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Temperature and pH control in composting of coffee and agricultural wastes
~
Pergamon
War Sci. Tech VoJ.40.No.l.pp.113-119.1999
Publishedby ElsevierScienceLtd on behalfof the IAWQ Pnnted in Great Britain. All rightsreserved 0273-1223/99 520.00 + 0.00
PII: 50273-1223(99)00371-6
TEMPERATURE AND pH CONTROL IN COMPOSTING OF COFFEE AND AGRICULTURAL WASTES W. A. Nogueira*, F. N. Nogueira* and D. C. Devens** • Av. Rio Branco. 1626/1201 .29055642 Viloria ES. Brazil •• Consorcio das Bacias dos Rios Santa Maria/Jucu, Viloria ES. Brazil
ABSTRACT One of the by-products of instant coffee production is a solid waste with high organic content. This waste is disposed of either by landfill sites or by burning as fuel. Both alternatives are considered economically and environmentally unsatisfactory due to the physico-chemical characteristics of the waste . This study investigates the applicability of the forced aeration composting process to a mixture of this coffee waste and agricultural wastes . Approximately 40 tons of waste was used to set up the static piles . Temperature was used as the main parameter to enable control of the aeration rate. Other parameters used to monitor the process were humidity. pH and carbon/n itrogen ratio. The results obta ined were satisfactory and the experiments lead to the production of a high quality compost presenting a carbon/nitrogen rano of the order of 13/1 to IS/I. «:i 1999 Published by Elsevier Science Ltd on behalf of the lAWQ. All nghts reserved
KEYWORDS Aerated piles; bio-stabilisation; composting; industrial solids wastes; wastes association. INTRODUCTION The industrial production ofinstant coffee generates a coffee solid waste during the processes of torrefaction and grinding of the coffee.beans. The two most common disposal methods used for this coffee waste are disposal to landfill sites, or burning as fuel in burners in the industry itself. This coffee waste contains high organic loads and acidity levels. The organic matter present in the waste when it is generated is still unstable and needs to be further degraded to enable a more adequate disposal to the environment. Furthermore, when used as fuel, the high organic contents and the inadequate burning conditions often lead to an inefficient burning process and to the excessive production of particulate matter, with direct implications for the quality of the air in the vicinity of the industry. Therefore, the disposal methods commonly used are not considered the best possible economic or environmental solutions to the problem. The composting process of aerated static piles has been used in many places as related by (Epstein et al., 1976; Stentiford et al., 1985), however, examples of its application to treating industrial solid wastes are not easily found in the literature. One of the important factors affecting the transformation of organic matter is the temperature of the process (Goldstein, 1987). In organic waste composting, the adequate physical distribution of large amounts of organic matter and the controlled conditions of the process, lead to the accumulation of the heat. The temperatures within the piles rise, reaching levels of up to 800C. The development of adequate temperature levels depends on a series of factors, such as the protein content of the waste, a low carbon/nitrogen ratio, humidity and others. Wastes which are submitted to grinding and 113
W. A. NOGUEIRA et al.
114
sievmg, obtaining a finer granulometry and becoming more homogeneous, produce piles with better distribution of temperature and lower heat loss. In forced aeration composting the heat flux is ascendant. The point of greater heat generation is at the base of the piles. The rate of aeration must be controlled as a function of the internal temperature of the piles, in order to obtain the adequate temperature distribution throughout the totality of the mass of waste being composted. The right association of wastes with different physical characteristics may lead to a mixture which enables a better temperature distribution through improved aeration.
