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Bioenergy potential of eight common aquatic weeds
Biological Wastes 34 (1990) 359-366
Bioenergy Potential of Eight Common Aquatic Weeds S. A. A b b a s i , P. C. N i p a n e y & G. D. S c h a u m b e r g * Salim Ali School of Ecology, Pondicherry (Central) University, Pondicherry 605 001, India (Received 10 June 1989; revised version received 2 July 1990; accepted 23 July 1990)
ABSTRACT Eight common aquatic weeds Salvinia molesta, Hydrilla verticillata, Nymphaea stellata, Azolla pinnata, Ceratopteris sp. Scirpus sp. Cyperus sp. and Utricularia reticulata were digested anaerobically to produce methane. The carbon to nitrogen ( C/N) ratio, carbon to phosphorus (C/P) ratio, and the volatile solids ( V S ) content of the weeds varied widely. No trend between these factors and the methane yield was discernable; the possible reasons are discussed. The energy potential of the weeds per unit area of the weed crop was worked out. Natural stands of salvinia, such as the one employed in the present investigation, would yieM energy (methane) of the order of 108 Kcal ha- 1 year- 1.
INTRODUCTION Salvinia (Salvinia molesta, Mitchell), hydrilla (Hydrilla verticillata, casps.), water lily (Nymphaea stellata, Wild.), azolla (Azolla pinnata), water sprite (Ceratopteris sp.), bulrush (Scirpus sp.), papyrus (Cyperus sp.), and bladder wort (Utricularia reticulata) are amongst the aquatic weeds c o m m o n l y occurring in tropical and subtropical regions of the world (Varshney & Rozoska, 1976; Gupta, 1979; Gopal, 1987). Salvinia, a free-floating water fern indigenous to South America, is currently recognised as the world's * Present address: Division of Natural Sciences,Sonoma State University, California 94928, USA. 359 Biological Wastes 0269-7483/90/$03'50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain
360
S. A. Abbasi, P. C. Nipaney, G. D. Schaumberg
most intransigent weed (Russell, 1987) mainly because of its propensity to grow very rapidly at the expense of other weeds, even water hyacinth (Eichhornia crassipes, Mart). Hydrilla is the most problematic amongst the submerged aquatic weeds; during summer months it grows several centimetres a day and can attain biomass densities of l 5 tonnes per hectare (Martyn & Snell, 1979). Water lily (N. stellata) is a rooted floating plant. It has been reported as inundating large areas of stagnant waters used other wise for drinking water supply, irrigation, pisciculture, or recreation in tropical countries like India (Gupta, 1979) and the Philippines (Gangstad, 1978). A. pinnata has become a menacing aquatic weed in Africa, Asia, Australia and South America. The small floating plants of A. pinnata can pass through intake screens and clog the pumps and control equipment of irrigation and drainage systems (Varshney & Rzoska, 1976; NAS, 1976). Water sprite (Ceratopteris thalictroides) is commonly found wild in swamps, rice paddies, and ditches throughout much of tropical and subtropical Asia, the East Indies, and Oceana (NAS, 1976). The infestation of Scirpus sp. has been reported in the Lakes of Indonesia (Scirpus grosses) and East Africa. It invades paddy fields to the detriment of the target crop in Eastern and Southern India (Gupta, 1979). Cyperus (C. papyrus, C. antiquorum) form vast stands in swamps, in shallow lakes, and along stream banks throughout Africa (NAS, 1976). In some African waters (for example Lake Victoria, the White Nile, the Congo, and the Okavango swamp) large clumps detach from the land and form floating islands that obstruct navigation and water flow (NAS, 1976). Kisumu Harbour on Lake Victoria, Kenya, is constantly plagued with floating islands (NAS, 1976). Uganda has about 6500km 2 of permanent swamp, much of it covered with C. papyrus. Cyperus has also invaded lakes and streams in the tropical Asia, including Kerala, India (NAS, 1976). Utricularia sp. has been reported in ponds, lakes and irrigation canals in the Indian subcontinent. It has also been reported elsewhere in the tropics (Gupta, 1979; NAS, 1976). The aquatic weeds, especially salvinia, azolla, hydrilla and Cyperus are highly productive, and can easily attain biomass yields of 10dryt ha-1 year- 1, or more (Reddy & DeBusk, 1985). It is difficult to control or destroy the weeds through chemical or biological agents owing to the high costs and environmental backlash (Abbasi & Nipaney, 1986; Abbasi et al., 1988). Periodical harvesting and utilisation is apparently the best strategy for keeping the weeds under control, and amongst the various utilisation options the one involving anaerobic digestion to produce energy (methane) appears to be the most promising (Abbasi, 1987; Abbasi et al., 1988). In this paper studies are presented on the anaerobic digestion of the seven weeds mentioned above. The energy potential of the weeds per unit area of
Methane energy from aquatic weeds
361
the weed crop has been worked out. Salvinia has also been studied under the same environmental conditions as the other seven weeds, for the sake of comparison.
