Sustainable Energy Production by Aerobic and Anaerobic Digestion of Bio-Waste

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

[email protected]

[email protected]

[email protected]

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.



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



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

http://www.oregon.gov/ENERGY/RENEW/Biomass/biogas.shtml

http://stg1.kar.nic.in/samanyamahiti/SMEnglish_0607/default.htm


http://www.geda.org.in/bio/bio_powegeanimal.htm

http://www.geda.org.in/bio/bio_powegeanimal.htm


http://stg1.kar.nic.in/samanyamahiti/SMEnglish_0607/default.htm

http://www.oregon.gov/ENERGY/RENEW/Biomass/biogas.shtml

Sustainable Energy Production by Aerobic and Anaerobic Digestion of Bio-Waste

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