Kinetic Studies of Biogas Evolved

from Water hyacinth

 

 

F. Shoeb, H. J. Singh

 

City Convent School

Miyan Bazar (East)

Gorakhpur – 273 001 (UP)

INDIA

E-mail: farzana@nde.vsnl.net.in

 

 

 

Paper Accepted for Oral Presentation

and Published in

 

 

AGROENVIRON 2000

 

 

2nd International Symposium on New Technologies

for Environmental Monitoring

and Agro - Applications

 

 

 

PROCEEDINGS

 

 

18 – 20 October 2000

Tekirdag / Turkey

 

 

 

 

 

 

 

KINETIC STUDIES OF BIOGAS EVOLVED FROM WATER HYACINTH

 

F. Shoeb*, H .J. Singh**

City Convent School, Miyan Bazar (East), Gorakhpur – 273 001 (UP), India

Email: farzana@nde.vsnl.net.in

 

 

ABSTRACT

 

Water Hyacinth - a native of South America is abundantly found in India, Bangladesh, South East Asia and in the Philippines Islands. Under favorable conditions a growth rate as high as 17.5 metric tons of wet Water hyacinth per hectare per day has been reported. Due to vegetative reproduction it spreads rapidly clogging drainage, ditches, shedding out other vegetations and interfering with shipping and recreation.

The concept of using aquatic plants for conversion to energy (methane) is gaining attention in tropical and sub tropical regions of the world where warm climate is conductive to the plant growth through out the year. Anaerobic digestion of organic matter is the oldest method for disposing the waste. The anaerobic digestion of animal, agricultural and industrial wastes has been widely studied. However, very little work has been done using aquatic plants particularly Water Hyacinth.

The present paper deals with the kinetics of gas produced from Water Hyacinth. The study was done in a batch fed digester. Attempts have been made to reach at an optimum condition for the production of maximum amount of gas by the addition of lower volatile fatty acids, Cow dung and inoculums etc. The important and useful results that was drawn from the study is that we can run the biogas plants even in the cold winter nights by using certain additives. After digestion of Water Hyacinth inoculums can be used as good manure for soil fertility, which is free from harmful chemicals, which is a boon for sustainable agriculture practices.

 

 

KEY WORDS: BIOMASS, BIOGAS, ANAEORABIC DIGESTION, WATER HYACINTH, KINETICS, BIOENERGY, NON CONVENTIONAL ENERGY, RENEWABLE ENERGY.

 

 

INTRODUCTION

 

The socio-economic development of a country largely depends on the availability and consumption of energy. The available sources of energy can be classified into two categories viz., non – renewable and renewable. The non -renewable Sources of energy are finite deposits of coal, natural gas, U235 and deuterium which are also called fossil fuels. The renewable sources of energy are photosynthesis, solar energy, hydroelectric and tidal powers. In the renewable account energy is being deposited everyday whereas in the non- renewable account energy deposits are continuously depleted by our withdrawals. Because of continuous depletion in the natural resources by an increased consumption of the energy, alternatives of fossil fuels has to be searched out. Therefore, bioenergy is the only alternative and cheap source of energy which can be made available to the rural areas of the country.

 

Biomass

 

The term biomass is understood to mean all land and water plants, their wastes or by-products resulting from the transformation of these plants. There are different kinds of biomasses like animal waste, crop and agricultural residues, forest products and forestry residues and industrial wastes. The others are marine and aquatic biomass. Aquatic macrophytes grow rapidly and are considered as nuisance plants. Among floating macrophytes, Water hyacinth (Eichornia crassipes), water lettuce and pennywort are found to be most productive compared to small leaf floating plants.

 

Water Hyacinth as a Source of Energy

 

Due to vegetative reproduction and extremely high growth rate Water hyacinth spread rapidly clogging drainage, ditches, shading out other aquatic vegetation and interfering with shipping and recreation. Water hyacinth has attracted the attention of scientists to use it as a potential biomass for he production of biogas because of its high growth yield and availability in large amount throughout the year and allover the world.

