpoint
of energy recovery. A comparison of advantages and disadvantages of different treatment
methods are presented in Table 5.
Table 5. Advantages and disadvantages of sludge treatment methods
(Kumar, 2000; Furness et al., 2000; Karayildirim et al.,2006; Bermejo et al., 2006;
Mahmood and Elliot, 2006;Monte et al., 2008)
Treatment
Method
Advantages
Disadvantages
Incineration
-
High reduction of sludge volume by about
90%
- Incineration process can be energy deficient
- Air pollution problems (NOx and SO
2
emissions)
- Dewatering/thickening of the sludge is
required
- Emission of chlorinated compounds
- High cost due to the increasing demand on
the flue gas cleaning
-
Nearly complete elimination of the organic
materials
-
Possible utilization for the ashes obtained
Pyrolysis
-
Non-burning process
- Dewatering/thickening of the sludge is
required.
- Less technical maturity for its application to
paper/pulp sludges
-
Production of a mixture of gaseous and
liquid fuels and a solid inert residue
-
Conversion of all sludge biomass fraction
into useful energy
-
Volume reduction by as much as 90% and
production of a sterile carbon char
Gasification
-
Higher efficiency of energy recovery
- Dewatering and drying of sludge is needed
- Not commercially developed for pulp and
paper sludge treatment
- Complexity of technology
-
Reduced environmental emissions
-
Ability to handle most inorganic compounds
found in sludge
-
Production of an inert solid waste
Direct
Liquefaction
-
Reaction occurs in aqueous phase, so that no
dewatering, thickening and drying of the
feedstock is required
- Not commercially developed
-
Production of a mixture of high calorific
value liquid fuels
-
Conversion of all sludge biomass fraction
into useful energy
Supercritical
Water Oxidation
-
Easily controlled by operator
- Corrosion and salt deposition in the
equipment which accelerates the
deterioration of the reactor
-
Reaction medium is water, so no
dewatering/drying required
-
High organic carbon destruction efficiencies
Anaerobic
Digestion
-
High energy recovery efficiency
- Slow process, long residence times
- Cannot accept shock loading and excessive
foaming is often a problem
-
Low operating temperature
-
No dewatering/drying required
