anaerobic digestion of pig manure with different agro-industrial wastes
TRANSCRIPT
Biological Wastes 28 (1989) 143-155
Anaerobic Digestion of Pig Manure with Different Agro-industrial Wastes
M. H. W o n g & Y. H. C h e u n g *
Department of Biology, Hong Kong Baptist College, 224 Waterloo Road, Kowloon, Hong Kong
(Received 26 March 1988; revised version received 19 August 1988; accepted 6 September 1988)
ABSTRACT
The present investigation studied the effectiveness of four types of agro- industrial wastes (cardboard, newspaper, sawdust and sugar-cane waste) as a carbon source in the batch anaerobic digestion of pig manure.
Pig manure with either sawdust or cardboard, especially sawdust, produced higher total volumes of biogas than with newspaper or sugar-cane waste. Mixtures in the ratio 4:1 pig manure:sawdust, and 4:1 and 3:1 pig manure: cardboard gave the highest yields of gas with the highest percentage of methane. The total solids, volatile solids and chemical oxygen demand of the digested materials showed that the organic material of the wastes had been substantially decreased by digestion.
The fertilizer values of the digested materials were tested by planting Lolium perenne (ryegrass) on sand amended with digested wastes. Pig manure with sugar-cane waste gave the highest crop yield, followed by manure plus cardboard or sawdust; pig manure with newspaper gave the lowest yield.
I N T R O D U C T I O N
Anaerobic digestion is now a common method for treating sewage sludge before final disposal or land application in many countries (Hobson et al., 1981) including Hong Kong. In the past, aerobic processes have been
* Present address: Environmental Protection Department, 1 lth FI., Empire Centre, Tsim Sha Tsui, Kowloon, Hong Kong.
143 Biological Wastes 0269-7483/89/$03-50 © 1989 Elsevier Science Publishers Ltd, England. Printed in Great Britain
144 M. H. Wong, Y. H. Cheung
favoured for the treatment of organic wastes, but anaerobic digestion is now being more widely used.
Pig manure is the major source of river pollution in the New Territories, Hong Kong (Binnie & Partners, 1974; Hoare, 1987). In June 1986, regulations were established for the enforcement of the Water Pollution Control Ordinance of Hong Kong enacted in 1980. Under this Ordinance, discharge of polluting effluent to water-control zones will require a licence. The current practice of flushing animal manure into rivers will soon be replaced by treatment of effluent in order to meet established water-quality objectives. Removal of pig manure while it is still in a semi-solid form would greatly reduce the organic load on treatment plants. However, the collected pig manure would also require proper treatment as it is putresible. This could be done by anaerobic digestion, and reduction of pathogenic bacteria is an important beneficial aspect of digestion (Farrah & Bitton, 1983; Olsen et aL, 1985).
Incorporation of waste materials such as rice straw to give an appropriate C :N ratio will increase gas production (Xu, 1984; Pathak et al., 1985; Wong, 1987). With the above background in mind, the objectives of the present study were to test anaerobic digestion of pig manure with four different kinds of agro-industrial wastes (cardboard, sawdust, sugar-cane waste and newspaper) available in Hong Kong, and assess the fertilizer value of the digested materials by measuring the dry matter production of ryegrass (Lolium perenne) growing on the materials.
METHODS
Collection and treatment of samples
The pig manure was collected from the Kadoorie Experiment Extension Farm in the New Territories of Hong Kong. Pigs there are fed on a diet of corn and wheat bran. Manure was collected while it was still dry and allowed to compost at ambient temperature (about 25 _ 4°C) for 2 weeks before use. Samples of newspaper, cardboard, sawdust, and sugar-cane waste were collected from domestic sources, supermarkets, a local carpentry and a herbal tea shop, respectively. All the waste materials were air-dried at ambient temperature (about 25 _+ 4°C) for 2 weeks. Due to its wetness, sugar- cane waste was further oven-dried (55°C for 24h). All the wastes, except sawdust, were cut into approximately 1.5 x 3 cm pieces before use.