In this context, this work has two main objectives: • •
Study the forced aeration composting method, using static piles, as a cost effective and environmentally sound alternative to the disposal of the coffee solid waste. Evaluate the benefits of associating different waste materials for the forced aeration composting process (Devens, 1995) . MATERlALS AND METHODS
Waste materials The main raw material used in this study was the solid waste generated by the production of instant coffee. This waste has a high organic content, high acidity levels and high humidity levels . Although the coffee residue presents high humidity levels, of the order of 80 to 85%, it does not start the fermentation process very easily. It also presents very fine granulometry and can be easily compacted, which may lead to problems in the distribution of air throughout the pile. Aiming to improve the initial physico-chemical characteristics of the waste under study, it was mixed with vegetable wastes before entering the composting process. The vegetable waste was shredded to produce smaller and more uniform pieces, this leads to a mixture of coffee and vegetable waste which is of fine granulometry and more homogeneous. Tables 1 to 3 show the main characteristics of the coffee residue, of the vegetable waste and of the mixture of these two wastes used for each of the four different experiments carried out. Table I. Characteristics of the coffee waste Pile
CIN Ratio
I II III
26/1 26/1 26/1 26/1
IV Table 2. Characteristics ofthe vegetable waste Pile
CIN Ratio
I II III IV
26/1 2011 2011 2011
Composting of coffee and agricultural wastes
liS
Table 3. Characteristics of the mixture of the two wastes Pile
I II III IV
Hwnidity
Organic Matter
(%)
(%)
(%)
83
94 94 95 93
2.52 2.68 2.61 2.60
74 80
75
Nitrogen
Relation CIN 26/1 22/1 22/1 24/1
Experimental set up This work was carried out in full scale installations, located in an open air area, being subject to natural influences of the environment (weather conditions). The area used was paved with concrete in order to enable the production of leachate to be closely monitored, and avoid direct contact and infiltration to the soil. The static piles were set up with trapezoidal cross sections, with top width varying from 0.90 to 3.5Om and base width varying from 3.40 to 6.5Om. The heights adopted for the piles ranged from 1.50 to 1.65m. Air was introduced into the piles through a PVC pipe with a diameter of 100mm placed at the centre of the base ofthe piles. The set up of the piles is of significant importance to the composting process. With the objective of compacting the piles, they were initially set up using alternate layers of coffee waste and vegetable waste separately. This set up was discarded after the first experiments as it did not produce the expected results. The other experiments were carried out using the previously described mixture of coffee waste and shredded vegetable waste. Another relevant aspect of the piles set up is the air diffusion at the base of the piles. In the first experiments, larger pieces of vegetable were put around"the perforated PVC tube to act as air diffusers. However, problems of air circulation were observed within the piles. In the following experiments, small stones were put over the PVC tube to act as air diffusers. The air circulation obtained was considered satisfactory for the purpose of the experiments. Experimental development The study was developed through four experiments indicated as I, II, III and IV. Each experiment indicates the use of a different proportion of coffee waste to vegetable waste in the mixture. These proportions are shown for each experiment in Table 4. Table 4. Composition of the piles in the four different experiments. Pile
I II III IV The static piles were set up with a trapezoidal cross section. The dimensions of the piles varied for each of the four experiments and are presented in Table 5. The indicated dimensions are related to the set up of the piles at the beginning ofthe experiments.
W. A. NOGUEIRA et al.
116
Table 5. Dimensions of the piles Pile I II III IV
Height 1.55 1.65 1.60 1.49
Dimensions (m Base 6.50 3.45 3.70 3.40
I
r
Top 3.50 0.95 0.90 1.00
Process control The control of the composting process was obtained through monitoring the following parameters: temperature, pH, humidity, carbon/nitrogen ratio. The internal temperature of the piles was measured daily during all the experimental period. The temperatures were measured using a digital thermometer. The reading points were defined in order to cover the whole volume of the pile. Thus it was possible to characterise the temperatures at the bottom, middle and top of the piles and observe the temperature gradients. Temperature was used as the main indicator of the fermentation process. The aeration rate was controlled manually, using the temperature as the control parameter. The pH and humidity monitoring was carried out through weekly samples taken in different points of the piles. The control of the carbon/nitrogen ratio was determined initially for each waste involved, with the purpose of defining the amount of each waste to be used. At the end of the experimental periods this ratio was again evaluated with the intent of determining the decomposition stage.