 

Anaerobic digestion of organic matter is the oldest method for disposing the waste. The anaerobic digestion of animal (Jain, Singh, Tauro, 1981; Shelat, 1983) agricultural (Dunlop, 1975; Jewell, Dell’ Orto, Fanfoni, Fast, Jackson, Kabrick, Gottung, 1981; Jewell, Dell’ Orto, Fanfoni, Fast, Gottung, Jackson, Kabrick, 1982; Jewell, 1981; Wujcik, Jewell, 1981; Edi Iswanto Wiloso, Triadi Basuki, Syahrul Aiman, 1995) and industrial wastes(Unni, Pillai, Singh, 1987)  has been widely studied. However, very little work has been done using aquatic plants particularly the Water hyacinth.

 

The kinetics for a blend of Water hyacinth – grass municipal wastes has been studied by Ghosh et al (Ghosh, Henry, Klass, 1980). Wolverton and Mc Donald ( Wolverton, Mc Donald, 1981) have suggested that anaerobic filter technique provides a large surface area for the anaerobic bacteria to establish and maintain an optimal balance of facultative, acid forming and methane producing bacteria. Unni et al (Unni, Pillai, Nigam, singh, Baruah, 1981) have reported that in the anaerobic digestion of Water hyacinth imperfect mixing of the content of the digester has no impact on the overall digestion rate. Vaidyanathan et al (Vaidyanathan, Kavadia, Shroff, Mahajan, 1985) have reported that higher methane production rate and lower retention time are obtained with ground Water hyacinth in comparison to chopped. Further they have found that the gas obtained from ground Water hyacinth had higher methane content.

 

During the present investigation a systematic study on the kinetics of biogas evolution from Water hyacinth is undertaken. The study was further elaborated by using a blend of Water hyacinth and Cow dung in different proportion and attempts have been made to reach at an optimum condition for the production of maximum amount of biogas.

 

 

EXPERIMENTAL

 

Kinetics of biogas evolved during the anaerobic degradation of Water hyacinth was done in a batch-fed digester. A schematic diagram of the experimental set up used is shown in figure 1. It consists of a 500 ml capacity glass bottle A which serves as a digester. The glass bottle A is connected to a gas collector B through a three way stop cock. The collector B is made from a glass tube of 1.0” i.d. and 4’ length and is connected to a water leveler C with the help of a polythene tubing. A narrow strip of graph paper was pasted on glass collector B and the later was calibrated between the two points B1 and B2. In order to do the calibration the leveler C was detached at the bottom and a rubber tube with a pinch- cock was fitted. Water was filled from the top of the column so that the water level could reach the point B1 was noted with cathetometer. 100 ml of water was taken out from column B by opening the pinch-cock at the bottom and the new level of water column was read. The difference of the two readings on the cathetometer would give the corresponding length measured on the glass column for 100 ml. Subsequent withdrawals of 100 mls were made and the positions of the levels on the glass columns B were determined and marked on the graph paper strip. The process was repeated till the water level reached just above the point B2.  The calibration was repeated three times. A plot between the water column height measured from the reference point B1 and volume was made. Plot shows a linear relationship between the column heights and the volume. After getting the calibration done the leveler C was attached with the glass column B at the bottom. The volume of gas evolved during a particular time interval was measured over water at atmospheric pressure by leveling the water column levels in columns B and C. The distance between the two levels was noted on the graph paper and converted into corresponding volumes by multiplying with the calibration factor. The digester was kept in an air thermostat maintained at a particular temperature. A series of batch-fed reactors were placed together for studying the kinetics under varying conditions. The following parameters have been studied:

 

 

Figure 1: Experiment Setup for Studying the Biogas Evolution

 

Effect of Dilution of Water Hyacinth

 

In a fixed amount of Water hyacinth water was added in the ratio (by wt.) Water hyacinth: water as 1:1, 1:3 and 1:5.

