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Comparative effects of peat and sawdust employed as bulking agents in composting
Bioresource Technology 44 (1993) 65-69
COMPARATIVE EFFECTS OF PEAT A N D SAWDUST EMPLOYED AS BULKING AGENTS IN COMPOSTING A . M . M a r t i n , a J. E v a n s , b D. P o r t e r a & T. R. P a t e l a "Department of Biochemistry, Memorial University of Newfoundland, St. John's, Nfld, Canada A 1B 3X9 bDepartment of Biology, Memorial University of Newfoundland, St. John's, Nfld, Canada A 1B 3X9 (Received 26 February 1992; revised version received 20 July 1992; accepted 20 August 1992)
Fisheries plants and poultry operations are sometimes located in remote areas. Generally, the organic wastes they produce have been traditionally dumped at sea or trucked to local landfill sites. However, there is an increasing need to find more ecologically-responsible alternatives to those practices, such as composting. For Canada, having one of the largest fisheries of all countries in the world, the composting of fisheries wastes has a special appeal. Northern regions such as Newfoundland, which possess a limited amount of good soil, can benefit from the production of a nutrient-rich product for soil enhancement. Although a lack of extensive agricultural production in those regions could limit the selection of bulking agents to be employed for composting, they generally possess peat resources and, in many cases, a forestry industry that produces waste organic materials in the form of sawdust, bark and wood chips. The feasibility of utilizing peat in the composting of fisheries wastes has been presented (Mathur et al., 1986). Martin and Chintalapati (1989) studied the preparation of fish offal-peat compost and its use as a fermentation substrate. This work presents further studies, conducted utilizing an experimental composting unit, in the composting of combinations of fisheries wastes and chicken manure with peat and sawdust.
Abstract An experimental composting system was employed to compost fish offal and a mixture of chicken manure and crab processing wastes. Peat, sawdust, and mixtures of peat and sawdust were employed as bulking agents for the composting processes. A mixture of chicken manure and crab waste was composted sooner than fish offal and relationships were observed between the type of bulk material employed and the composition of the composted material. Key words: Chicken manure, composting, crab wastes, fish offal, peat, sawdust. INTRODUCTION Composting is generally a simple and inexpensive treatment, which has more public acceptability than the dumping of organic wastes in the ocean or in fresh water reservoirs, or the landfilling of sewage sludge. Miller (1991) presented an extended study of the biodegradation of solid wastes by composting and of the physical and chemical factors affecting the composting process. Mathur (1991) studied the nature of compostable materials and the requirements for an optimal composting process, such as the carbon to nitrogen (C/N) ratio, pH, moisture content, and temperature. The effect of temperature on the composting of sewage sludge was presented by Nakasaki et al. (1985). The above mentioned works of Mathur ( 1991 ) and Miller ( 1991 ) are comprehensive reviews of the many factors involved in composting operations. Some of the problems encountered in the application of composting methods are due to the inefficient design of the composting operation (Ser6s-Aspax & AlcafiizBaldellou, 1985). Motte and Griffis (1980) reported that mixtures of organic materials with adequate nitrogen content, and with similar levels of moisture, carbon and bulk densities, could compost at different rates.
METHODS Experimental compost bins Experimental compost bins were designed so that several mixtures of organic wastes could be simultaneously composted under conditions that replicated those within large-scale, commerical composting windrows. Each bin was 2"5 m long and 1.25 m high and wide. The bottom and sides were insulated with 5 cm of plastic foam. The interior of each was subdivided into equal spaces by insulated partitions to minimize the possibility of heat transfer between sections. This allowed the simultaneous composting of different waste mixtures. A layer of bricks and plastic mesh held the waste load about 5 em above the floor of
Bioresource Technology 0960-8524/93/S06.00 © 1993 Elsevier Science Publishers Ltd, England. Printed in Great Britain 65
A. M. Martin, J. Evans, D. Porter, T. R. Patel
66
process was an heterogeneous mix of different materials, analysis of these mixes was not considered to be of value. It was only at the end, after the composting process had taken place, that sampling could be conducted with minimal errors and the analysis of the chemical composition could be meaningful. The initial water contents of the batches of compost were those of the raw materials making up the mixtures, no water being added, and were approximately the same for each mixture (averaging 66 + 9%). The mixtures were blended and placed in the bins with care to obtain a similar degree of compaction.
the bin, allowing air to circulate below the composting mass. Aeration to the composting mass was due to the natural vertical convective flow of air through it. The fibrous and granular characteristics of the bulk material employed aided the air flow circulation by creating vertical channels. Other alternatives to aeration such as blowing of air or the presence of ducts were not considered because they would allow heat losses and dessication, which could reduce the rate of composting in cold climates (Crawford, 1983). The top of the compost was covered by a hinged roof, which kept out the rain and snow, but was uninsulated. Figure 1 shows a schematic representation of a section of the experimental composting bin used in this work.
