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Recycling of biogas-plant effluent through aquatic plant (Lemna) culture
Bioresource Technology 41 (1992) 213-216
Recycling of Biogas-Plant Effluent through Aquatic Plant ( Lemna ) Culture R R. Balasubramanian & R. Kasturi Bai* School of Energy, Environment and Natural Resources, Madurai Kamaraj University, Madurai-625 021, India (Received 10 June 1'991 ; revised version received 9 September 1991 : accepted 15 October 1991 }
Abstract 7he production of the aquatic plant, common duckweed (Lemna) on slurry obtained from a K VIC model biogas plant fed on cattle waste, was investigated. The plants grew well in a 1% concentration of biogas plant effluent. The mean biomass yield was 1.06 g dry mass m - : day-i. The crude protein content of this biomass was 16"40+_2"05% (dry we(~ht). The nutrient composition of the plant was the same as plants grown on other media. Key words: KV1C model biogas plant, digested biogas plant slurry, recycling, Lemna, biomass production. INTRODUCTION Aquatic plants are considered a nuisance since they spread and clog-up lakes and waterways. Many aquatic plants are capable of prolific growth and produce large amounts of biomass. As a result, there is a growing interest in the potential use of aquatic plants for nutrient removal from waste or polluted water and recycling of the resulting biomass for production of gaseous fuel, feed, fibre and other products. In particular, the protein content of aquatic plants is similar in chemical composition to leaf protein from crop plants (Boyd, 1971, 1974) and rich in amino acids (Anon, 1976) similar to hen-egg protein (Maciejewska-potapozykowa et al., 1970), especially in the common duckweed, Lemna. The protein content of the duckweed was 16-30% of dry weight (Allenby, 1968; FAO, 1979; Amado et al., 1980). Duckweed were able to grow successfully on wastewater to convert degradable pollutants *Author to whom correspondence should be addressed.
directly into protein-rich fodder, while the remaining effluent water was suitable for irrigation (Oron et al., 1987). In the present study, KVIC (Khadi and Village Industries Commission) model gobar (cattle waste) gas plant effluent was utilized for biomass production using Lemna, in outdoor mini ponds. The efficiency of harvesting the nutrients of biogas slurry through Lemna was assessed taking into consideration the biomass production and the nutrient contents of the plants.
METHODS Biogas plant slurry The effluent was collected from a 4 m 3 capacity KVIC digester running on cattle dung, situated in the premises of the School of Energy Sciences (Lat 9°53'N; Long 78°E). The retention time was 30 days. The effluent was periodically collected and analysed.
Aquatic plant Lemna was collected from a pond near the University campus.
Mass culture of aquatic plants in the biogas plant slurry Mass culture experiments were carried out in (duplicate) 300 litre-capacity mini ponds, 120 × 60 × 46 cm in size out-doors. Initially 150 g (208 g m --~) of Lemna plants were introduced to each pond containing 1% slurry in a total volume of 250 I. On every fifth day, slurry was added (wet weight) at 1% concentration level. The water loss was replaced every day with tap water to maintain a constant volume. The plants grown above 150 g were harvested at 5 day intervals.
213 Bioresource Technology 0960-8524/92/S05.00 © 1992 Elsevier Science Publishers Ltd, En,,land Printed in Great Britain
214
P. R. Balasubramanian, R. Kasturi Bai
Analyses The following parameters were analysed at every 5 day interval, pH was measured using an Elico/ Li 10T pH meter. Conductivity measurements were done using a Systronics 304 digital conductivity meter. Available and total phosphorus, sodium, potassium, calcium, ammonia nitrogen, biochemical oxygen demand, total Kjeldhal nitrogen and crude protein were analysed according to 'Standard Methods' (APHA, 1985). The potassium, godium and calcium contents were estimated using a Systronics Flame Photometer 'mediflame 127'. Temperature measurements were made using a thermometer in the range of 0-100°C. Solar insolation was measured by using 'Surya Mapi'. Biomass of plants was estimated as the wet weight of the plants. Plants were collected by nylon net and gently pressed to remove excess water and weighed. Total solids (TS) content was estimated by drying at 105°C to constant weight. Volatile solids (VS) content was estimated by heating the sample at 500°C for 6 h. The nutrient contents of the plants given were for air-dried samples at room temperature of 30 _+3°C.
