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Geotechnical News • March 2013
37
WASTE GEOTECHNICS
ing property of the bottom ash will be
presented later in detail.
The fly ash collected from the air
pollution control system is primarily
composed of fine dust with particles
sizes ranging from 50 to 250
µ
m.
The colour of fly ash usually varies
from gray to black. Table 2 shows a
representative measurement of the
heavy metal content in the fly ash [2].
It can be seen that the fly ash contains
high levels of Pb, Zn, Cu, Cr, and Cd.
In addition, Ye et al. (2007) showed
that the fly ash also contains persistent
organic pollutants such as dioxin and
polycyclic aromatic hydrocarbons [9].
Therefore, the fly ash can be catego-
rized as a hazardous waste, and it
should be collected, stored, and trans-
ported separately from the bottom ash
using the methods set by the Pollution
Control Standard for Municipal Solid
Waste Incineration (GB18485-2001)
as stipulated by China’s Ministry
of Environmental Protection. The
standard requires that all the fly ash be
disposed of safely in a landfill with a
double liner system.
Chemical composition and leaching
toxicity of bottom ash
Table 3 shows the chemical composi-
tion of the bottom ash collected from
five major cities in China, including
Beijing, Shanghai, Guangzhou, Shen-
zhen, and Kunming [3]. It can be seen
that the primary chemical composi-
tions of the ash include SiO
2
, Al
2
O
3
,
CaO, and Fe
2
O
3
. A certain amount of
SiO
2
and Al
2
O
3
in the bottom ash are
in amorphous form and have potential
activity.
As seen in Table 3, the bottom ash
contains some heavy metals and
soluble salts, so its leaching toxicity
should be evaluated before disposal or
utilization. Table 4 shows a represen-
tative result for the samples collected
from Shanghai from 2004 to 2006
[4]. It can be seen that the leaching
concentrations of heavy metals in the
bottom ash is lower than the limit val-
ues regulated in the national standard
(GB5086 [5]), thus bottom ash is not
considered a hazardous material and
can be placed in a landfill directly.
Engineering properties of bottom
ash
The disposal or utilization of bottom
ash requires an understanding of its
engineering properties, including the
particle size distribution, unit weight,
water permeability, shear strength,
etc. Table 5 shows representative data
of the particle size distribution for
bottom ash [6]. The particle sizes of
the bottom ash range from 0.07 to 10
mm, and 70-85% of the particles are
as large as sand. It was found that the
particle size distribution depends on
the type of MSW incinerator produc-
ing ashes. Compared with that from
the mass burning incinerator, the bot-
tom ash generated from the fluidized
bed incinerator has a higher fraction of
particle sizes in the range of 2-10 mm
and a lesser fraction in the range of
0.45-0.9 mm.
Table 6 shows a comparison of
engineering properties between the
bottom ash and a gravely sand with a
similar particle size distribution [7].
The specific gravity and density of the
bottom ash are smaller than that of
the sand. The bottom ash has a higher
water absorption capacity than the
sand. The water permeability of the
bottom ash is in the same order of 10
-4
cm/s as that of the sand. The friction
angle of the bottom ash ranges from
40
°
to 45
°
and is greater than that of
the sand. The greater friction angle is
attributed to the irregular shape and
rough surface of bottom ash.[8].
Table 2. Content of heavy metals in fly ash (mg/kg).
Heavy
metals
Pb
Zn
Cu
Cr
Cd
Hg
Content
(mg/kg)
3084 4745.6 587.6 160.8 125.0
5.8
Table 3. Chemical composition of the bottom ash collected
from five major cities in China.
City
SiO
2
Al
2
O
3
CaO Fe
2
0
3
MgO K
2
O Na
2
O P
2
O
5
TiO
2
S and others
Beijing
56.7 13.8 9.7 6.1 2.9 2.6 1.9 2.48 0.9
2.92
Shanghai
39.1 16.4 24.3 6.8 2.2 2.00 2.4 2.30 0.87
3.63
Guangzhou 53.55 13.56 14.34 3.07 1.06 1.69 0.67 3.12 1.18
7.73
Shenzhen 48.07 10.47 17.77 5.95 1.14 1.97 1.30 3.40 1.22
8.71
Kunming 46.22 11.56 18.74 15.33 0.94 1.87 1.03 2.40 0.82
1.09
Figure 1. Bottom ash without bulk
materials.