CHAPTER ONE

1.0 INTRODUCTION

In today’s energy demanding life style, need for exploring and exploiting new

sources of energy which are renewable as well as eco-friendly is a must. In rural

areas of developing countries various cellulosic biomass (cattle dung, agricultural

residues, etc.) are available in plenty which have a very good potential to cater to

the energy demand, especially in the domestic sector. In India alone, there are an

estimated over 250 million cattle and if one third of the dung produced annually

from these is available for production of biogas, more than 12 million biogas

plants can be installed (Kashyap et al., 2003).

Biogas technology offers a very attractive route to utilize certain categories of

biomass for meeting partial energy needs. In fact proper functioning of biogas

system can provide multiple benefits to the users and the community resulting in

resource conservation and environmental protection. (www.need.org)

The high demand for fuelwood which is a big business in places like Nigeria, has

monumentally grown with the rise in the population of the world, leading to the

loss of trees and forests, through a process called deforestation. However, people

seem to forget that the wood emanates from the trees in the forest, and despite

certain regulations concerning the use of woods, the activities of illegal loggers

cannot be entirely supervised or curtailed. The deforestation level is higher than

the forestation efforts, and this has resulted in environmental degradation. The

greatest hindrance to the observance of these regulations is the absence of

1

alternative source of fuel such as the kerosene and other petroleum domestic fuels

(www.biogas-renewablenergy.info/biogas_resources.html)

Biogas is a gaseous fuel containing 60% methane, 40% carbon dioxide and small

amount of hydrogen sulphide, nitrogen and hydrogen. It is obtained from

biomass-plant and animal materials, by the process of anaerobic (absence of

oxygen) digestion or fermentation, of which the in-feed to the biogas plant

includes: urban waste (garbage), urban refuse, agricultural waste, cow dung-case

study. Biogas is used as biofuels which originates from biogenic material, can be

used for cooking, heating, generating electricity and running a vehicle.

(http://ezinearticles.com/?Biogas-Technology)

1.1 PROBLEM STATEMENT

Because of the population explosion, and related energy demands of deforestation

level due to the over use of fuelwood by the felling of trees and reduction of

forest, are higher than forestation efforts, they have resulted to environmental

degradation.

Another major cause of the environmental degradation, which has become the

greatest threat to the health of the environment and the economy of the

underdeveloped worlds, and most especially the developing and the developed

worlds, is the use of fossil fuels. These fossil fuels are non-renewable, although

the major source of the energy and income of the economy of many countries

such as Nigeria. But with the discovery and utilisation of biogas, which is a

gaseous fuel obtained from biomass by the process of anaerobic digestion, most

problems (e.g. air, land, water pollution) and associated with the fossil fuels, are

being resolved.

2

1.2 AIM AND OBJECTIVES

The aim of this research project is to design; fabricate and test run a bench scale

bioreactor for the production of biogas and the objectives for the research are:

i. Design and fabricate a bioreactor using available and non-expensive material.

ii. Test run a bioreactor using cow dung, to obtain biogas as a biofuel to be

recommended as a substitute for the non-renewable and expensive domestic fuels

Kerosene.

iii. To calculate and record the cumulative volumes of biogas being produced after

the anaerobic digestion.

1.3 JUSTIFICATIONS

The justifications of this research project are as follows:

i. Anaerobic production of biogas does not produce any offensive smell, causing

reduction of pollution, hence making it environmentally friendly.

ii. It is cost efficient.

iii. The raw materials are cheap and readily available.

iv. It reduces greenhouse effect.

v. Greatly increases the fertilizer value of the manure and protects water source.

vi. Biogas is a renewable energy resulting from biomass which replaces fossil

energy.

vii. Electricity resulting from biogas can be sold to the electricity distributors.

viii. For agronomic advantages, transformation of liquid manure into fertilizer, more

easily assimilated by the plants, with reduction in the odours and disease causing

agents.

3

1.4 SCOPES

The scopes for this research are as follows:

i. Design the bioreactor

ii. Fabricate the bioreactor

iii. Test run with cow dung.