reported to be greater in more dilute manure (Novak & Franklova, 1980), or the plants were unable to use all the nutrients released during mineralization and the nutrients supplied at 5 day intervals. The BOD of the water was between 17 and 20 mg 1-l, which showed the suitability of using this water for further uses. The biomass yield increased by up to 67% on all the harvesting days (at 5 day intervals), except on the fifteenth day. The yield exceeded 100% increase on the tenth, thirtieth and thirty-fifth days of the culture period. The regression analysis of data on cumulative biomass production of L e m n a plant in 1% concentration of biogas plant slurry showed that growth was significant (Fig. 1) if the plants were harvested at 5 day intervals. Plants started decaying after the forty-fifth day. It was observed that the same culture re-established if the water was partially drained out and the tank refilled with fresh water at 40 day intervals with simultaneous application of slurry at 5 day intervals. The mean biomass production was 1.06 g dry mass m-2 day- J. Earlier reports showed that the yield of common duckweed ranges between 0"1
RESULTS AND DISCUSSION The ambient temperature during the course of this study from 16 May 1989 to 3 July 1989 was between 35 ° and 38°C. The water temperature was between 30 ° and 34.5°C. The solar insolation around 12:00 h was between 400 and 920 W m-2.
The nutrient content of the digested slurry is given in Table 1. The preliminary experiments using this slurry carried out in tiffs laboratory in mini ponds with batch type of culture showed the highest production of biomass (150% increase) within 6 days in 1% slurry concentration. The plants utilized nearly 100%, 55% and 45% of ammonia nitrogen, available potassium and phosphorus, in 6 days, respectively. The present experiment was continued with a semicontinuous type of feeding slurry at 5 day intervals. The plant utilized ammonia nitrogen completely after each application of biogas slurry, whereas phosphorus, potassium, sodium and calcium showed accumulation from the initial levels of 1.42+_0.15, 6.00 + 0.00, 40.00 _+0.50 and 18.00 +_0.00 mg lto 11.10_+1.50, 20"00_+0-00, 69.00_+1.00 and 30-00_+ 1.22 mg 1-~ respectively on the fiftieth day. This might be due to mineralization, because the organic matter mineralization has been
Table l. The nutrient content of the digested biogas plant slurry (values are the mean+ SD of 30 samples) (nutrient values are a percentage of TS) 10'50 ± 0"46 1.41 _+0"33 0'60 _+0"25 0'30+__0"15 1.30-+0-24
Total solids (TS) Total Kjeldhal nitrogen (TKN) Ammonia nitrogen (NH4-N) Phosphorus (P) Potassium (K)
1200 Y = 22.4 x ÷ -18,9 r = 0 98 100C z o_ Z
o
80C
o o.
~r
600!
20O o 0!
I
I
1
I
10
20
30
t.O
50
DAYS
Fig. 1. The regression analysis (e) of data on cumulative biomass production of Lemna plant in 1% concentration of biogas plant effluent, (o) cumulative biomass production (values are the means of duplicates).
Digester effluent in Lemna cultivation
215
Table 2. Nutrient composition of Lemna plant grown in 1% slurry (values are the mean
_~: SD of 30 samples) (nutrient and VS are a percentage of TS)
Total solids (TS)
Volatile solid (VS)
TKN
3.37 _+0-17
75"20 +2'27
2"63 _+0"3l
P
K
Na
0.70 2.82 0"70 _+0-20 _+0"63 +0"14
Ca 0"65 -+0"08
Crude protein" 16"40 _+2"05
"(?rude protein = TKN × 6'25.
and 12.0 (average 3-8) g dry m a s s m -2 day -~ in nutrient-enriched medium (Reddy et al., 1983), and in natural stands it was around 0.5-4-2 ( 1-41 g dry mass m -2 day -] (Ryther, 1979). In diluted livestock waste, the production was nearly 12 g dry mass m -2 day-~ during the winter season (Lincolin et al., 1986). The crude protein content of plants reached the maximum level of 19.16% between the twenty-fifth and thirtieth day of the culture period. The nutrient analysis of the harvested plants showed (Table 2) the uptake of nutrients from the medium and plants had the same nutritional composition as plants grown on other media 'Allenby, 1968; FAO, 1979 Sutton & Ornes, 1975; Wolverton & McDonald, 1978; Chawla, 1986; Oron et al., 1987; Oron & Willers, 1989). These plants can be utilized in the feed formulations of fish, (Devaraj et al., 1981) ducks ,'Maciejewska-potapozykowa et al., 19701 and poultr3' (Muzaffarov et al., 1968). The L e m n a plants grown by utilizing biogas-plant effluent were tested as a supplementary feed with cattle dung in biogas production (Balasubramanian & Kasturi Bai, 19911 by anaerobic batch fermentation. Cattle dung and L e m n a at 3:1 ratio, w/w, (solid and water ratio at 1:2, w/v) produced 0"9 1 of methane g-I volatile solid reduced as against 1.1 1 of methane g-~ volatile solid reduced on cattle dung alone. The present findings indicate that the digested biogas plant slurry could be efficiently used in the mass culture of aquatic plant ( L e m n a ) biomass and these harvested plants have a rich nutrient content and could be recycled again for biogas production, or as feeds. ACKNOWLEDGEMENT The authors thank the Department of Non Conventional Energy Sources (DNES) New Delhi, for financial assistance.