Preparation of seeding material
A portion of the fresh manure was composted for 5 days at ambient temperature (about 25 _ 4°C). It was turned over once in the middle of the
Digestion of pig manure with other wastes 145
composting period. The moisture of the composted manure was then adjusted to about 90% using distilled water. The diluted slurry was placed in a digester (as described below) for 30 days at 37°C before being used as seeding material.
Experimental set-up
The digestion apparatus consisted of a 4-1itre glass jar with an airtight lid with a metal tube connected to tygon tubing (Wong, 1987). Calibrated glass columns (5 cm x 1-5 m), partly immersed in a water bath, were used to collect gas samples (HMSO, 1977a) by connecting them to the tygon tubings. The water in the bath was acidified with hydrochloric acid to pH 4 and kept at 35 + 2°C by a stirrer heater. The control of water level in the columns was by means of a vacuum pump with a water tap.
Analysis of biogas
The gas production was recorded directly from the calibrated glass columns. Identification and quantitative analysis of methane in the biogas was done with a Perkin-Elmer Gas Chromatograph-Mass Spectrograph equipped with a thermal conductivity detector and a glass column (3 mm x 2m) packed with Molecular Sieve 5A.
Analysis of the wastes
The following methods were used for the analysis of the wastes before and after digestion: moisture content (oven, 105°C), total volatile solids content (muffle furnace, 600°C), organic carbon (Walkley & Black, 1934), total nitrogen (semi-kjeldahl method, Allen et al., 1974), total phosphorus (Watanabe & Olsen, 1962) and chemical oxygen demand (HMSO, 1977b).
Evaluation of fertilizing values of digested materials
The digested materials from Experiments 1 and 2 were air-dried at 25 + 4°C, and 30 g (dry weight) of each digested material was mixett:~ith 1 kg of silver sand previously washed with distilled water and dried thoroughly under the sun. Each mixture was placed separately into a plastic pot (20 cm diameter, 8 cm depth). Silver sand alone (1 kg) was also prepared in the control pots. Ten grams of ryegrass seeds (Lolium perenne) from Carolina Biological Supply Company were sown on each pot. Distilled water (about 10 ml) was added to keep the pot wet. Then each pot was wrapped in a clear plastic bag for seed germination to take place. The pots were placed outdoors and 4 days after sowing the seeds were unwrapped. With daily watering, the grass
146 M. H. Wong, Y. H. Cheung
was grown under the sun for 3 weeks. The aerial portion of grass in each pot was then harvested. The dry matter content of grass grown in digested wastes from both experiments (105°C oven), and total nitrogen content (semi-kjeldahl method, Allen et al., 1974) of grass grown in digested wastes from Experiment 1 only, were determined.
Experiment 1. Gas production of pig manure plus different wastes
Composted pig manure and different waste materials in the ratio of 1:1 (dry weight basis) were mixed with 50 ml of the seeding material in a digester (total weight 188 g, total volume 1.4 litres). The moisture in each digester was adjusted to 85% by adding distilled water. There were five experimental groups including pig manure alone, each in triplicate. All the digesters were placed in a water bath at 35 _ 2°C for a period of 45 days. The daily gas production was recorded.
Experiment 2. Effects of different mixing ratio of pig manure to wastes on gas production
Sawdust and cardboard were chosen, as these two wastes mixed with pig manure gave the highest yields of biogas according to the results of the first experiment. The ratios of pig manure to the wastes to be tested were 1:1, 2:1, 3:1 and 4:1 (dry weight basis, total weight 188 g, total volume 1.4 litres). There were five treatments including pig manure alone, each in duplicate. The volume of biogas produced was recorded every day for 45 days. The methane content in the biogas was measured once every 3 days.
RESULTS AND DISCUSSION
The gas productions from various treatments in Experiment I are shown in Fig. 1. Pig manure alone (control) and pig manure with newspaper or sugar- cane waste had the lowest average daily gas productions, throughout the experimental period, although the control group seemed to produce a slightly higher yield in the first 10 days. Pig manure with cardboard or sawdust, especially sawdust, gave higher gas yields with a slow increase in production at the beginning to reach a maximum at about 27-36 days.
