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Basic characteristics of low -temperature carbon products
from waste sludge
Y． SHINOGI★ , H. YOSHIDA
★ ★
, T. KOIZUMI★ , M. YAMAOKA★ , T.
SAITO★
* National Institute for Rural Engineering, 2-1-6 Kannondai,
Tsukuba, Ibaraki, 305-8609, Japan
** Rural Infrastructure Bureau, Ministry of Agriculture, Forestry
and Fisheries, 1-2-1 Kasumigaseki,
Chiyoda-ku, Tokyo, Japan
ABSTRACT
There is a growing need to recycle resources effectively and
therefore it is important to develop technologies for reuse of waste
materials, e.g. composting. Although compost offers a good potential,
its use is not widespread as compost technologies are not as yet
practical in Japan. We have been proposing carbonization though work
in this area is just starting and there is little or no reliable data
available. To develop this technology, is important to accumulate and
store every little data that becomes available. This study was done to
investigate the basic properties of carbon products from sludge,
and compared them with mainly those of wooden charcoal. The results
indicated that carbon products from sludge are more porous and
contain some useful minerals such as nitrogen, phosphorus and
potassium, etc. and harmful substances exceeding the acceptance level
were not observed. Such information is essential for building a
`zero-emission society`.
KEY WORDS;
carbonization; carbon product; recycling
INTRODUCTION
It is difficult to dispose of waste products especia lly in developed
countries such as Japan. Also, it is essential to recycle resources
effectivel y, such as by reusing waste products.
Waste sludge from animals and humans has been reported to contain
various useful minerals such as nitrogen, phosphorus and potassium,
so it makes sense to reuse these as resources effectivel y. Although
compost is a potential way, it is not widely accepted for various
reasons in Japan. It is necessary to develop such recycling
technologies to build a "zero -emission society".
We have been proposing carbonization, which is said that it reduces
weight and volume, and can also reduce foul odors and makes the
product easy to handle. It is expected to be used for soil amendments
and as a material for filtering water, among others.
In Japan, wooden charcoal is widely used. While various studies
have been carried out on charcoal, there have been few studies on
other kinds of carbon products. It is important to accumulate research
data and utilize them properl y.
This paper reports on the basic properties of carbon products mainly
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from waste sludge, with the focus on low -temperature carbon products
in view of energy cost. A comparison with charcoal is discussed, and
some useful results were obtained.
METHODOLOGY
Carbonization
Carbonization is different from incineration in terms of ox ygen
suppl y. In this study, Toshiba’s TEAC carbonization plant was used.
The maximum temperature of the plant is 400 ℃ and vessel capacity is
about 500 L.
Four kinds of material were used as original materials: sugarcane
straw (dried and crushed residue), rice husks (dried), cattle waste
(dewatered), and sewage sludge ( from Sequencing Batch Reactor
Activated Sludge Process and dewatered). They were packed in cans
and heated in the plant. Carbonization temperature was 380 ℃ and the
energy process was constant; this carbonization temperature was not
particularly high in view of the energy cost.
Analysis
Various physical and chemical anal yses were carried out.
1) Physical analysis
Volume and weight were measured before and after carbonization.
The reduction percentage was calculated using Eq uation (1).
Reduction percentage for weight or volume (%) = (Input weight
(Volume) - Carbon weight (Volume))/Input weight (Volume)*100 -----Eq. (1)
Density was calculated as a specific volume. In addition, the
maximum water holding capacity was observed.
2) Chemical analysis
pH and electric conductivity (EC) were measured. Carbon products
were soaked in distilled water and boiled for five minutes and
measured by pH and EC sensors, respectively. In addition, NC anal ysis
(NC Anal yzer) and chemical analysis were carried out for solute water
of the carbon products.
3) Industrial analysis
Constitutional anal ysis, element anal ysis, surface area anal ysis by
nitrogen gas and porous distribution analysis by mercury were carried
out.
4) Metal substance anal ysis
Heavy metals and harmful substances were checked.
RESULTS AND DISCUSSION
Characteristics
Table 1 shows the results of carbonization. The water content of the
sludge before carbonization is higher, therefore the elapsed time is
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longer. The reduction of weight and volume vary for each material.
For example, there was a 93% reducti on of weight by carbonization for
sewage sludge. The reduction of weight and volume was high.
Table 2 shows the main characteristics of the carbon products.
