i

DESIGN OF A KITCHEN WASTE BASED

MINI BIO-GAS UNIT FOR A SCHOOL:

“A Case Study of St. Henry’s College

Kitovu”

ORTEGA IAN

2015.

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KYAMBOGO UNIVERSITY

FACULTY OF ENGINEERING

DEPARTMENT OF MECHANICAL AND PRODUCTION ENGINEERING

DESIGN OF A KITCHEN WASTE BASED MINI BIO-

GAS UNIT FOR A SCHOOL: “A CASE STUDY OF ST. HENRY’S

COLLEGE KITOVU”

A Project Report Submitted to the Department of Mechanical and Production Engineering of

Kyambogo University as a Partial Fulfillment for the Award of Bachelor of Engineering in

Mechanical and Manufacturing Engineering.

By

ORTEGA IAN

(11/U/11049/EMD/PD)

Supervisor:

MR. SSEMPEBWA RONALD

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DECLARATION

I Ortega Ian declare to the best of my knowledge and belief that this Report is purely my original

project work under the supervision of Mr. Ssempebwa Ronald unless otherwise as stated in the

references. It has never been presented or submitted to any institute of higher learning for the

award of any academic qualification.

Signed……………………..

Date………………………..

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DEDICATION

This project proposal is dedicated with special appreciation to my two mothers, Florence

Ndagire, Elizabeth Birabwa and my late father, Kevin Aliro Ogen.

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ACKNOWLEDGMENTS

First and foremost, great thanks go to the Almighty God because without His mercy and Love,

that He has enabled me write this Report.

I would like to extend my appreciation to everyone who enabled me directly or indirectly to

successfully complete this Report.

I wish to express my sincere gratitude to my project supervisor, Mr.Ssempebwa Ronald, for his

great supervisory role during the preparation of this Report document especially for willingly

giving his time and attention without withholding anything from me.

I would also like to thank the following lecturers who have gave me assistance; Dr. Ssengonzi

Bagenda and Ms. Akello Lilian.

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ABSTRACT

Majority of Ugandan Schools use firewood for cooking (Oluka, 2014). The same schools also

have a good number of kitchen waste generated per day from the students. The researcher carried

out a study that designed a kitchen waste based bio-gas unit for a School that will see the school

better manage its kitchen and food waste and in return reduce the costs spend in procuring

firewood (Ainebyoona, 2014).

Firewood is very costly as a source of energy, both to the environment and to the school. Yet

coupled with this, is the lack of an efficient way for schools to dispose of their kitchen waste and

its proper management. The production of biogas from kitchen waste thus ensures an efficient

way of managing kitchen waste and also offsets some of the costs incurred from the procurement

of firewood for cooking (Fulford, 1988).

The objectives of the study were to study the different existing biogas digester technologies. The

study also determined the sizes of different components for the most efficient digester and

established the rational design specifications of the most efficient design. A financial evaluation

for the appropriate design for extracting biogas from Kitchen waste was carried out. The research

employed quantitative data analysis and was carried out for a period based on a planned work

schedule.

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TABLE OF CONTENTS

DECLARATION ............................................................................................................................. i

DEDICATION ................................................................................................................................ ii

ACKNOWLEDGMENTS ............................................................................................................. iii

ABSTRACT ................................................................................................................................... iv

TABLE OF FIGURES .................................................................................................................. vii

CHAPTER ONE: INTRODUCTION ............................................................................................. 1

1.1 Background ...................................................................................................................... 1

1.2 Historical Background of Biogas Technology In Uganda ............................................... 2

1.3 Problem Statement ........................................................................................................... 3

1.4 Objectives of the Study .................................................................................................... 4

1.4.1 General Objective ..................................................................................................... 4

1.4.2 Specific Objectives ................................................................................................... 4

1.5 Research Questions .......................................................................................................... 4

1.6 Scope of the Study............................................................................................................ 4

1.6.1 Geographical Scope .................................................................................................. 4

1.6.2 Subject Scope ............................................................................................................ 4

1.6.3 Time Scope ............................................................................................................... 4

1.7 Justification of the Study .................................................................................................. 4

1.8 Significance of the Study ................................................................................................. 5

CHAPTER TWO: LITERATURE REVIEW ................................................................................. 6

2.1 Introduction ...................................................................................................................... 6

2.2 Introduction To Biogas..................................................................................................... 6

Process of Biogas Production ...................................................................................................... 6

Hydrolysis ................................................................................................................................ 6

Acetogenisis............................................................................................................................. 6

Methanogenesis ....................................................................................................................... 7

Factors Affecting Biogas Generation .......................................................................................... 8

2.3 Bio Digester Models In Uganda ....................................................................................... 9

2.3.1 Floating Drum Digester ............................................................................................ 9

2.3.2 Fixed Dome Digester (CAMARTEC Design) .......................................................... 9

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2.3.3 Plastic Digester Designs ......................................................................................... 10

2.3.4 A Bio Digester Model Of Interest ........................................................................... 11

2.4 Design specifications In the Production of Bio Gas ...................................................... 13

CHAPTER THREE: METHODOLOGY ..................................................................................... 15

3.1 Overview ............................................................................................................................. 15

3.2 Research Design .................................................................................................................. 15

3.3 Data Source ......................................................................................................................... 15

3.3.1 Primary Sources ............................................................................................................ 15

3.3.2 Secondary Sources ........................................................................................................ 15

3.4 Data Collection Methods ..................................................................................................... 15

3.4.1 Use of Interviews .......................................................................................................... 15

3.4.2 Observation ................................................................................................................... 16

3.4.3 Library Research and Use of Internet ........................................................................... 16

3.4.4 Comparative Analysis Method ..................................................................................... 16

3.5 Design Parameters (James Kuria, 2008) ............................................................................. 16

3.8 Data Processing, Presentation and Analysis ....................................................................... 18

CHAPTER FOUR: PRESENTATION AND DISCUSSION OF FINDINGS ............................. 19

4.1 Overview ............................................................................................................................. 19

4.2 Digester Model For Consideration and Ranking................................................................. 19

4.3 Available Feedstock/Waste In The School ......................................................................... 21

4.3.1 Kitchen Waste ............................................................................................................... 21

4.3.2 Animal Waste ............................................................................................................... 23

4.3.3 Human Waste ............................................................................................................... 23

4.4 Energy Needs/ Requirements of The School ...................................................................... 23

4.5 Design of a Digester To Produce Biogas ............................................................................ 24

