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The influence of aeration on the composting of poultry manure-ground corncob mixtures
J. agric. Engng Res. (1970) 15 (1) 11-16
The Influence
of Aeration
Manure-ground
on the Composting Corncob
of Poultry
Mixtures
R. G. BELL* The influence of the rate of aeration on 1.5 m columns of composting mixtures of 2 parts poultry manure and I part ground corncob was investigated. The results indicated that the optimum aeration rate for the production of a stable sanitary compost was 4 litres of air/m”/min for every 10 cm of cornposting material up to a maximum depth of about 2.4 m.
1.
Introduction
The disposal of litter-free poultry manure collected from caged laying systems has become a problem because of the enormous size of many commercial operations. Even when sufficient land is available for manure disposal the manure must be stored during those periods when land application is inadvisable or not possible. 3 Some form of manure treatment is therefore necessary to prevent manure storage facilities from becoming public health or pollution hazards. Cornposting has long been used as a method of stabilizing organic wastes. Rapid stabilization can be effected by composting if the material undergoing decomposition has a moisture content near 50 y&” a course structure to allow air circulation and a carbon to nitrogen ratio of about 30 to 1.2 The generation of heat is a phenomenon which is associated with the composting process. Under appropriate conditions the temperature of actively composting materials will rise apprecirise is a function of both microbial activity and ably. ’* * The magnitude of such a temperature the rate of heat loss6 Therefore, within a given set of conditions the temperature of a composting mixture can be used as a measure of the cornposting process. Temperature is easily determined and is of great significance in the public health aspects of composting as a waste treatment process.5.7 Fresh poultry manure is poorly qualified as a compostable material as it has a moisture content near 750,:, a carbon to nitrogen ratio of about 10: 1 and is composed of many fine particles. In preliminary experiments it was found that mixtures of 2 parts by weight of fresh poultry manure and 1 part of dry ground corncob could be readily composted. The use of corncob as a diluent decreased the moisture content, increased the carbon to nitrogen ratio and improved the texture of the composting mixture. The present study was undertaken to investigate the influence of aeration on the cornposting of poultry manure-ground corncob mixtures. The results it was hoped would prove to be useful criteria for the design of an efficient pilot cornposting plant.
2.
Materials and methods
A composter (Fig. I) was constructed from 1 cm thick polyvinyl chloride (PVC) sheet and a I .6.5 m length of PVC pipe with an internal diameter of 25 cm and a wall thickness of 1 cm. The ends of the composter were removable and consisted of a square of PVC sheet bolted to a PVC flange cemented to the end of the pipe section (see detail of Fig. I). A silicone grease liquid gasket between the flange and the PVC square ensured that the ends were gastight. Ammonia was removed from the exhaust gas stream by a sulphuric acid trap. The exhaust gases were then mixed with natural gas from the domestic supply and ignited at a jet. This simple after-burner effectively destroyed the odiferous compounds present in the exhaust gas. The flow of air through the composter was measured periodically using a wet test meter (Precision Instruments, Chicago, Ill., U.S.A.) situated between the composter and the after-burner. ’ Department of Microbiology, University of Guelph, Guelph, Ontario, Canada II
12
COMPOSTING
OF
POULTRY
MANURE-GROUND
_,--
--(meter
CORNCOH
MIS-I
C’RLIS
/-\
Exhoust,,H’
Thermometers
169cm
2 inlet
Composting
y _30 cm
mixture
Silicone grease \ v \
\
M I I
l-i
1 I
Fig. 1. Cornposter used to study the influence of aeration on the composting of poultry manure
The corncob used in this series of experiments was a kiln-dried animal bedding material which was milled on a Papec Feed Mill (Papec Machine Co., Shortsville, N.Y.) to pass a f in screen. A composting mixture of 2 parts manure to 1 part ground corncob was prepared. The composter was filled to a depth of 1.5 m with approximately 30 kg of the cornposting mixture. After the top had been securely bolted down the air flow was adjusted to the desired rate and the cornposting process allowed to proceed. Daily temperature readings were taken at all depths. Regular turning of material being composted in windrows has been found to be beneficial to the composting process so on every third day the composter contents were removed, mixed and replaced in such a manner that the compost column was inverted. Aeration was then continued and another set of temperature readings taken. 3.