Almost all carbon products were alkaline (pH>7), which is the same as
that of charcoal (Abe, 1988), probably due to detachment of alkali
metals (Sugai M, 1991). EC is larger than that of charcoal. Especially,
carbon from cattle waste is largest, while that from sewage sludge is
smallest. The maximum water holding capacity is largest for the
carbon from cattle waste and smallest for that from sewage sludge.
This means that the carbon product from cattle waste can hold about
three times its own volume of water.
Foul odors such as ammonium were not observed. Consequentl y,
carbonization enables waste pro ducts to be handled easily because it
reduces the weight, volume and odors.
Engineering properties
Table 3 shows the engineering properties. The surface area of
sugarcane straw is relatively large, though less than that of charcoal
(average surface area of charcoal is 300-400 m2/g (Abe, 1988)).
The porous volume is smaller and the average porous diameter is
larger than that of charcoal. The constitution of the carbon products
varies widel y. Generally, the ash percentage is larger and the fixed
carbon percentage is smaller than those of charcoal. In addition, the
carbon element is smaller than that of charcoal. Anyhow, carbon
products are porous.
Carbon from sugarcane has a different constitution from the other
materials (high fixed carbon and low ash r atio) and higher carbon
ratio.
Table 4 shows the chemical properties of carbon products. Total
carbon (T-C) is smaller than that of charcoal and total nitrogen (T-N)
is smaller than that of the compost product. P 2O5, CaO, MgO and K2O
vary greatl y accordin g to the original materials. Generall y, CaO is
larger than that of the
charcoal and compost product. Carbon from sludge contains a larger
amount of MgO but smaller amount of K 2O than the compost product.
Figure 1 shows the t ype of nitrogen of each carbon product. The t ype
of organic and inorganic nitrogen differs from carbon products.
Organic nitrogen is dominant for cattle waste carbon, while inorganic
nitrogen is dominant for sewage sludge and sugarcane carbon.
Maximum total nitrogen is about 0.5 (mg/g).
Figure 2 shows soluble phosphate for each carbon product. Phosphate
ion is biggest for the carbon from cattle waste, and smallest for
charcoal. Generall y, soluble phosphate could be expected from these
kinds of carbon products.
Figure 3 shows soluble potassium ion from each carbon products.
Potassium ion is largest for cattle waste carbon, and smallest for
sewage sludge carbon. Generall y, potassium ion could be also expected
from these kinds of carbon products. However, it is necessary to
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clarify the dissolution mechanism of these elements.
Harmful substances
Table 5 shows some harmful substances including heavy metals. The
results showed there are no harmful level (based on the Japanese
standard) of heavy metals and harmful substances. However, the l evel
of zinc (Zn) requires careful attention, as in this standard the zinc
level is lax. In addition, copper (Cu) is also a major problem, and
though it was not observed in this study it should be checked
carefully.
CONCLUSIONS
Experiments were carried out to clarify the basic properties of low
temperature carbon products from waste sludge. The following results
were obtained:
1. Carbonization reduces the weight and volume of waste products.
2. Carbon products are rather light. Generall y, they can co ntain large
amounts of water, and may be used as soil amendments to improve
permeabilit y and water holding capacit y.
3. Some useful minerals such as nitrogen, phosphorus and potassium
are contained in each t ype of carbon product.
4. Harmful substances exceeding the acceptable level were not
observed.
REFERENCES
ABE F. ,1988. Thermochemical Study of Forest Biomas (Japanese
with English summary). Bull. of Forest and Forest Product Research
Institute, No.352, pp. 1-95
SUGAI M. ,1991. Charcoal and wooden acid liquid for protection of
Environment (Japanese). Ienohikari Publishing Co., Ltd., pp. 32 -39
1
2
Table 1 Results of Carbonization
Water content(%)
Material
10*
270
76
83
Sewage sludge
83
495
93
86
220
44
78
300
90
9*
Cattle sludge
81
83
*; Air dried condition.