4.5.1 Two Cases Of Digesters For Consideration ................................................................. 26

4.5.2 Design Formulas: .......................................................................................................... 27

4.6 Final Design ........................................................................................................................ 28

4.7 Economic Analysis .............................................................................................................. 30

CHAPTER FIVE: ......................................................................................................................... 32

CONCLUSION AND RECOMMENDATIONS ......................................................................... 32

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5.0 Conclusions .................................................................................................................... 32

5.1 Recommendations .......................................................................................................... 32

REFERENCES ............................................................................................................................. 33

APPENDIX ................................................................................................................................... 35

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TABLE OF FIGURES

1 Uganda National Household Surveys (UNHS)............................................................................ 2

2 Composition of Biogas ................................................................................................................ 6

3 C/N Ratio of Some Organic Materials ......................................................................................... 7

4 Weighting Table......................................................................................................................... 20

5 School Menu .............................................................................................................................. 21

6 Foodwaste Generated Over 7 day period ................................................................................... 22

7 Energy Requirements of The School ......................................................................................... 24

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CHAPTER ONE: INTRODUCTION

1.1 Background

According to the ministry of Education, Uganda has at least 22,500 primary schools

(government-owned and private) and at least 2,000 secondary schools. If each of the schools

uses at least 12 trucks of firewood annually to feed its charges, then Uganda is effectively

losing a small forest to raise at least 294,000 trucks of firewood, assuming all the schools had

at least 700 leaners (Education, 2009).

Uganda has a forest cover of nearly three million hectares or 15.2 per cent of its total land

mass, according to recent statistics from the Food and Agriculture Organisation (FAO). It

should be more, if it was not for the fact that between 1990 and 2010, Uganda lost 37.1 per

cent of its forest cover, or about 1.8 million hectares (Mongabay, 2000) .

Currently, Uganda loses 2.2 per cent of its forest cover per year, with officials attributing the

losses mostly to encroachment on forest land for farming or settlement due to a high

population growth, as well as cutting of trees for fuel wood like firewood and charcoal.

Because most schools in Uganda really on firewood for their energy requirements as far as

cooking and heating of water are concerned, this poses a major problem for the environment

and the health risks involved in cooking using firewood. Thus, schools are playing an indirect

role in the depletion of forests, while at the same time posing a risk to those who cook and

those in the vicinity of the kitchens where firewood is the means of fuel.

Yet with all these problems, schools have a lot of Kitchen waste at the end of the day to

dispose. The increasing quantities of waste lead to increased threats to the environment as

already witnessed in town areas and cities.

Firewood and charcoal are still the most commonly-used sources of energy for cooking even

in Ugandan households. According to the Uganda National Household Survey Report

2009/2010, up to 95 per cent of the households still used wood fuels (GovUGA, 2012).

―Firewood was most commonly used by the rural household (86 per cent) while charcoal is

commonly used by urban households [70 per cent],‖ said the report. ―It is worth noting that

the proportions of households that used electricity for cooking was still very low which could

be due to the high tariffs charged per unit.‖ Biogas production from Kitchen waste presents

itself as a cheaper and safe alternative.

Household consumption of firewood and charcoal (Mill. Shs)

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Table 1 Uganda National Household Surveys (UNHS).

Item 1996/97 2002/03 2005/06 2009/10

Charcoal 4,076 6,936 9,345 98,699

Firewood 13,967 20,677 23,425 310,440

Total 18,043 27,613 32,770 409,139

Table shows the value of household expenditure on firewood and charcoal as estimated from

the Uganda National Household Surveys (UNHS). The total nominal value increased to

409.1 billion in 2009/10 from Shs. 32.8 billion in 2005/06. The value of charcoal and

firewood consumption went up by more than 10 times during the same period.

Kitchen wastes are organic materials which are easily bio-degradable. They are a potential

raw material for biogas production. Generally Kitchen waste is treated as waste and thrown

which acts as the key factor for the pollution. The pollution leads to number of diseases

which affect human health. Energy production from waste is becoming more popular these

days. It has mainly two direct advantages. One, the disposal waste is reduced as it is utilized.

Another, energy is generated (Klinghoffer, 2013) (Young, 2010)

Thus, generating biogas from kitchen waste achieves many goals. First, the schools are able

to reduce on the costs spent on energy. Secondly, the threat on the environment caused by

deforestation is reduced. The other point is that the health risks imposed by cooking with

firewood are minimized and lastly, the byproducts after the production of biogas with kitchen

waste act as a good fertilizer thus increasing agricultural production. We also don’t forget,

thus in the end, it becomes easy for schools to utilize their kitchen waste in an efficient way

that saves them money. A recent study commissioned by the Global Village Energy

Partnership (GVEP) in (Uganda’s) Wakiso district shows that schools spend up to 400,000

UGX (US$158) per month on fuel for cooking meals and heating water, with urban schools

spending twice this amount. Thus an average secondary school spends at least Shs.4 million

per term for the cost of 4 trucks on firewood required to cook the school means (Afedraru,

2014). This puts significant strain on the school. Producing biogas from kitchen waste is one

important step towards the re-allocation of school funding for more productive uses.

1.2 Historical Background of Biogas Technology In Uganda

The first biogas plant in Uganda was built by the Church Missionary Society in Mbarara

district in the early 1950s and emphasis was on treatment of sewage (Odogola, Wilfred 1992,

p.2). In the 1960s, some missionaries built one demonstration plant in Kotido district

(Kikuuma, Andrew 2001, 2). However, the first documented study that was very extensive

was a PhD thesis by Boshoff (1969/70) then based at Makerere University. This digester

built at Kabanyolo Farm did generate gas but it didn’t go outside Kabanyolo (Simoga, Zap,

2000, 56). In 1974, Silverman made a baseline study of biogas in the central region of the

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country and recommended biogas as viable in Uganda (Odogola, Wilfred 1992, p.2).

Although Silverman made the recommendation, implementation was never reached due to

the unfavourable political climate at the time (Kikuuma, Andrew 2001, 2).

In 1985, the Chinese biogas team carried out a feasibility study and concluded that the

technology was most viable in small-scale private diary farms with easy access to feedstock

(Odogola, Wilfred 1992, p.3). In 1989, the government showed interest in the technology

and several demonstration farms were constructed in Karamoja district (Kikuuma, Andrew

2001, 2). FAO carried out another study sanctioned by the Ministry of Energy, which led to

the creation of the National Biogas programme in Uganda. They recommended a Chinese-

type design to be built at secondary schools as a bio-latrine using cow-dung but with

possibilities of incorporating human manure. A number of secondary schools consequently

received these plants and these include Budo, Namagunga, Mwiri and Tororo Girls.