Results and discussion
The temperature changes observed in columns of the poultry manure corncob mixture aerated at 24, 48,96 and 192 1 of air/m2/min are shown in Figs 2, 3, 4 and 5 respectively. The initial level of the surface of the compost column for each 3 day incubation period is marked on the x-axis by an upright arrow and the final surface level is indicated by an inverted arrow. If composting is to prove a satisfactory method by which to treat poultry manure prior to storage it is desirable that the end product should be both free of pathogenic micro-organisms and in a biologically stable condition. Salmonella sp. have been observed to be destroyed within 2 days in the a-mesothermic (55-65” C) zone of cornposting refuse.’ If the temperature of a composting mixture fails to rise above 30” C it can be considered to be almost completely stabilized.6
R.
G.
BELL
I?
LO !ieight
above
false
floor,
cm
0
20
40
60
Height
Fig. 2.
0
20
40 Height
Temperature
60
changes in
80
above
100
false
floor,
40
60
80
Height
above
false
I00
I20
I40
80
above
false
floor,
IO@ floor,
I?0
‘40
cm
cm
cornpostingpoultry manure aerated at 24 llm2/min
120
140
cm
Height
above
false
floor,
cm
Dav
2oT ’ ’ 0
’
1
20
Height
Fig. 3. Temperature
’
1
40
1
1
1
1+1+ ., I 100
60
80
above
false
floor,
I
cm
Day
+I+
IjIll 20
1 I40
changes in composting poultry manure aerated at 48 I/m2/min
Dw
0
1
120
40 Height
60
80
above
false
100
I20
floor,
cm
y
70
Day
c
60
(0) 7 R
:
50
(XI (0)
9
I
I
k 2 z 0
20
Temperature
?O
40 Height
I 40 Height
Fig. 4.
0
140
/
I 1
I
60
80
above
false
,
60
80
above
false
100 floor,
I b 100 floor,
I20
140
cm
changes in composting poultry manure aerated af 96 I/m’/min
PO cm
140
14
COMPOSTING
Height
above
OF
false
floor,
POULTRY
MANURE-GROUNO
cm
Height
C‘ORbiC‘OH
above
false
floor,
MIX7LR[:S
cm
Day
Height
Fig. 5. Temperature
above
false
floor,
cm
changes in composting poultry manure aerated at 192 I/m”/min
It is proposed to use the depth of compost at temperatures in excess of 55°C (Table 1) as a sanitary measure and a measure of the column heat in excess of 30” C (Table II) as an indication of the stability of the compost. TABLE
I
Height in cm of compost column at 55” C and above
Aeration rute, Ilm’/min
48 53 25 45 46 48 53 48 53
48
96
192
96 93 53 II5 88 19 65 21 0
117 103 33 92 90 0 75 0 0
0 0 68 0 0 0 0 0 0
-
-
-
TABLE 11
Area under temperature curves above 30” C
I
&y
Aeration rate. llm21min
~_________
1 2 3 4 5 6 7 8 9
24
48
96
192
20 20 16 19 19 20 21 19 20
39 38 27 39 35 19 31 13 3
43 38 27 38 33 16 19 4 3
17 21 27 16 13 9 13 8 5
K.
G.