REFERENCES Allenby, K. G. (19681. Some analysis of aquatic plants and waters. Hydrobiologia, 32,486-90. Amado, R., Muller-Heimeyer, R. & Marti, U. (198(t). Proteingehalt, aminosaure-zusammensetzung and neutralzucker gehalt von lemnaceen. Veroff. Geobot. Inst. ETtt. Stiftung RubeL Zurich 70, Heft, 103-17. Anon. ( 19761. Making Aquatic Weeds" UsejS~k Some Perspectives for Developing Countries. National Academy of Sciences, pp. 175. APHA (19851. Standard Methods for the Examinations o]" Water and Waste Water, 16th edn., American Public Health Association/American Water Works Association/ Water Pollution Control Federation, Washington, DC. Balasubramanian, E R. & Kasturi Bai, R. (19911. Recycling of biogas plant slurry grown Lemna sp. plants for fuel production. In, Association of Microbiologists of India, 31st Annual Conference Proceedings. TNAU, Coimbatore, India (in press). Boyd, C. E. (19711. Leaf protein form aquatic plants. IBI' Handbook, 20, 44-9. Boyd, C. E, (19741. 7. Utilization of aquatic plants. In Aquatic Vegetation and Its Use and Control, ed. D. S. Mitchell, UNESCO, Paris, pp. 107-14. Chawla, O. P. (19861. Advances in Biogas Technology, ICAR, Publications and Information Division, New Delhi, pp. 144, Devaraj, K. V., Krishna Rao, D. S. & Keshavappa, G. Y. ( 1981 ). Utilization of duckweed and waste cabbage leaves in the formulation of fish feed. Mysore J. Agric, Sci., 15, 132-5. FAO (19791. Handbook of Utilization of Aquatic Plants: a Review of Worm Literature, ed. E. C. S. Little. Food and Agriculture Organization of the United Nations. FAO Technical paper No. 87, Rome, p. 176. Lincolin, E. E, Koopman, B., Bagnall, L. O. & Nordstedt, R. A. (1986). Aquatic system for fuel and feed production from livestock waste. J. Agric. Eng., 33, 159-69. Maciejewska-potapozykowa, W., Konopska, L. & Narymska, E. (19701. Proteins in duckweed (Lemna minor). Acta Soc. Bot. Pol., 39(2), 251-5. Muzaffarov, A. M., Taubaev, T. & Abdiev, M. (19681. The utilization of duckweed for poultry feed. Uzb. Biol. Zh., 12(31, 44-6. Novak, B. & Franklova, B. (1980). Treatment of liquid cattle manure: mineralization and stabilization of organic substances. Agric. Wastes, 2,303-12. Oron, G. & Willers, H. (19891. Effect of wastes quality on treatment efficiency with duckweed. Water Sci. Technol., 21,639-45. Oron, G., de-Vegt, A. & Porath, D. (19871. The role of the operation regime in waste water treatment with duckweed. Water Sci. Technol., 19, 97-105.
216
P. R. Balasubramanian, R. Kasturi Bai
Reddy, K. R., Sutton, D. L. & Bowes, G. (1983). Freshwater aquatic plant biomass production in Florida. In, Soil and Crop Science Society of Florida Proceedings, 42, 28-40. Ryther, J. H. (1979). Cultivation of macroscopic marine algae and fresh water aquatic weeds. Rep. U.S Dep. Energy Contr. No. EY-76-S-O2-294B, p. 74.
Sutton, D. L. & Ornes, W. H. (1975). Phosphorus removal from static sewage effluent using duckweed. J. Environ. Qual., 4(3), 367-70. Wolverton, B. C. & McDonald, R. C. (1978). Nutritional composition of water hyacinth grown on domestic sewage. Econ. Bot., 32(4), 363-70.