In the early 90s, an estimated 120-170 biogas units were constructed in the country. Out of

these, probably 50%-60% are still operational (Odogola, Wilfred 1992, p.3).

Of all these studies in Uganda, none of the remarkable studies have been made regarding use

of kitchen waste. Literature regarding use of kitchen waste only or vegetable wastes only as

input for biogas generation is difficult to find. All of the plants installed use cattle dung as

feedstock and about 80 percent of them have also been connected with toilet to add the

human excreta as feedstock. However, none have been using the other organic wastes for this

purpose. Therefore, the use of organic wastes, of which the Vegetable and Kitchen Waste

(VKW) comprises the main part, for the production of biogas is an environment-friendly

technology both in the urban as well as rural areas (Lama, 2013). When applied, it will

benefit the schools in Uganda at the same time it will initiate at source management of

municipal solid waste in urban areas. It will decrease firewood, fossil fuel as well as

chemical fertilizer demand thus saving the foreign currency of the country and discouraging

deforestation (Abbasi, 2011)

1.3 Problem Statement

Firewood is very costly as a source of energy, both to the environment and to the school. Yet

coupled with this, is the lack of an efficient way for schools to dispose of their kitchen waste and

its proper management. The production of biogas from kitchen waste thus ensures an efficient

way of managing kitchen waste and also offsets some of the costs incurred from the procurement

of firewood for cooking.

4

1.4 Objectives of the Study

1.4.1 General Objective

To design a Kitchen-waste based mini-bio gas unit for St. Henry’s College Kitovu, a secondary

school in Masaka District of Uganda.

1.4.2 Specific Objectives

To study the different existing biogas digester technologies.

To determine the rational design specifications of the most efficient design

To do a financial evaluation for the appropriate design for extracting biogas from Kitchen

waste.

1.5 Research Questions

What forms of biogas digester models are being used at the moment?

What are the rational design specifications that will optimize the production of the

biogas?

How long will it take for a biogas unit for benefit the school as far as the investments are

concerned?

1.6 Scope of the Study

1.6.1 Geographical Scope

The project confined itself to St.Henry’s College Kitovu, a secondary school in Masaka

District.

1.6.2 Subject Scope

The main focus of the design of an appropriate kitchen-waste based bio-gas unit was

limited to the design concepts and calculations and the computer aided drawings of the

digester components.

1.6.3 Time Scope

The project was conducted with in a period of seven months that is from November to

May 2014/2015. From this time after approval of the design concepts and procedures, then the

preparations for fabricating the digester may be done.

1.7 Justification of the Study

1. There is lack of proper management of Kitchen Waste in Most Schools

2. The Cost of firewood impacts greatly on the school expenditure, yet alternative sources of

energy such as electricity from the main-grid are very expensive (G. Oelert, 1987).

3. Cooking with firewood results in indoor air pollution. Indoor air pollution is widely

recognized to be a ubiquitous problem linked with the burning of solid biomass fuels inside the

5

kitchens. ―Use of inefficient fuel wood stoves promotes uncontrolled exposure to smoke leading

to a growing number of respiratory disorders.

1.8 Significance of the Study

Mitigation of forest depletion, biodiversity impact, CO2 emissions and global warming

all of which are associated with firewood as a source of energy. A single biogas unit is

estimated to directly help conserve 3 tons of fuel wood annually through fuel switching,

resulting in substitution of unsustainably harvested biomass and maintenance of forest

habitat, with associated biodiversity benefits and local benefits to soil stability and to the

dry season stream flows in the region (Khoiyangbam, 2011). Since fuel wood is generally

harvested unsustainably, this reduction translates into the prevented release of 5.5 tons of

carbon dioxide into the atmosphere annually from an average biogas digester, resulting in

environmental benefits from a reduced contribution to global warming and associated

impacts (Nijaguna, 2006).

Schools will be able to save on energy costs from firewood. The design can also be

adopted for homes as a substitute for charcoal, LPG, and electricity purchases for

cooking and lighting in urban and peri-urban areas (Sasse L, 1991).

Improved health from indoor environment in Kitchen because of substitution of wood

with biogas because of its smoke-free odour nature.

The spent waste material that emerges at the end of the biogas process, the slurry, is a

high nutrient organic fertilizer that surpasses raw manure, and can be applied either

directly or in conjunction with composted agricultural residue (Journal, 2011). If

composted properly, the slurry will give higher fertilizer yields and increase overall crop

yield and production, thereby augmenting income and restoring soil fertility in areas

where soil degradation is prevalent; simultaneously, as it replaces chemical fertilizers, the

slurry saves the money previously spent on chemical fertilizers (Discussion, 2012).

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CHAPTER TWO: LITERATURE REVIEW

2.1 Introduction

This chapter reviews the literature of the different biogas extraction mechanisms, process of

biogas production and the biogas potential of different feeds and appropriate conditions for peak

levels from these feeds.

2.2 Introduction To Biogas

Biogas is the inflammable gas produced from the anaerobic fermentation of the bio degradable

substance due to the activity of the methanogenic bacteria. This gas is mainly composed of the

methane (CH4), carbon dioxide (CO2), water vapor etc. (Rai, 2009)

2 Composition of Biogas (Ref: www.wikipedia.org)

Substances Symbol Percentage

Methane 50-70

Carbon dioxide 30-40

Hydrogen 5-10

Nitrogen 1-2

Water Vapor O 0.3

Hydrogen Sulphide Traces

Process of Biogas Production

There are three steps or the process involved in the production or the activity of the gas

(Mudhoo, 2012).