METHODS
Sampling of the weeds The weeds (whole plants) were collected from ponds, irrigation canals and paddy fields in and around Calicut, India. They were washed liberally with water to remove attached coarse sediments. They were then washed with EDTA solution (50 g litre-~), followed by deionised water, to remove dust particles absorbed on the plant surface (Abbasi, 1988). After draining offthe water, the weeds were carefully wiped with filter paper. From the weeds thus washed and wiped, whole plants were randomly picked for analysis. They were weighed ('wet weight'), dried at 105°C in a hot-air oven to a constant weight ('dry weight'), and stored in sealed polythene bags. The rest of the plants were weighed and chopped for feeding the digesters.
Chemical analysis of the weeds The volatile solid (VS) content was estimated by the combustion of the dried weeds at 620°C. Total kjeldahl nitrogen, and phosphorus were determined spectrophotometrically (Rand et al., 1985) using a Hitachi 220 spectrophotometer. Total organic carbon (TOC) was determined by the method of Piper (1966). Copper and nickel were determined by atomic absorption spectrometry using a IL 951 double beam atomic absorption/emission spectrometer (Rand et al., 1985).
Anaerobic digestion of the weeds All-glass air-tight digesters of 1000ml capacity attached to gas sampling units were employed. Duplicate digesters were charged with 100 g chopped weed and 250 ml water. The digesters were housed in a chamber maintained at 37 +_ loc. The methane content of the biogas was determined using a Tracor 590 gas-liquid chromatograph.
R E S U L T S A N D DISCUSSION The chemical analysis of the weeds is presented in Table 1. The water contents of the weeds were high, which is typical of aquatic plants (Gupta,
S. A. Abbasi, P. C. Nipaney, G. D. Schaumberg
362
~ °~°~ ~ - ~ -
6 6 6 o 6 ~
"c
e~
oooooo~ <
6 6 6 6 6 ~
,,,a 0
,,e <
o6 6 coB o6 6 coB o o ~
,.---k
E
,.Q
ZZ II II
Methane energy from aquatic weeds
363
1979). The VS, nitrogen, phosphorus, copper and nickel contents varied widely amongst the weeds, but no trend was discernible. Likewise the CfN and C/P ratios also varied widely but showed no relationship with each other; for example azolla and Utricularia had similar C/N ratios but their C/P ratios differed markedly. Considering the heterogeneous nature of the digester feed, the agreement TABLE 2 Biogas Production and Energy Equivalent of Different Aquatic Weeds in Batch Digestion
Plant
Digestion time (days)
Biogas production
Methane production
1 kg -1 dr)' weight
1 kg -1 VS
1 kg -1 dr)' weight
1 kg -1 VS
Energy equivalent (kcal kg- 1 dr)' weight)
Salvinia
15 20 25 35
175 240 258 347
207 284 306 410
103 142 152 205
122 168 180 242
928 1279 1370 1847
Azolla
15 20 25 35
64 83 111 167
79 102 137 205
41 54 72 108
51 66 88 132
369 487 649 973
Ceratopteris 15 20 25 35
75 101 131 225
93 125 163 280
55 74 96 164
68 91 119 204
496 667 865 1478
Scirpas
15 20 25 35
18 26 38 63
23 32 48 78
15 22 32 54
20 27 41 66
135 198 288 487
Cyperas
15 20 25 35
19 25 33 54
24 31 42 67
11 14 19 30
13 17 24 38
99 126 171 270
Utricula ria
15 20 25 35
64 85 108 182
78 104 132 222
38 51 64 108
46 62 79 132
342 460 577 973
15 20 25
76 92 106