Activities during the composting period
The composting period started when the wastes were mixed and was terminated when the temperature within the bin dropped to a stable level. There was no mixing or turning of the wastes during the composting period. Throughout the composting period, temperature readings were taken using Omega RTD temperature probes (Omega Technologies, Stanford, CT) inserted into the centres of the piles. The readings were recorded continuously in a Mac IIx computer (Apple Computer Inc., Cupertino, CA), using a LabView 2 virtual instrument software package (National Instruments, Austin, TX). Samples of the compost were taken weekly for chemical analysis from the cores of the compost piles.
Waste mixtures
Six different types of waste mixtures were composted, each being a mixture of carbon-rich material (in three sources; sawdust, peat and a mixture of equal amounts of sawdust and peat), and nitrogen-rich material (from two sources; fish offal and a 2:1 mixture of chicken manure and Queen crab shells), as shown in Table 1. The materials for composting were obtained from various locations in the province of Newfoundland. The peat utilized was highly decomposed Sphagnum peat from a bog in St Shotts. The chicken manure, crab wastes and sawdust were collected from sawmills and processors in the area of Trinity. Fish offal was provided by Ocean Products, Foxtrap. This study intended to test real waste products being created on-site in several areas of this Province, and the principle of minimizing operational costs was applied, with the intention of a further scale-up of the process. Therefore, a minimum pretreatment of the samples was conducted. Table 2 provides information about the approximate composition of the raw materials employed in this work. Because what was present at the beginning of the
Analytical methods
Moisture The moisture content was determined according to the method of the Association of Official Analytical Chemists (AOAC 7.003, 1980). Total carbohydrate (TCH) T C H concentration was determined by the anthrone reagent method (Whistler & Wolfrom, 1962).
6
5cm
I N 3
, I
--+---T~R 1.25 m
-T--
Iscm
I
Fig. l. Schematic representation of a section of an experimental compost bin (sectional view) (1, wall, of 1"2 cm clapboard over 1-2 cm pressboard; 2, 5 cm plastic foam insulation; 3, bricks; 4, 2 mm D plastic mesh; 5, temperature probe; 6, hinged roof; 7, section representing the circulation of air by natural convection through channels created in the composting mass, facilitated by the bulking material).
Peat and sawdust in composts Total lipids The total lipids content was determined using the method of Bligh and Dwyer (1959). Total nitrogen A modified micro-Kjeldahl method (AOAC 47.021, 1980) was employed to determine the total nitrogen content. Ash The AOAC Method 14.006 (1980) was used to find the ash content of the samples. pH The pH of the samples was determined using a combination electrode with a breakage-resistant bulb by inserting it directly into the sample. At least two replicates of each experiment were conducted, and statistical analyses computed. The t-test for significance was also applied to some of the data (Box etal., 1978). RESULTS AND DISCUSSION The relationship between the dry weights of carbon and nitrogen in composts has been discussed by Poincelot (1975), who reported that carbon to nitrogen (C/N) ratios between 26 and 35 have been observed to
Table 1. Initial composition of compost mixtures
Compost mixture
Composition"
1
25% fish offal, 75% sawdust 25% fish offal, 37-5% sawdust, 37.5% peat 25% fish offal, 75% peat 20% chicken manure, 10% crab waste, 70% sawdust 20% chicken manure, 10% crab waste, 35% sawdust, 35% peat 20% chicken manure, 10% crab waste, 70% peat
2 3 4 5 6
"Approximate dry weight composition.
