Environmental Engineering Reference
In-Depth Information
taBle 32.19
sludge concentration-density-Percent of solids relationships (at 20°c)
Percent
sludge (w
s
)
concentration
(g/l)
sludge density
(kg/m
3
)
concentration
(g/l)
Percent
sludge (w
s
)
sludge density
(kg/m
3
)
0.5
5.0045
1000.90
5
0.4996
1000.90
1
10.0180
1001.80
10
0.9982
1001.80
2
20.0723
1003.61
20
1.9928
1003.60
3
30.1629
1005.43
30
2.9839
1005.40
4
40.2901
1007.25
40
3.9714
1007.20
5
50.4541
1009.08
50
4.9554
1009.00
6
60.6551
1010.92
60
5.9359
1010.80
7
70.8933
1012.76
70
6.9129
1012.60
8
81.1688
1014.61
80
7.8864
1014.40
9
91.4820
1016.47
90
8.8565
1016.20
10
101.8330
1018.33
100
9.8232
1018.00
15
154.1624
1027.75
150
14.6056
1027.00
20
207.4689
1037.34
200
19.3050
1036.00
of mesophilic technology. However, when heat regeneration is used (between sludge outflow and
inflow) the parasitic energy demand of thermophilic and mesophilic processes are equal (Zupancic
and Ros 2003). The mesophilic sludge digestion process can sustain OLR of 3.0-3.5 kg of volatile
solids per m
3
of digester per day (kg m
−3
/day), whereas a thermophilic process can sustain OLR of
up to 8.0 kg m
-3
/day. This offers a digester size reduction of at least 50%, which can be beneficial in
construction costs.
The biogas yield (SBP) of anaerobic sludge digestion (mesophilic or thermophilic) is between
400 and 600 L per kg of volatile solids inserted (L/kg), depending on sludge composition. This
yields 24-36 L per PE per day of biogas; 7-10 W per PE of power potential and 2-3 W per PE of
electric power potential. Biogas production rate varies in mesophilic digesters from 0.7 to 1.1 cubic
meters per volume of digester per day (m
3
∙m
−3
∙d
−1
); in thermophilic from 1.3 to 1.7 m
3
m
−3
/d ay.
Biogas composition is at 60-80% methane, the rest is mostly CO
2
. H
2
S is present in relatively low
concentrations (up to 0.4%). A specific problem in sludge biogas may be the presence of siloxanes
(Dewil et al. 2006). They originate from municipal wastewater, especially from cosmetic and
detergent products containing silicon based compounds and which are washed out to wastewater
from human use. At high temperatures of combustion, siloxanes are oxidized to SiO
2
which remains
on the surface of the machine parts. There it causes abrasion of the pistons and that results in CHP
breakdown. How much this phenomena affects the power generation fuel cells has not yet been
investigated. To avoid this problem biogas must be treated and siloxanes removed. There are several
treatments available (Schweigkofer and Niesser 2001); one of the modern treatment possibilities is
removal with membranes (Ajhar and Melin 2006). Resent research is also focused from removal on
siloxanes from the sludge before anaerobic digestion (Appels et al. 2008).
32.5.2.5 Further treatment and use of digested sludge
(Biosolids) and supernatant Water
After anaerobic digestion sludge is usually mechanically dewatered (25-40% of dry solids), the
solid and liquid fraction are then further treated. Landfilling of biosolids is prohibited in most
EU countries (EEP 1986), so other options must be accounted for. The most environment-friendly
option would be fertilizer use. Contrary to farm waste, which is usually suitable for fertilizer use,
biosolids from municipal sludge may contain trace elements that may prohibit fertilizer use (Singh
and Agrawal 2008). Especially high concentrations of heavy metals such as Cu, Mn, Pb and Zn
