The people living in poor and rural areas of developing counties always
facing the problem of basic energy therefore renewable energy sources are
necessary to mitigate this problem. In villages, people use firewood as a fuel
for heating and cooking purposes but it produces pollution and smoke that are
dangerous for human health. Firewood also contributes in deforestation which
causes distortion or imbalance in the ecosystem. Rural areas inhabitants can
use biogas as a replacement for cooking gas to reduce their gas bill. Biogas
consists of methane CH4 (55-70%), CO2 (25-50%), H2O
(1-5%), H2S (0-0.5%), N2 (0-5%) and NH3
(0-0.05%). Methane gas produce from a biogas digester in anaerobic condition
(absence of oxygen) however, operating a large digester in a densely populated
places and insufficient areas can be challenging due to inefficient waste
management systems. On the other hand, using a small-scale portable biogas
digester could overcome these problems. In this work, low cost portable biogas
digester is designed and evaluated for biogas production from natural sources
available in India such as cow dung, food waste and plant waste.This design, while applicable for a typical rural
Indian family and can also be utilized globally.
A.
Benefits of Biogas
·
Biogas systems produce clean
energy for household use. After an initial investment on biogas plant there is
no need to spend more money on fuel and no more smoke and pollution produce
from wood or charcoal.
·
Biogas makes cooking quicker
and easier than cooking with firewood.
·
A farm with a biogas system
is a cleaner and safer place, it kills the bacteria in livestock manure.
·
Biogas system produces
excellent and safe fertilizers for the agriculture purpose.
·
Biogas systems can play
major role against global warming by allowing to burn methane from organic
waste, instead of letting it escape into the atmosphere where it adds to the
greenhouse effect.
B.
Biogas
Digester
Biogas produce by decomposition of organic waste in a biogas digester. The size
of a digester can vary according to its application and capacity from a small
household system to a large commercial plant having thousands cubic meters.
Farmers mostly use cow dung as a feedstock to feed into digester. Two simple
biogas digester designs have been developed, the fixed dome type digester and
the floating dome digester. The digestion process is same in both digesters but
the gas collection method is different. In the floating type dome, the water
sealed cover is provided in the digester in order to stop the gas leakage when
the floating dome rises as gas is produced and acts as a storage chamber,
whereas the fixed dome type has a lower gas storage capacity and requires good
sealing in order to prevent gas leakage when the high pressure developed by gas
in the digester. Both the digesters have been designed for use with animal
waste, food waste and plant waste. The biomass is fed into the digester via the
inlet pipe and undergoes fermentation process under anaerobic condition in the
digestion chamber. The temperature is quite important factor in digestion process
because bacteria do their work best at temperatures between 30– 60oC. It takes
from 2 to 8 weeks to digest the biomass, depending on the temperature. The
extra slurry or decomposed slurry is removed at the outlet for use as a
fertilizer.
Due to lack of space in the home for biogas
digester, the portable biogas digester
can be installed into the building or building rooftop. The size of digester
will depend on how much biogas needed to meet daily cooking (and lighting)
requirements, the availability and amount of biomass and water available on
place. Portable biogas digesters are built using FRP (Fiber reinforced plastic)
and it can be fixed in a small area. Water jacket models prevent biogas from
leakage are most efficient among different designs. Portable biogas plants
having two portions, one is the digester where fermentation process occurs and
another portion where biogas is collected and stored. Both the portions are separated
using water jacket, which is also acting as safety relief when excess gas is
produced. As the regular supply of water is essential for operation, the
rainwater harvesting could help for the biogas plant.
C.
Operational
Parameters
Operation of a biogas plant is affected by the
following factors:
·
Temperature
The methane forming bacteria works best in temperature
ranges 20-60oC. Digestion at higher temperature proceeds more
rapidly than at lower temperature. The gas production decreases sharply below
20oC and almost stops at 10oC
·
Pressure
The minimum
pressure in the digester should be 6-10cm of water column i.e., 1.2 bar which
is ideal for proper functioning. It should never be allowed to exceed 40-50 cm
of water column. Excess pressure inhibits release of gas from slurry and
leakage in masonry.