67
produce an efficient and rapid composting process. However, higher ratios have been reported to produce biostable final products (Gotaas, 1956). Working with chicken manure and sawdust composting processes, Galler and Davey (1971) reported that the batches having the lower C/N ratios (more manure) had a higher oxygen consumption that extended over a longer period. It appeared that the carbon in the manure was more available to the microorganisms than the carbon in the sawdust, implying that the processes with the low C/N ratios may actually have had higher ratios of available carbon to nitrogen (Motte & Griffis, 1980). Similar results were reported by Willson & Hummel (1975) when composting straw with dairy manure. In the present work, the ratios of the materials to be composted were determined using a practical approach, considering the potential availability of the materials in areas where the composting process will be developed on a larger scale. Figure 2 (I) shows the C/N ratios for the raw materials employed in this work. As expected, the bulking materials had higher C/N ratios than the animal wastes. The C/N ratios for the composting mixes are presented in Fig. 2 (II). The presence of sawdust produced higher values of C/N than the presence of peat. However, in all cases the calculated values for the C/N ratios were higher than 35. Numerous methods or indicators could be applied for determining the maturity of a compost (Jimrnez & Garc/a, 1989), but none work equally well for a variety of composts. Among the better indicators, the selfheating test is one of the most important, indicating that further degradation is no longer occurring when the temperature falls to a stable level (Johnson et al., 1991 ). This was the criterion that indicated the end of the composting process. At approximately 20 days after starting, all the mixtures showed an increase in temperature to similar values in the range of thermophilic activity (defined by temperatures higher than 37°C), fluctuating around a mean value of 55°C (the mean ambient temperature was about 0°C). After 70 days the temperature of the mixtures with chicken manure and crab shells began to fall rapidly. Irrespective of the bulking material or combination of bulking materials employed, these
Table 2. Approximate composition of raw materials used in the composting processes"
Component
Ash Lipids Nitrogen
Chicken manure (solid)b
Chicken manure (slurry) h
Chicken manure '~
Crab waste
Fish offal
Peat
Sawdust c
28 2 4.48
5 0"3 0"93
25.6 -3-6
22.4 d, 1"8i 8-1~
16.9a.e 1"3f 8"2a
4"3g 4'8g 0.7 g
2"3 -0' 1
aPercentage dry weight. bFAO (1982). 'Toth (1973). CtMathur ( 1991 ). eOnly includes P, K, Ca, Mg, Fe, AI. fStott, undated. ~Martin and Chintalapati (1989).
e
A. M. Martin, J. Evans, D. Porter, T. R. Patel
68
samples completed the composting process in approximately 108 days, as shown by a decrease in their temperatures to ambient values in that period of time. Temperatures in experiments with fish wastes remained in the thermophilic range much longer. In general, samples with fish offal remained at higher temperatures for at least 30 more days. That chicken
c/N
7 / /
200
50
/ / /
4O
30 20 10 0
A
B
C
D
E
200
1"1" lOO 50
1
2
3
4
5
6
Fig. 2. (I) Carbon/nitrogen ratios of raw materials employed: A, chicken manure; B, crab waste; C, fish offal; D, peat; E, sawdust (from Mathur, 1991). (II) Initial carbon/ nitrogen ratios of composting mixtures ( 1-6: see Table 1 for key to mixture composition).
Table 3. Final proximate composition of composted mixtures a
Compost mixture 1 2 3 4 5 6
Lipids (%) Nitrogen (%) 0"51+0.01 3"21+0.03 8.94+0-09 0"40+0.06 1"96+0.08 5"05 + 0"07
1"07+0.03 1"71+0.04 1"63+0.08 1.29+0.12 1"50+0"09 2.46+ 0"00
Ash (%) 2"45+0.88 23-95+3'70" 28"11 +5.20 a 4"38+0"38 33"50+1"00 b 34.12 + 2"73b
"Percentage of dry weight. Mean values of three determinations + standard deviations. Values in the same column with the same superscript are not statistically different (P > 0-05).
manure compost has a higher rate of biodegradation than fish offal compost could be due to the presence in the former of cellulolytic microorganisms from the digestive tract of chickens, animals which are generally exposed to carbohydrates in their feeds. However, fish generally do not metabolize carbohydrates, and it would be expected that carbohydrases would not be present in significant amounts in fish offal. The pH of the samples showed a smooth rising trend, from a mean value of 5"73 + 1.59 at the beginning of composting to a final mean value of 6.30 + 0.64. Tables 3 and 4 show the final compositions of the composted mixtures. In Table 3, it is interesting to observe the apparent relationship between the final lipid concentration in the samples and the concentration of peat in the composting mixture. A hypothetical cause for this outcome could be the presence of biological inhibitors in the peat (Martin & White, 1985), which could have specifically prevented lipolytic activity in the composting processes studied. A similar effect was observed regarding the nitrogen content of the samples. Two explanations could be given for this; one is the contribution of nitrogen from the peat (Martin & Chintalapati, 1989). The other is a confirmation of the report by Mathur et al., (1986) that, by their adsorption properties, the acidic peat fibres are able to retain NH3, which otherwise would escape from the composting process. There is also a direct relationship between the peat concentration in the composting mixtures and the ash concentration in the composted samples, as a consequence of peat having a higher ash content than sawdust (Fuchsman, 1980). Indeed, different waste materials, for example oily or non-oily fish, might give different results. However, the main discussion of this paper refers to the comparative effects of the bulking agents employed on the ash, lipid and nitrogen contents of the composted product. As long as the same raw material is employed, the results, although different for each kind of waste, should show the same trend, being a function of the bulking agent used. Samples of the finished composts were hydrolysed and the total carbohydrate contents (TCH) of the hydrolysates produced were determined (Table 4). This parameter is useful if the hydrolysate is intended to be employed for biotechnological applications
Table 4. Final composition and pH of composted mixtures a
Compost mixture
Moisture (%)
TCH (g/litre)b
pH
1 2 3 4 5 6
67"90 + 0.35 62"86 + 1.39a 62.16 ___0.51 a 68.93 + 0"35h 66.96 + 1.38b 68.07 + 0.56 b
18-68 + 0.48 31.74 + 1.25~ 32.17 + 0.71 a 13'23 + 1-45 19.15 + 1.52b 22.79 + 0.73 b
6"79 + 0"05 7-02 + 0.10 5.28 +0-02 6"60 + 0.06 5-89 + 0-02 6.24 + 0.04
aMean values of three determinations + standard deviations. Values in the same column with the same superscript are not statistically different (P > 0"05). bTCH of compost hydrolysates.