RESULTS The evolution of pH levels was similar for all experiments. During the first ten days a slight decrease in pH was observed. During this period the levels went from the initial 5.0 to 4.5. After the tenth day, all piles registered a steady increase in pH, reaching 7.5 at the thirtieth day of experiments. Higher increase rates were observed for piles III and IV, with pH levels reaching 7.0 before the twentieth day. Final humidity levels were of the order of 50% except for pile II. Due to strong rains between the fourth and thirteenth day of experiments with pile II, the humidity observed on the thirtieth day for this pile was 60%. Table 6. Analytical results of the composts
The results of analysis carried out to verify different parameters in samples of the final composts are shown in Table 6. Table 7 shows the levels of the main parameter required for organic fertilisers of two different categories (Good and Excellent). The levels obtained for the composts for pH, elN ratio, sulphur and potassium are within the levels required for a good fertiliser. The results for phosphorus are low when
Composting of coffee and agricultural wastes
117
compared to the required values. The calcium values are just below the required values except for Pile II, which reaches the required values for a good organic fertiliser. Table 7. Characteristics of organic fertilisers PARAMETER
~ --_. ---- r-- .----- ---I I
Excellent
PH Carbon/Nitrogen --_. - .. Phosphorus (%)
Good
7.5
6.5-7.5
8/1 to 12/1
12/1 to 18/1
I
-_ _-_._ - ---------_.-
> 1.5 --- _._-_.. - --_._-_. _._--- --- -0.5-1.5 - ---
Calcium (%) Sulphur (%) - - - - -. -
. - -.-
- --
> 3.0
1.5-3.0
>0.5
0.2-0.5
f - - - - - -- - - - -- -
> 1.5
Potassium (%)
0.5-1.5
(KiIhI, 1985) Figures 1-4 present the temperatures observed during the experimental period. The variation observed show a natural tendency of elevation during the first 15 days, and decline as the transformation process takes place. 75
-r----------------------,
70
~ 65
~ 60 .3 ~
8. 55 E
~
50 45 40
+---.......- - - _ -__- - - _ - - _ - - ~ o
5
10
15
20
25
30
Time (day)
Figure I. Temperature variation in the centre and top - Pile I. 75
r----------------------,
70 ~
£
65
~ 60
.3 ~
8. 55 E
~
I~TOP -+-Cenlrll I
50 45 40
---+---_--.. . . - - _ + - - - - l
+.---.......
o
5
10
15
20
25
Time (day)
Figure 2. Temperature variation in the centre and top - Pile II.
30
W. A. NOGUEIRA eral.
118 75
r--------------------..,
70
~ 65
..
.a
60
~
~
55
'"
50
E ~
45 40
- - _ - - - _ - -.......
o
5
10
15
---+____-1
20
25
30
Time (day)
Figure 3. Temperature variation in the centre and top - Pile III. 75
r----------------.------,
!:r
70
TOP
-+-Centre
I
65
~60 e ~
55
8-
~ 50
~
45
. . . . .- -_---_--__I
40 ~--_---+__-o 10 15
20
25
30
Time (day) Figure 4. Temperature variation in the centre and top- Pile IV.
For all experiments, thermophilic temperatures (45-60°C) were observed after the second day of experiments. For experiments I, III and IV, termophilic temperatures were observed during a period of22 to 28 days. After this period temperatures within the piles began to drop to levels below 45°C. The temperatures observed for pile III showed a drop at the fifth day of experiments with the temperature remaining low until the tenth day when it began to rise again. This was due to adverse weather conditions and strong rains during several days. In all experiments, the temperatures observed in the centre of the piles were slightly lower than those observed at the top region ofthe piles, where the highest temperatures occurred.
All piles registered similar temperatures for their central region and the average temperature observed for the central region of all piles was of 53°C. The highest temperature in this region was observed in the centre region ofpile III, where the temperature reached 71°C. For the top region, the higher temperatures were observed in pile III (72°C). The average temperature observed in the top region of all piles was 58°C. CONCLUSIONS This study presents the results obtained from the composting of organic solid waste using the positive aeration piles method.