4

CHAPTER TWO

2.0 LITERATURE REVIEW

Biogas simply refers to a gas produced by the biological

breakdown/disintegration of organic matter in the absence of oxygen. Organic

waste such as dead plant and animal material, animal dung, and kitchen waste can

be converted into a gaseous fuel known as the ‘Biogas’. Biogas originates from

biogenic material (i.e resulting from biological activities) and is a type of biofuel,

which is generally used to describe secondary renewable fuels which are obtained

by thermo chemical processing or the bioconversion of biomass especially

carbonaceous waste materials.

Biogas is a gaseous fuel obtained from biomass by the process of anaerobic

digestion or fermentation, when bacteria degrade biological material in the

absence of oxygen. The in-feed or raw material to the biogas plant or bioreactor

for the production of biogas includes:

i. Manure (for example cow dung)

ii. Urban waste(garbage)

iii. Green waste

iv. Sewage

v. Agricultural waste.

Biogas comprises primarily methane (CH4), and carbon dioxide CO2 and may

have small amount of hydrogen sulphide (H2S), moisture and siloxanes. The

gases methane, hydrogen and carbon monoxide (CO) can be combusted or

5

oxidized with oxygen. This energy release allows s biogas to be used as a fuel.

Biogas can be used as fuel in any country for any heating purpose, such as

cooking. It can also be used in anaerobic digesters where it is typically used in a

gas engine to convert the energy in the gas into electricity and heat. Just like the

natural gas, the biogas can also be compressed and used to power motor vehicles.

In the UK, for example, biogas is estimated to have the potential of replacing

around 17% of vehicle fuel. Biogas is a renewable fuel, so it qualifies for

renewable energy subsidies in some parts of the world. Biogas can also be

cleaned and upgraded to natural gas standards when it becomes bio-methane.

Figure 2.1 shows the cycle of how biogas is produced, its sources and how it is

being utilized.

Figure 2.1: The Biogas cycle

6

2.1 ORGANIC WASTE

In order to haven an in-depth of biogas, it is very vital to have a good

understanding of organic waste; what they are, their examples, and their

environmental impacts.

An organic waste therefore, is anything that comes from plants or animals that is

biodegradable, which are substances that will decay relatively quickly as a result

of the action of bacteria and break down into elements such as carbon that are

recycled naturally.(Encarta Dictionaries, 2009).

2.1.1 Types of waste

Several types of organic waste can be used:

i. Agri-food industries waste: They are very charged organic effluents and are

potentially strong in methane. This includes the Fruit and vegetable by-products,

and canteen waste,

ii. Green waste: Waste of the communities - shearing of grass, sheets etc and have

seasonal character.

iii. Wastewater treatment plants sludge : They are divided into the Primary and

secondary sludge.

iv. Grease: They could be obtained from restaurant vats of degreasing and are

potentially strong in methane.

Other examples of wastes are: Eggshells, rice, beans, cheese, bones, frozen

refrigerated food, paper towels etc

2.1.2 Impacts of Organic Waste

Organic waste has both negative and positive impacts to the environment and its

entire component (biotic and abiotic). The following are some specific areas of

the impact of organic waste.

7

a. Contamination

Much of the land used for waste disposal cannot be reused in the future because

of contamination. This occurs when rubbish in landfills is compressed and the air

is squeezed out. The rubbish breaks down anaerobically (without oxygen), which

means that acids are produced. The acids affect other rubbish items, such as

plastic, to create a toxic mix known as leachate. Leachate collects at the bottom of

landfills where it then seeps into the ground water and from there into the

waterways.

b. Landfill space

In many areas the land allocated to waste disposal is rapidly filling up.

Approximately half of all household waste is organic. Most of this waste can be

recycled through composting – turning waste materials into a rich soil supplement

for use in your garden. By composting, not only can you help to reduce the

amount of waste that goes into landfill but you can also help to reduce

contamination and greenhouse gasses.

c. Greenhouse gases

As organic waste decomposes in landfill it produces the greenhouse gases,

methane and carbon dioxide.

These greenhouse gases contribute worldwide climate change. Most landfill gas

is made up of 54% methane and 40% carbon dioxide. Methane is twenty four

times more damaging as a greenhouse gas than carbon dioxide. Scientists predict

that climate change will impact on all our lives, especially in the areas of

agriculture and human health.