Hydrolysis

Acetogenisis

Methanogenesis

Hydrolysis

It is the first step involved in the process also known as the liquefaction. In this process the

fermentive bacteria converts’ insoluble complex organic into the soluble organic compound and

also complex polymer is converted into the simple monomer. (World, 2008) Examples are

cellulose is converted into the sugar, amino acid and fatty acid. This is being the important step,

is also the rate limiting step. Industrially this problem is overcome by use of the chemical

reagent. (Mathur AN and Rathore, 1992)

Acetogenisis

In this process the product from the first process is converted into the simple organic acid,

carbon dioxide and hydrogen. Major acids which are produced during this process are Acetic

acid (CH3COOH), propionic acid (CH3CH2COOH), (CH3CH2CH2COOH), and ethanol

(C2H5OH). (Mathur AN and Rathore, 1992)

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The reaction involve is

GlucoseEthanol + Carbon dioxide

Methanogenesis

Methane is produced by the action of the bacteria called methanogens bacteria. There are two

methods of the production of the methane, first is by the cleavage of acetic acid to generate the

carbon dioxide and methane. Second process is the reduction of carbon dioxide with the

hydrogen. Methane production is higher from the second process, but it is limited by the amount

of the hydrogen in the digester. (Mathur AN and Rathore, 1992)The reaction involve in this

process are:

Acetic ( ) Methane ( ) + Carbon dioxide ( )

Ethanol + Carbon dioxide Methane + Acetic acid

Carbon dioxide + Hydrogen Methane + Water

3 C/N Ratio of Some Organic Materials (Ref: www.norganics.com)

S.N. Raw Materials C/N Ratio

1 Duck dung 8

2 Human Excreta 8

3 Chicken dung 10

4 Goat dung 12

5 Pig dung 18

6 Sheep dung 19

7 Cow dung/ Buffalo dung 24

8 Water Hyacinth 25

9 Elephant dung 43

10 Straw (Maize) 60

11 Straw (rice) 70

12 Straw (Wheat) 90

13 Saw dust Above 200

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Factors Affecting Biogas Generation

• Carbon to Nitrogen (C/N) ratio: Carbon (as carbohydrates) and nitrogen (as

protein, ammonium nitrates etc.) are the main food of anaerobic bacteria. If the

C/N ratio is very high, nitrogen will be consumed rapidly and the rate of reaction

will be decreased. On the other hand if the C/N ratio is very low, nitrogen will be

liberated and accumulated in the form of ammonia. The ammonia can kill or

inhibit the growth of bacteria specially methane producers. In general a ratio of in

range of 20-30:1 is considered the best for anaerobic digestion. (Mathur AN and

Rathore, 1992)

• pH value: Both over acidic and over alkaline than certain limits are harmful to

Methanogenesis organisms. The optimum biogas production is achieved when the

pH value of the input mixture to the digester is between 6 and 7. (Mathur AN and

Rathore, 1992)

• Temperature: Enzymatic activity of bacteria largely depends upon temperature,

which is critical factor for methane production. The bacteria work best at a

temperature of 35°C to 38°C. (Mathur AN and Rathore, 1992)

• Loading Rate: The digester load is primarily dependent upon four factors-

substrate, temperature, volumetric burden and type of plant. The correct rate of

loading is important for efficient gas production. (Mathur AN and Rathore, 1992)

• Retention Time: It depends on the type of feedstock and the temperature. The

retention time is calculated by dividing total capacity of the digester by the rate at

which organic matter is fed into it. (Mathur AN and Rathore, 1992)

• Total Solid Content: For proper solubility of organic materials, the ratio between

solid and water should be 1:1 on unit volume basis when the domestic wastes are

used. If the slurry mixture is too diluted, the solid particles can precipitate at the

bottom of digester and if it is too thick, the flow of gas can be impeded. In both

cases gas production will be less than optimum production. (Mathur AN and

Rathore, 1992)

• Toxicity: Mineral ions, heavy metals and the detergents are some of the toxic

materials that inhibit the normal growth of the pathogens in the digester. Small

quantity of mineral ions like sodium, potassium, calcium, magnesium and sulphur

stimulates the growth of bacteria while very heavy concentration of these ions

will have toxic effect. (Mathur AN and Rathore, 1992)

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• Pressure: It has been reported that better production of biogas takes place at

lower pressures. (Mathur AN and Rathore, 1992)

2.3 Bio Digester Models In Uganda

2.3.1 Floating Drum Digester

In this design, the digester chamber is made of brick

masonry in cement mortar. A mild steel drum is placed

on top of the digester to collect the biogas produced

from the digester (Walekhwa, 2009). Thus, there are

two separate structures for gas collection and

production. With the introduction of the fixed drum

plant, the floating drum plants have become obsolete in

the country due to a comparatively high investment

and maintenance costs, along with other design

constraints. (Bikash Pandey, 2007)

2.3.2 Fixed Dome Digester (CAMARTEC Design)

This is the most widely used in the country and is known as the CAMARTEC design named for

the government research institute in Arusha, Tanzania where it was first developed. It consists of

an underground brick masonry compartment (fermentation chamber) with a dome on top for gas

storage. Here the dome and the fermentation chamber are combined as one unit. This has

eliminated the costlier mild steel gas holder (floating drum type) which is susceptible to

corrosion (Bikash Pandey, 2007).

(Bikash Pandey, 2007) 2

(Bikash Pandey, 2007) 1

10

(Bikash Pandey, 2007) 3

2.3.3 Plastic Digester Designs

This design consists of a digester bag made of thick gauge polythene tube between

0.5 and 1.0 meter in diameter which is placed in a trench. The inlet and outlet are

typically made with 4 inch diameter PVC pipes tied at the end of the digester bag

with rubber bands from car tubes (Ilukor, 1986). Gas produced is collected in a

separate reservoir ―balloon‖ also made with a polythene tube. Gas is carried from the

digester first to the balloon then to the kitchen with half inch PVC plastic pipe. The

reservoir balloon is hung down from a beam and gas pressure is maintained by tying

brick weights to its end.

11

One of the organizations that was actively promoting these designs was Integrated

Rural Development Initiative (IRDI). The designs were costing around Shs.340,000

including the cement channel constructed for the slurry to flow. These bio plants

turned out to be less popular than expected. Problems coupled with them included

low gas pressure, and correspondingly slow cooking times and insufficient brightness

of lights. Users also found that there was frequent clogging both at the inlet and outlet

pipes. However the biggest problem was the fragility of the polythene bags which

lasted no more than three years with some being damaged within six months. The

plastic membrane could easily be punctured by sticks or pointed stones and could also

be damaged by cows or other animals inadvertently stepping on them or easily

sabotaged by disgruntled neighbors (Bikash Pandey, 2007)

(Bikash Pandey, 2007) 5

2.3.4 A Bio Digester Model Of Interest

ARTI has developed a compact biogas plant which uses waste food rather than

dung/manure as feedstock, to supply biogas for cooking. The plant is sufficiently

compact to be used by urban households, and about 2000 are currently in use – both

in urban and rural households in Maharashtra. A few have been installed in other

parts of India and even elsewhere in the world.