BLLL
1s
The increasing stability of the material being composted is indicated by the decrease in the temperature of the column as the process progresses (Figs 2-4 and Table II). Following mixing and inversion of the compost columns a distinct increase in temperature was observed at the 3 lowest aeration rates. Since the operating conditions after mixing remained unchanged the temperature increase must result from increased microbial activity caused by the introduction of fresh substrates into the vicinity of the composting micro-organisms. The rapid fall in temperature above the 60 cm level in the compost column aerated at 24 I/m’/ tnin (Fig. 2, left) probably indicates that only enough oxygen was provided to support sanitary composting in the bottom 60 cm of the column. This hypothesis is supported by the observation that the upper part of the column smelt strongly of lower fatty acids which is indicative of the occurrence of anaerobic metabolism in that region. After the first inversion (Fig. 2, right) the tr-mesothermic zone, about 50 cm wide, occurred at a level which corresponded to the level where the temperature fell below 30” C the third day, i.e. the end of the composting zone. The gradual advance of the a-mesothermic zone down the column reflects the colonization of the now adequately aerated former anaerobic region by aerobic composting micro-organisms. After the second inversion (Fig. 2, bottom) the a-mesothermic zone was again situated at a level corresponding to the end of the former composting zone. From Table I it can be seen that an aeration rate of 14 I/m”/min was only adequate to maintain a compost column about 50 cm high at a sanitizing temperature. The almost constant heat of the column throughout the composting process (Table II) indicates that readily decomposable organic matter was still present after 9 days. These observations suggest that the aeration rate was inadequate to stabilize and sanitize a 1.5 m column of compost. When the aeration rate was increased to 48 l/m’/min (Fig. 3, left), both the a-mesothermic zone (Table I) and the heat of the column (Table II) were approximately double those produced by aeration at 24 I/m”/min. As a consequence of the increased cooling effect of the stronger air current the start of the a-mesothermic zone was displaced from the 1I cm to the 24 cm level. The a-mesothermic zone (Fig. 3, left) ended very abruptly at the I20 cm level which strongly suggests that the oxygen supply became exhausted at that level. By the ninth day the column had almost cooled down which suggests that the remaining organic matter was well stabilized. The results recorded for the 96 l/m”/min aeration (Fig. 4) were very similar to those recorded for the 48 l/m”/min aeration (Fig. 3). The a-mesothermic zone was extended almost to the top of the column. The abrupt fall in temperature at the surface (Figs 4 and5) is believed to be a surface effect and not to be a consequence of oxygen depletion. While the aeration was sufficiently high to provide an adequate supply of oxygen throughout the entire length of the column the evaporative and convection cooling effects were not large enough to prevent the column from heating LIP. At the highest rate of aeration, 192 I/m2/min (Fig. 5), the cooling effect of the air current was large enough to prevent any substantial rise in the temperature of the column. The temperature changes recorded in Fig. 5, Zeft differ from all the other results in that the column became hotter as incubation progressed. This would occur if the heat produced by microbial activity was only slightly greater than the heat loss. Initially the temperature would rise slowly, but as the temperature increased so would the rate of microbial activity. The difference between heat production and heat loss would increase so the temperature of the column would continue to rise. However. as the readily utilizable substrate became exhausted the heat production would decrease until it was smaller than the heat loss so the column would cool down. This stage was probably reached about the time of the first inversion (Fig. 5, right) after which the column continued to cool. Visual examination of the compost after 9 days’ incubation showed that the poultry manureground corncob mixture had undergone very little decomposition. The low temperature of the compost on the ninth day was the result of the cooling effect of the air current and not a reflection on the stability of the compost. Too much air as well as too little can reduce both the sanitary and sterilizing efficiency of the composting process. The results of these aeration experiments suggest that to obtain maximum efficiency frOm 21
16
COMPOSTING
OF
POULTRY
MANURE-GROUND
CORNCOB
MIXTURtS
unit both aeration rate and depth of compost must be considered. The optimum aeration rate to compost mixtures of 2 parts fresh poultry manure and 1 part ground corncob is 4 1 of air/m2/min for every 10 cm of compost up to a maximum depth of about 2.4 m.
composting
Acknowledgements
This investigation was supported by funds from the Ontario Department of Food and Agriculture, grant no. 695-04 from the University of Guelph Research Advisory Board, and grant no. A5730 from the National Research Council of Canada. The author wishes to thank Dr. C. T. Corke of this department for his advice and comments on the preparation of this manuscript. REFERENCES ’ Bowes, P. C. Spontaneous heating and ignition in stored palm-kernels. J. Sci. Fd Agric., 1951, 2 (2) 79 2 Braun, R. Utilization of organic industrial wastes by composting. Compost Sci., 1962, 3 (3) 34 3 Cooper, G. S.; Ketcheson, J. W.; Webber, L. R. Agriculture as a contributor to pollution. A.I.C. Rev., 1969, 24 (3) 9 4 Kneiss, I. F. Combined sludge-garbage composting. Compost Sci., 1962, 3 (2) 13 ’ Knoll, K. H. The influence of various composting processes on non-sporeforming pathogenic bacteria. International Research Group on Refuse Disposal (IRGRD) Inf. Bull., 1963, No. 19 6 Neise, G. Experiments to determine the degree of decomposition of refuse compost by its self-heating capability. IRGRD Inf. Bull., 1963, No. 17 ’ Strauch, D. Requirements of veterinary hygiene in the removal of urban refuse. 1RGRD Inf. Bull., 1964, No. 20 8 Walker, I. K.; Williamson, H. M. The spontaneous ignition of wool. J. appl. Chem., 1957, 7 (8) 468
The Influence
of Aeration
Manure-ground
on the Composting Corncob
of Poultry
Mixtures
R. G. BELL* The influence of the rate of aeration on 1.5 m columns of composting mixtures of 2 parts poultry manure and I part ground corncob was investigated. The results indicated that the optimum aeration rate for the production of a stable sanitary compost was 4 litres of air/m”/min for every 10 cm of cornposting material up to a maximum depth of about 2.4 m.