Composting of coffee and agricultural wastes
119
The variation in the humidity of the wastes observed during the experiments occurred in a manner that favoured the composting process. Due to the initial characteristics of the coffee waste, there was a certain concern as to the negative influences of the high levels of humidity of the coffee residue and of its very fine granulometry. However, with the adequate control of the aeration system and the correction of the granulometry by mixing with vegetable wastes, it was possible to work within the appropriate humidity levels. There was no production of leachate during any of the experiments, indicating that the composting process took place under the expected conditions. The pH and temperature variations observed were very uniform. The maximum levels observed remained within the ideal levels for the development of the composting process. No adverse effects were observed as consequence of pH and temperature levels observed within the piles. Using the above described installations and operating conditions, it was possible to obtain a compost with a carbon/nitrogen ratio of the order of 13/1 to 15/1, all of which are within the range of nitrogen mineralization. The internal temperature distribution observed in the piles is characteristic of aerobic decomposition processes. Temperature control within the piles using an adequate aeration system, proved to be simple to obtain and very useful in the composting process. The good efficiencies obtained in the transformation of the organic matter was greatly favoured by the adequate temperature control. Although the coffee waste has a high capacity to be compacted, this did not affect the composting process. The possible negative effects were minimised by mixing vegetable matter with the coffee waste. The three materials used to cover the piles, coffee waste, eucalyptus waste or organic compost, all presented good results as thermal isolators during the composting process. The vegetable wastes used as air diffusers at the base of the pile were compacted by the weight of the composting mass, leading to poor air diffusion. This set up should be avoided as it delays the composting process and leads to poor results. The distribution of the waste in separate layers when setting up the piles is not recommended. The different wastes should be shredded to small pieces and mixed into a homogeneous mass. This enables a more effective action of the micro-organisms involved in the decomposition process. The final results obtained in this experimental work show the great possibility of wastes association, in this case, coffee and vegetables wastes, using the composting process through the static pile, with forced positive aeration. The results obtained demonstrated the applicability of the composting process by static piles using forced aeration to the coffee residue. The initial main concerns were in relation to the physico-chemical characteristics of this residue. However, the negative effects which might be caused by high humidity and fine granulometry of the coffee residue were avoided by mixing coffee residue and vegetable wastes.
REFERENCES Devens, D. C. (1995) . Aplicafiio do Processo de Compostagem com Aerafiio ForcadaPositiva aos ResiduosS6lidosda Industria de Cafe$chivel. MSc thesis, Centro Tecnol6g ico, Universidade Federal do Espirito Santo, Vitoria ES, Brazil. Epstein, E., WiUson, G. B., Burge, W. D., Mullen, D. C. and Enkiri, N. K. (1976). A forced aeration system for composting wastewater sludge. J. Water Pollution ControlFederation, 48(4), 688. Goldstein, N. (1987) . Technology evaluation at compost sites. In: Bio Cycle, MaylJune, 28-33. Kilhl, E. J. (1985) . Fertilizantes agricolas. In: Agronomica CeresLTDA, S30 Paulo, pp. 492. Stentiford, E. I., Taylor, P. I., Leton, T. G. and Mara, D. D. (1985). Forced aeration composting of domestic refuse and sewage sludge. J. WaterPollutionCantrolFederation, 23-32.
Pergamon
War Sci. Tech VoJ.40.No.l.pp.113-119.1999
Publishedby ElsevierScienceLtd on behalfof the IAWQ Pnnted in Great Britain. All rightsreserved 0273-1223/99 520.00 + 0.00
PII: 50273-1223(99)00371-6
TEMPERATURE AND pH CONTROL IN COMPOSTING OF COFFEE AND AGRICULTURAL WASTES W. A. Nogueira*, F. N. Nogueira* and D. C. Devens** • Av. Rio Branco. 1626/1201 .29055642 Viloria ES. Brazil •• Consorcio das Bacias dos Rios Santa Maria/Jucu, Viloria ES. Brazil
ABSTRACT One of the by-products of instant coffee production is a solid waste with high organic content. This waste is disposed of either by landfill sites or by burning as fuel. Both alternatives are considered economically and environmentally unsatisfactory due to the physico-chemical characteristics of the waste . This study investigates the applicability of the forced aeration composting process to a mixture of this coffee waste and agricultural wastes . Approximately 40 tons of waste was used to set up the static piles . Temperature was used as the main parameter to enable control of the aeration rate. Other parameters used to monitor the process were humidity. pH and carbon/n itrogen ratio. The results obta ined were satisfactory and the experiments lead to the production of a high quality compost presenting a carbon/nitrogen rano of the order of 13/1 to IS/I. «:i 1999 Published by Elsevier Science Ltd on behalf of the lAWQ. All nghts reserved