Dr. Anand Karve (President of ARTI) developed a compact biogas system that uses

starchy or sugary feedstock (waste grain flour, spoilt grain, overripe or misshapen

fruit, no edible seeds, fruits and rhizomes, green leaves, kitchen waste, leftover food,

etc). (Bikash Pandey, 2007)

Operation

The smaller tank is the gas holder and is inverted over the larger one which holds the

mixture of decomposing feedstock and water (slurry). At inlet feeding matter should

12

be ground or pulped and mix with 2 to 3 bucket full of water. So, an inlet is provided

with much smaller amount of solid matter than the residue from a manure-based

plant, and ARTI recommend that the liquid is mixed with the fedstock and recycled

into the plant. (Bikash Pandey, 2007)

A pipe takes the biogas to the kitchen, where it is used with a biogas stove. Such

stoves are widely available in India which has a long tradition of using manure-based

biogas plants. The gas holder gradually rises as gas is produced, and sinks down again

as the gas is used for cooking. Weights can be placed on the top of the gas holder to

increase the gas pressure. (Bikash Pandey, 2007)

Advantages

• The immediate benefit from owning a compact biogas system is the savings in

cost.

• It is an environmentally friendly cooking system.

• The size and cost of this system is relatively lower.

• It is an extremely user friendly system, because it requires daily only a couple of

kg feedstock, and the disposal of daily just 5 liters of effluent slurry.

13

• A single plant produces sufficient biogas to at least halve the use of LPG or

kerosene for cooking in a household, as well as a small amount of solid residue

which can be used as fertilizer. (Bikash Pandey, 2007)

Disadvantages

• The biogas plant can become acidic and fail if it is over-fed, and this is a

particular challenge with a plant using highly digestible organic materials.

• Plant’s heat insulation is not considered.

• Since heat insulation is not considered, it cannot be used in region where weather

fluctuates more. (Bikash Pandey, 2007)

2.4 Design specifications In the Production of Bio Gas

Each of the following describes the factors that must be taken into consideration in generating

the final design of the bio-gas digester that’s appropriate for the purpose (Singh, 2008).

1. Physical Conditions

The performance of a biogas plant is dependent on the local conditions in terms of

climate, soil conditions, the substrate for digestion and building material availability. The

design must respond to these conditions. In areas with generally low temperatures,

insulation and heating devices may be important. If bedrock occurs frequently, the design

must avoid deep excavation work. The amount and type of substrate to be digested have a

bearing on size and design of the digester and the inlet and outlet construction. The

choice of design will also be based on the building materials which are available reliably

and at reasonable cost. (Bikash Pandey, 2007)

2. Availability: G.I. sheet, R.B.C and polythene drum, all are easily available in the market.

But Polythene drum are easily available in the form of drum in the market. (Bikash

Pandey, 2007)

3. Strength (pressure holding capacity): R.B.C and G.I. sheet has got high pressure

bearing capacity than polythene drum. (Bikash Pandey, 2007)

4. Leakage: Concrete may have major leakage problem if fabrication is done in poor

management way . So ratio of cement water should be well maintained. G.I sheet may

have leakage problem through joint like rivet joint. But there is less leakage chance for

polythene if adhesive are properly stocked on the joints. (Bikash Pandey, 2007)

5. Durability: Durability of R.B.C is higher than polythene drum and G.I sheet.

6. Fabricability: Polythene drum plant fabrication is easier than R.B.C and G.I sheet due to

less labor cost and less machining parts respectively.

14

7. Solid Waste Reuse: Polythene drum such as sprit drum and paint drum are reused

material. R.B.C can also be reuse but G.I sheet need to fabricate. (Bikash Pandey, 2007)

8. Cost: R.B.C and G.I sheet fabrication cost is higher than Polythene drum.

15

CHAPTER THREE: METHODOLOGY

3.1 Overview

This chapter contains how the research was conducted and the major description of the methods

which was used to obtain data, tools and instruments used in the design of the kitchen-waste

based biogas unit.

3.2 Research Design

The research of this project was designed basing largely on the study of different biogas

production mechanisms with greater attention to the ―above-ground‖ designs, factors that affect

biogas production and digester design parameters and considerations to achieve efficient output.

3.3 Data Source

In this design, research was based on both the primary and secondary sources of information

3.3.1 Primary Sources

This was directly obtained from different players in the bio-gas energy industry in Uganda,

various designs already in use in different homesteads and in schools. This data is efficient in

that it offers adequate information as it centers on the live events from the field through

interviews.

3.3.2 Secondary Sources

This data type was obtained from mechanical engineering text books like mechanical

engineering design, related designs. Other sources to be included are the previous reports done

already by colleagues in the same field of engineering. The secondary sources were from text

books, reports, journals, articles from books of professional engineers and the use of internet.

The data in the literature review is intended to highlight on what other scholars had disposed in

relation to the research study.

3.4 Data Collection Methods

The researcher employed the use of interviews and the observation method, as instruments for

data collection.

3.4.1 Use of Interviews

The interviews were structured, for instance asking each informant (cooks, students and school

authorities) similar questions, all rotating around energy requirements and waste generated so as

to get the total capacity and select how much biogas can be generated per hour. The reliability of

information gathered will be of high essence since similar questions will be asked to many

different key players and this will give in-depth information about particular cases of interest to

me. However, am anticipating that the research may be limited by the responses given, because

the respondents might assume that they are under investigation hence being cautious.

16

On the other hand informal interviews were used where non structured questions will be

followed but a frame work of key points around which investigative discussions were built. It

was used to gather information from lecturers and design Engineers. This method was used to

gather information about the prices of the materials and some devices involved in the design

from which financial analysis of the design will be established. Some of the questions to students

included how much food waste every student gives out per day and how much firewood is used

per day and the equivalent of this as far as biogas requirements.

3.4.2 Observation

Through the on-site observation, the data in relation to the operation kitchen waste based biogas

design was collected. This helped me to know the quantity of kitchen waste generated per day

and hence formed a basis for the choice of the energy output and efficiency for this design.

3.4.3 Library Research and Use of Internet

This acted as a supplementary method of information gathering. It was used to crosscheck the

information and constants got using other techniques. Here, I dug deep to get more clear

information about the background of the study and the other design concepts. This kind of

research involved the use of technical reports, personal documents, project dissertation reports

and design journals and surfing from the internet.