1.
Introduction
The disposal of litter-free poultry manure collected from caged laying systems has become a problem because of the enormous size of many commercial operations. Even when sufficient land is available for manure disposal the manure must be stored during those periods when land application is inadvisable or not possible. 3 Some form of manure treatment is therefore necessary to prevent manure storage facilities from becoming public health or pollution hazards. Cornposting has long been used as a method of stabilizing organic wastes. Rapid stabilization can be effected by composting if the material undergoing decomposition has a moisture content near 50 y&” a course structure to allow air circulation and a carbon to nitrogen ratio of about 30 to 1.2 The generation of heat is a phenomenon which is associated with the composting process. Under appropriate conditions the temperature of actively composting materials will rise apprecirise is a function of both microbial activity and ably. ’* * The magnitude of such a temperature the rate of heat loss6 Therefore, within a given set of conditions the temperature of a composting mixture can be used as a measure of the cornposting process. Temperature is easily determined and is of great significance in the public health aspects of composting as a waste treatment process.5.7 Fresh poultry manure is poorly qualified as a compostable material as it has a moisture content near 750,:, a carbon to nitrogen ratio of about 10: 1 and is composed of many fine particles. In preliminary experiments it was found that mixtures of 2 parts by weight of fresh poultry manure and 1 part of dry ground corncob could be readily composted. The use of corncob as a diluent decreased the moisture content, increased the carbon to nitrogen ratio and improved the texture of the composting mixture. The present study was undertaken to investigate the influence of aeration on the cornposting of poultry manure-ground corncob mixtures. The results it was hoped would prove to be useful criteria for the design of an efficient pilot cornposting plant.
2.
Materials and methods
A composter (Fig. I) was constructed from 1 cm thick polyvinyl chloride (PVC) sheet and a I .6.5 m length of PVC pipe with an internal diameter of 25 cm and a wall thickness of 1 cm. The ends of the composter were removable and consisted of a square of PVC sheet bolted to a PVC flange cemented to the end of the pipe section (see detail of Fig. I). A silicone grease liquid gasket between the flange and the PVC square ensured that the ends were gastight. Ammonia was removed from the exhaust gas stream by a sulphuric acid trap. The exhaust gases were then mixed with natural gas from the domestic supply and ignited at a jet. This simple after-burner effectively destroyed the odiferous compounds present in the exhaust gas. The flow of air through the composter was measured periodically using a wet test meter (Precision Instruments, Chicago, Ill., U.S.A.) situated between the composter and the after-burner. ’ Department of Microbiology, University of Guelph, Guelph, Ontario, Canada II
12
COMPOSTING
OF
POULTRY
MANURE-GROUND
_,--
--(meter
CORNCOH
MIS-I
C’RLIS
/-\
Exhoust,,H’
Thermometers
169cm
2 inlet
Composting
y _30 cm
mixture
Silicone grease \ v \
\
M I I
l-i
1 I
Fig. 1. Cornposter used to study the influence of aeration on the composting of poultry manure
The corncob used in this series of experiments was a kiln-dried animal bedding material which was milled on a Papec Feed Mill (Papec Machine Co., Shortsville, N.Y.) to pass a f in screen. A composting mixture of 2 parts manure to 1 part ground corncob was prepared. The composter was filled to a depth of 1.5 m with approximately 30 kg of the cornposting mixture. After the top had been securely bolted down the air flow was adjusted to the desired rate and the cornposting process allowed to proceed. Daily temperature readings were taken at all depths. Regular turning of material being composted in windrows has been found to be beneficial to the composting process so on every third day the composter contents were removed, mixed and replaced in such a manner that the compost column was inverted. Aeration was then continued and another set of temperature readings taken. 3.
