DURING
early 1970s the government decided to give thrust
to the generation of gobar gas (renamed after
1982 as biogas) by anaerobic digestion of gobar
(cow dung) for meeting fuel needs of rural population.
This mode was expected to bring economic uplift
and social welfare of the rural sector. Both family
size and community size models were launched and
appropriately subsidised. The task for the development
and propagation of the mode was entrusted to PAU
and the Department of Agriculture, Punjab.
During the initial 17 years (up to 1988) after
the launch of the project, the total number of
biogas plants built were 16,878. The September
1988 heavy rain and consequent floods damaged
more than 3,000 biogas plant which needed total
rebuilding.
In March, 1990, an assessment of the community-size
biogas plant (CBPs) was carried out and it was
found that till then 120 CBPs were sanctioned
and out of which only 68 had been completed and
commissioned. The performance efficiency of the
68 commissioned since was eight CBPs "closed"
or "non-functional", 15 operating at
"connection capacity (CC)" of less than
25 per cent, 43 operating at the CC of 50 per
cent. For this plight of the CBPs, as obtained,
not only the poor management but also non-existence
of suitable infrastructure needed for technology
improvement, were considered to be responsible.
Thereafter, there has not been any significant
change in the situation even though the "management
of CBPs" was passed on to PEDA (Punjab Energy
Development Agency).
Now the Punjab Government was to put a CBP at
Kaljharani village in Bathinda district which
would be part of the total rural employment project.
For the project at present Rs 50 lakh has been
earmarked and the project comprises setting up
a modern dairy complex and a centralised solar
power plant. The CBP will be fed by the dung resulting
from the dairy complex. It is hoped PEDA will
put up the CBP of improved technology than hitherto
adopted for the CBPs so that it gives the targeted
performance. It is proposed to examine the improved
models which have been found successful in India.
Biogas
technology
Anaerobic digestion of cow dung is essentially
a two-phase process. In the first phase, the acid-forming
bacteria hydrolyse and ferment the organic compounds
(such as carbohydrates, lipids, proteins, etc.)
to form organic acids, alcohols and gases of carbondioxide,
nitrogen and traces of hydrogen sulphide. The
environment in which the above activity occurs
is "acidic" with pH ranging from 5.1
to 6.8 and bulk of digestion resulting in the
reduction in volume takes place during this phase.
The bacteria of this phase are "less sensitive
to changes in temperatures".
In the second phase, a "consortium of bacteria"
work upon the organic acids produced during the
earlier phase to produce methane gas. This group
of bacteria is relatively slow performing and
gives optimum performance i.e. amount of methane
gas production when the prevailing temperature
is around 30°C and the environment alkaline
(pH ranging from 7.2 to 7.4). If there is rise
or fall of temperature more than 10°C from
the optimum value listed above, the bacteria stop
working and methane gas production stops altogether.
In the model adopted for the CBPs so far i.e.
the KVIC (Khadi Village Industries Corporation)
Model, which comprises a masonry tank to accommodate
cow dung slurry, and a floating steel cylinder
for storage of biogas produced. The set up provides
anaerobic digestor with Œdetention period‚
of around 60 days. In this model both phases take
place in one vessal. The gas produced is at low
pressure and theoretically should have mix ratio
of methane (60 per cent) and carbondioxide (40
per cent). The peak efficiency of the "best
designed and operated model" seldom exceeds
40 per cent. The effluent from the digestor is
black in colour, having offensive odour and large
percentage of moisture (more than 80 per cent)
and it is very difficult to remove water from
the slurry or its "drying" in pits.
The methane gas content has been found to vary
with feed material and also during a day. It may
go down to 50 per cent even. The chlorofic value
of the gas has been found to be around 650 BTUs
per ft3 or 5735 K Cal per m3.
As already mentioned, both phases are accommodated
in a single chamber and it is presumed that in
most of the area and for most of the time alkaline
environment is obtained and acidic activity is
limited near the inlet of the cow-dung feed or
slurry. Each time fresh slurry is added a shock
results in the alkaline environment. So it is
essential to carefully regulate the frequency
of loading (to once per day) and also to control
the dose, mix ratio and rate of loading. The dose
is kept between 2 and 3 per cent of the total
volume of slurry held in the digester so that
alkaline environment is not disturbed and an equilibrium
state prevails for methanogenic stage activity
to take place.
If the equilibrium is disturbed in the digester
foaming trouble develops in the digester. This
may be on account of the presence of inhibitors
in the feed (like ammonia, antibiotics, etc.).
Excess ammonia may be obtained if pig dung is
added and the antibiotics may be from the dung
of sick animals under treatment.
Though some attempts have been made to prevent
drop of temperature in the digestor during winter
to prevent a drop in the methane gas production,
yet these have been with limited success.
Characteristics
of a good CBP
A good biogas plant must optimise the following
four functions:
i) gas production rate
ii) methane gas concentration in the biogas produced
iii) stability of the process
iv) lower overall cost of production of biogas.
For this two-phased digester system in which phases
I and II are achieved in two separate digesters,
which has been found to optimise all the four
functions, is considered next.
Conclusion
It is hoped that PEDA learns a lesson from previously
obtained performance of the CBPs, and would not
go in for the same i.e. KVIC model, but should
introduce a "state-of-the-art" design
at the Kaljharani complex.
Biogas
(Gobar) Technology top