Peat and sawdust in composts (Martin & Chintalapati, 1989). A consistent result was observed, indicating that the presence of peat in the initial mixture tended to increase the T C H of the hydrolysates. This result is compatible with the lower concentration of cellulose in peat than in wood. Cellulose is considered to be a carbohydrate that is difficult to hydrolyse, while the peat biomass composition is more complex and includes water-soluble and easilyhydrolysable carbohydrates (Fuchsman, 1980). In conclusion, no correlations were found between the final composition of the composted material and the characteristics of the wastes employed as nitrogen sources. It appears, however, that the composition of the final product was largely affected by the type of bulking material used, given the relationships found between them. The results regarding the process durations and their independence from the kind of bulking material employed tends to confirm the above mentioned observation that carbon in lignocellulosic types of bulking agents has a low microbial availability, and the degradation of these agents in the composting process is slow. Consequently, the differences in lengths of the composting processes, if any, should be a function of the wastes employed as nitrogen sources, as was observed in this work.
ACKNOWLEDGEMENTS The authors would like to acknowledge the Natural Sciences and Engineering Research Council of Canada, and Seabright Corporation, St. John's, Newfoundland, for providing funding for this work. The authors would like to thank Mr Paul Bemister, Biochemistry Department, Memorial University of Newfoundland, for his assistance.
REFERENCES AOAC (1980). Official Methods of Analysis, 13th edn, ed. W. Horwitz. Association of Official Analytical Chemists, Washington, DC. Bligh, E. G. & Dwyer, W. J. (1959). A rapid method of total lipid extraction and purification. Canadian Journal of Biochemistry and Physiology, 37, 911-17. Box, G. E. P., Hunter, W. G. & Hunter, J. S. (1978). Statistics for Experimenters -- An Introduction to Design, Data Analysis, and Model Building. John Wiley, New York, pp. 21-56. Crawford, J. H. (1983). Composting of agricultural wastes. A review. Process Biochemistry, 18, 14-18. FAO (1982). Compendium of technologies used in the treatment of residues of agriculture, fisheries, forestry and related industries. FAO Agricultural Services Bulletin 33 Rev. I. Food and Agriculture Organization of the United Nations, Rome, pp. 78-8 i. Fuchsman, C. H. (1980). Peat." Industrial Chemistry and Technology. Academic Press, New York.
