8/7/2019 Sustainable Energy Production by Aerobic and Anaerobic Digestion of Bio-Waste
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“Sustainable Energy Production by Aerobic and
Anaerobic Digestion of Bio-Waste”Pavan Kumar P N, Manu Kumar S, Ashish K K
E & C Department, MSRIT Bangalore, INDIA
Abstract- This paper is intended to address the most important
topic of today's world, „Energy Shortage‟. One way of solving it
is to make all the rural areas in the country, self sufficient in
their power needs. Since agriculture and animal husbandry are
two main occupations of the rural masses, and since they
contribute to tonnes of waste every month, we present two novel
ideas - aerobic and anaerobic digestion of bio-waste to generate
the required electric power for the village. The aerobic digestion
pit achieves a high thermal energy output. The anaerobic
digester generates methane using a custom bio-waste processor
that increases the output of resulting biogas (mostly CH4). We
present the use of methanogens that increase the amount of
methane produced by ~18%. We also note that by using hyper-
thermophiles, we can increase the temperature of the aerobic
digester to a maximum of 950C to increase thermal output.
The resulting methane can be used to generate electricity. The
heat from the aerobic digester can be used to either generate
electricity or increase the temperature of anaerobic reaction
which further increases the methane output.
Keywords — Aerobic Decomposition Plant, Anaerobicdecomposition plant, Methanogens, Bio-Waste Processor.
I. I NTRODUCTION
Agriculture has to this day remained as one of our
country’s major occupations, and the farmer is the backbone
of our country. Hence, the government has provided many
benefits and subsidiaries to the rural population, power or
electricity at lower rates being one of them. But, in the last 2-3
years, the country has been facing heavy “power shortages”
due to irregular rain patterns, and longer summers.The solutions we intend to implement in these villages is that
of processing of bio-waste found in and around the rural
geography and extract energy from it and use it to generateelectricity.
The proposed idea aims at simplicity and efficiency while
being clean and cost efficient to adopt. The idea aims atconverting all waste available in the rural areas efficiently to
energy. Separate digesters are used for plant and animal
waste for efficient conversion into energy and mathematical
proofs for these are given later in the article.
II. CONCEPT AND RESULTS
Plant and animal waste are collected from in and around the
rural area in consideration and are piled up for use in the
reactors/digesters.
We intend to use separate digestion processes for plant and
animal waste i.e. aerobic process for decomposition of plantwaste and anaerobic process for decomposition of animal
waste respectively.
The main intention for using different processes is that theseparate digestion pits aim to achieve better efficiency
individually in converting wastes to energy, as animal waste
do not efficiently decompose under aerobic conditions as they
do under anaerobic conditions, and vice versa for the plant
waste.
A. Working of the Aerobic Decomposition Plant
Plants decompose under two processes - aerobic and
anaerobic processes. Aerobic process is where the glucose andother carbohydrates are oxidized in presence of a lot of
oxygen to give CO2and H2O. Anaerobic process is where
glucose is oxidized in insufficient oxygen to give methane as a
byproduct along with other gases.
Chemically, anaerobic decomposition employs an electron
transport chain, with inorganic molecules other than oxygen
used as a final electron acceptor.
An example for the intermediate process can be
glucose + 3SO42-
+ 3H+ → 6HCO
3-+ 3SH
-, ΔG
0'= - 453 kJ.
The terminal electron acceptors (sulfate SO42-
) have smaller
reduction potentials than O2, i.e. meaning that less energy is
released per oxidized molecule of primary electron donor in
the above reactions) than in aerobic respiration (i.e. the
process of aerobic decomposition is less energetically
efficient).
The equation for the oxidative decomposition of glucose is
given as
C6H12O6 + 6O2 → 6CO2 + 6H2O + 686kcal
Since the molar mass of glucose is 180 gm, 1000kg (1tonne ) of assorted plant waste contains 5555.55 moles of
glucose.
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The above reaction, being exothermic, liberates 686 kcal
(2872.15kJ) of heat energy on an average for each mole of glucose that is consumed. Hence, the amount of chemical
energy (heat) present in the assorted waste is 1.595 x 107
kJ.