3.4.4 Comparative Analysis Method

This method was used to compare the different oil extraction types used in industries and at

home based on their functional requirements and effects.

The sizes of the components for the biogas unit and the design specifications were determined by

the use of Calculations and formulas. These formulas about the design were got from text books

and design manuals.

They were used in establishing the financial analysis of the biogas unit design that were chosen

for example calculating the payback period.

Computer aided design programs like solid works and AutoCAD were used to develop the

drawings for the unit components.

3.5 Design Parameters (James Kuria, 2008)

Design Parameter Weighting Factor (1-5)

Physical Conditions 2

Availability 4

Capacity (Gas Production) 5

Strength (Pressure Holding Capacity) 5

Leakage 1

Durability 5

Fabricability 2

Cost 4

17

Maintenance and Servicing 3

Safety 5

Human Factors 4

From the Weighting Factor table shown above, it is evident that some design specifications

heavily outweigh and hold more importance than others. The design specifications that had a

weighting factor of five, therefore being of utmost important, are capacity, product cost, safety,

durability, human factors, health issues, and environmental conditions. As a result, those design

specification were be always considered when choosing a final design.

Objectives, Methods of data collection and their Description.

S/N Objective Method of data

collection

Description

1 To study the different existing

biogas digester models in Uganda.

Desk Research They was used to obtain data

concerning different literature

that exists when it comes to

digesters currently in use in

Uganda.

Observation This provided first-hand data on

the advantages and

disadvantages of the different

digesters.

Interviews This provided data about the

different digester models in

Uganda.

2 To determine the rational design

specifications of the most efficient

design

Desk Research

and Observation

Using internet and books, the

dimensions of the different

components of the digester were

determined

3 To do a financial evaluation for the

appropriate design for extracting

biogas from Kitchen waste.

Desk research

and Interviews

Using internet and books, the

payback period and the return on

investments were calculated.

18

3.7 Tools used for data collection

• Use of computer aided engineering design tools such as solid edge and solid works

software to develop engineering drawings , components and a model

During the process of data collection, the following tools were employed in attaining data that is

vital for the analysis of the design parameters of the various components of the biogas digester.

a) Trolley-type weighing scale (Scale with accuracy of up to 1000Kgs); this will be used

in the determination of the weight of the food waste generated per day.

b) Tape measure (Scale with accuracy of up to 15 Metres); this was vital in the

determination of the dimensions (lengths and breadths) of the gas holder and slurry tanks.

c) A Timer (accuracy of up to seconds); this was used to measure the time required to

cook food and heat water.

3.8 Data Processing, Presentation and Analysis

The data from respondents was collected, recorded and discussed for the better choice of the

design specifications.

The method applied in the analysis was quantitative in nature; interview responses and that got

through observation noted. After collecting of this data, it was analysed extracting meaningful

information hence easing the establishment of the financial analysis.

The process of data analysis mainly constituted of two major processes and they include; editing

and tabulation.

Editing involved crosschecking errors and omissions in the instruments in order to ensure

accuracy, uniformity and completeness. I ensured that each of the research instruments was clear,

logical and hence achievement of comprehensive responses.

Tabulation was the last stage of data processing where counting and adding of all answers to

particular questions about production capacities was done for the whole study. It involved

allocating individual answers of individual respondents to particular questions. Under tabulation,

computer packages were used like EXCEL to handle the quantitative data processing.

19

CHAPTER FOUR: PRESENTATION AND DISCUSSION OF FINDINGS

4.1 Overview

This chapter presents the findings on the different studies made, the various components

considered in the design of the kitchen-waste based biogas digester and the intended design

under study.

4.2 Digester Model For Consideration and Ranking

Weighting factors were evaluated for the different biogas digesters currently in use in Uganda.

These were based on the design parameters under consideration. These were the 11 design

parameters upon which each digester model was ranked;

Physical Conditions

Availability

Capacity (Gas Production)

Strength (Pressure Holding Capacity)

Leakage

Durability

Fabricability

Cost

Maintenance and Servicing

Safety

Human Factors

As stipulated from the methodology, each of these carried a specific weight from 1 to 5

according to how important it was in the design. Results from the tabulated analysis are shown

below for the four different biogas digester designs.

20

4 Weighting Table

Floating

Drum

Digester

Carmatec

(Fixed

Dome)

Plastic

Digester

Design

ARTI

Physical

Conditions

0 2 0 0

Availability

4 4 4 4

Capacity

(Gas

Production)

0 5 0 0

Leakage

0 0 1 1

Durability

0 5 0 5

Fabricability

2 2 2 2

Cost

4 4 4 4

Maintenance

and

Servicing

0 3 0 0

Safety

5 5 5 5

Human

Factors

0 4 0 4

Strength

(Pressure

Holding

Capactity)

0 5 0 5

TOTAL 15 39 16 30

Note: A 0 here means it fails to meet the required specifics, it doesn’t imply the absence of such

a quality, but simply implies that is scores low on that end.

According to the ranking system, CARMATEC which is a Floating Drum Digester design is the

best upon which to base our design scoring 39, ARTI comes it closely at 39 while the other two

designs score very low at 15 and 16 respectively thus ruling them out for design consideration.

CARMATEC is chosen as the final choice for the following reasons. It has a 24 hour biogas

output (maximum gas output daily) which is appropriate for the energy needs of the school. It

can sustain very high pressures. In addition, it has a service free maintenance for over 30 years.

21

4.3 Available Feedstock/Waste In The School

It was found out that the waste in the school was of three major types;

Kitchen Waste

Human Waste

Animal Waste

4.3.1 Kitchen Waste

Kitchen waste is made up of the following:

Food Waste/ food leftovers

Waste Water

Peelings (Matooke, Sweet potatoes etc)

This included the food leftovers from the servings of Breakfast, Lunch and Supper. Data here

was collected for a period of 7 days in order to get an accurate estimate of the average waste

generated per day.