69
Galler, W. S. & Davey, C. B. (1971). High rate poultry manure composting with sawdust. Livestock Waste Management and Pollution Abatement, ASAE Publication. PROC-271, pp. 159-62. Gotaas, H. B. (1956). Composting, Sanitary Disposal and Reclamation of Organic Wastes. Monograph Series, No. 31, World Health Organization, Geneva. Jim6nez, E. I. & Garcia, V. P. (1989). Evaluation of city refuse compost maturity: a review. Biological Wastes, 27, 115-42. Johnson, K. W., Malterer, T. J. & Levar, T. E. (1991). The evaluation of peat-based fish waste composts for use as horticultural container substrates. Paper presented at the Fisheries By-Products Composting Conference, University of Wisconsin Sea Grant Program, Madison, WI, October 21-23. Martin, A. M. & Chintalapati, S. P. (1989). Fish offal-peat compost extracts as fermentation substrate. Biological Wastes, 27, 281-8. Martin, A. M. & White, M. D. (1985). Growth of the acidtolerant fungus Scytalidium acidophilum as a potential source of single-cell protein. Journal of Food Science, 50, 197-200. Mathur, S. P. (1991). Composting processes. In Bioconversion of Waste Materials to Industrial Products, ed. A. M. Martin. Elsevier Applied Science, London, pp. 147-83. Mathur, S. P., Daigle, J.-Y., Levesque, M. & Dinal, H. (1986). The feasibility of preparing high quality compost from fish scrap and peat with seaweeds or crab scrap. Biological Agriculture and Horticulture, 4, 27-38. Miller, E C. (1991). Biodegradation of solid wastes by composting. In Biological Degradation of Wastes, ed. A. M. Martin. Elsevier Applied Science, London, pp. 1-30. Motte, C. R. & Griffis, C. L. (1980). Variations in the composting process for different organic carbon sources. Agricultural Wastes, 2, 215-23. Nakasaki, K., Shoda, M. & Kubota, H. (1985). Effect of temperature on composting of sewage sludge. Applied Environmental Microbiology, 50, 1526-30. Poincelot, R. P. (1975). The Biochemistry and Methods of Composting. The Connecticut Agricultural Experimental Station, New Haven, Bulletin 754. Quoted by Motte, C. & Griffis, C. L. (1980). Variations in the composting process for different organic carbon sources. Agricultural Wastes, 2,215-23. Ser6s-Aspax, A. & Alcafiiz-Baldeilou, J. M. (1985). Application of pyrolysis-gas chromatography to the study of composting process of barley straw and pear-tree wood. Journal of Analytical Applied Pyrolysis, 8, 415-26. Stott, C. (undated). Seafood Products Resource Guide. Blacksburg, Sea Grant Extension Division, Virginia Polytechnical Institute and State University, p. 149. Toth, S. J. (1973). In Symposium: Processing Agricultural and Municipal Wastes, ed. G. E. Inglett. AVI Publishing Co., Westfort, CN, p. 172. Whistler, R. L. & Wolfrom, M. L. (1962). Methods in Carbohydrate Chemistry, Vol. 1. Analyses and Preparation of Sugars. Academic Press, New York, 589 pp. Wilson, G. B. & Hummel, J. W. (1975). Conservation of nitrogen in dairy manure during composting. Managing Livestock Wastes. ASAE Publication, PROC-275, 490, 491 and 496. Quoted in Motte, C. R. & Griffis, C. L. (1980). Variations in the composting process for different organic carbon sources. Agricultural Wastes, 2, 215-23.
COMPARATIVE EFFECTS OF PEAT A N D SAWDUST EMPLOYED AS BULKING AGENTS IN COMPOSTING A . M . M a r t i n , a J. E v a n s , b D. P o r t e r a & T. R. P a t e l a "Department of Biochemistry, Memorial University of Newfoundland, St. John's, Nfld, Canada A 1B 3X9 bDepartment of Biology, Memorial University of Newfoundland, St. John's, Nfld, Canada A 1B 3X9 (Received 26 February 1992; revised version received 20 July 1992; accepted 20 August 1992)
Fisheries plants and poultry operations are sometimes located in remote areas. Generally, the organic wastes they produce have been traditionally dumped at sea or trucked to local landfill sites. However, there is an increasing need to find more ecologically-responsible alternatives to those practices, such as composting. For Canada, having one of the largest fisheries of all countries in the world, the composting of fisheries wastes has a special appeal. Northern regions such as Newfoundland, which possess a limited amount of good soil, can benefit from the production of a nutrient-rich product for soil enhancement. Although a lack of extensive agricultural production in those regions could limit the selection of bulking agents to be employed for composting, they generally possess peat resources and, in many cases, a forestry industry that produces waste organic materials in the form of sawdust, bark and wood chips. The feasibility of utilizing peat in the composting of fisheries wastes has been presented (Mathur et al., 1986). Martin and Chintalapati (1989) studied the preparation of fish offal-peat compost and its use as a fermentation substrate. This work presents further studies, conducted utilizing an experimental composting unit, in the composting of combinations of fisheries wastes and chicken manure with peat and sawdust.