8

2.1.3 Benefits of Compositing

i. Compost improves drainage in clay soils and helps sandy soils retain water.

ii. Compost assists plant growth and disease resistance.

iii. Compost also helps to absorb and filter runoff, protecting streams from erosion

and pollution.

iv. Composting reduces unwanted insects, limiting the need for commercial

herbicides or pesticides, therefore preventing runoff pollution.

v. You won't have to bag and drag garden waste to the kerb for collection or pay to

have it trucked to the tip.

2.1.4 Composition of Cow Dung

The composition of Cow dung slurry is given in the table 2.1.

Table 2.1: Table showing the composition of Cow dung

Component Percentage

Nitrogen (N2) 1.8-2.4%

Phosphorus (P2O5) 1.0-1.2%

Potassium (K2O) 0.6-0.8%

Organic humus 50-75%

About one cubic foot of gas may be generated from one pound of cow manure at

around 28°C.

2.2 BIOMASS

Biomass is any organic matter—wood, crops, seaweed, animal wastes— that can

be used as an energy source. Biomass is probably our oldest source of energy

after the sun. For thousands of years, people have burned wood to heat their

homes and cook their food.

9

Biomass gets its energy from the sun. All organic matter contains stored energy

from the sun. During a process called photosynthesis, sunlight gives plants the

energy they need to convert water and carbon dioxide into oxygen and sugars.

These sugars called carbohydrates, supply plants and the animals that eat plants

with energy. Foods rich in carbohydrates are a good source of energy for the

human body.

Biomass is a renewable energy source because its supplies are not limited. We

can always grow trees and crops, and waste will always exist.

2.2.1 Types of Biomass

We use four types of biomass today—wood and agricultural products, solid

waste, landfill gas and biogas, and alcohol fuels.

i. Wood and Agricultural Products

Most biomass used today is home grown energy. Wood—logs, chips, bark, and

sawdust—accounts for about 49 percent of biomass energy. But any organic

matter can produce biomass energy. Other biomass sources include agricultural

waste products like fruit pits and corncobs.

Wood and wood waste, along with agricultural waste, are used to generate

electricity. Much of the electricity is used by the industries making the waste; it is

not distributed by utilities, it is cogenerated. Paper mills and saw mills use much

of their waste products to generate steam and electricity for their use. However,

since they use so much energy, they need to buy additional electricity from

utilities.

Increasingly, timber companies and companies involved with wood products are

seeing the benefits of using their lumber scrap and sawdust for power generation.

This saves disposal costs and, in some areas, may reduce the companies’ utility

10

bills. In fact, the pulp and paper industries rely on biomass to meet half of their

energy needs. Other industries that use biomass include lumber producers,

furniture manufacturers, agricultural businesses like nut and rice growers, and

liquor producers.

ii. Solid Waste

Burning trash turns waste into a usable form of energy. One ton (2,000 pounds) of

garbage contains about as much heat energy as 500 pounds of coal. Garbage is

not all biomass; perhaps half of its energy content comes from plastics, which are

made from petroleum and natural gas.

Power plants that burn garbage for energy are called waste-to-energy plants.

These plants generate electricity much as coal-fired plants do, except that

combustible garbage—not coal—is the fuel used to fire their boilers. The

electricity from garbage, costs more than that from coal and other energy sources.

The main advantage of burning solid waste is that it reduces the amount of

garbage dumped in landfills by 60 to 90 per cent, which in turn reduces the cost

of landfill disposal. It also makes use of the energy in the garbage, rather than

burying it in a landfill, where it remains unused.

iii. Landfill Gas

Landfill gas is a complex combination of different gases created by the action of

microorganisms within a landfill, which is a carefully engineered depression in

the ground (or built on top of the ground, resembling a football stadium) into

which wastes are put. The aim of having this landfill is to avoid any hydraulic (or

water-related) connection between the wastes and the environment, most

especially the groundwater (www.ejnet.org/landfill). Bacteria and fungi are not

picky eaters. They eat dead plants and animals, causing them to rot or decay. A

11

fungus on a rotting log is converting cellulose to sugars to feed itself. Although

this process is slowed in a landfill, a substance called methane gas is still

produced as the waste decays.