 

Effect of Addition of Lower Volatile Fatty acids

 

Acetic acid was chosen as a representative lower volatile fatty acid. Different amount of acetic acid was added in the system containing Water hyacinth.

 

Effect of Addition of Cow Dung

 

Different amounts of Cow dung (Gobar) in the range of 2-10 gm were added in the systems containing 50 gm of chopped Water hyacinth in 250 ml of water. The kinetics has also been followed by keeping the total volatile solid (VS) content of Water hyacinth and Cow dung (Gobar) fixed.

 

Effect of Inocculum

 

Kinetics has also been studied by using inoculums of different incubation period. In order to prepare inocculum the following procedure was adopted. About 20 liters of slurry was made by mixing Cow dung and water in the ratio 4:5. About 2 litres of the slurry was distributed in 6 glass bottles of about 2.5 litres capacities. The glass bottles were fitted with a rubber cork having one hole. In the hole a glass tube was inserted which remained above the layer of the slurry. The other end was connected with Teflon tubing, the outlet of which was dipped in a container filled with water. With this set up the gas produced during the incubation period could bubble through water but no air would enter into the slurry thus, maintaining the anaerobic condition. This arrangement was made for all the six bottles. These bottles were kept in an air thermostat maintained at 40° C.  After the expiry of the desired incubation period the bottles were opened and the contents were filtered through 60-mesh sieve. The filtrate was used as inoculums of 5, 15, 20, 30 and 40 days incubation periods were prepared and the following systems were made for detailed investigation.

 

 

Further, varying amounts of inoculums of 15 and 20 days were taken at a fixed amount of Water hyacinth and

the kinetics was followed.

 

pH Measurement

 

The change in the pH of the biomass during anaerobic degradation was measured. The experimental setup consists of a 500 ml capacity conical flask. 250 ml slurry of a particular composition was added into it. The flask was capped with a rubber cork having two holes. Through one hole the electrode (combination type) was inserted and was dipped in the slurry. The electrode was connected to a pH metre. In the other hole a corning glass tube was fitted in such a way that the end of it remained above the slurry. The other end was connected to a gas collector. The flask was placed in an air thermostat maintained at 40º C. The production of biogas and pH were noted simultaneously as a function of time.

 

 

RESULTS AND DISCUSSION

 

Results plotted in fig. 2 show that blending of Water hyacinth in water is critical for producing maximum amount of biogas in a minimum time period. The ratio of 1:5 by weight for Water hyacinth and water has been found to produce maximum amount of gas. The Water hyacinth has a leafy structure. If sufficient amount of water is not added the biomass would not be soaked enough to go through the degradation process efficiently and hence a less amount of biogas is produced. Thus, detailed kinetic studies were performed using a blend of Water hyacinth and water in the ratio of 1:5 by weight. The results obtained from Cow dung slurry under the same condition are also plotted in fig. 2. The results show that Water hyacinth produce more gas per gm biomass fed as compared to Cow dung. This is true if the results are plotted per gm volatile solid because the percentage of total volatile solid in Water hyacinth and Cow dung has been determined to be almost the same. This is due to the fact that substrates for methanogenic bacteria are readily available from leafy plants such as Water hyacinth. The production rate of biogas from the same amount of Cow dung and Water hyacinth is plotted in fig.3. The results show that gas production rate from Water hyacinth is higher as compared to pure Cow dung slurry. However, the time period for attaining the maximum production rate is longer (about 40 – 50 days) for Water hyacinth as compared to Cow dung (20 – 30 days). This is due to the fact that bacteria needed for biogas production in the case of Water hyacinth takes a longer time period to grow whereas in ruminants waste such as Cow dung pathogens are already present and bacterial growth takes a little time for biogas production.

 

 

Figure 2: Effect of Dilution of Biomass on Gas Evolution Kinetics.

