Environmental Engineering Reference
In-Depth Information
and it is employed worldwide as the oldest and most important process for sewage sludge
stabilization and treatment (Ponsá et al., 2008). While anaerobic digestion is commonly
practiced in the municipal sector, it has not gained popularity in the pulp and paper industry
mainly because of its long sludge residence time requirement of 20-30 days (Elliot and
Mahmood, 2007). There is currently no full-scale anaerobic digestion facility in the pulp and
paper sector for the digestion of solid residues. Nevertheless, there is recent technological
advancement that potentially can make anaerobic digestion of pulp/paper sludge more
feasible by the development and establishment of pretreatment of sludge prior to anaerobic
digestion to accelerate the hydrolysis of sludge.
3.
D
ISCUSSION AND
C
OMPARISON OF
T
REATMENT
M
ETHODS
The operating conditions (temperature, pressure, atmosphere and products, etc.) vary
among the methods discussed in the preceding sections, as summarized in Table 4. For
example, incineration, gasification and SCWO methods utilize air or oxygen while the
remaining methods are conducted under oxygen depleted or anaerobic conditions. Some
direct liquefaction processes employ hydrogen gas to obtain better product yields and results.
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.
Table 4. Summary of Comparison of Secondary Sludge Treatment Methods
Comparison
Parameter
Incinerat
ion
Direct
Liquefaction
Anaerobic
Digestion
Pyrolysis
Gasification
SCWO
Preheating/
Drying
required
Yes
(25% solids,
~150
o
C)
No
Yes
No
No
No
37 (Mesophilic)
55
(Thermophilic)
Operating
Temp (
o
C)
400-
600
850-950
400-800
800-1400
250-400
Operating
Pressure
Ambient or
slightly above
Ambient
Ambient
4-20 MPa
22 MPa
Ambient
H
2
-
reducing
N
2
- inert
Operating
atmosphere
Oxygen
depleted
Air or
oxygen
Air or
oxygen
Air
Anaerobic
Primary
Energy
Products
Oil, Gas and
Char
Oil, Gas and
Char
Biogas,
Methane
Steam
Syngas
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.