94 113 130
48 58 66
59 71 81
433 523
Hydrilla
S. A. Abbasi, P. C. Nipaney, G. D. Schaumberg
364
between the results obtained for the pairs of digesters was good; the average difference in the daily gas production being 0.2 litres kg-1 weed. The results on biogas, methane, and energy equivalent of methane production by different weeds as a function of HRT are summarised in Table 2. The energy equivalent was calculated on the basis of the reported (Meyers, 1983) calorific value of methane (213 kcallitres-1 at NTP). Several factors are known to influence anaerobic digestion and methane yield from biomass, including C/N ratio (Meynell, 1976; Chynoweth & Srivastava, 1980; Ghosh et al., 1985), C/P ratio (Ghosh, 1981), nature of organic matter constituting VS (Ghosh et al., 1981), iron (Speece & Perkin, 1985), vitamins (Scherer & Sahm, 1981), trace nutrients (Scherer & Sahm, 1981), and natural inhibitants (Ghosh et al., 1981) present in the biomass. An examination of Tables 1 and 2 indicates that no single factor dominates the gas yield. Often, the C/N ratio is used as an index of the suitability of organic feeds (Stafford et al., 1980; TERI, 1985) for methanogenesis, but there is no agreement in the literature on the 'ideal' C/N ratio. The ratios proposed as 'ideal' for C/N vary from 12 (Ghosh et al., 1981) to 72 (DeRenzo, 1977). The present studies, in which all the substrates fall within the 'ideal' range and yet have widely disparate methane generating abilities, support the suggestion that several factors besides C/N and C/P ratios together contribute to biogas and methane yield. Table 3 presents figures for energy yield from the weeds as a function of crop area and annual biomass production in typical natural stands. The calculations have been limited to the weeds of which annual productivity figures are available in the literature. Salvinia would yield energy of the order TABLE 3 Energy Equivalent of Weeds per Unit Crop Area per Year a
Plant
Medium
Productivity (t ha- 1 year- 1) (dr), weight)
Salvinia molesta Natural ponds
57
Azolla pinnata
Nutrient medium
11
Scirpus Hydrilla verticillata Nymphaea sp.
-Nutrient medium
2-5 25-0 15
--
12h
ReJerence
Energy equivalent (kcal ha- i year- 1)
Abbasi & Nipaney, 10'53 x 10 v 1989 Reddy & DeBusk, 1.07 × 107 1985 Lakshman, 1987 0-12-1.22 x 107 Reddy & Bagnall, 1.16 × 1 0 7 1981 Boyd, 1968 0'40 × 1 0 7
Calculated for weeds of which annual productivity has been reported in the literature. h The annual productivity calculated on the basis of 1"8 t/ha (dry weight) standing crop.
Methane energyfrom aquatic weeds
365
of 108 kcal ha - ~ y e a r - 1, while azolla, Scirpus, hydrilla, and Nymphaea have energy potentials of the order of 107 kcal h a - ~ y e a r -
ACKNOWLEDGEMENTS The authors are grateful to Dr K. Venkatasubramanian, Vice-Chancellor, Pondicherry (Central) University, and Dr D o . Ferish, Dean, Division of Natural Sciences, Sonoma State University, California, for inspiring guidance. Thanks are also due to M r Scott Heidike, BS student, for valuable assistance. The financial support was in part by the Department of NonConventional Energy Sources and in part by the Council of Scientific and Industrial Research, New Delhi.