The School Menu is as below

5 School Menu

Days Breakfast Lunch Evening Tea Supper

Monday Soya and Sugar

in porridge

accompanied

with a bun

Posho, fried

beans and

cabbages (fried)

Bla

ck T

ea

Posho and fried

beans

Tuesday Soya and Sugar

in porridge

accompanied

with a bun

Posho and fried

beans

Rice (pan oiled)

and fried beans

Wednesday Milk and Sugar

in porridge

accompanied

with a bun

Posho, fried

beans and friend

cabbages

Posho and fried

beans

Thursday Soya and Sugar

in porridge

accompanied

with a bun

Posho and fried

peas

Posho and friend

beans

Friday Milk and Sugar

in porridge

accompanied

with a bun

Posho and fried

beans

Rice (pan oiled)

and fried beans

Saturday Soya and Sugar Sweet Posho and fried

22

in porridge

accompanied

with a bun

potatoes/posho,

fried beans and

fried cabbages

beans

Sunday Milk and Sugar

in porridge

accompanied

with a bun

Matooke with

G.nuts or rice

with meat

Posho and fried

beans

After a 7 day survey, the following was noted;

6 Foodwaste Generated Over 7 day period

All In Kilograms TOTAL TOTAL

FOOD

WASTE

PER

DAY

Breakfast Lunch Supper

Monday

50 180 170 400 Cooked

3 15 10 28 143 Uneaten

40 40 35 115 Leftovers

Tuesday

49 180 290 519 Cooked

5 10 2 17 122 Uneaten

36 34 35 105 Leftovers

Wednesday

50 180 182 412 Cooked

5 12 15 32 153 Uneaten

40 41 40 121 Leftovers

Thursday

51 180 180 411 Cooked

4 13 15 32 149 Uneaten

40 35 42 117 Leftovers

Friday

50 180 290 520 Cooked

5 10 3 16 125 Uneaten

39 40 30 109 Leftovers

Saturday

49 160 160 369 Cooked

3 30 30 63 172 Uneaten

35 34 40 109 Leftovers

Sunday

50 290 185 525 Cooked

5 5 29 39 154 Uneaten

40 35 40 115 Leftovers

TOTAL

FOOD

WASTE IN

A WEEK

1018

23

AVERAGE

FOOD

WASTE

145.4

It is thus established that a total of 1018kg of food waste is generated per 7 days. This gives an

average of 145.4kg of food waste per day.

4.3.2 Animal Waste

The school has a farm that consists of pigs and cows.

COWS PIGS TOTAL animals

NUMBER 3 36 37

BIOGAS YIELD

*For Pig Slurry (0.25-

0.50 per kg)

*For Cattle Slurry

(0.20-0.30 per kg)

3*0.20=0.6cubic

metres of biogas per

day

36*0.25=9 cubic

metres of biogas per

day

TOTAL YIELD 0.6 + 0=9.6 cubic metres of biogas per day

4.3.3 Human Waste

The school is made up of 1100 students. According to the complete biogas handbook, the daily

biogas yield from a human being weighing 50kg is 0.028 of biogas per day taking the daily

amount of excrement to be 0.50kg and that of urine to be 1kg. Thus 1100 students can produce at

least 30.8 of biogas per day.

4.4 Energy Needs/ Requirements of The School

The energy needs are divided into three categories:

Cooking needs

1100 students require 0.3 *1100=330 of gas per day. Thus, to meet the cooking

requirements, the system must be designed to meet this capacity.

Heating

Lighting

One Lamp requires at least 0.1 /h of gas.

24

7 Energy Requirements of The School

Number Number Of Lights Biogas Requirement

Classrooms 25 4 lights (fluorescent

tube per

classroom)=4*25=100

lights

100*0.1 /h=10

Dormitories 11 6 lights 6*0.1 /h=0.6

Laboratories 4 4*4=16 lights 16*0.1 /h=1.6

Dining Hall 1 10 lights 10*0.1 /h=1

TOTAL GAS

REQUIRED 24

day

(10+0.6+1.6+1)=13.2

*24 hours=316.8

/h

Suitable digesting temperature 20 to 35 °C

Retention time 40 to 100 days

Biogas energy 6kWh/m3 = 0.61 L diesel fuel

Biogas generation 0.3 – 0.5 m3 gas/m

3 digester volume per

day

Human yields 0.02 m3/person per day

Cow yields 0.4 m3/Kg dung

Gas requirement for cooking 0.3 to 0.9 m3/person per day

Gas requirement for one lamp 0.1 to 0.15m3/h

Biogas guideline data. Adapted from WERN 1

4.5 Design of a Digester To Produce Biogas

The biogas digester design model has 8 basic components:

1. Mixing Pit or Inlet: This is where manure and water are measured and mixed before

feeding them into the digester. It is equipped with (a) sluice gate usually made of wood to

control or allow for the proper mixture of water and manure before the release of the

mixture into the digester, and (b) cover –which can be made of recycled corrugated G.I

sheet.

25

2. Inlet Pipe: This serves as a conveyor of the manure-water mixture or slurry from the

mixing pit to the digester. It is a straight slanting pipe, using prefabricated concrete

culvert 8 inches minimum inside diameter.

3. Digester/Gas Storage: This is where the slurry is allowed to ferment through bacterial

action and where gas is being stored. It is a water and air-tight structure , some features of

the digester are;

a) The flooring of the digester is concave or saucer-type where the inorganic solids and

parasite eggs settle and collect.

b) The wall is made of concrete hollowblocks with water-proofing plaster. The

inlet/outlet pipes fit midway the wall height.

c) Ring Beam, which acts as the ―foundation‖ of the dome. Made of reinforced plastic

concrete; it indicates correct slurry level when the digester is being filled initially.

The gas storage is fixed into the digester. It is that portion above the ringbeam or the space inside

the dome. The dome is made of reinforced concrete and is plastered twice and finally sealed with

paraffin or wax for complete proofing.

4. Outlet Chamber: It serves 2 important functions. (a) Where the effluent residue is taken

out; and (b) where the slurry is forced out when the gas pressure within digester/gas

storage exceeds atmospheric pressure.

The chamber consists of three parts:

(a) Outlet Pit---is circular in shape, made of concrete hollowblocks with plastering, and

having a volume to 1/3 of the volume of the digester/cylinder ( ).

(b) Outlet pipe—is prefabricated round concrete pipe with 8-inch inside diameter (same

as inlet pipe).

(c) Cover---to keep rain water, debris and children from falling into the pit. It can be

made recycled G.I. sheet.

5. Removable Manhole: It provides access to the digester for cleaning, inspection and

maintenance. It is made of concrete and is water-sealed. Asphalt material is used for

gasket seal.