 

×, 50gm Water Hyacinth + 50 ml Water , 50gm Water Hyacinth + 100 ml Water¡, 50 gm. Water Hyacinth + 250 ml. Water, Ф Cow Dung 50 gm+ 250ml Water

 

 

Figure 3: Rate of Biogas Production from Water Hyacinth and

 

Cow Dung with 50 gm Biomass in 250 ml Water.•, Water Hyacinth, × , Cow Dung

 

The addition of lower volatile fatty acid such as acetic acid has a remarkable effect on the total amount of gas produced during a particular time interval as well as on the gas production rate. The results plotted in fig. 4 show that an amount as little as 0.6 ml (10% by volume) of acetic acid in 250 ml of the blend of 50 gm of Water hyacinth and water increases the gas production by about one and a half time. The rate of production of biogas is also increased by more than twice as shown in fig. 5. However, addition of a larger amount of acetic acid (2 ml of 10% by volume) reduces the gas production rate as shown in fig. 5. This may be assigned to the fact that most of the biogas comes from the methyl group of the lower volatile fatty acid and the process is accomplished under methanogenic condition (Klass, 1984). The survival of the methanogenic bacteria largely depends on the pH of the medium. A larger addition of acetic acid thus makes the medium more acidic reducing the activity of methanogens and hence a lower rate of gas production has been observed. This is in accordance with the general observation made by Mc Carty and  Mc Kinney (Mc Carty, Mc Kinney, 1961) and Andrews (Andrews, Sanit, 1969) that methanogenesis is facilitated in the presence of lower volatile fatty acids in a certain range of concentration.

 

 

Figure 4: Effect of Addition of Acetic Acid on Biogas Production from Water Hyacinth.

 

×, 50gm Water Hyacinth + 50 ml Water, 50gm Water Hyacinth  + 2.0 ml Acetic Acid (10 % by

volume)  + 250 ml Water ¡, 50 gm. Water Hyacinth + 0.6 ml. Acetic Acid (10 % by volume)+ 250 ml Water

 

 

Figure 5: Effect of addition of acetic acid on Biogas Production Rate from Water Hyacinth.

 

 l, 50 gm Water Hyacinth + 250 ml water; ¡, 50 gm Water Hyacinth + 2.0 ml Acetic Acid (10 % by

 volume) + 250 ml water; ×, 50 gm. Water Hyacinth + 0.6 ml. Acetic Acid (10 % by volume)+ 250 ml water

 

Results are also plotted to show the effect of mixing of Cow dung in the blend of Water hyacinth and water. It is found that the addition of Cow dung increases the gas production as well as it lowers the induction period. This may be assigned to the fact that the total volatile solid content of the system increases with the increased amount of Cow dung. The lowering of the induction period in the case of Cow dung + Water hyacinth system is due to the fact that the bacteria responsible for the degradation of biomass is facilitated by the addition of Cow dung.

 

Thus, the studies performed on the systems keeping the total volatile solid content fixed in the blend of Water hyacinth + Cow dung + water, the ratio of 2:3 by weight of Water hyacinth to Cow dung is found to be most suitable for the biogas production. The results plotted show that the production rate of such a system is more than double as compared to the Water hyacinth or the Cow dung systems taken separately under identical conditions.

 

Inoculums of different incubation periods have remarkable effect on biogas production. The total amount of gas produced at the end of decomposition is higher with inoculums of 15 and 20 days as shown in fig. 6. However, the rates of production of biogas while using inoculums of 20 and 30 days are higher as shown in fig. 7. Thus, it is inferred that inoculums of incubation period of 20 and 30 days are the best for the maximum production of biogas during a particular time period. It attains a maximum rate of 0.25 liter/day during the 5th day of gas production in comparison to a much longer time period of 20 to 30 days at a much lower rate of 0.045 litre / day in the case of Cow dung + Water hyacinth system with a 50 gm of biomass in each case. The total amount of gas produced depends on the amount of volatile fatty acid decomposed, which in turn would depend on the number of methanogenic bacteria present in the system. If the inoculums are kept for a longer period the amount of substrate does not increase further. However, there is a possibility of decreasing number of methanogenic bacteria in the inoculums of longer period because they are active only during a particular time period (Hobson, Bousfield, Summers, 1983). The less the bacterial count the lesser would be the gas production. The results plotted show that the gas production increases as the amount of inoculums is increased. The tendency of the curves towards a constant value shows the total amount of the gas available from the systems chosen for detailed studies. The values are different for each system and increase as the amount of inocculum is increased. This is probably due to the fact that in the inocculum prepared from Cow dung slurry some lower volatile fatty acids are also present and a large amount of inocculum would have greater amount of such acids which in turn produces biogas during methanogenesis. However, the rate of production of biogas does not depend on the amount of inocculum used.