REFERENCES Abbasi, S. A. (1987). Renewable energy from aquatic biomass. In Proceedings of the 1986 International Congress on Renewable Energy Sources, CSIC, Madrid, pp. 60-9. Abbasi, S. A. (1988). Environmental analysis of cerium. Inter. J. Environ. Anal. Chem., 34, 181 90. Abbasi, S. A. & Nipaney, P. C. (1986). Infestation of the fern genus Salvinia: its status and control. Environmental Conservation, 13, 235-41. Abbasi, S. A. & Nipaney, P. C. (1989). Productivity of Salvinia molesta (Mitchell). Unpublished work, communicated to Aquatic Botany. Abbasi, S. A., Nipaney, P. C. & Soni, R. (1988). Aquatic weeds: distribution, impact and control. Journal of Scientific and Industrial Research, 47, 650-61. Boyd, C. E. (1968). Freshwater plants: a potential source of protein. Economic Botany, 22, 359. Chynoweth, D. P. & Srivastava, V. J. (1980). Methane production from marine biomass. In International Symposium on Biogas, Microalgae, and Livestock Wastes--1980, Taipei, Taiwan, ROC, 15-17 September, pp. 266. DeRenzo, D. J. (1977). Bioconversion of Solid Waste and Sewage Sludge in Energy from Bioconversion of Waste Material. Noyes Data, Dark Ride, NJ. Gangstad, E. O. (1978). Weed Control Methods for River Basin Management, CRC Press Inc., Florida. Ghosh, S. (1981). Biomethanation of peat. In International Peat Symposium, Benidji State University, Benidji, Minnesota. Ghosh, S., Klass, D. L. & Chynoweth, D. P. (1981). Bioconversion of Macrosystis pyrifera to methane. J. Chem. Tech, Biotechnol., 31, 791-807. Ghosh, S., Henry, M. P. & Christopher, R. W. (1985). Hemicellulose conversion by anaerobic digestion. Biomass, 6, 257. Gopal, B. (1987). Water Hyacinth, Aquatic Plant Studies L Elsevier Science Publishers, The Netherlands, pp. 1-471.
366
S. A. Abbasi, P. C. Nipaney, G. D. Schaumberg
Gupta, O. P. (1979). Aquatic Weeds: their Menace and Control---a Textbook and Manual Today and Tomorrow Printers and Publishers, New Delhi, pp. 1-279. Lakshman, G. (1987). Ecotechnological opportunities for aquatic plants--a survey of utilization options. In Proceedings of Aquatic Macrophytes for Water Treatment and Resource Recovery, ed. K. R. Reddy & W. H. Smith. Magnolia Publishing, Orlando, FL, p. 49. Martyn, R. D. & Snell, W. W. (1979). Lake Conroe aquatic vegetation survey I aerial color infrared photography--base line map--1979. MP-1502, 2M-2-82. Meyers, R. A. (1983). Handbook of Energy Technology and Economics, John Wiley & Sons, New York, pp. 257. Meynell, P. J. (1976). Methane: Planning a Digester, Prism, Detroit. National Academy of Sciences (1976). Making aquatic weeds useful: some perspectives for developing countries. Report, Board on Science and Technology for International Development, Commission of Interational Relations, Washington, DC, p. 175. Piper, C. S. (1966). Soil and Plant Analysis. Inter Science Publications, New York. Rand, M. C., Greenberg, A. R. & Taras, M. J. (1985). Standard Methods for the Examination of Water and Wastewater, 16th edn, APHA, Washington, DC. Reddy, K. R. & Bagnall, L. O. (1981). Biomass production of aquatic plants used in agricultural drainage water treatment. In International Gas Research Conference Proceedings. Govt. Institute, Inc., Rockville, MD, p. 660. Reddy, K. R. & DeBusk, W. F. (1985). Growth characteristics of aquatic macrophytes cultured in nutrient-enriched water: II. Azolla, duckweed and salvinia. Economic Botany, 39, 200. Russell, A. (1987). Guinness Book of Worm Records. Bantom Books, New York, pp. 105. Scherer, P. & Sahm, H. (1981). Effect of trace elements and vitamins on the growth of Methanosarcina barkerL Acta Biotechnologica, 1, p. 57. Speece, R. E. & Perkin, G. F. (1985). Nutrient requirements for anaerobic