KEYWORDS Aerated piles; bio-stabilisation; composting; industrial solids wastes; wastes association. INTRODUCTION The industrial production ofinstant coffee generates a coffee solid waste during the processes of torrefaction and grinding of the coffee.beans. The two most common disposal methods used for this coffee waste are disposal to landfill sites, or burning as fuel in burners in the industry itself. This coffee waste contains high organic loads and acidity levels. The organic matter present in the waste when it is generated is still unstable and needs to be further degraded to enable a more adequate disposal to the environment. Furthermore, when used as fuel, the high organic contents and the inadequate burning conditions often lead to an inefficient burning process and to the excessive production of particulate matter, with direct implications for the quality of the air in the vicinity of the industry. Therefore, the disposal methods commonly used are not considered the best possible economic or environmental solutions to the problem. The composting process of aerated static piles has been used in many places as related by (Epstein et al., 1976; Stentiford et al., 1985), however, examples of its application to treating industrial solid wastes are not easily found in the literature. One of the important factors affecting the transformation of organic matter is the temperature of the process (Goldstein, 1987). In organic waste composting, the adequate physical distribution of large amounts of organic matter and the controlled conditions of the process, lead to the accumulation of the heat. The temperatures within the piles rise, reaching levels of up to 800C. The development of adequate temperature levels depends on a series of factors, such as the protein content of the waste, a low carbon/nitrogen ratio, humidity and others. Wastes which are submitted to grinding and 113
W. A. NOGUEIRA et al.
114
sievmg, obtaining a finer granulometry and becoming more homogeneous, produce piles with better distribution of temperature and lower heat loss. In forced aeration composting the heat flux is ascendant. The point of greater heat generation is at the base of the piles. The rate of aeration must be controlled as a function of the internal temperature of the piles, in order to obtain the adequate temperature distribution throughout the totality of the mass of waste being composted. The right association of wastes with different physical characteristics may lead to a mixture which enables a better temperature distribution through improved aeration.
In this context, this work has two main objectives: • •
Study the forced aeration composting method, using static piles, as a cost effective and environmentally sound alternative to the disposal of the coffee solid waste. Evaluate the benefits of associating different waste materials for the forced aeration composting process (Devens, 1995) . MATERlALS AND METHODS
Waste materials The main raw material used in this study was the solid waste generated by the production of instant coffee. This waste has a high organic content, high acidity levels and high humidity levels . Although the coffee residue presents high humidity levels, of the order of 80 to 85%, it does not start the fermentation process very easily. It also presents very fine granulometry and can be easily compacted, which may lead to problems in the distribution of air throughout the pile. Aiming to improve the initial physico-chemical characteristics of the waste under study, it was mixed with vegetable wastes before entering the composting process. The vegetable waste was shredded to produce smaller and more uniform pieces, this leads to a mixture of coffee and vegetable waste which is of fine granulometry and more homogeneous. Tables 1 to 3 show the main characteristics of the coffee residue, of the vegetable waste and of the mixture of these two wastes used for each of the four different experiments carried out. Table I. Characteristics of the coffee waste Pile
CIN Ratio
I II III
26/1 26/1 26/1 26/1
IV Table 2. Characteristics ofthe vegetable waste Pile
CIN Ratio
I II III IV
26/1 2011 2011 2011
Composting of coffee and agricultural wastes
liS
Table 3. Characteristics of the mixture of the two wastes Pile
I II III IV
Hwnidity
Organic Matter
(%)
(%)
(%)
83
94 94 95 93
2.52 2.68 2.61 2.60
74 80
75
Nitrogen
Relation CIN 26/1 22/1 22/1 24/1
Experimental set up This work was carried out in full scale installations, located in an open air area, being subject to natural influences of the environment (weather conditions). The area used was paved with concrete in order to enable the production of leachate to be closely monitored, and avoid direct contact and infiltration to the soil. The static piles were set up with trapezoidal cross sections, with top width varying from 0.90 to 3.5Om and base width varying from 3.40 to 6.5Om. The heights adopted for the piles ranged from 1.50 to 1.65m. Air was introduced into the piles through a PVC pipe with a diameter of 100mm placed at the centre of the base ofthe piles. The set up of the piles is of significant importance to the composting process. With the objective of compacting the piles, they were initially set up using alternate layers of coffee waste and vegetable waste separately. This set up was discarded after the first experiments as it did not produce the expected results. The other experiments were carried out using the previously described mixture of coffee waste and shredded vegetable waste. Another relevant aspect of the piles set up is the air diffusion at the base of the piles. In the first experiments, larger pieces of vegetable were put around"the perforated PVC tube to act as air diffusers. However, problems of air circulation were observed within the piles. In the following experiments, small stones were put over the PVC tube to act as air diffusers. The air circulation obtained was considered satisfactory for the purpose of the experiments. Experimental development The study was developed through four experiments indicated as I, II, III and IV. Each experiment indicates the use of a different proportion of coffee waste to vegetable waste in the mixture. These proportions are shown for each experiment in Table 4. Table 4. Composition of the piles in the four different experiments. Pile
I II III IV The static piles were set up with a trapezoidal cross section. The dimensions of the piles varied for each of the four experiments and are presented in Table 5. The indicated dimensions are related to the set up of the piles at the beginning ofthe experiments.
