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Possible losses of plant nutrients from biogas slurry
Agricultural Wastes 12 (1985) 317-320
Short Communication Possible Losses of Plant Nutrients from Biogas Slurry
ABSTRACT One of the major problems in the e[ficient running of biogas plants & proper management of their slurry due to the increased volume of cattle dung ]t'om 1 to 1.6-1.7 times. There[ore, a laboratory study was undertaken to determine the contents of some plant nutrients in the liquid and semi-solid portions. A known weight of" slurry was centrifuged at 6000 rpm for hall an hour and the liquid and semi-solid portions were separated and weighed. The slurry as a whole, liquid and semi-solid portions, was analysed[or its N, P, K, Zn, Cu, Mn and Fe content. It was observed that if the slurryjrom the biogas plant is collected directly into the underground manure pits and the excessive liquid is allowed to leach down, then the losses of K, Zn, N, Cu, Mn, Fe and P would be 36.5, 25.0, 21.5, 14.8, 5.9, 4.9 and 2.7 %, respectively. Moreover, the slurry could be [urther decomposed and applied easily in the fields.
INTRODUCTION The energy crisis and the rising price.s of oil have required scientists to look into alternative sources of energy from natural resources. Cattle dung has an exploitable heating value but, at the same time, its manurial value cannot be ignored. In India more than 1200 million tonnes of dung become available each year, of which about 400 million tonnes is burnt directly (Johal & Shukla, 1980). If this dung could be utilized for the production of biogas then, as well as its utilization as a source of energy, the treated slurry could be used as a source of organic manure. The heat 317 Agricultural Wastes 0141-4607/85/$03.30 © Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain
318
A. P. Gupta, V. K. Gupta, R. S. Antil, Jagan Nath
efficiency of cattle dung is also increased by about 11 ~o by converting it into biogas (Mulmute, 1980). Despite an adequate subsidy for the installation of biogas plants, they have not been generally installed because of several problems. The major drawback is in the utilization or disposal of the treated slurry due to increased water content. The slurry is partially decomposed but needs further decomposition before its use as an organic manure. Sun drying of slurry leads to volatilization losses of plant nutrients and a crust is also formed on the surface which inhibits drying. For its complete decomposition it has to be collected in underground uncemented pits and the liquid portion is bound to leach down from these pits and cause losses of plant nutrients. Little information is available in the literature regarding such losses from biogas slurry. In the work reported here, therefore, the biogas slurry was separated into liquid and semi-solid forms and analyzed to assess the possible losses of some plant nutrients if the liquid portion were removed.
METHODS Fresh effluent slurry from three biogas plants (Gobar digesters) was collected. A weighed quantity of slurry was centrifuged at 6000 rpm for 30 min and the liquid was separated and weighed. The semi-solid portion was dried in an oven at 60 _+ 2 °C to constant weight to determine the moisture content. The dried slurry was ground to pass through a 2 mm sieve in order to obtain a uniform sample of the material. The liquid and dried semi-solid portions were digested in a H 2S 0 4 : H C 1 0 4 (9:1) mixture. Nitrogen in the digest was estimated by the method of Lindner (1944). Phosphorus was determined by the method of Koenig & Johnson (1942). Total K was estimated with a flamephotometer and Zn, Mn, Cu and Fe with an atomic absorption spectrophotometer. The concentrations of nutrients in the semi-solid portion and slurry were calculated to their original wet state.
RESULTS A N D DISCUSSION The ratio of the semi-solid to liquid portions was from 1:1.6 to 1:1.7. The moisture content of the semi-solid portion after centrifugation was 84-4 ~o
319
Losses oJ plant nutrients Jrom biogas slurry
to 84"6 ~o. The cattle dung contains a b o u t 80 ~o water, which is similar to the water content of the semi-solid portion of the slurry, and the increased water in the whole slurry was caused by the addition of diluted dung to the digester. Because of this increased water content, transportation of slurry to the fields is more difficult than transportation of fresh manure. The plant nutrient contents of the liquid, semi-solid and whole slurry are shown in Table 1. If the slurry as a whole is put in underground pits then the liquid portion will leach down through the sides and bottoms of the pits and so TABLE !