digestion. In Biotechnological Advances in Processing Municipal Wastes for Fuels and Chemicals, ed. A. A. Antonopoulos. ANL/CNSV-TM-167, Argonne National Laboratory, Argonne, Illinois, p. 195. Stafford, D. A., Hawkes, D. L. & Horton, T. (Eds) (1980).Methane production from waste organic matter. In Environmental Conditions for the Control of Digester Performances, Chapter 8. CRC Press Inc., Bocca Raton, FL, p. 149. Stafford, D. A., Hawkes, D. L. & Horton, T. (1980). Environmental Conditions for the Control of Digester Performances. CRC Press Inc., Bocca Raton, Florida, pp. 149-67. Tata Energy Research Institute (TERI) (1985). Biogas Technology: an information package. Tata Energy Documentation and Information Centre, Bombay, p. 189. Varsheny, C. K. & Rzoska, J. (1976). Aquatic Weeds in South East Asia. Dr W. Junk, BV Publishers, The Hague.
Bioenergy Potential of Eight Common Aquatic Weeds S. A. A b b a s i , P. C. N i p a n e y & G. D. S c h a u m b e r g * Salim Ali School of Ecology, Pondicherry (Central) University, Pondicherry 605 001, India (Received 10 June 1989; revised version received 2 July 1990; accepted 23 July 1990)
ABSTRACT Eight common aquatic weeds Salvinia molesta, Hydrilla verticillata, Nymphaea stellata, Azolla pinnata, Ceratopteris sp. Scirpus sp. Cyperus sp. and Utricularia reticulata were digested anaerobically to produce methane. The carbon to nitrogen ( C/N) ratio, carbon to phosphorus (C/P) ratio, and the volatile solids ( V S ) content of the weeds varied widely. No trend between these factors and the methane yield was discernable; the possible reasons are discussed. The energy potential of the weeds per unit area of the weed crop was worked out. Natural stands of salvinia, such as the one employed in the present investigation, would yieM energy (methane) of the order of 108 Kcal ha- 1 year- 1.
INTRODUCTION Salvinia (Salvinia molesta, Mitchell), hydrilla (Hydrilla verticillata, casps.), water lily (Nymphaea stellata, Wild.), azolla (Azolla pinnata), water sprite (Ceratopteris sp.), bulrush (Scirpus sp.), papyrus (Cyperus sp.), and bladder wort (Utricularia reticulata) are amongst the aquatic weeds c o m m o n l y occurring in tropical and subtropical regions of the world (Varshney & Rozoska, 1976; Gupta, 1979; Gopal, 1987). Salvinia, a free-floating water fern indigenous to South America, is currently recognised as the world's * Present address: Division of Natural Sciences,Sonoma State University, California 94928, USA. 359 Biological Wastes 0269-7483/90/$03'50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain
360
S. A. Abbasi, P. C. Nipaney, G. D. Schaumberg
most intransigent weed (Russell, 1987) mainly because of its propensity to grow very rapidly at the expense of other weeds, even water hyacinth (Eichhornia crassipes, Mart). Hydrilla is the most problematic amongst the submerged aquatic weeds; during summer months it grows several centimetres a day and can attain biomass densities of l 5 tonnes per hectare (Martyn & Snell, 1979). Water lily (N. stellata) is a rooted floating plant. It has been reported as inundating large areas of stagnant waters used other wise for drinking water supply, irrigation, pisciculture, or recreation in tropical countries like India (Gupta, 1979) and the Philippines (Gangstad, 1978). A. pinnata has become a menacing aquatic weed in Africa, Asia, Australia and South America. The small floating plants of A. pinnata can pass through intake screens and clog the pumps and control equipment of irrigation and drainage systems (Varshney & Rzoska, 1976; NAS, 1976). Water sprite (Ceratopteris thalictroides) is commonly found wild in swamps, rice paddies, and ditches throughout much of