Abstract An experimental composting system was employed to compost fish offal and a mixture of chicken manure and crab processing wastes. Peat, sawdust, and mixtures of peat and sawdust were employed as bulking agents for the composting processes. A mixture of chicken manure and crab waste was composted sooner than fish offal and relationships were observed between the type of bulk material employed and the composition of the composted material. Key words: Chicken manure, composting, crab wastes, fish offal, peat, sawdust. INTRODUCTION Composting is generally a simple and inexpensive treatment, which has more public acceptability than the dumping of organic wastes in the ocean or in fresh water reservoirs, or the landfilling of sewage sludge. Miller (1991) presented an extended study of the biodegradation of solid wastes by composting and of the physical and chemical factors affecting the composting process. Mathur (1991) studied the nature of compostable materials and the requirements for an optimal composting process, such as the carbon to nitrogen (C/N) ratio, pH, moisture content, and temperature. The effect of temperature on the composting of sewage sludge was presented by Nakasaki et al. (1985). The above mentioned works of Mathur ( 1991 ) and Miller ( 1991 ) are comprehensive reviews of the many factors involved in composting operations. Some of the problems encountered in the application of composting methods are due to the inefficient design of the composting operation (Ser6s-Aspax & AlcafiizBaldellou, 1985). Motte and Griffis (1980) reported that mixtures of organic materials with adequate nitrogen content, and with similar levels of moisture, carbon and bulk densities, could compost at different rates.
METHODS Experimental compost bins Experimental compost bins were designed so that several mixtures of organic wastes could be simultaneously composted under conditions that replicated those within large-scale, commerical composting windrows. Each bin was 2"5 m long and 1.25 m high and wide. The bottom and sides were insulated with 5 cm of plastic foam. The interior of each was subdivided into equal spaces by insulated partitions to minimize the possibility of heat transfer between sections. This allowed the simultaneous composting of different waste mixtures. A layer of bricks and plastic mesh held the waste load about 5 em above the floor of
Bioresource Technology 0960-8524/93/S06.00 © 1993 Elsevier Science Publishers Ltd, England. Printed in Great Britain 65
A. M. Martin, J. Evans, D. Porter, T. R. Patel
66
process was an heterogeneous mix of different materials, analysis of these mixes was not considered to be of value. It was only at the end, after the composting process had taken place, that sampling could be conducted with minimal errors and the analysis of the chemical composition could be meaningful. The initial water contents of the batches of compost were those of the raw materials making up the mixtures, no water being added, and were approximately the same for each mixture (averaging 66 + 9%). The mixtures were blended and placed in the bins with care to obtain a similar degree of compaction.
the bin, allowing air to circulate below the composting mass. Aeration to the composting mass was due to the natural vertical convective flow of air through it. The fibrous and granular characteristics of the bulk material employed aided the air flow circulation by creating vertical channels. Other alternatives to aeration such as blowing of air or the presence of ducts were not considered because they would allow heat losses and dessication, which could reduce the rate of composting in cold climates (Crawford, 1983). The top of the compost was covered by a hinged roof, which kept out the rain and snow, but was uninsulated. Figure 1 shows a schematic representation of a section of the experimental composting bin used in this work.
Activities during the composting period
The composting period started when the wastes were mixed and was terminated when the temperature within the bin dropped to a stable level. There was no mixing or turning of the wastes during the composting period. Throughout the composting period, temperature readings were taken using Omega RTD temperature probes (Omega Technologies, Stanford, CT) inserted into the centres of the piles. The readings were recorded continuously in a Mac IIx computer (Apple Computer Inc., Cupertino, CA), using a LabView 2 virtual instrument software package (National Instruments, Austin, TX). Samples of the compost were taken weekly for chemical analysis from the cores of the compost piles.
Waste mixtures
Six different types of waste mixtures were composted, each being a mixture of carbon-rich material (in three sources; sawdust, peat and a mixture of equal amounts of sawdust and peat), and nitrogen-rich material (from two sources; fish offal and a 2:1 mixture of chicken manure and Queen crab shells), as shown in Table 1. The materials for composting were obtained from various locations in the province of Newfoundland. The peat utilized was highly decomposed Sphagnum peat from a bog in St Shotts. The chicken manure, crab wastes and sawdust were collected from sawmills and processors in the area of Trinity. Fish offal was provided by Ocean Products, Foxtrap. This study intended to test real waste products being created on-site in several areas of this Province, and the principle of minimizing operational costs was applied, with the intention of a further scale-up of the process. Therefore, a minimum pretreatment of the samples was conducted. Table 2 provides information about the approximate composition of the raw materials employed in this work. Because what was present at the beginning of the
Analytical methods
Moisture The moisture content was determined according to the method of the Association of Official Analytical Chemists (AOAC 7.003, 1980). Total carbohydrate (TCH) T C H concentration was determined by the anthrone reagent method (Whistler & Wolfrom, 1962).
6
5cm
I N 3
, I
--+---T~R 1.25 m
-T--
Iscm
I
Fig. l. Schematic representation of a section of an experimental compost bin (sectional view) (1, wall, of 1"2 cm clapboard over 1-2 cm pressboard; 2, 5 cm plastic foam insulation; 3, bricks; 4, 2 mm D plastic mesh; 5, temperature probe; 6, hinged roof; 7, section representing the circulation of air by natural convection through channels created in the composting mass, facilitated by the bulking material).