Recycling of Biogas-Plant Effluent through Aquatic Plant ( Lemna ) Culture R R. Balasubramanian & R. Kasturi Bai* School of Energy, Environment and Natural Resources, Madurai Kamaraj University, Madurai-625 021, India (Received 10 June 1'991 ; revised version received 9 September 1991 : accepted 15 October 1991 }
Abstract 7he production of the aquatic plant, common duckweed (Lemna) on slurry obtained from a K VIC model biogas plant fed on cattle waste, was investigated. The plants grew well in a 1% concentration of biogas plant effluent. The mean biomass yield was 1.06 g dry mass m - : day-i. The crude protein content of this biomass was 16"40+_2"05% (dry we(~ht). The nutrient composition of the plant was the same as plants grown on other media. Key words: KV1C model biogas plant, digested biogas plant slurry, recycling, Lemna, biomass production. INTRODUCTION Aquatic plants are considered a nuisance since they spread and clog-up lakes and waterways. Many aquatic plants are capable of prolific growth and produce large amounts of biomass. As a result, there is a growing interest in the potential use of aquatic plants for nutrient removal from waste or polluted water and recycling of the resulting biomass for production of gaseous fuel, feed, fibre and other products. In particular, the protein content of aquatic plants is similar in chemical composition to leaf protein from crop plants (Boyd, 1971, 1974) and rich in amino acids (Anon, 1976) similar to hen-egg protein (Maciejewska-potapozykowa et al., 1970), especially in the common duckweed, Lemna. The protein content of the duckweed was 16-30% of dry weight (Allenby, 1968; FAO, 1979; Amado et al., 1980). Duckweed were able to grow successfully on wastewater to convert degradable pollutants *Author to whom correspondence should be addressed.
directly into protein-rich fodder, while the remaining effluent water was suitable for irrigation (Oron et al., 1987). In the present study, KVIC (Khadi and Village Industries Commission) model gobar (cattle waste) gas plant effluent was utilized for biomass production using Lemna, in outdoor mini ponds. The efficiency of harvesting the nutrients of biogas slurry through Lemna was assessed taking into consideration the biomass production and the nutrient contents of the plants.
METHODS Biogas plant slurry The effluent was collected from a 4 m 3 capacity KVIC digester running on cattle dung, situated in the premises of the School of Energy Sciences (Lat 9°53'N; Long 78°E). The retention time was 30 days. The effluent was periodically collected and analysed.
Aquatic plant Lemna was collected from a pond near the University campus.
Mass culture of aquatic plants in the biogas plant slurry Mass culture experiments were carried out in (duplicate) 300 litre-capacity mini ponds, 120 × 60 × 46 cm in size out-doors. Initially 150 g (208 g m --~) of Lemna plants were introduced to each pond containing 1% slurry in a total volume of 250 I. On every fifth day, slurry was added (wet weight) at 1% concentration level. The water loss was replaced every day with tap water to maintain a constant volume. The plants grown above 150 g were harvested at 5 day intervals.
213 Bioresource Technology 0960-8524/92/S05.00 © 1992 Elsevier Science Publishers Ltd, En,,land Printed in Great Britain
214
P. R. Balasubramanian, R. Kasturi Bai
Analyses The following parameters were analysed at every 5 day interval, pH was measured using an Elico/ Li 10T pH meter. Conductivity measurements were done using a Systronics 304 digital conductivity meter. Available and total phosphorus, sodium, potassium, calcium, ammonia nitrogen, biochemical oxygen demand, total Kjeldhal nitrogen and crude protein were analysed according to 'Standard Methods' (APHA, 1985). The potassium, godium and calcium contents were estimated using a Systronics Flame Photometer 'mediflame 127'. Temperature measurements were made using a thermometer in the range of 0-100°C. Solar insolation was measured by using 'Surya Mapi'. Biomass of plants was estimated as the wet weight of the plants. Plants were collected by nylon net and gently pressed to remove excess water and weighed. Total solids (TS) content was estimated by drying at 105°C to constant weight. Volatile solids (VS) content was estimated by heating the sample at 500°C for 6 h. The nutrient contents of the plants given were for air-dried samples at room temperature of 30 _+3°C.
