Environmental Engineering Reference
In-Depth Information
Comparing with other typical industrial sludges containing 16-35% of total dry solids
(Oral et al., 2005; Rato Nunes, et al., 2008), the original secondary sludges from municipal
wastewater treatment plant or a pulp and paper mill usually contains a much higher ratio of
water (98-99%). A comparison of municipal and pulp and paper secondary sludge
characteristics is presented in Table 1. Similarities exist between municipal and pulp and
paper waste activated sludge, which would suggest that technologies used in the municipal
wastewater sector could be transferred into the pulp and paper industry. General processes
and technologies of sludge reduction technologies through process changes (e.g. operational
control, and return activated sludge treatment) or through post-treatment (e.g., incineration,
carbonization gasification, pyrolysis, supercritical water oxidation (SCWO) and
aerobic/anaerobic digestion, etc.) have been recently reviewed by Mahmood & Elliot (2006).
While changing/optimizing the sludge producing process would certainly reduce the amount
of secondary sludge generation and thus alleviate the issues of sludge waste management,
secondary sludge post-treatment technologies and in particular those aimed at energy
recovery are the focus of this chapter.
Table 1. Comparison of municipal and pulp and paper activated sludge
(modified from Elliott and Mahmood, 2007)
Municipal
Pulp/Paper
Total dry solids (TS) (%)
0.8-1.2
1.0-2.0
Volatile solids (%TS)
59-68
65-97
N (%TS)
2.4-5.0
3.3-7.7
P (%TS)
0.5-0.7
0.5-2.8
Fe (g/kg_TS)
0
0.33-2.2
PH
6.5-8.0
6.0-7.6
Heating value
(MJ/kg_TS)
19-23
22-25
To evaluate different post-treatment options and in order to provide a means of
comparison, it is advantageous to compare the energy efficiency for each option if possible. A
definition of net energy efficiency may be outlined as follows (Xu and Lancaster, 2008). The
net energy efficiency or “Energy Output/Input Ratio” can be defined as the ratio of energy
content of the objective products to the energy input to produce it, as show in Eq. (1). For
simplification of the discussion, several assumptions may be adopted: (1) Since the feedstock
used is waste biomass or waste sludge, it can be considered as the feed of “ZERO” energy
value, (2) the heat loss of the reactor or process is negligibly small assuming well insulation
and (3) the energy consumption by other auxiliary operations (e.g., feedstock preparation and
feeding, products separation and recovery, etc.) may be neglected. From the above
assumptions, for many hydrothermal treatment processes for the treatment of sludge, the
energy input may be approximated by the energy required to dewater/thicken the sludge from
its original TS content (e.g. 1-2% for secondary pulp/paper sludge) to a suitable TS content
for the process use (e.g 25% TS or above), and to heat the dewatered sludge from room
temperature (
T
rm
) to the specified reaction temperature (
T
).