Peat and sawdust in composts Total lipids The total lipids content was determined using the method of Bligh and Dwyer (1959). Total nitrogen A modified micro-Kjeldahl method (AOAC 47.021, 1980) was employed to determine the total nitrogen content. Ash The AOAC Method 14.006 (1980) was used to find the ash content of the samples. pH The pH of the samples was determined using a combination electrode with a breakage-resistant bulb by inserting it directly into the sample. At least two replicates of each experiment were conducted, and statistical analyses computed. The t-test for significance was also applied to some of the data (Box etal., 1978). RESULTS AND DISCUSSION The relationship between the dry weights of carbon and nitrogen in composts has been discussed by Poincelot (1975), who reported that carbon to nitrogen (C/N) ratios between 26 and 35 have been observed to
Table 1. Initial composition of compost mixtures
Compost mixture
Composition"
1
25% fish offal, 75% sawdust 25% fish offal, 37-5% sawdust, 37.5% peat 25% fish offal, 75% peat 20% chicken manure, 10% crab waste, 70% sawdust 20% chicken manure, 10% crab waste, 35% sawdust, 35% peat 20% chicken manure, 10% crab waste, 70% peat
2 3 4 5 6
"Approximate dry weight composition.
67
produce an efficient and rapid composting process. However, higher ratios have been reported to produce biostable final products (Gotaas, 1956). Working with chicken manure and sawdust composting processes, Galler and Davey (1971) reported that the batches having the lower C/N ratios (more manure) had a higher oxygen consumption that extended over a longer period. It appeared that the carbon in the manure was more available to the microorganisms than the carbon in the sawdust, implying that the processes with the low C/N ratios may actually have had higher ratios of available carbon to nitrogen (Motte & Griffis, 1980). Similar results were reported by Willson & Hummel (1975) when composting straw with dairy manure. In the present work, the ratios of the materials to be composted were determined using a practical approach, considering the potential availability of the materials in areas where the composting process will be developed on a larger scale. Figure 2 (I) shows the C/N ratios for the raw materials employed in this work. As expected, the bulking materials had higher C/N ratios than the animal wastes. The C/N ratios for the composting mixes are presented in Fig. 2 (II). The presence of sawdust produced higher values of C/N than the presence of peat. However, in all cases the calculated values for the C/N ratios were higher than 35. Numerous methods or indicators could be applied for determining the maturity of a compost (Jimrnez & Garc/a, 1989), but none work equally well for a variety of composts. Among the better indicators, the selfheating test is one of the most important, indicating that further degradation is no longer occurring when the temperature falls to a stable level (Johnson et al., 1991 ). This was the criterion that indicated the end of the composting process. At approximately 20 days after starting, all the mixtures showed an increase in temperature to similar values in the range of thermophilic activity (defined by temperatures higher than 37°C), fluctuating around a mean value of 55°C (the mean ambient temperature was about 0°C). After 70 days the temperature of the mixtures with chicken manure and crab shells began to fall rapidly. Irrespective of the bulking material or combination of bulking materials employed, these
Table 2. Approximate composition of raw materials used in the composting processes"
Component
Ash Lipids Nitrogen
Chicken manure (solid)b
Chicken manure (slurry) h
Chicken manure '~
Crab waste
Fish offal
Peat
Sawdust c
28 2 4.48
5 0"3 0"93
25.6 -3-6
22.4 d, 1"8i 8-1~
16.9a.e 1"3f 8"2a
4"3g 4'8g 0.7 g
2"3 -0' 1
aPercentage dry weight. bFAO (1982). 'Toth (1973). CtMathur ( 1991 ). eOnly includes P, K, Ca, Mg, Fe, AI. fStott, undated. ~Martin and Chintalapati (1989).
e
A. M. Martin, J. Evans, D. Porter, T. R. Patel
68
samples completed the composting process in approximately 108 days, as shown by a decrease in their temperatures to ambient values in that period of time. Temperatures in experiments with fish wastes remained in the thermophilic range much longer. In general, samples with fish offal remained at higher temperatures for at least 30 more days. That chicken
c/N
7 / /
200
50
/ / /
4O
30 20 10 0
A
B
C
D
E
200
1"1" lOO 50
1
2
3
4
5
6
Fig. 2. (I) Carbon/nitrogen ratios of raw materials employed: A, chicken manure; B, crab waste; C, fish offal; D, peat; E, sawdust (from Mathur, 1991). (II) Initial carbon/ nitrogen ratios of composting mixtures ( 1-6: see Table 1 for key to mixture composition).