W. A. NOGUEIRA et al.
116
Table 5. Dimensions of the piles Pile I II III IV
Height 1.55 1.65 1.60 1.49
Dimensions (m Base 6.50 3.45 3.70 3.40
I
r
Top 3.50 0.95 0.90 1.00
Process control The control of the composting process was obtained through monitoring the following parameters: temperature, pH, humidity, carbon/nitrogen ratio. The internal temperature of the piles was measured daily during all the experimental period. The temperatures were measured using a digital thermometer. The reading points were defined in order to cover the whole volume of the pile. Thus it was possible to characterise the temperatures at the bottom, middle and top of the piles and observe the temperature gradients. Temperature was used as the main indicator of the fermentation process. The aeration rate was controlled manually, using the temperature as the control parameter. The pH and humidity monitoring was carried out through weekly samples taken in different points of the piles. The control of the carbon/nitrogen ratio was determined initially for each waste involved, with the purpose of defining the amount of each waste to be used. At the end of the experimental periods this ratio was again evaluated with the intent of determining the decomposition stage.
RESULTS The evolution of pH levels was similar for all experiments. During the first ten days a slight decrease in pH was observed. During this period the levels went from the initial 5.0 to 4.5. After the tenth day, all piles registered a steady increase in pH, reaching 7.5 at the thirtieth day of experiments. Higher increase rates were observed for piles III and IV, with pH levels reaching 7.0 before the twentieth day. Final humidity levels were of the order of 50% except for pile II. Due to strong rains between the fourth and thirteenth day of experiments with pile II, the humidity observed on the thirtieth day for this pile was 60%. Table 6. Analytical results of the composts
The results of analysis carried out to verify different parameters in samples of the final composts are shown in Table 6. Table 7 shows the levels of the main parameter required for organic fertilisers of two different categories (Good and Excellent). The levels obtained for the composts for pH, elN ratio, sulphur and potassium are within the levels required for a good fertiliser. The results for phosphorus are low when
Composting of coffee and agricultural wastes
117
compared to the required values. The calcium values are just below the required values except for Pile II, which reaches the required values for a good organic fertiliser. Table 7. Characteristics of organic fertilisers PARAMETER
~ --_. ---- r-- .----- ---I I
Excellent
PH Carbon/Nitrogen --_. - .. Phosphorus (%)
Good
7.5
6.5-7.5
8/1 to 12/1
12/1 to 18/1
I
-_ _-_._ - ---------_.-
> 1.5 --- _._-_.. - --_._-_. _._--- --- -0.5-1.5 - ---
Calcium (%) Sulphur (%) - - - - -. -
. - -.-
- --
> 3.0
1.5-3.0
>0.5
0.2-0.5
f - - - - - -- - - - -- -
> 1.5
Potassium (%)
0.5-1.5
(KiIhI, 1985) Figures 1-4 present the temperatures observed during the experimental period. The variation observed show a natural tendency of elevation during the first 15 days, and decline as the transformation process takes place. 75
-r----------------------,
70
~ 65
~ 60 .3 ~
8. 55 E
~
50 45 40
+---.......- - - _ -__- - - _ - - _ - - ~ o
5
10
15
20
25
30
Time (day)
Figure I. Temperature variation in the centre and top - Pile I. 75
r----------------------,
70 ~
£
65
~ 60
.3 ~
8. 55 E
~
I~TOP -+-Cenlrll I
50 45 40
---+---_--.. . . - - _ + - - - - l
+.---.......
o
5
10
15
20
25
Time (day)
Figure 2. Temperature variation in the centre and top - Pile II.