Table 1 shows the properties of the wastes before and after digestion. The pH values of all the experimental groups before digestion were comparable. However, all the digested materials turned acidic, with the exception of pig manure plus sawdust. The lowest pH of 5.09 of pig manure plus newspaper indicated that acidity would hinder gas production. Additions of wastes
Digestion of pig manure with other wastes
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Fig. 1. Daily gas productions from various agro-industriai wastes added to pig manure, (a) pig manure alone, and with (b) newspaper, (c) cardboard, (d) sawdust and (e) sugar-cane waste.
raised the C :N ratio of pig manure (Table 1). The Total Solids content in each treatment fell greatly. The initial and final percentages of total organic carbon, total phosphorus and volatile solids were comparable. There was a slight increase in the final percentage of total nitrogen in all the digested materials. The COD value of digested pig manure alone was half the original value while digestion with waste materials further reduced the final values.
All waste-amended pots gave a higher dry matter production of grass than
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Digestion of pig manure with other wastes 149
TABLE 2 Growth of Rye-grass in Various Digested Materials (from
Experiment 1, each Value is a Mean of three Replicates)
Yield of dry matter Nitrogen (g/pot) (% DM)
Silver sand alone 1.50 0.60 Pig manure (PM) alone 5"07 0"32 PM + newspaper 3.80 0.28 PM + cardboard 4-71 0-43 PM + sugar-cane waste 5.18 0.71 PM + sawdust 3.40 0.41
the control pots. Pots amended with digested materials derived from pig manure plus sugar-cane waste had the highest yield and the highest nitrogen content in the leaves (Table 2).
The average daily biogas productions from various ratios of cardboard- amended pig manure were substantially lower than those of sawdust- amended pig manure (Figs 2 and 3). The reason why cardboard was less
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150 M.H. Wong, ~ H . Cheung
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Digestion o f pig manure with other wastes 151
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effective in the second experiment than the first experiment was unknown. In general, the average daily gas production from the digesters containing sawdust and different proportions of pig manure showed a gradual increase to a maximum between the 20th and 30th day, then gas production was maintained at a high level to the end of the experiment. Gas productions in the digesters containing different proportions of cardboard fluctuated throughout the experimental period and a general pattern was not observed among the four treatment groups. Higher methane percentages were obtained from digesters with added sawdust than from those with cardboard (Figs 4 and 5). In general, hig h methane contents were found in gas from the sawdust-containing groups after 20 days and methane was maintained at a level above 75% until the end of the experiment. Digesters containing cardboard had a general trend towards a gradual increase and then a steady decline of methane percentages after reaching the maximum between 20 and 30 days. Table 3 shows the average daily gas productions and methane percentages. A ratio of 4:1 of pig manure: sawdust and 3:1 and 4:1 of pig manure :cardboard seemed to optimize gas yields and methane percentages.
Table 4 lists the C: N ratio of the feeding materials before digestion and the properties of the materials after digestion. Additions of the wastes raised the C :N ratio. Pig manure digested with cardboard became more acidic
152 M. H. Wong, Y. H. Cheung
(pH 5-34-6.06) than that digested with sawdust (pH 7.02-7"03). There was a substantial reduction of dry solids while volatile solids remained more or less the same in both cases.
The sawdust treatments had higher volatile solids destructions (16-54-52.81%) than all except the 2:1 ratio cardboard treatments. A large proportion of COD was removed by digesting pig manure with the two kinds of wastes (from 8540mg/g of pig manure alone to 643-3451 rag/g).
TABLE 3 The Summary Results of Average Daily Gas Production and Methane Percentage from Experiment 2 (each Value is a Mean of Triplicates)
Average gas Methane (ml/g volatile solid) (%)
Pig manure: sawdust 1:1 3'59 58"30 2:1 3.31 60-01 3:1 2.47 58-65 4:1 4.39 57.56
Pig manure :cardboard 1 : 1 0.64 14.80 2:1 0"83 24"29 3:1 0-70 42"69 4:1 0-74 40.80
All the digested wastes containing different proportions of sawdust and cardboard gave a higher production of rye-grass than the control group (silver sand alone). Pig manure digested with cardboard gave higher yield than manure digested with sawdust. However, no obvious trend was observed among the various treatments within each type of waste material (Table 5).