On an average <REF>, about 4 tons of plant waste can be
collected and piled per month and the net chemical energy
present in 4 tonnes of plant waste is 63.8 x 106
kJ.
If this energy is completely converted to electrical energy,
63.8 x 106 kJ / (3.6 x 106 J/kWh) = 17722.22 kWh of electric power can be generated per month.
To achieve aerobic decomposition, we use a specially
designed digester. Plant waste from all sources in and nearby
the village are collected and introduced into the digestion pit.The waste starts decomposing due to the presence of
mesophilic bacteria in the waste and once the temperature
reaches 40°C, thermophilic bacteria enter the decomposition
process and raise the temperature to an optimal level of 65°C.
The increase in temperature occurs because of the breaking
down of complex glucose molecules into simpler moleculeslike Carbon dioxide and water. This process also releases
enormous quantities of energy which occurs in the form of
heat.
For the above reaction to occur, sufficient oxygen and
water must be supplied to the digestion pit and hence an air
pipe is used to supply sufficient oxygen. Since the air can cooldown the digester and slow down the reaction, it is heated by
encircling it around the digester and then the hot air is sent in.
Water inlets are also provided. Correct levels of humidity (60
- 70%) has to be maintained from the reaction to take place.
Also since the reaction liberates CO2, it collects on the bottom
of the pit as it relatively heavy. This is let out using valves andtubes along the bottom of the digester. Sensors are used to
monitor CO2 and humidity levels and proper actions are take
if there are any inconsistencies.
Fig. 1. Construction of the Aerobic digestion plant
The digester also has stirrers keep the reactant material in
constant motion so as to distribute O2 freely throughout thedigester and prevent any occurrence of anaerobic digestion in
it. The stirrer is powered by vertical shaft wind turbines as the
winds in rural areas are low level winds and are sufficiently
string.
The carbon-nitrogen ration of the plant waste has also to be
kept in consideration as, If the compost mix is too low in
nitrogen, it will not heat up. If the nitrogen proportion is toohigh, the compost may become too hot, killing the compost
microorganisms, or it may go anaerobic, resulting in a foul-
smelling mess. The usual recommended range for C/N ratios
at the start of the composting process is about 30/1, but thisideal may vary depending on the bioavailability of the carbon
and nitrogen. As carbon gets converted to carbon dioxide (and
assuming minimal nitrogen losses) the C/N ratio decreases
during the composting process, with the ratio of finished
compost typically close to 10/1.Grass clippings and other
green vegetation tend to have a higher proportion of nitrogen(and therefore a lower C/N ratio) than brown vegetation such
as dried leaves or wood chips.[1]
The maximum temperature reached in this process is
~650C and the thermophilic bacteria decompose less
efficiently at temperatures > 650C and around 75
0C, the
bacteria start to die. If higher temperatures aredesired, hyperthermophilic bacteria (specifically of bacteria of
the genus Thermus) can be used to get temperatures up to
950C, but the practicality of their usage has to be further
studied.
B. Working of Anaerobic Digestion Plant
Animal waste, predominantly cow dung is collected from
the village, and dumped in the bio-waste processor.C6H12O6 → 3CO2 + 3CH4
The principle behind this bio-waste processor is to providea favourable medium for the culture of microorganisms
ensuring the efficient conversion of dung to methane. The
slurry (10% dung in water) is introduced into the bioreactor
from the inlet on the left of the bioreactor. To allow for maximum extraction of methane, rotor blades are used to stir
the contents of the pit continuously
Steam is supplied through the inlet valve to maintain the
temperature and pressure inside the bioreactor. The
temperature and pressure gauge continuously monitors the
pressure and temperature changes within the bioreactor. Theoptimum temperature that is to be maintained in the bioreactor
is between 38-40 degrees Celsius and the optimum pressure is
1atm. If the temperature or the pressure goes below the
optimum value, steam is supplied through the valve to
maintain the conditions inside the reactor. The used up sludge
is taken out through the opening provided at the bottom of the
bioreactor.
In order to increase the production of methane (main
component of biogas), Methanogens or methane producing
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Archaea are added to the bioreactor after a small amount of
waste has been decomposed.