·
Solid to moisture ratio in
the biomass
The water and solid mass should be in equal
proportion. If water content is too high in the slurry, the gas production will
drop. If water content is too low, acids will form and it hinder the
fermentation process.
·
pH value
During methane forming stage, pH value 6.5 to 7.5 should
be maintained. In the acid forming stage, pH value may be around 6 or less.
·
Feeding rate
Optimum feed rate should be maintained in the
digester. At higher feeding rate the retention period of biomass will be less
and undigested Slurry may come out.
·
Carbon to nitrogen ratio
The optimum C/N ratio is 30:1 for maximum
microbiological activity.
·
Seeding
In order to accelerate the fermentation process, a
small amount of digested slurry containing a methane forming bacteria is added
to the freshly charged slurry. This process is known as seeding. Seeding helps to
start and accelerate the digestion process.
·
Mixing or stirring
Mixing of slurry get uniformity in substrate concentration
and temperature, prevents the deposition of solids at the bottom and break down
the scum layer.
MATERIALS
AND METHODS
A.
Source of Feedstock
Cow dung is
solely used as the prime source for biogas production. Cow dung generate from
3–5 cattle/day can run a simple 8–10 m3 biogas plant which is able to produce
1.5–2 m3 biogas per day which is sufficient for the family 6–8 persons, can
cook meal for 2 or 3 times or may light two lamps for 3 hrs or run a
refrigerator for all day and can also operate a 3-KW motor generator for 1 hr
[6]. In this paper cow dung has been taken for studies. Food waste can also be
used in the digester which includes vegetables, fruits and other items. The
treatment process of food waste products gives hazardous waste. The usage of
chemicals is one of the main reasons for this.
B.
Sample Collection
A sample of 200
kg cow dung has been taken for biogas production and equal proportion of water
which mix together in the digester and forms semi solid state called slurry.
The slurry formation is shown in fig.1.
Fig. 1 Slurry formation
C.
Cow Dung Characterization
Cow dung is not
a waste product but do purify all the waste present in the nature [7]. The
study was conducted on a cow dung sample collected from dairy farm. The waste
was mixed with equal quantity of tap water approximately 200L to form slurry.
D.
Experimental Setup
A completely
recycled anaerobic reactor made from two polyethylene water tank (used for
drinking water storage). The floating type gas reactor consists of a water tank
as a digester having capacity 400L and another water tank for gas storage
having capacity 250L as shown in fig.2. As the gas produces in the digester and
collected in the another tank which act as floating dome, raises up due to
pressure created by biogas. This digestion system for AD of cow dung consists
of with arrangement for feed, recirculation is made by 400L container, biogas
collection and measurement by using 250L container, 90mm T joint, 90mm male
adapter, 90mm cap, 90mm PVC pipe, Elbow 63mm, 63mm male adapter 2, 63mm check
nut, 63mm PVC pipe, 12.5mm male adapter, 12.5 galvanized elbow, PVC adhesive
seal, 50mm Ball valve, 63,50 and 12.5mm barrel piece, Funnel and Teflon tape.
Fig. 2 Experimental
setup
E.
Reactor Operation
The cow dung
slurry was fed to the reactor from the inlet with the help of funnel and the
floating drum was placed in the digester. Before keeping the floating drum in
the digester the gas valve was opened in order to allow the air come out from
the gas chamber. To make the fermentation process fast, the place was selected
so that the sun radiation can fall directly on the digester. The fermentation process under anaerobic
condition produced biogas which was collected in the gas chamber.
CONCLUSION
After the
thorough study on the performance of biogas digester and evolution of
acidogenic reactor, the following collusion has been reached. As a result of
the treatment of cow dung under anaerobic digestion, the useful bi product,
bio-gas has been produced with a considerable rate of decrease in the values of
pH, acidity and alkalinity. Through the successful anaerobic processing of cow
dung inside the reactor, methanogen gradually converts the organic acids into
the methane gas and carbon dioxide, which indicates that the waste has better
anaerobic biodegradability. Thus achieves a waste of resource utilization. The
results show that reactor can treat animal waste with high contaminated load.