Composition of Some Plant Nutrients in Different Portions of Biogas Slurry and Losses in the Liquid Portion Nutrients
N P K Zn Mn Fe Cu
Composition (mg kg- 1) Liquid SemiWhole solid slurry
490.0 22-5 2 213"0 4'9 1-1 8.8 0.7
3 038"0 1 366"0 6 549"0 25"3 29.9 289-1 6.2
1434.0 520-0 3 819.0 12.4 11.8 112.6 2.7
Losses in liquid portion (mg kg- 1 wet slurry)
308.5 14.2 1393.4 3.1 0.7 5.5 0.4
The values are the average of three different samples. cause losses of nutrients. The slurry added in such pits will lose moisture and can be used as organic manure in the fields at the required time and there will be no problems in its transportation as water added to form the digester feedstock will have leached away. This will, however, lead to some loss of nutrients and the losses of plant nutrients were calculated on the basis of the quantity of the liquid separated out at 6000 rpm in a centrifuge. The losses, in milligrams per kilogram of whole slurry, are shown in Table l and the per cent losses in Fig. 1. The loss of K was the greatest, followed by N, the losses of the other nutrients being insignificant. However, when the percentage losses of individual nutrients were calculated, it was observed that the loss of Zn was next to that of K, but that its total content is very low. Based on these results, it can be concluded that the outlet of a biogas
320
A. P. Gupta, V. K. Gupta, R. S. Antil, Jagan Nath 40 35 30
~2s 0
~ 20
Zn
N
¢J
~ ~0 5 Fig. 1. Losses of nutrients in the liquid from biogas slurry.
plant must be attached to uncemented underground pits. The effluent slurry can be stored in these pits and the excess liquid will leach down and thus help in transportation of the residue to the fields. Moreover, before it is applied in the fields, it will further decompose.
REFERENCES Johal, H. S. & Shukla, A. N. (1980). Adoption of Gobar gas plants in rural areas. Khadi Gramodyog 26, 542-7. Koenig, R. A. & Johnson, C. R. (1942). Colorimetric determination of phosphorus in biological materials. Indust. Eng. Chem. (Analytical), 14, 155-6. Lindner, R. C. (1944). Rapid analytical method for some of the more common inorganic constituents of plant tissues. Pl. Physiol. 19, 76-89. Mulmute, M. G. (1980). Institutional finance for Gobar gas scheme. Khadi Gramodyog, 26, 395-404.
A. P. Gupta, V. K. Gupta, R. S. Antil & Jagan Nath Department o f Soils, Haryana Agricultural University, Hissar-125004 (Haryana), India
Short Communication Possible Losses of Plant Nutrients from Biogas Slurry
ABSTRACT One of the major problems in the e[ficient running of biogas plants & proper management of their slurry due to the increased volume of cattle dung ]t'om 1 to 1.6-1.7 times. There[ore, a laboratory study was undertaken to determine the contents of some plant nutrients in the liquid and semi-solid portions. A known weight of" slurry was centrifuged at 6000 rpm for hall an hour and the liquid and semi-solid portions were separated and weighed. The slurry as a whole, liquid and semi-solid portions, was analysed[or its N, P, K, Zn, Cu, Mn and Fe content. It was observed that if the slurryjrom the biogas plant is collected directly into the underground manure pits and the excessive liquid is allowed to leach down, then the losses of K, Zn, N, Cu, Mn, Fe and P would be 36.5, 25.0, 21.5, 14.8, 5.9, 4.9 and 2.7 %, respectively. Moreover, the slurry could be [urther decomposed and applied easily in the fields.
INTRODUCTION The energy crisis and the rising price.s of oil have required scientists to look into alternative sources of energy from natural resources. Cattle dung has an exploitable heating value but, at the same time, its manurial value cannot be ignored. In India more than 1200 million tonnes of dung become available each year, of which about 400 million tonnes is burnt directly (Johal & Shukla, 1980). If this dung could be utilized for the production of biogas then, as well as its utilization as a source of energy, the treated slurry could be used as a source of organic manure. The heat 317 Agricultural Wastes 0141-4607/85/$03.30 © Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain
318
A. P. Gupta, V. K. Gupta, R. S. Antil, Jagan Nath
efficiency of cattle dung is also increased by about 11 ~o by converting it into biogas (Mulmute, 1980). Despite an adequate subsidy for the installation of biogas plants, they have not been generally installed because of several problems. The major drawback is in the utilization or disposal of the treated slurry due to increased water content. The slurry is partially decomposed but needs further decomposition before its use as an organic manure. Sun drying of slurry leads to volatilization losses of plant nutrients and a crust is also formed on the surface which inhibits drying. For its complete decomposition it has to be collected in underground uncemented pits and the liquid portion is bound to leach down from these pits and cause losses of plant nutrients. Little information is available in the literature regarding such losses from biogas slurry. In the work reported here, therefore, the biogas slurry was separated into liquid and semi-solid forms and analyzed to assess the possible losses of some plant nutrients if the liquid portion were removed.