 

 

 

 

 

 

 

 

 

 

 

 

Figure 6: Effect of Incubation Period of Inocculum on Gas Evolution Kinetics

 

¡, WH 50gm          +             Inocculum 250ml – Incubation Period 5 days

r, WH 50gm          +             Inocculum 250ml – Incubation Period 15 days

×, WH 50gm           +             Inocculum 250ml – Incubation Period 20 days

£, WH 50gm          +             Inocculum 250ml – Incubation Period 30days

Ф, WH 50gm          +             Inocculum 250ml – Incubation Period 40days

 

 

 

 

 

 

 

 

 

 

 

Figure 7: Biogas Production Rate (Conditions are the same as in figure 6)

 

The pH change during the course of the degradation of Water hyacinth is shown in fig. 8. During the initial period of degradation there is a decrease in the pH showing thereby that first stage of anaerobic degradation leads to the production of acids. The results plotted in figure 8 also show that most of the biogas in the system containing 50 g Water hyacinth/250 ml water is produced once the pH of the system stabilizes around 7.0.

 

 

 

 

 

 

 

 

 

 

Figure 8: pH Change and Gas Evaluation During Anaerobic Degradation of  Water Hyacinth.

 

System contains 50 gm W.H. + 250 ml water

 

 

 

 

The kinetic date has been fitted with the help of an integrated form of an equation.

 

 

                                                                                                                                            [1]

 

 

Where a, b and c are constants. The analysis of the kinetic data with the help of the above equation was done in two parts.

 

Ø       The system undergoing its own incubation for the generation of microbes, and

Ø       The systems in which microbes of different incubation periods were added which reduced the incubation period of biogas generation to almost zero.

 

In the first case microbial growth is the limiting factor. Under such a condition cV << 1, where c is related to the number of microbes present. In such a situation Eq. [1] can be written as

 

 

                                                                                                                                                                    [2]

 

 

which could be reduced to

 

 

                                                                                                                                                                                                                     [3]

 

 

Plots of (1/V) (dV/dt) vs. V are made for the systems viz., (1) Water hyacinth + water, (2) Cow dung slurry and (3) Water hyacinth + Cow dung + water as shown in figs. 9-11. Straight lines are obtained which yielded the values of constants a and b as listed in Table 1. A close examination of the results show that a is related to the amount of substrate available for conversion into methane and b is related to the total amount of biodegradable substance available from feed for acidogenesis/hydrolysis leading to the substrates for conversion into biogas. In the case of pure Water hyacinth and pure Cow dung under the same condition the value of a is almost the same. However, the value of b in the case of Water hyacinth is almost two times less. This is in accordance with the analysis of Water hyacinth and Cow dung. The easily biodegradable matter particularly hemi-cellulose is higher in Water hyacinth than Cow dung (Robbins, Armold, Weiel, 1983). Further, the lignin content, which is not easily degradable, is higher in Cow dung slurry and thus yielding a lower value of b.