The present study has indicated that pig manure digested with sawdust and cardboard was superior to manure plus sugar-cane waste or newspaper in terms of total volume of biogas produced and percentage of methane contained in the biogas. The optimal mixing ratios (pig manure:waste materials) were 4:1 for sawdust, 3:1 and 4:1 for cardboard. The organic- loadings of the mixtures were substantially reduced after digestion. Growth of rye-grass on sand with digested materials showed that the digested materials possessed fertilizer values. The possibilities of collecting and mixing pig manure with agro-industrial wastes on a commercial scale should, therefore, be explored in order to mitigate the water pollution problem and at the same time produce a high grade fertilizer.
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154 M. H. Wong, Y. 14. Cheung
TABLE 5 Growth of Rye-grass in Various Digested Materials (from
Experiment 2, each Value is a Mean of three Replicates)
g/pot
Silver sand alone 0"74
Pig manure :sawdust 1:1 2"66 2:1 1"74 3:1 2.30 4:1 1.12
Pig manure :cardboard 1:1 2"95 2:1 2.46 3:1 3'37 4:1 2"06
A C K N O W L E D G E M E N T S
We would like to thank Mr P. S. Leung and Mr P. Wong for technical assistance, and Mr S. S. Chan for drawing the diagrams. The research grant (85/86-09) from Hong Kong Baptist College is gratefully acknowledged.
R E F E R E N C E S
Allen, S. E., Grimshaw, H. M., Parkinson, J. A. & Quarmby, C. (1974). Chemical Analysis of Ecological Materials. Blackwell Scientific Publications, Oxford.
Binnie & Partners (1974). New Territories Stream Pollution Study. Hong Kong Government Press, Hong Kong.
Farrah, S. R. & Bitt'on, G. (1983). Bacterial survival and association with sludge flocks during aerobic and anaerobic digestion of waste-water sludge under laboratory conditions. Applied and Environmental Microbiology, 45, 174-81.
HMSO (1977a). Amenability of sewage sludge to anaerobic digestion, 1977. In Methods for the Examination of Waters and Associated Materials. HMSO, London.
HMSO (1977b). Chemical oxygen demand (dichromate value) of polluted and waste waters, 1977. In Methods for the Examination of Waters and Associated Materials. HMSO, London.
Hoare, R. W. M. (1987). Redressing the balance--The problem of agricultural wastes in Hong Kong. Resources and Conservation, 13, 63-74.
Hobson, P. N., Bousfield, S. & Summers, R. (1981). Methane Generation from Human, Animal, and Agricultural Wastes. Applied Science Publishers, London.
Digestion of pig manure with other wastes 155
Olsen, J. E., Jorgensen, J. B. & Nansen, P. (1985). On the reduction of Mycobacterium paratuberculosis in bovine slurry subjected to batch mesophilic anaerobic digestion. Agricultural Wastes, 13, 273-80.
Pathak, B. S., Jain, A. K. & Dev, D. S. (1985). Biogasification of cattle dung and cattle dung-rice straw mixture at different solid concentrations. Agricultural Wastes, 13, 251-9.
Walkley, A. & Black, I. A. (1934). An examination of the Degtjareff method for determining soil organic matter and a proposed modification of the chromic acid titration method. Soil Science, 37, 29-38.
Watanabe, F. S. & Olsen, S. R. (1962). Colorimetric determination of phosphorus in water extracts of soil. Soil Science, 93, 183-8.
Wong, M. H. (1987). Biogas production from rice straw and pig manure using batch fermentation at 38°C. In BioenvironmentalSystems, Vol. 3, ed. D. L. Wise. CRC Press, Boca Raton, Florida.
XU, Y. (1984). The importance of stalks used as raw material in biogas fermentation and the matters needing attention. China Biogas, 1, 24-5 (in Chinese, with English summary).