Table 3. Final proximate composition of composted mixtures a
Compost mixture 1 2 3 4 5 6
Lipids (%) Nitrogen (%) 0"51+0.01 3"21+0.03 8.94+0-09 0"40+0.06 1"96+0.08 5"05 + 0"07
1"07+0.03 1"71+0.04 1"63+0.08 1.29+0.12 1"50+0"09 2.46+ 0"00
Ash (%) 2"45+0.88 23-95+3'70" 28"11 +5.20 a 4"38+0"38 33"50+1"00 b 34.12 + 2"73b
"Percentage of dry weight. Mean values of three determinations + standard deviations. Values in the same column with the same superscript are not statistically different (P > 0-05).
manure compost has a higher rate of biodegradation than fish offal compost could be due to the presence in the former of cellulolytic microorganisms from the digestive tract of chickens, animals which are generally exposed to carbohydrates in their feeds. However, fish generally do not metabolize carbohydrates, and it would be expected that carbohydrases would not be present in significant amounts in fish offal. The pH of the samples showed a smooth rising trend, from a mean value of 5"73 + 1.59 at the beginning of composting to a final mean value of 6.30 + 0.64. Tables 3 and 4 show the final compositions of the composted mixtures. In Table 3, it is interesting to observe the apparent relationship between the final lipid concentration in the samples and the concentration of peat in the composting mixture. A hypothetical cause for this outcome could be the presence of biological inhibitors in the peat (Martin & White, 1985), which could have specifically prevented lipolytic activity in the composting processes studied. A similar effect was observed regarding the nitrogen content of the samples. Two explanations could be given for this; one is the contribution of nitrogen from the peat (Martin & Chintalapati, 1989). The other is a confirmation of the report by Mathur et al., (1986) that, by their adsorption properties, the acidic peat fibres are able to retain NH3, which otherwise would escape from the composting process. There is also a direct relationship between the peat concentration in the composting mixtures and the ash concentration in the composted samples, as a consequence of peat having a higher ash content than sawdust (Fuchsman, 1980). Indeed, different waste materials, for example oily or non-oily fish, might give different results. However, the main discussion of this paper refers to the comparative effects of the bulking agents employed on the ash, lipid and nitrogen contents of the composted product. As long as the same raw material is employed, the results, although different for each kind of waste, should show the same trend, being a function of the bulking agent used. Samples of the finished composts were hydrolysed and the total carbohydrate contents (TCH) of the hydrolysates produced were determined (Table 4). This parameter is useful if the hydrolysate is intended to be employed for biotechnological applications
Table 4. Final composition and pH of composted mixtures a
Compost mixture
Moisture (%)
TCH (g/litre)b
pH
1 2 3 4 5 6
67"90 + 0.35 62"86 + 1.39a 62.16 ___0.51 a 68.93 + 0"35h 66.96 + 1.38b 68.07 + 0.56 b
18-68 + 0.48 31.74 + 1.25~ 32.17 + 0.71 a 13'23 + 1-45 19.15 + 1.52b 22.79 + 0.73 b
6"79 + 0"05 7-02 + 0.10 5.28 +0-02 6"60 + 0.06 5-89 + 0-02 6.24 + 0.04
aMean values of three determinations + standard deviations. Values in the same column with the same superscript are not statistically different (P > 0"05). bTCH of compost hydrolysates.
Peat and sawdust in composts (Martin & Chintalapati, 1989). A consistent result was observed, indicating that the presence of peat in the initial mixture tended to increase the T C H of the hydrolysates. This result is compatible with the lower concentration of cellulose in peat than in wood. Cellulose is considered to be a carbohydrate that is difficult to hydrolyse, while the peat biomass composition is more complex and includes water-soluble and easilyhydrolysable carbohydrates (Fuchsman, 1980). In conclusion, no correlations were found between the final composition of the composted material and the characteristics of the wastes employed as nitrogen sources. It appears, however, that the composition of the final product was largely affected by the type of bulking material used, given the relationships found between them. The results regarding the process durations and their independence from the kind of bulking material employed tends to confirm the above mentioned observation that carbon in lignocellulosic types of bulking agents has a low microbial availability, and the degradation of these agents in the composting process is slow. Consequently, the differences in lengths of the composting processes, if any, should be a function of the wastes employed as nitrogen sources, as was observed in this work.
ACKNOWLEDGEMENTS The authors would like to acknowledge the Natural Sciences and Engineering Research Council of Canada, and Seabright Corporation, St. John's, Newfoundland, for providing funding for this work. The authors would like to thank Mr Paul Bemister, Biochemistry Department, Memorial University of Newfoundland, for his assistance.
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