Basic HTML Version
38
Geotechnical News • March 2013
www.geotechnicalnews.com
WASTE GEOTECHNICS
Utilization of MSW incineration
bottom ash as civil engineering
materials
As discussed previously, the bottom
ash generated from MSW incinera-
tion has the engineering properties
of aggregates and is low in leaching
toxicity. Thanks to these characteris-
tics, bottom ash has been utilized as
civil engineering materials in China
in recent years. Such utilization has
many advantages including mitigating
the stress of disposing bottom ash in
limited landfill space, thereby reduc-
ing the cost of landfilling operations,
and providing a substitute material
for the shortage of natural aggregates
for civil engineering. However, the
adverse impact of using bottom ash
in civil engineering projects include
the risk of environment pollution,
which should be evaluated in advance.
MSW incineration plant operators
are trying their best to find safe and
economic approaches to utilize bottom
ash. Presently, there are mainly three
approaches which are detailed herein.
Using bottom ash as filling
materials for subgrades
Using bottom ash as filling materials
for road subgrades and embankments
has been a major means of utilizing
bottom ash in China. First, particles
greater than 10 mm are removed by a
screening technique, and then metals
and other substances that are harm-
ful to the environment are removed
through a magnetic separation tech-
nique. The resultant bottom ash is then
similar to a lightweight aggregate in
terms of engineering properties. The
filling materials can be easily com-
pacted to the prescribed density for
roadwork. Engineering practice shows
that subgrades filled with bottom
ash have sufficient bearing capacity,
stability, and permeability, meeting
engineering requirements [10] .
Using bottom ash as daily cover
material for MSW landfills
Regardless of the rapid increase in
the incineration of MSWs, landfilling
remains the primary disposal method
in China. In many cities, newly built
incineration plants are located near
the landfill sites. Landfill operations
require a lot of fine soil for the daily
and intermediate cover of wastes.
Daily cover is used to prevent wastes
from blowing away, to prevent ani-
mals from getting into the waste, and
to mitigate the spreading of odour.
Some landfill operators are starting to
utilize bottom ash as daily cover mate-
Table 4. Leaching toxicity of heavy metals for the bottom ash
generated in Shanghai.
Heavy
metals
Sample
collected in
October
2006 (mg/L)
Sample
collected in
August 2005
(mg/L)
Sample
collected in
March 2004
(mg/L)
Limit
regulated
in GB5086
(mg/L)
Hg
0.000014
ND
ND
0.05
Pb
ND
ND
ND
3
Cd
ND
ND
ND
0.3
Cr
0.048
0.021
ND
10
CR
+6
0.020
0.009
ND
1.5
Cu
0.524
0.405
0.049
50
Zn
ND
0.004
0.008
50
Be
ND
ND
ND
0.1
Ba
0.102
0.356
0.449
100
Ni
ND
ND
ND
10
As
ND
ND
ND
1.5
F
0.040
0.40
0.35
50
CN
-
ND
ND
ND
1.0
Note: ND indicates a value lower than the minimum detection limit of the
instruments.
Table 5. Particle size distribution of bottom ash (%).
Type of MSW
Incinerator
Particle size (mm)
2~10 0.9~2 0.45~0.9 0.07~0.45 <0.07
Mass burning
incinerator
15.07 15.74
27.52
40.88
0.78
Fluidized bed
incinerator
31.41 12.38
18.23
35.48
2.51
Table 6. Comparison of engineering properties between
bottom ash and sand.
Material
Specific
gravity
Maximum
dry
density
(kg/m
3
)
Water
absorption
capacity
(%)
Water
permeability
(cm/s)
Internal
friction
angle (
0
)
Bottom ash 1.50-2.44 1264-1760 4.1-17.0
10
-4
40-45
Gravely
sand
2.65
1900
<2
10
-4
32-45