Results and discussion
The temperature changes observed in columns of the poultry manure corncob mixture aerated at 24, 48,96 and 192 1 of air/m2/min are shown in Figs 2, 3, 4 and 5 respectively. The initial level of the surface of the compost column for each 3 day incubation period is marked on the x-axis by an upright arrow and the final surface level is indicated by an inverted arrow. If composting is to prove a satisfactory method by which to treat poultry manure prior to storage it is desirable that the end product should be both free of pathogenic micro-organisms and in a biologically stable condition. Salmonella sp. have been observed to be destroyed within 2 days in the a-mesothermic (55-65” C) zone of cornposting refuse.’ If the temperature of a composting mixture fails to rise above 30” C it can be considered to be almost completely stabilized.6
R.
G.
BELL
I?
LO !ieight
above
false
floor,
cm
0
20
40
60
Height
Fig. 2.
0
20
40 Height
Temperature
60
changes in
80
above
100
false
floor,
40
60
80
Height
above
false
I00
I20
I40
80
above
false
floor,
IO@ floor,
I?0
‘40
cm
cm
cornpostingpoultry manure aerated at 24 llm2/min
120
140
cm
Height
above
false
floor,
cm
Dav
2oT ’ ’ 0
’
1
20
Height
Fig. 3. Temperature
’
1
40
1
1
1
1+1+ ., I 100
60
80
above
false
floor,
I
cm
Day
+I+
IjIll 20
1 I40
changes in composting poultry manure aerated at 48 I/m2/min
Dw
0
1
120
40 Height
60
80
above
false
100
I20
floor,
cm
y
70
Day
c
60
(0) 7 R
:
50
(XI (0)
9
I
I
k 2 z 0
20
Temperature
?O
40 Height
I 40 Height
Fig. 4.
0
140
/
I 1
I
60
80
above
false
,
60
80
above
false
100 floor,
I b 100 floor,
I20
140
cm
changes in composting poultry manure aerated af 96 I/m’/min
PO cm
140
14
COMPOSTING
Height
above
OF
false
floor,
POULTRY
MANURE-GROUNO
cm
Height
C‘ORbiC‘OH
above
false
floor,
MIX7LR[:S
cm
Day
Height
Fig. 5. Temperature
above
false
floor,
cm
changes in composting poultry manure aerated at 192 I/m”/min
It is proposed to use the depth of compost at temperatures in excess of 55°C (Table 1) as a sanitary measure and a measure of the column heat in excess of 30” C (Table II) as an indication of the stability of the compost. TABLE
I
Height in cm of compost column at 55” C and above
Aeration rute, Ilm’/min
48 53 25 45 46 48 53 48 53
48
96
192
96 93 53 II5 88 19 65 21 0
117 103 33 92 90 0 75 0 0
0 0 68 0 0 0 0 0 0
-
-
-
TABLE 11
Area under temperature curves above 30” C
I
&y
Aeration rate. llm21min
~_________
1 2 3 4 5 6 7 8 9
24
48
96
192
20 20 16 19 19 20 21 19 20
39 38 27 39 35 19 31 13 3
43 38 27 38 33 16 19 4 3
17 21 27 16 13 9 13 8 5
K.
G.
BLLL
1s
The increasing stability of the material being composted is indicated by the decrease in the temperature of the column as the process progresses (Figs 2-4 and Table II). Following mixing and inversion of the compost columns a distinct increase in temperature was observed at the 3 lowest aeration rates. Since the operating conditions after mixing remained unchanged the temperature increase must result from increased microbial activity caused by the introduction of fresh substrates into the vicinity of the composting micro-organisms. The rapid fall in temperature above the 60 cm level in the compost column aerated at 24 I/m’/ tnin (Fig. 2, left) probably indicates that only enough oxygen was provided to support sanitary composting in the bottom 60 cm of the column. This hypothesis is supported by the observation that the upper part of the column smelt strongly of lower fatty acids which is indicative of the occurrence of anaerobic metabolism in that region. After the first inversion (Fig. 2, right) the tr-mesothermic zone, about 50 cm wide, occurred at a level which corresponded to the level where the temperature fell below 30” C the third day, i.e. the end of the composting zone. The gradual advance of the a-mesothermic zone down the column reflects the colonization of the now adequately aerated former anaerobic region by aerobic composting micro-organisms. After the second inversion (Fig. 2, bottom) the a-mesothermic zone was again situated at a level corresponding to the end of the former composting zone. From Table I it can be seen that an aeration rate of 14 I/m”/min was only adequate to maintain a compost column about 50 cm high at a sanitizing temperature. The almost constant heat of the column throughout the composting process (Table II) indicates that readily decomposable organic matter was still present after 9 days. These observations suggest that