New regulations require landfills to collect methane gas for safety and

environmental reasons. Methane gas is colourless and odourless, but it is not

harmless. The gas can cause fires or explosions if it seeps into nearby homes and

is ignited. Landfills can collect the methane gas, purify it, and use it as fuel.

Methane, the main ingredient in natural gas, is a good energy source. Most gas

furnaces and stoves use methane supplied by utility companies. In 2003, East

Kentucky Power Cooperative began recovering methane from three landfills. The

utility now uses the gas at five landfills to generate 16 megawatts of electricity—

enough to power 7,500 to 8,000 homes.

Today, a small portion of landfill gas is used to provide energy. Most is burned

off at the landfill. With today’s low natural gas prices, this higher-priced biogas is

rarely economical to collect. Methane, however, is a more powerful greenhouse

gas than carbon dioxide. It is better to burn landfill methane and change it into

carbon dioxide than release it into the atmosphere.

Methane can also be produced using energy from agricultural and human wastes.

Biogas digesters are airtight containers or pits lined with steel or bricks. Waste

put into the containers is fermented without oxygen to produce a methane-rich

gas. This gas can be used to produce electricity, or for cooking and lighting. It is a

safe and clean-burning gas, producing little carbon monoxide and no smoke.

Biogas digesters are inexpensive to build and maintain. They can be built as

family-sized or community-sized units. They need moderate temperatures and

moisture for the fermentation process to occur. For developing countries, biogas

12

digesters may be one of the best answers to many of their energy needs. They can

help reverse the rampant deforestation caused by wood-burning, and can reduce

air pollution, fertilize over-used fields, and produce clean, safe energy for rural

communities. Examples of biomass used as the raw materials for the production

of biogas are shown in Figure 2.2.

Figure 2.2: Types of Biomass.

Almost half of the biomass used today comes from burning wood and wood

scraps such as saw dust. More than one-third is from biofuels, principally ethanol,

that are used as a gasoline additive. The rest comes from crops, garbage, and

landfill gas.

Industry is the biggest user of biomass. Over 51 percent of biomass is used by

industry. Electric utilities use 11 percent of biomass for power generation.

Biomass produces 0.7 percent of the electricity we use.

Transportation is the next biggest user of biomass; almost 24 percent of biomass

is used by the transportation sector to produce ethanol and biodiesel.

13

The residential sector uses 11 percent of the biomass supply. About one-tenth of

American homes burn wood for heating, but few use wood as the only source of

heat.

2.2.2 Using Biomass Energy

Usually we burn wood and use its energy for heating. Burning, however, is not

the only way to convert biomass energy into a usable energy source. There are

four ways:

i. Fermentation: There are several types of processes that can produce an

alcohol (ethanol) from various plants, especially corn. The two most

commonly used processes involve using yeast to ferment the starch in the

plant to produce ethanol. One of the newest processes involves using

enzymes to break down the cellulose in the plant fibres, allowing more

ethanol to be made from each plant, because all of the plant tissue is

utilized, not just the starch.

ii. Burning: We can burn biomass in waste-to-energy plants to produce

steam for making electricity, or we can burn it to provide heat for

industries and homes.

iii. Bacterial Decay: Bacteria feed on dead plants and animals, producing

methane. Methane is produced whenever organic material decays.

Methane is the main ingredient in natural gas, the gas sold by natural gas

utilities. Many landfills are recovering and using the methane gas

produced by the garbage.

iv. Conversion: Biomass can be converted into gas or liquid fuels by using

chemicals or heat. In India, cow manure is converted to methane gas to

14

produce electricity. Methane gas can also be converted to methanol, a

liquid form of methane.

2.2.3 Biomass and the Environment

Environmentally, biomass has some advantages over fossil fuels such as coal and

petroleum. Biomass contains little sulphur and nitrogen, so it does not produce

the pollutants that can cause acid rain. Growing plants for use as biomass fuels

may also help keep carbon dioxide levels balanced. Plants remove carbon dioxide

—one of the greenhouse gases—from the atmosphere when they grow.