reported to be greater in more dilute manure (Novak & Franklova, 1980), or the plants were unable to use all the nutrients released during mineralization and the nutrients supplied at 5 day intervals. The BOD of the water was between 17 and 20 mg 1-l, which showed the suitability of using this water for further uses. The biomass yield increased by up to 67% on all the harvesting days (at 5 day intervals), except on the fifteenth day. The yield exceeded 100% increase on the tenth, thirtieth and thirty-fifth days of the culture period. The regression analysis of data on cumulative biomass production of L e m n a plant in 1% concentration of biogas plant slurry showed that growth was significant (Fig. 1) if the plants were harvested at 5 day intervals. Plants started decaying after the forty-fifth day. It was observed that the same culture re-established if the water was partially drained out and the tank refilled with fresh water at 40 day intervals with simultaneous application of slurry at 5 day intervals. The mean biomass production was 1.06 g dry mass m-2 day- J. Earlier reports showed that the yield of common duckweed ranges between 0"1
RESULTS AND DISCUSSION The ambient temperature during the course of this study from 16 May 1989 to 3 July 1989 was between 35 ° and 38°C. The water temperature was between 30 ° and 34.5°C. The solar insolation around 12:00 h was between 400 and 920 W m-2.
The nutrient content of the digested slurry is given in Table 1. The preliminary experiments using this slurry carried out in tiffs laboratory in mini ponds with batch type of culture showed the highest production of biomass (150% increase) within 6 days in 1% slurry concentration. The plants utilized nearly 100%, 55% and 45% of ammonia nitrogen, available potassium and phosphorus, in 6 days, respectively. The present experiment was continued with a semicontinuous type of feeding slurry at 5 day intervals. The plant utilized ammonia nitrogen completely after each application of biogas slurry, whereas phosphorus, potassium, sodium and calcium showed accumulation from the initial levels of 1.42+_0.15, 6.00 + 0.00, 40.00 _+0.50 and 18.00 +_0.00 mg lto 11.10_+1.50, 20"00_+0-00, 69.00_+1.00 and 30-00_+ 1.22 mg 1-~ respectively on the fiftieth day. This might be due to mineralization, because the organic matter mineralization has been
Table l. The nutrient content of the digested biogas plant slurry (values are the mean+ SD of 30 samples) (nutrient values are a percentage of TS) 10'50 ± 0"46 1.41 _+0"33 0'60 _+0"25 0'30+__0"15 1.30-+0-24
Total solids (TS) Total Kjeldhal nitrogen (TKN) Ammonia nitrogen (NH4-N) Phosphorus (P) Potassium (K)
1200 Y = 22.4 x ÷ -18,9 r = 0 98 100C z o_ Z
o
80C
o o.
~r
600!
20O o 0!
I
I
1
I
10
20
30
t.O
50
DAYS
Fig. 1. The regression analysis (e) of data on cumulative biomass production of Lemna plant in 1% concentration of biogas plant effluent, (o) cumulative biomass production (values are the means of duplicates).
Digester effluent in Lemna cultivation
215
Table 2. Nutrient composition of Lemna plant grown in 1% slurry (values are the mean
_~: SD of 30 samples) (nutrient and VS are a percentage of TS)
Total solids (TS)
Volatile solid (VS)
TKN
3.37 _+0-17
75"20 +2'27
2"63 _+0"3l
P
K
Na
0.70 2.82 0"70 _+0-20 _+0"63 +0"14
Ca 0"65 -+0"08
Crude protein" 16"40 _+2"05
"(?rude protein = TKN × 6'25.
and 12.0 (average 3-8) g dry m a s s m -2 day -~ in nutrient-enriched medium (Reddy et al., 1983), and in natural stands it was around 0.5-4-2 ( 1-41 g dry mass m -2 day -] (Ryther, 1979). In diluted livestock waste, the production was nearly 12 g dry mass m -2 day-~ during the winter season (Lincolin et al., 1986). The crude protein content of plants reached the maximum level of 19.16% between the twenty-fifth and thirtieth day of the culture period. The nutrient analysis of the harvested plants showed (Table 2) the uptake of nutrients from the medium and plants had the same nutritional composition as plants grown on other media 'Allenby, 1968; FAO, 1979 Sutton & Ornes, 1975; Wolverton & McDonald, 1978; Chawla, 1986; Oron et al., 1987; Oron & Willers, 1989). These plants can be utilized in the feed formulations of fish, (Devaraj et al., 1981) ducks ,'Maciejewska-potapozykowa et al., 19701 and poultr3' (Muzaffarov et al., 1968). The L e m n a plants grown by utilizing biogas-plant effluent were tested as a supplementary feed with cattle dung in biogas production (Balasubramanian & Kasturi Bai, 19911 by anaerobic batch fermentation. Cattle dung and L e m n a at 3:1 ratio, w/w, (solid and water ratio at 1:2, w/v) produced 0"9 1 of methane g-I volatile solid reduced as against 1.1 1 of methane g-~ volatile solid reduced on cattle dung alone. The present findings indicate that the digested biogas plant slurry could be efficiently used in the mass culture of aquatic plant ( L e m n a ) biomass and these harvested plants have a rich nutrient content and could be recycled again for biogas production, or as feeds. ACKNOWLEDGEMENT The authors thank the Department of Non Conventional Energy Sources (DNES) New Delhi, for financial assistance.