 

 

 

 

 

 


 

Figure 9: Plot of 1/V (dV/dt) Vs V for the System Containing Water Hyacinth 50gm in 250ml Water

 

 

Figure 10: Plot of 1/V (dV/dt) Vs V for the System Containing Cow Dung 50gm in 250ml Water

 

 

 

 

Figure 11: Plot of 1/V (dV/dt) Vs V for the System Containing Water Hyacinth and Cow Dung in the Ratio of 2:3 in 250ml Water


 
Table 1: Values of a and b for Various Systems

 

                      System

Water hyacinth  +    Cow dung      +             Water                             a                             b

         (gm)                      (gm)                            (ml)

 

 

 

 

 

50

0

250

0.07

0.051

20

30

250

0.09

0.048

0.0

50

250

0.08

0.105

 

The second situation arises when the microbes are already present in plenty such as the systems containing inoculums. In such a situation cV >> 1 where the availability of substrate by acidogenesis / hydrolysis is the limiting factor. Thus, Eq. [1] reduces to the form

 

 

                                                                                                                                                                                                                                                 [4]

                               

Where a’ = a/c and b’ = b/c

 

Plots of dV/dt vs. V are plotted. Straight lines are obtained showing the validity of Eq. [4] in the case of systems utilizing inocculums of different incubation periods. From these plots values of a’ and b’ are evaluated and listed in Tables 2 a-c for various systems.

 

Table 2a: Values of Constants a' and b' Using Inocculums of Different Incubation Periods

Water Hyacinth = 50 gm; Amount of  Inocculum = 250 ml

Incubation period

(Days)

a’

b’

 

 

 

15

0.22

0.056

20

0.27

0.075

30

0.23

0.074

40

0.23

0.075

Table 2b: Values of Constants a' and b' at Varying Amount of Inocculums of 15 days Incubation Periods, Water Hyacinth = 50 gm

 

Amount of Inocculum

(ml)

a’

b’

 

 

 

100

0.142

0.045

200

0.190

0.045

250

0.220

0.056

300

0.240

0.060

Table 2c: Values of Constants a' and b' at Varying Amount of Inocculums of 20 days Incubation Period,

Water Hyacinth = 50 gm

 

 

Amount of Inocculum

(ml)

 

a’

 

b’

 

 

 

100

0.192

0.060

200

0.258

0.075

250

0.272

0.075

300

0.308

0.075

 

The values of a’ are almost the same for the inoculums of different incubation periods provided the amount of inoculums used is the same. This is due to the fact that the numbers of microbes present in the system are sufficient high to convert the substrate available into biogas. However, increasing the amount of inocculum at a fixed incubation period increases the values of a’ and b’. This may be assigned to the fact that along with the microbes some substrates are present because inoculums are taken from Cow dung slurry, the content of which increases with increasing amount.

 

CONCLUSIONS

 

1.        The total amount of gas produced from Water hyacinth is about one and a- half time higher than the Cow dung per gm volatile solid.

 

2.        A blend of Water hyacinth and Cow dung in the ratio of 2:3 by weight is most suitable for biogas production.

 

3.        Addition of very little amount of lower volatile fatty acid particularly acetic acid facilitates the gas production. This finding is very much helpful at the village level for the farmers using biogas plants. They face a great difficulty in production during the cold winter nights. During the same season most of the farmers in the sugar belt crush cane to produce GUD or thick syrup. If the leftover of the process of making Gud or cane sugar juice is kept for fermentation for a few days the content will be highly rich in acetic acid. The addition of this left over would circumvent the problem of lower gas production during the cold winter nights and biogas plants could run successfully during all the seasons.

 

4.       The rate of production of biogas from Water hyacinth is higher as compared to Cow dung slurry. However, the fermentation process takes a longer time period in the case of Water hyacinth. The kinetic studies performed with Water hyacinth + inocculum show that gas production rate increases twelve times in a very short period of five days in comparison to Cow dung + Water hyacinth (20 – 40 days) systems.

 

5.       The digested slurry can be used as useful chemical free eco-friendly manure.

 

 

REFERENCES

 

 

1.        Andrews, J.F., Sanit., J., 1969, Eng. Div., Proc. Amer. Soc. Civ. Eng., 95, p. 95.

2.        Dunlop, C.E., 1975, In: Single Cell Protein II (Eds. S.R. Tannenbaum and D.I.C. Wans), MIT Press, Cambridge, Mass .,pp. 244-262.