2.3 RENEWABLE RESOURSES

Renewable energy is energy which comes from natural resources such as

sunlight, wind, rain, tides, and geothermal heat, which are renewable (naturally

replenished) and the renewable power capacities with respect to years are

represented on Figure 2.3. About 16% of global final energy consumption comes

from renewables, with 10% coming from traditional biomass, which is mainly

used for heating, and 3.4% from hydroelectricity. New renewables (small hydro,

modern biomass, wind, solar, geothermal, and biofuels) accounted for another 3%

and are growing very rapidly.(Renewables 2011: Global Status Report) The share

of renewables in electricity generation is around 19%, with 16% of global

electricity coming from hydroelectricity and 3% from new renewables.

(Renewables 2011: Global Status Report).

Wind power is growing at the rate of 30% annually, with a worldwide installed

capacity of 238 gigawatts at the end of 2011, and is widely used in Europe, Asia,

and the United States. (Alex M., 2011). At the end of 2011 the photovoltaic (PV)

capacity worldwide was 67 GW, and PV power stations are popular in Germany

15

and Italy. Solar thermal power stations operate in the USA and Spain, and the

largest of these is the 354 megawatt (MW) SEGS power plant in the Mojave

Desert. The world's largest geothermal power installation is the Geysers in

California, with a rated capacity of 750 MW. Brazil has one of the largest

renewable energy programs in the world, involving production of ethanol fuel

from sugarcane, and ethanol now provides 18% of the country's automotive fuel.

Ethanol fuel is also widely available in the USA. (World Energy Assessment

(2001), Renewable energy technologies). While many renewable energy projects

are large-scale, renewable technologies are also suited to rural and remote areas,

where energy is often crucial in human development. As of 2011, small solar PV

systems provide electricity to a few million households, and micro-hydro

configured into mini-grids serves many more. Over 44 million households use

biogas made in household-scale digesters for lighting and/or cooking and more

than 166 million households rely on a new generation of more-efficient biomass

cook stoves. United Nations' Secretary-General Ban Ki-moon has said that

renewable energy has the ability to lift the poorest nations to new levels of

prosperity. (Renewables 2011)

Climate change concerns, coupled with high oil prices, peak oil, and increasing

government support, are driving increasing renewable energy legislation,

incentives and commercialization. New government spending, regulation and

policies helped the industry weather the global financial crisis better than many

other sectors. According to a 2011 projection by the International Energy

Agency, solar power generators may produce most of the world’s electricity

within 50 years, dramatically reducing the emissions of greenhouse gases that

harm the environment (Ben S., 2011).

16

Figure 2.3: Global renewable power capacity excluding hydro

(www.wikipedia.org)

Renewable energy flows involve natural phenomena such as sunlight, wind, tides,

plant growth, and geothermal heat, as the International Energy Agency explains

that the renewable energy is derived from natural processes that are replenished

constantly. In its various forms, it derives directly from the sun, or from heat

generated deep within the earth. Included in the definition is electricity and heat

generated from solar, wind, ocean, hydropower, biomass, geothermal resources,

and biofuels and hydrogen derived from renewable resources.

(Renewables, 2010)

Renewable energy provides 19% of electricity generation worldwide. Renewable

power generators are spread across many countries, and wind power alone

already provides a significant share of electricity in some areas: for example, 14%

in the U.S. state of Iowa, 40% in the northern German state of Schleswig-

17

Holstein, and 20% in Denmark. Some countries get most of their power from

renewables, including Iceland and Paraguay (100%), Norway (98%), Brazil

(86%), Austria (62%), New Zealand (65%), and Sweden (54%).

(Renewables, 2010)

Solar hot water makes an important contribution to renewable heat in many

countries, most notably in China, which now has 70% of the global total (180

GWth). Most of these systems are installed on multi-family apartment buildings

and meet a portion of the hot water needs of an estimated 50–60 million

households in China. Worldwide, total installed solar water heating systems meet

a portion of the water heating needs of over 70 million households. The use of

biomass for heating continues to grow as well. In Sweden, national use of

biomass energy has surpassed that of oil. Direct geothermal for heating is also

growing rapidly. Transport fuels. Renewable biofuels have contributed to a

significant decline in oil consumption in the United States since 2006. The 93

billion liters of biofuels produced worldwide in 2009 displaced the equivalent of

an estimated 68 billion liters of gasoline, equal to about 5% of world gasoline

production. (www.wikipedia.org)

2.3.1 Mainstream forms of Renewable Energy

The following energies are examples of renewable energy.

i.Wind power.

ii. Hydro Power.

iii. Solar Energy

iv. Biomass

v. Biofuels

vi. Geothermal energy.