Table 2 Characteristics of Carbon Product
Carbon
PH
EC
(H2O)
(mS/cm)
Maximum water
product
7
8
Reduction of weight(%) Reduction of volume(%)
Sugarcane straw
Rice husk
3
4
5
6
Elapsed time(min)
Density
holding capacity (t/m3)

(%)

Sugarcane straw
6.8
0.38
497
0.23
Sewage sludge *
8.5
0.09
57
0.65
Rice husk
8.7
0.22
232
0.31
Cattle waste *
8.2
2.90
294
0.32
Charcoal 1
9.8
0.14
117
0.34
Charcoal 2
*: Dewatered
7.0
0.08
184
0.35
Table 3
Material
Constitution（％）
Total
Average
ture
Area per
por ous Porous
(℃)
unit weight diameter volume
(㎡/g)
Carbon
Engineering Property
Tempera S u r f a c e
(Å)
Main element（％）
Carbon Hydro-
Water
Ash
(m3/t)
Volati-
Fixed
lity
Carbon
(C)
gene
Oxygen
(O)
Rice straw
380
5.6
231
0.03
0.89
38.5
19.6
41.0
47.3
2.9
11.0
9.8
Rice husk
380
3.5
227
0.02
0.63
42.8
26.1
30.5
45.6
2.8
8.6
8.7
Cattle waste
380
2.2
235
0.01
0*
25.6*
44.6*
29.9*
49.2
7.8
17.5
8.2
Sewage sludge
380
2.4
273
0.02
2.4
44.9
22.7
30.0
38.3
3.0
12.7
8.5
9
10
11
（Per dry weight）
Table 4 Chemical Properties
T-C
Original Material
(％)
T-N
(％)
C/N
P 20 5
Ratio (mg/100g)
CEC
CaO
MgO
K2O
Phosphate
(me/100g)
(mg/100g)
(mg/100g)
(mg/100g)
Absorption
coefficient
(mg/100g)
Sugarcane straw
68.1
0.04
1580
650
8.8
560
270
590
1100
Sewage sludge
38.3
5.20
007
6000
0.5
5200
1100
750
-160
Trash from canal *
43.4
2.50
017
3500
9.2
3700
710
840
520
Cattle waste(380℃)
62.1
0.10
620
42
39.0
3250
1430
473
2060
―
**Compost product
12
13
14
15
pH
(H)
30.9
2.06
15
2350
882
64.2-92.5
0.10- 0.74
―
20-170
1.0-16.3
140-1220
30-240
***Charcoal
*; Reference, **: Based on data from Ministry of Agriculture, Forestry and Fisheries, Japan
***: Calculated based on data from Forestry Agency, Ministry of Agriculture, Forestry and Fisheries, Japan
50
2060
―
170-450
―
1
Table 5 Harmful Substances
Heavy metal (mg/kg)
Tempe
Material
Cd
As
T-Hg
380
ND
ND
Sugarcane straw
380
ND
Cattle waste
300
rature
Harmful substance (mg/)
Zn
Cd
Si
P
Pb
Cr
ND
17
ND
ND
ND
0.01
ND
ND
0.20
52
ND
ND
ND
ND
0.28
ND
ND
83
ND
ND
ND
1.30
ND
ND
500
ND
ND
ND
5.0▲
50▲
2.0▲
1800★
0.3■
1.0■
1.0■
As
T-Hg
Al-Hg
PCB
ND
ND
ND
ND
ND
ND
ND
ND
ND
0.01
ND
ND
0.0007
ND
ND
ND
ND
ND
ND
ND
ND
3.0■
1.5■
1.5■
0.005■
ND■
(℃)
Rice husk
Sewage
380
sludge
Standard
2
3
4
5
less
less
less
less
less
less
less
less
less
less
than
than
than
than
than
than
than
than
than
than
▲: Law on Prevention of Soil Deterioration, Environment Agency, Japan
★: Recommendation Standard on Organic Nutrition, Ministry of Agriculture, Forestry and Fisheries, Japan
■: Law on Industrial Waste Management including Heavy Metals, Ministry of Generals , Japan
less than
0.03■
less than
0.5
Nitrogen (mg/g)
N on-O rganic N itrate(m g/g)
0.4
O rganic N itrate(m g/g)
0.3
0.2
0.1
0.0
Sugarcane
C attle w aste
Trash at D rainage canal
Phosphate Ion (mg/g)
1
2
3
4
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
C harcoal
0.76
0.68
S ugarcane
C attle w aste
0.09
0.12
Trash at D rainage
canal
H um an sludge
0.01
F ig.2　P hosphorate Ion
Potassium Ion (mg/g)
5
6
10
7.80
8
6
4
3.40
2.60
2
0.20
0.68
0
S ugarcane
7
H um an sludge
Fig.1　T ype of N itrogen.
C attle w ast
Trash at
H um an sludge
D rainage canal
F ig.3 P o tassiu m Io n
C harcoal
C harcoal