30
W. A. NOGUEIRA eral.
118 75
r--------------------..,
70
~ 65
..
.a
60
~
~
55
'"
50
E ~
45 40
- - _ - - - _ - -.......
o
5
10
15
---+____-1
20
25
30
Time (day)
Figure 3. Temperature variation in the centre and top - Pile III. 75
r----------------.------,
!:r
70
TOP
-+-Centre
I
65
~60 e ~
55
8-
~ 50
~
45
. . . . .- -_---_--__I
40 ~--_---+__-o 10 15
20
25
30
Time (day) Figure 4. Temperature variation in the centre and top- Pile IV.
For all experiments, thermophilic temperatures (45-60°C) were observed after the second day of experiments. For experiments I, III and IV, termophilic temperatures were observed during a period of22 to 28 days. After this period temperatures within the piles began to drop to levels below 45°C. The temperatures observed for pile III showed a drop at the fifth day of experiments with the temperature remaining low until the tenth day when it began to rise again. This was due to adverse weather conditions and strong rains during several days. In all experiments, the temperatures observed in the centre of the piles were slightly lower than those observed at the top region ofthe piles, where the highest temperatures occurred.
All piles registered similar temperatures for their central region and the average temperature observed for the central region of all piles was of 53°C. The highest temperature in this region was observed in the centre region ofpile III, where the temperature reached 71°C. For the top region, the higher temperatures were observed in pile III (72°C). The average temperature observed in the top region of all piles was 58°C. CONCLUSIONS This study presents the results obtained from the composting of organic solid waste using the positive aeration piles method.
Composting of coffee and agricultural wastes
119
The variation in the humidity of the wastes observed during the experiments occurred in a manner that favoured the composting process. Due to the initial characteristics of the coffee waste, there was a certain concern as to the negative influences of the high levels of humidity of the coffee residue and of its very fine granulometry. However, with the adequate control of the aeration system and the correction of the granulometry by mixing with vegetable wastes, it was possible to work within the appropriate humidity levels. There was no production of leachate during any of the experiments, indicating that the composting process took place under the expected conditions. The pH and temperature variations observed were very uniform. The maximum levels observed remained within the ideal levels for the development of the composting process. No adverse effects were observed as consequence of pH and temperature levels observed within the piles. Using the above described installations and operating conditions, it was possible to obtain a compost with a carbon/nitrogen ratio of the order of 13/1 to 15/1, all of which are within the range of nitrogen mineralization. The internal temperature distribution observed in the piles is characteristic of aerobic decomposition processes. Temperature control within the piles using an adequate aeration system, proved to be simple to obtain and very useful in the composting process. The good efficiencies obtained in the transformation of the organic matter was greatly favoured by the adequate temperature control. Although the coffee waste has a high capacity to be compacted, this did not affect the composting process. The possible negative effects were minimised by mixing vegetable matter with the coffee waste. The three materials used to cover the piles, coffee waste, eucalyptus waste or organic compost, all presented good results as thermal isolators during the composting process. The vegetable wastes used as air diffusers at the base of the pile were compacted by the weight of the composting mass, leading to poor air diffusion. This set up should be avoided as it delays the composting process and leads to poor results. The distribution of the waste in separate layers when setting up the piles is not recommended. The different wastes should be shredded to small pieces and mixed into a homogeneous mass. This enables a more effective action of the micro-organisms involved in the decomposition process. The final results obtained in this experimental work show the great possibility of wastes association, in this case, coffee and vegetables wastes, using the composting process through the static pile, with forced positive aeration. The results obtained demonstrated the applicability of the composting process by static piles using forced aeration to the coffee residue. The initial main concerns were in relation to the physico-chemical characteristics of this residue. However, the negative effects which might be caused by high humidity and fine granulometry of the coffee residue were avoided by mixing coffee residue and vegetable wastes.
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