tropical and subtropical Asia, the East Indies, and Oceana (NAS, 1976). The infestation of Scirpus sp. has been reported in the Lakes of Indonesia (Scirpus grosses) and East Africa. It invades paddy fields to the detriment of the target crop in Eastern and Southern India (Gupta, 1979). Cyperus (C. papyrus, C. antiquorum) form vast stands in swamps, in shallow lakes, and along stream banks throughout Africa (NAS, 1976). In some African waters (for example Lake Victoria, the White Nile, the Congo, and the Okavango swamp) large clumps detach from the land and form floating islands that obstruct navigation and water flow (NAS, 1976). Kisumu Harbour on Lake Victoria, Kenya, is constantly plagued with floating islands (NAS, 1976). Uganda has about 6500km 2 of permanent swamp, much of it covered with C. papyrus. Cyperus has also invaded lakes and streams in the tropical Asia, including Kerala, India (NAS, 1976). Utricularia sp. has been reported in ponds, lakes and irrigation canals in the Indian subcontinent. It has also been reported elsewhere in the tropics (Gupta, 1979; NAS, 1976). The aquatic weeds, especially salvinia, azolla, hydrilla and Cyperus are highly productive, and can easily attain biomass yields of 10dryt ha-1 year- 1, or more (Reddy & DeBusk, 1985). It is difficult to control or destroy the weeds through chemical or biological agents owing to the high costs and environmental backlash (Abbasi & Nipaney, 1986; Abbasi et al., 1988). Periodical harvesting and utilisation is apparently the best strategy for keeping the weeds under control, and amongst the various utilisation options the one involving anaerobic digestion to produce energy (methane) appears to be the most promising (Abbasi, 1987; Abbasi et al., 1988). In this paper studies are presented on the anaerobic digestion of the seven weeds mentioned above. The energy potential of the weeds per unit area of
Methane energy from aquatic weeds
361
the weed crop has been worked out. Salvinia has also been studied under the same environmental conditions as the other seven weeds, for the sake of comparison.
METHODS
Sampling of the weeds The weeds (whole plants) were collected from ponds, irrigation canals and paddy fields in and around Calicut, India. They were washed liberally with water to remove attached coarse sediments. They were then washed with EDTA solution (50 g litre-~), followed by deionised water, to remove dust particles absorbed on the plant surface (Abbasi, 1988). After draining offthe water, the weeds were carefully wiped with filter paper. From the weeds thus washed and wiped, whole plants were randomly picked for analysis. They were weighed ('wet weight'), dried at 105°C in a hot-air oven to a constant weight ('dry weight'), and stored in sealed polythene bags. The rest of the plants were weighed and chopped for feeding the digesters.
Chemical analysis of the weeds The volatile solid (VS) content was estimated by the combustion of the dried weeds at 620°C. Total kjeldahl nitrogen, and phosphorus were determined spectrophotometrically (Rand et al., 1985) using a Hitachi 220 spectrophotometer. Total organic carbon (TOC) was determined by the method of Piper (1966). Copper and nickel were determined by atomic absorption spectrometry using a IL 951 double beam atomic absorption/emission spectrometer (Rand et al., 1985).
Anaerobic digestion of the weeds All-glass air-tight digesters of 1000ml capacity attached to gas sampling units were employed. Duplicate digesters were charged with 100 g chopped weed and 250 ml water. The digesters were housed in a chamber maintained at 37 +_ loc. The methane content of the biogas was determined using a Tracor 590 gas-liquid chromatograph.
