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.
- Water Hyacinth 50gm + Inocculum 250ml (Incubation Period –5 days)
- Water Hyacinth 50gm + Inocculum 250ml (Incubation Period –15 days)
- Water Hyacinth 50gm + Inocculum 250ml (Incubation Period –20 days)
- Water Hyacinth 50gm + Inocculum 250ml (Incubation Period –30 days)
- Water Hyacinth 50gm + Inocculum 250ml (Incubation Period –40 days)
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.