REFERENCES Allenby, K. G. (19681. Some analysis of aquatic plants and waters. Hydrobiologia, 32,486-90. Amado, R., Muller-Heimeyer, R. & Marti, U. (198(t). Proteingehalt, aminosaure-zusammensetzung and neutralzucker gehalt von lemnaceen. Veroff. Geobot. Inst. ETtt. Stiftung RubeL Zurich 70, Heft, 103-17. Anon. ( 19761. Making Aquatic Weeds" UsejS~k Some Perspectives for Developing Countries. National Academy of Sciences, pp. 175. APHA (19851. Standard Methods for the Examinations o]" Water and Waste Water, 16th edn., American Public Health Association/American Water Works Association/ Water Pollution Control Federation, Washington, DC. Balasubramanian, E R. & Kasturi Bai, R. (19911. Recycling of biogas plant slurry grown Lemna sp. plants for fuel production. In, Association of Microbiologists of India, 31st Annual Conference Proceedings. TNAU, Coimbatore, India (in press). Boyd, C. E. (19711. Leaf protein form aquatic plants. IBI' Handbook, 20, 44-9. Boyd, C. E, (19741. 7. Utilization of aquatic plants. In Aquatic Vegetation and Its Use and Control, ed. D. S. Mitchell, UNESCO, Paris, pp. 107-14. Chawla, O. P. (19861. Advances in Biogas Technology, ICAR, Publications and Information Division, New Delhi, pp. 144, Devaraj, K. V., Krishna Rao, D. S. & Keshavappa, G. Y. ( 1981 ). Utilization of duckweed and waste cabbage leaves in the formulation of fish feed. Mysore J. Agric, Sci., 15, 132-5. FAO (19791. Handbook of Utilization of Aquatic Plants: a Review of Worm Literature, ed. E. C. S. Little. Food and Agriculture Organization of the United Nations. FAO Technical paper No. 87, Rome, p. 176. Lincolin, E. E, Koopman, B., Bagnall, L. O. & Nordstedt, R. A. (1986). Aquatic system for fuel and feed production from livestock waste. J. Agric. Eng., 33, 159-69. Maciejewska-potapozykowa, W., Konopska, L. & Narymska, E. (19701. Proteins in duckweed (Lemna minor). Acta Soc. Bot. Pol., 39(2), 251-5. Muzaffarov, A. M., Taubaev, T. & Abdiev, M. (19681. The utilization of duckweed for poultry feed. Uzb. Biol. Zh., 12(31, 44-6. Novak, B. & Franklova, B. (1980). Treatment of liquid cattle manure: mineralization and stabilization of organic substances. Agric. Wastes, 2,303-12. Oron, G. & Willers, H. (19891. Effect of wastes quality on treatment efficiency with duckweed. Water Sci. Technol., 21,639-45. Oron, G., de-Vegt, A. & Porath, D. (19871. The role of the operation regime in waste water treatment with duckweed. Water Sci. Technol., 19, 97-105.
216
P. R. Balasubramanian, R. Kasturi Bai
Reddy, K. R., Sutton, D. L. & Bowes, G. (1983). Freshwater aquatic plant biomass production in Florida. In, Soil and Crop Science Society of Florida Proceedings, 42, 28-40. Ryther, J. H. (1979). Cultivation of macroscopic marine algae and fresh water aquatic weeds. Rep. U.S Dep. Energy Contr. No. EY-76-S-O2-294B, p. 74.
Sutton, D. L. & Ornes, W. H. (1975). Phosphorus removal from static sewage effluent using duckweed. J. Environ. Qual., 4(3), 367-70. Wolverton, B. C. & McDonald, R. C. (1978). Nutritional composition of water hyacinth grown on domestic sewage. Econ. Bot., 32(4), 363-70.