R E S U L T S A N D DISCUSSION The chemical analysis of the weeds is presented in Table 1. The water contents of the weeds were high, which is typical of aquatic plants (Gupta,
S. A. Abbasi, P. C. Nipaney, G. D. Schaumberg
362
~ °~°~ ~ - ~ -
6 6 6 o 6 ~
"c
e~
oooooo~ <
6 6 6 6 6 ~
,,,a 0
,,e <
o6 6 coB o6 6 coB o o ~
,.---k
E
,.Q
ZZ II II
Methane energy from aquatic weeds
363
1979). The VS, nitrogen, phosphorus, copper and nickel contents varied widely amongst the weeds, but no trend was discernible. Likewise the CfN and C/P ratios also varied widely but showed no relationship with each other; for example azolla and Utricularia had similar C/N ratios but their C/P ratios differed markedly. Considering the heterogeneous nature of the digester feed, the agreement TABLE 2 Biogas Production and Energy Equivalent of Different Aquatic Weeds in Batch Digestion
Plant
Digestion time (days)
Biogas production
Methane production
1 kg -1 dr)' weight
1 kg -1 VS
1 kg -1 dr)' weight
1 kg -1 VS
Energy equivalent (kcal kg- 1 dr)' weight)
Salvinia
15 20 25 35
175 240 258 347
207 284 306 410
103 142 152 205
122 168 180 242
928 1279 1370 1847
Azolla
15 20 25 35
64 83 111 167
79 102 137 205
41 54 72 108
51 66 88 132
369 487 649 973
Ceratopteris 15 20 25 35
75 101 131 225
93 125 163 280
55 74 96 164
68 91 119 204
496 667 865 1478
Scirpas
15 20 25 35
18 26 38 63
23 32 48 78
15 22 32 54
20 27 41 66
135 198 288 487
Cyperas
15 20 25 35
19 25 33 54
24 31 42 67
11 14 19 30
13 17 24 38
99 126 171 270
Utricula ria
15 20 25 35
64 85 108 182
78 104 132 222
38 51 64 108
46 62 79 132
342 460 577 973
15 20 25
76 92 106
94 113 130
48 58 66
59 71 81
433 523
Hydrilla
S. A. Abbasi, P. C. Nipaney, G. D. Schaumberg
364
between the results obtained for the pairs of digesters was good; the average difference in the daily gas production being 0.2 litres kg-1 weed. The results on biogas, methane, and energy equivalent of methane production by different weeds as a function of HRT are summarised in Table 2. The energy equivalent was calculated on the basis of the reported (Meyers, 1983) calorific value of methane (213 kcallitres-1 at NTP). Several factors are known to influence anaerobic digestion and methane yield from biomass, including C/N ratio (Meynell, 1976; Chynoweth & Srivastava, 1980; Ghosh et al., 1985), C/P ratio (Ghosh, 1981), nature of organic matter constituting VS (Ghosh et al., 1981), iron (Speece & Perkin, 1985), vitamins (Scherer & Sahm, 1981), trace nutrients (Scherer & Sahm, 1981), and natural inhibitants (Ghosh et al., 1981) present in the biomass. An examination of Tables 1 and 2 indicates that no single factor dominates the gas yield. Often, the C/N ratio is used as an index of the suitability of organic feeds (Stafford et al., 1980; TERI, 1985) for methanogenesis, but there is no agreement in the literature on the 'ideal' C/N ratio. The ratios proposed as 'ideal' for C/N vary from 12 (Ghosh et al., 1981) to 72 (DeRenzo, 1977). The present studies, in which all the substrates fall within the 'ideal' range and yet have widely disparate methane generating abilities, support the suggestion that several factors besides C/N and C/P ratios together contribute to biogas and methane yield. Table 3 presents figures for energy yield from the weeds as a function of crop area and annual biomass production in typical natural stands. The calculations have been limited to the weeds of which annual productivity figures are available in the literature. Salvinia would yield energy of the order TABLE 3 Energy Equivalent of Weeds per Unit Crop Area per Year a
Plant
Medium
Productivity (t ha- 1 year- 1) (dr), weight)
Salvinia molesta Natural ponds
57
Azolla pinnata
Nutrient medium
11
Scirpus Hydrilla verticillata Nymphaea sp.