METHODS Fresh effluent slurry from three biogas plants (Gobar digesters) was collected. A weighed quantity of slurry was centrifuged at 6000 rpm for 30 min and the liquid was separated and weighed. The semi-solid portion was dried in an oven at 60 _+ 2 °C to constant weight to determine the moisture content. The dried slurry was ground to pass through a 2 mm sieve in order to obtain a uniform sample of the material. The liquid and dried semi-solid portions were digested in a H 2S 0 4 : H C 1 0 4 (9:1) mixture. Nitrogen in the digest was estimated by the method of Lindner (1944). Phosphorus was determined by the method of Koenig & Johnson (1942). Total K was estimated with a flamephotometer and Zn, Mn, Cu and Fe with an atomic absorption spectrophotometer. The concentrations of nutrients in the semi-solid portion and slurry were calculated to their original wet state.
RESULTS A N D DISCUSSION The ratio of the semi-solid to liquid portions was from 1:1.6 to 1:1.7. The moisture content of the semi-solid portion after centrifugation was 84-4 ~o
319
Losses oJ plant nutrients Jrom biogas slurry
to 84"6 ~o. The cattle dung contains a b o u t 80 ~o water, which is similar to the water content of the semi-solid portion of the slurry, and the increased water in the whole slurry was caused by the addition of diluted dung to the digester. Because of this increased water content, transportation of slurry to the fields is more difficult than transportation of fresh manure. The plant nutrient contents of the liquid, semi-solid and whole slurry are shown in Table 1. If the slurry as a whole is put in underground pits then the liquid portion will leach down through the sides and bottoms of the pits and so TABLE !
Composition of Some Plant Nutrients in Different Portions of Biogas Slurry and Losses in the Liquid Portion Nutrients
N P K Zn Mn Fe Cu
Composition (mg kg- 1) Liquid SemiWhole solid slurry
490.0 22-5 2 213"0 4'9 1-1 8.8 0.7
3 038"0 1 366"0 6 549"0 25"3 29.9 289-1 6.2
1434.0 520-0 3 819.0 12.4 11.8 112.6 2.7
Losses in liquid portion (mg kg- 1 wet slurry)
308.5 14.2 1393.4 3.1 0.7 5.5 0.4
The values are the average of three different samples. cause losses of nutrients. The slurry added in such pits will lose moisture and can be used as organic manure in the fields at the required time and there will be no problems in its transportation as water added to form the digester feedstock will have leached away. This will, however, lead to some loss of nutrients and the losses of plant nutrients were calculated on the basis of the quantity of the liquid separated out at 6000 rpm in a centrifuge. The losses, in milligrams per kilogram of whole slurry, are shown in Table l and the per cent losses in Fig. 1. The loss of K was the greatest, followed by N, the losses of the other nutrients being insignificant. However, when the percentage losses of individual nutrients were calculated, it was observed that the loss of Zn was next to that of K, but that its total content is very low. Based on these results, it can be concluded that the outlet of a biogas
320
A. P. Gupta, V. K. Gupta, R. S. Antil, Jagan Nath 40 35 30
~2s 0
~ 20
Zn
N
¢J
~ ~0 5 Fig. 1. Losses of nutrients in the liquid from biogas slurry.
plant must be attached to uncemented underground pits. The effluent slurry can be stored in these pits and the excess liquid will leach down and thus help in transportation of the residue to the fields. Moreover, before it is applied in the fields, it will further decompose.
REFERENCES Johal, H. S. & Shukla, A. N. (1980). Adoption of Gobar gas plants in rural areas. Khadi Gramodyog 26, 542-7. Koenig, R. A. & Johnson, C. R. (1942). Colorimetric determination of phosphorus in biological materials. Indust. Eng. Chem. (Analytical), 14, 155-6. Lindner, R. C. (1944). Rapid analytical method for some of the more common inorganic constituents of plant tissues. Pl. Physiol. 19, 76-89. Mulmute, M. G. (1980). Institutional finance for Gobar gas scheme. Khadi Gramodyog, 26, 395-404.
A. P. Gupta, V. K. Gupta, R. S. Antil & Jagan Nath Department o f Soils, Haryana Agricultural University, Hissar-125004 (Haryana), India