6. Gas Outlet Pipe: It is located through the manhole sleeve. It is of 1-inch G.I. pipe.

7. Stirrer/Mixer: This is a mechanical device inside the digester used to stir the fermenting

slurry to stimulate gas production and to break the ―scum‖ layer forming at the surface of

the slurry. It is fabricated from G.I. pipes and flat bars. (The only component that requires

welding.)

8. Backfill: It serves to protect and insulate the concrete dome from the sun (dry and heat)

and provides rain water runoff. Soil and gravel with 70% and 30% proportion

respectively recommended.

26

4.5.1 Two Cases Of Digesters For Consideration

From available data collected; it is clear that to design a digester, there will be two cases to be

considered and upon which a choice can be made;

Case A: (Basing on Energy Requirements)

1. How much gas does the school need daily? (Overriding Consideration)

2. What digester volume is needed to produce this amount of gas?

3. How much daily volume of feedstock will be required?

4. What is the cost involved? (Feasibility/Viability Study)

In this case the biogas digester being designed must have a capacity of at least (330+316.8) m3/h

of biogas per day. Thus, to installed capacity had to be 646.8 m3 per day which is roughly

estimated to 650 m3 per day. Therefore, the researcher then set out to design a biogas digester of

the CARMATEC type with 650 m3 biogas per day.

Case B: (Basing on Available Feedstock)

27

1. How much raw materials and organic wastes are available? (Overriding Consideration)

2. What digester volume is needed to handle these materials?

3. What is the amount of gas expected?

4. How will this gas be utilized?

5. What is the cost involved? (Feasibility/Viability study)

Basing on available feedstock, the digester design must have an installed capacity of 20 cubic

metres. 9.6 coming from the livestock while the balance is met by the kitchen waste.

4.5.2 Design Formulas:

For structural stability and efficient performance, the design of a Chinese biogas model is

governed by certain mathematical formulas which are as follows:

1. h/d=1/3 That is, the diameter of the digester is three times its height.

2. /d=1/5 That is, the distance from the ringbeam to the manhole is one fifth the diameter.

3. That is, the distance from the bottom center to the wall bottom is one-tenth the

diameter.

4. =1/3 That is, the volume of the outlet chamber, is one third the slurry chamber

volume .

5. + That is, the slurry volume, Vs is equal to the volume of the digester

below the ringbeam.

6. Height of Inlet/Outlet pipes=1/2h That is, the inlet and outlet pipes are placed one-half

the height of the wall.

7. Vslurry=0.85Vt That is, the slurry volume is 85% the total digester volume, Vt.

Vdome=0.15Vt That is, the gas chamber volume (dome) is 15% of the total digester

volume, Vt. This volume relationship allows for gas pressure sufficient enough to force

the slurry to the outlet chamber.

8. Mixing pit volume should be slightly larger than the daily charge.

9. Manhole dimensions are standard for all volumes of digesters.

28

4.6 Final Design

To choose the final design, two approaches were compared (A and B). Case A would

base on the energy requirements while Case B would base on the available feedstock.

Thus, Case B was chosen as the final approach.

29

30

(Engineers, 2010)

The biogas plant size has a volume of 20cubic metres produced per day. Thus, the

components of the final design are as follows:

A. 264cm

B. 176cm

C. 233cm

D. 86cm

E. 203cm

F. 199cm

G. 293cm

H. 115cm

I. 137cm

J. 203cm

4.7 Economic Analysis

The techno-economic analysis for the project was used to evaluate the favourability and

profitability of the biogas investment project. The economic indicators considered were the

investment costs, the liquidation yield, the total operational costs, total income and revenues,

cost comparison, cost annuity comparison, profitability, static Pay Back Period (PBP), Net

Present Value (NPV) and Internal Rate of Return (IRR).

Procedure For Financial Evaluation

The procedure for financial evaluation is determined by Finck and Oelert (1985).

31

1985).

The cost of a Carmatec model in Uganda is estimated to be $1000 which is about Ugshs

3,002,000 at an exchange rate of IUSD=Ugshs3002.0.

*** The fixed dome design biogas plant has no moving or corroding parts and does not entail

significant repair and maintenance costs. Furthermore, all structures are made with masonry or

concrete and last for a very long time, unlike with tubular designs.

Payback Period=Cost of Project/Saving Per Month

Saving Per Month=Expenditure Before Installation of Biogas Plant-Expenditure After

Installation

Expenditure Per Term (3 Months):

The school uses 14 Lorries of firewood per term at a cost of Ugshs450, 000 per lorry.

This means an expenditure of Ugshs6, 300,000.

The school also uses 5000kg of briquettes which is a total of 125 bags at a cost of 4

million Uganda shillings per term.

Total expenditure on fuel per term is 6,300,000+4,000,000=Ugshs10, 300,000. A year has three

terms thus the school spends Ugshs30, 900,000 per year on fuel.

Savings Made Per Term

The school needs 330 of gas per day for cooking and heating purposes. The biogas generated

from the available feedstock is 20 per day. Percentage saved in form of gas is

⁄ *100=6.06% which is roughly estimated to 6%.

Thus 6% is saved by the school on fuel. Expressing this in a form of cash saved per term

6%* Ugshs10, 300,000=Ugshs 618,000 saved on fuel per term which comes to a saving of

Ugshs1, 854,000 per year.

Payback Period=10,300,000/1,854,000=5.55 years estimated to 6 years. Thus it will take 6

years to recover the money invested in the project.

32

CHAPTER FIVE:

CONCLUSION AND RECOMMENDATIONS

5.0 Conclusions

The project is feasible since it saves the school at least 6% on fuel expenses. And the payback

period is 6 years which implies that in 6 years the school will have recovered all the money

invested. The digester has a lifetime of 30 years thus; the excess years will be the profitable

periods assuming zero operation and maintenance costs.

• Different existing biogas digester technologies currently in use were studied. Carmatec

was found to be the most appropriate for our kitchen-waste design

• Rational design specifications and selection of the most efficient design was made.

• A financial evaluation for the appropriate design of extracting biogas from kitchen waste

was carried out. The project was found to be both feasible and viable basing on expected

rates of return and payback period of 6 years.

5.1 Recommendations

A study should be done on the biogas potentials of various kitchen and food wastes and

their combinations.

Food waste and biogas production for various digester models should be compared. This

will help in knowing which digester model really gives the best output when using food

waste.

Further study could then be carried out by adjusting suitable values of the factors like

C/N ratio, pH value and temperature by available methods.

33

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35

APPENDIX