-Nutrient medium
2-5 25-0 15
--
12h
ReJerence
Energy equivalent (kcal ha- i year- 1)
Abbasi & Nipaney, 10'53 x 10 v 1989 Reddy & DeBusk, 1.07 × 107 1985 Lakshman, 1987 0-12-1.22 x 107 Reddy & Bagnall, 1.16 × 1 0 7 1981 Boyd, 1968 0'40 × 1 0 7
Calculated for weeds of which annual productivity has been reported in the literature. h The annual productivity calculated on the basis of 1"8 t/ha (dry weight) standing crop.
Methane energyfrom aquatic weeds
365
of 108 kcal ha - ~ y e a r - 1, while azolla, Scirpus, hydrilla, and Nymphaea have energy potentials of the order of 107 kcal h a - ~ y e a r -
ACKNOWLEDGEMENTS The authors are grateful to Dr K. Venkatasubramanian, Vice-Chancellor, Pondicherry (Central) University, and Dr D o . Ferish, Dean, Division of Natural Sciences, Sonoma State University, California, for inspiring guidance. Thanks are also due to M r Scott Heidike, BS student, for valuable assistance. The financial support was in part by the Department of NonConventional Energy Sources and in part by the Council of Scientific and Industrial Research, New Delhi.
REFERENCES Abbasi, S. A. (1987). Renewable energy from aquatic biomass. In Proceedings of the 1986 International Congress on Renewable Energy Sources, CSIC, Madrid, pp. 60-9. Abbasi, S. A. (1988). Environmental analysis of cerium. Inter. J. Environ. Anal. Chem., 34, 181 90. Abbasi, S. A. & Nipaney, P. C. (1986). Infestation of the fern genus Salvinia: its status and control. Environmental Conservation, 13, 235-41. Abbasi, S. A. & Nipaney, P. C. (1989). Productivity of Salvinia molesta (Mitchell). Unpublished work, communicated to Aquatic Botany. Abbasi, S. A., Nipaney, P. C. & Soni, R. (1988). Aquatic weeds: distribution, impact and control. Journal of Scientific and Industrial Research, 47, 650-61. Boyd, C. E. (1968). Freshwater plants: a potential source of protein. Economic Botany, 22, 359. Chynoweth, D. P. & Srivastava, V. J. (1980). Methane production from marine biomass. In International Symposium on Biogas, Microalgae, and Livestock Wastes--1980, Taipei, Taiwan, ROC, 15-17 September, pp. 266. DeRenzo, D. J. (1977). Bioconversion of Solid Waste and Sewage Sludge in Energy from Bioconversion of Waste Material. Noyes Data, Dark Ride, NJ. Gangstad, E. O. (1978). Weed Control Methods for River Basin Management, CRC Press Inc., Florida. Ghosh, S. (1981). Biomethanation of peat. In International Peat Symposium, Benidji State University, Benidji, Minnesota. Ghosh, S., Klass, D. L. & Chynoweth, D. P. (1981). Bioconversion of Macrosystis pyrifera to methane. J. Chem. Tech, Biotechnol., 31, 791-807. Ghosh, S., Henry, M. P. & Christopher, R. W. (1985). Hemicellulose conversion by anaerobic digestion. Biomass, 6, 257. Gopal, B. (1987). Water Hyacinth, Aquatic Plant Studies L Elsevier Science Publishers, The Netherlands, pp. 1-471.
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S. A. Abbasi, P. C. Nipaney, G. D. Schaumberg
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