Fig. 2. Parts of the Anaerobic digestion Plant (Bio Waste Processor)
Methanogens are microorganisms that produce methane as a
metabolic by-product in anoxic conditions. They are common
in wetlands, where they are responsible for marsh gas, and in
the guts of animals such as ruminants and humans, where theyare responsible for the methane content of flatulence.
This is achieved as a result of the consecutive biochemical
breakdown of polymers to methane and carbon dioxide in an
environment in which varieties of microorganisms whichinclude fermentative microbes (acidogens); hydrogen-
producing, acetate-forming microbes (acetogens); and
methane-producing microbes (methanogens) harmoniously
grow and produce reduced end-products. Anaerobes play
important roles in establishing a stable environment at various
stages of methane fermentation.
According to our calculations, the addition of methanogens
will enhance the methane production by around 5%.
1) Theory and calculations: In the bio-waste digester we
are using, 65% of the biogas is composed of methane.
Typical digester gas, with a methane concentration of 65% ,
contains about 600 Btu (0.1758kWh) of energy per cubicfoot(0.02832m
3) [ 2 ]
Hence, an average of 6.2076 kWh per m3
of biogas can be
extracted.
On an average a cow gives about 24kg of dung per day, out of
which we can effectively collect 20 kg of dung per day which
in turn gives 1m3
of bio gas on decomposition. But, theaddition of methanogens increase production of bio gas
increases by 5%.
Hence, 20kg of dung gives 1.05 m3
of biogas.
If an average of 900 cattle is assumed to be present per village,
then 5670 kWh of electric power can be extracted every day,
i.e. 170100 kWh of electric power per month.If 45% efficiency is assumed in the conversion to electrical
energy, 76545 kWh of electric energy per month (or 2551.5
kWh per day) can be practically realized.
Thus, 94267 kWh of power can be generated per month
which is the sum of power obtained from the Aerobic and
Anaerobic digestion plants.
III. CONCLUSIONS
Our concept is expected to be good enough to satisfy the
energy needs of the village in consideration. This can beextended to all rural villages in the future after it is proved
successful in the village under consideration. The implication
of the idea suits everyone and is novel as explained in the
above sections. The idea is eco-friendly, simple and efficient
while being acceptable to the rural community in question.
Environmentally, the energy obtained is clean and hence isa better alternative to other conventional ways of energy
generation which are known to be either non-eco-friendly or
being inefficient. New concepts like introduction of
methanogens and the concept of extracting energy from
aerobic decomposition are new, elegant, simple and efficient.
As indicated in “Potential Impact” section, these first timeideas may go a long way in the fight against the current fossil-
fuel crisis.
The most notable point of this idea is that it is by far a
better alternative to the currently existing energy solutions. If
the proposed idea is implemented, and if it is successful, all
villages in India may in the near future be self sufficient intheir energy needs. A complete proof is provided in which the
mathematics indicate that the plant can efficiently generate the
required energy, far outweighing the energy required for the
running of the plant. The government of India may even
consider not supplying energy to these self-sufficient villages.This also means that more energy is available to the cities and
towns, leading to the country’s economic growth.
But more importantly, it has the great potential to act as a
driving force in the never ending quest for a cleaner, greener
and hence a better tomorrow.
ACKNOWLEDGMENT
Sincere thanks to Principal, MSRIT, to all our friends and
family members, and to all who contributed directly or
indirectly for the success of the project.
R EFERENCES
[1] Composting101.com, Carbon to Nitrogen Ratios.http://www.composting101.com/c-n-ratio.html
[2] Conversions obtained from Bio-Energy in Oregon website
http://www.oregon.gov/ENERGY/RENEW/Biomass/biogas.shtml[3] Rural Development and Panchayat Raj System, Karnataka, INDIA
Census Information
http://stg1.kar.nic.in/samanyamahiti/SMEnglish_0607/default.htm[4] China’s Methane power plant
http://www.npr.org/templates/story/story.php?storyId=89657242
[5] Gujrat Energy Development Authority, Biogas programmehttp://www.geda.org.in/bio/bio_powegeanimal.htm