the aeration rate was inadequate to stabilize and sanitize a 1.5 m column of compost. When the aeration rate was increased to 48 l/m’/min (Fig. 3, left), both the a-mesothermic zone (Table I) and the heat of the column (Table II) were approximately double those produced by aeration at 24 I/m”/min. As a consequence of the increased cooling effect of the stronger air current the start of the a-mesothermic zone was displaced from the 1I cm to the 24 cm level. The a-mesothermic zone (Fig. 3, left) ended very abruptly at the I20 cm level which strongly suggests that the oxygen supply became exhausted at that level. By the ninth day the column had almost cooled down which suggests that the remaining organic matter was well stabilized. The results recorded for the 96 l/m”/min aeration (Fig. 4) were very similar to those recorded for the 48 l/m”/min aeration (Fig. 3). The a-mesothermic zone was extended almost to the top of the column. The abrupt fall in temperature at the surface (Figs 4 and5) is believed to be a surface effect and not to be a consequence of oxygen depletion. While the aeration was sufficiently high to provide an adequate supply of oxygen throughout the entire length of the column the evaporative and convection cooling effects were not large enough to prevent the column from heating LIP. At the highest rate of aeration, 192 I/m2/min (Fig. 5), the cooling effect of the air current was large enough to prevent any substantial rise in the temperature of the column. The temperature changes recorded in Fig. 5, Zeft differ from all the other results in that the column became hotter as incubation progressed. This would occur if the heat produced by microbial activity was only slightly greater than the heat loss. Initially the temperature would rise slowly, but as the temperature increased so would the rate of microbial activity. The difference between heat production and heat loss would increase so the temperature of the column would continue to rise. However. as the readily utilizable substrate became exhausted the heat production would decrease until it was smaller than the heat loss so the column would cool down. This stage was probably reached about the time of the first inversion (Fig. 5, right) after which the column continued to cool. Visual examination of the compost after 9 days’ incubation showed that the poultry manureground corncob mixture had undergone very little decomposition. The low temperature of the compost on the ninth day was the result of the cooling effect of the air current and not a reflection on the stability of the compost. Too much air as well as too little can reduce both the sanitary and sterilizing efficiency of the composting process. The results of these aeration experiments suggest that to obtain maximum efficiency frOm 21
16
COMPOSTING
OF
POULTRY
MANURE-GROUND
CORNCOB
MIXTURtS
unit both aeration rate and depth of compost must be considered. The optimum aeration rate to compost mixtures of 2 parts fresh poultry manure and 1 part ground corncob is 4 1 of air/m2/min for every 10 cm of compost up to a maximum depth of about 2.4 m.
composting
Acknowledgements
This investigation was supported by funds from the Ontario Department of Food and Agriculture, grant no. 695-04 from the University of Guelph Research Advisory Board, and grant no. A5730 from the National Research Council of Canada. The author wishes to thank Dr. C. T. Corke of this department for his advice and comments on the preparation of this manuscript. REFERENCES ’ Bowes, P. C. Spontaneous heating and ignition in stored palm-kernels. J. Sci. Fd Agric., 1951, 2 (2) 79 2 Braun, R. Utilization of organic industrial wastes by composting. Compost Sci., 1962, 3 (3) 34 3 Cooper, G. S.; Ketcheson, J. W.; Webber, L. R. Agriculture as a contributor to pollution. A.I.C. Rev., 1969, 24 (3) 9 4 Kneiss, I. F. Combined sludge-garbage composting. Compost Sci., 1962, 3 (2) 13 ’ Knoll, K. H. The influence of various composting processes on non-sporeforming pathogenic bacteria. International Research Group on Refuse Disposal (IRGRD) Inf. Bull., 1963, No. 19 6 Neise, G. Experiments to determine the degree of decomposition of refuse compost by its self-heating capability. IRGRD Inf. Bull., 1963, No. 17 ’ Strauch, D. Requirements of veterinary hygiene in the removal of urban refuse. 1RGRD Inf. Bull., 1964, No. 20 8 Walker, I. K.; Williamson, H. M. The spontaneous ignition of wool. J. appl. Chem., 1957, 7 (8) 468