3.        Edi Iswanto Wiloso, Triadi Basuki, Syahrul Aiman, 1995. Utilization of Agricultural Wastes in Indonesia, Proceedings of the UNESCO –University of Tsukuba International Seminar on Traditional Technology for Environmental Conservation and Sustainable Development in the Asian Pacific Region, (Eds. Kozo Ishizuka, Shigeru Hisajima, Danyl R.J. Macer), Tsukuba Science City, Japan, 11-14 December, pp. 134-138.

4.        Ghosh, S., Henry, M.P., Klass, D.L., 1980, Conversion of Water Hyacinth-coastal Bermuda grass MSW sludge blends to methane, Biotechnology and Bioengineering Symp. No. 10, pp.163-187.

5.        Hobson, P.N., Bousfield, S., Summers, R., 1983, Methane Production from Agricultural and Domestic Waste, Applied Science Publishers Limited, London, Chapter 3.

6.        Jain, M.K., Singh, R., Tauro, P., 1981, Anaerobic digestion of cattle and sheep wastes, Agricultural Wastes, Vol. 3, pp. 65-73.

7.        Jewell, W.J., 1981, New Approaches in anaerobic digester design, Proceedings of the International Gas Research Conference, Los Angeles, California, 30 September, pp. 796-808.

8.        Jewell, W.J., Dell’ Orto, S., Fanfoni, S., Fast, S.J., Gottung, E.J., Jackson, D.A., Kabrick, R.M., 1982, Agricultural and high strength wastes, Proceedings of the Second International Symposium on Anaerobic Digestion, (Ed. Hughes D.E.) Elsevier Biomed:, Amsterdam, The Netherlands, pp. 151-168.

9.        Jewell, W.J., Dell’ Orto, S., Fanfoni, S.J., Fast, S.J., Jackson, D.A., Kabrick, R.M., Gottung, E.J., 1981, Crop residues conversion to biomass by dry fermentation, Paper No. 81-3573, Presented at the Winter meeting of the Amer. Soc.of Agri. Engineers, 15-18 Dec.

10.     Klass, D.L., 1984, SCIENCE, Vol. 223, No. 4640, pp. 1021-1028.

11.     Mc Carty, P.L., Mc Kinney, R.E., 1961, J. Water Pollution Control Fed., 33, p. 223.

12.     Robbins, J.E., Armold, M.T., Weiel, J.E., 1983, Anaerobic Digestion of Cellulose Dairy Cattle Manure Mixture, Agricultural Wastes, Vol. 8, pp. 105-118.

13.     Shelat, R.N., 1983, Performance Studies on Biogas plant Models, Alternative Energy Sources V, Part D: Biomass/Hydrocarbons/Hydrogen (Ed. T.N. Veziroglu), Elsevier Science Publishers B.V., Amsterdam, The Netherlands, pp. 93-102.

14.     Unni, B.G., Pillai, K.R., Nigam, J.N., Singh, H.D., Baruah, J.N., 1981, Steady state kinetics of biogas production from Water Hyacinth in unstirred reactor, Proceedings of the National Seminar on biogas Technology, USG Publishers and Distributors, Ludhiana, India, 9-11July, pp. 27-33.

15.     Unni, B.G., Pillai, K.R., Singh, H.D., 1987, Production of biogas from sugarcane press mud- laboratory scale and pilot plant studies, Energy Management, Vol. 11, no. 1, pp. 23-31.

16.     Vaidyanathan, S., Kavadia, K.M., Shroff, K.C., Mahajan, S.P., 1985, Biogas production in batch-fed and semi continuous digester using Water Hyacinth, Biotechnology and Bioengineering, Vol. XXVII, pp. 905-908.

17.     Wolverton, B.C., Mc Donald, R.C., 1981, Energy from Vascular Plant Waste Water Treatment Systems, Economic Botany, 35(2), pp. 224-232.

18.     Wujcik, W.J., Jewell, W.J., 1980, Dry anaerobic fermentation, Biotechnology and Bioengineering Symposium No. 10, pp. 43-65.