18

2.4 NON-RENEWABLE SOURCES

A non-renewable resource is a natural resource which cannot be produced,

grown, generated, or used on a scale which can sustain its consumption rate, once

depleted there is no more available for future needs. Also considered non-

renewable are resources that are consumed much faster than nature can create

them. Fossil fuels (such as coal, petroleum, and natural gas), nuclear power

(uranium) and certain aquifers are examples shown and on figure 2.4. In contrast,

resources such as timber (when harvested sustainably) or metals (which can be

recycled) are considered renewable resources.

Natural resources such as coal, petroleum (crude oil) and natural gas take

thousands of years to form naturally and cannot be replaced as fast as they are

being consumed. Eventually natural resources will become too costly to harvest

and humanity will need to find other sources of energy. Natural resources such as

coal, petroleum (crude oil) and natural gas take thousands of years to form

naturally and cannot be replaced as fast as they are being consumed. Eventually

natural resources will become too costly to harvest and humanity will need to find

other sources of energy. (McClain et al, 2007)

Figure 2.7: Examples of Energy sources. (www.wikipedia.org)

19

2.5 BIOGAS

Biogas is a gas that has a composition of about 50-70% Methane (CH4), 30-50%

Carbondioxide (CO2) with the remaining gases being: H2, O2, H2S, N2 and water

vapor, generated from the anaerobic digestion of organic waste. To ensure

optimal Bio-gas production, the three groups of micro-organisms must work

together. In case of too much organic waste, the first and second groups of micro-

organisms will produce a lot of organic acid which will decrease the pH of the

reactor, making it unsuitable for the third group of micro-organisms. This will

result in little or no gas production. On the other hand, if too little organic waste

is present, the rate of digestion by micro-organisms will be minimal and

production of Bio-gas will decrease significantly. Mixing could aid digestion in

the reactor but, too much mixing should be avoided as this would reduce bio-gas

generation. Table 2.2 below shows the amount of biogas generated from animal

waste and agriculture residue. (www.biogasworks.org)

Table 2.2: Amount of bio-gas generated from animal waste and agriculture

residue animal. (www.apo-tokyo.org/BiogasGP3.pdf)

Animal Gas produced L/kg-solid

Pig 340-550Cow 90-310Chicken 310-620Horse 200-300Sheep 90-310Straw 105Grasses 280-550Peanut shell 365Water Hyacinth 375

20

2.6 THE ANAEROBIC DIGESTION PROCESS.

Anaerobic biodegradation of organic material proceeds in the absence of

oxygenand the presence of anaerobic microorganisms. AD is the consequence of

a series ofmetabolic interactions among various groups of microorganisms. It

occurs in threestages, hydrolysis/liquefaction, acidogenesis and methanogenesis.

The first group of microorganism secretes enzymes, which hydrolyses polymeric

materials tomonomers such as glucose and amino acids. These are subsequently

converted bysecond group i.e. acetogenic bacteria to higher volatile fatty acids,

H2 and aceticacid. Finally, the third group of bacteria, methanogenic, convert H2,

CO2, andacetate, to CH4. These stages are described in detail below. The AD is

carried out in large digesters as shown in Figure 2.8 that are maintained at

temperatures ranging from 30° C -65° C.

Figure 2.8: The digesters at Tilburg Plant in The Netherlands

(http://www.steinmuller-valorga.fr/en)

The following are the various stage by stage anaerobic digestion processes, which

is also illustrated in the figure 2.9 below.

21

2.6.1 Hydrolysis/liquefaction

In the first stage of hydrolysis (figure 2.9), or liquefaction, fermentative bacteria

convert the insoluble complex organic matter, such as cellulose, into soluble

molecules such as sugars, amino acids and fatty acids. The complex polymeric

matter is hydrolyzed to monomer, e.g., cellulose to sugars or alcohols and

proteins to peptides or aminoacids, by hydrolytic enzymes, (lipases, proteases,

cellulases, amylases, etc.) secreted by microbes. The hydrolytic activity is of

significant importance in high organic waste and may become rate limiting. Some

industrial operations overcome this limitation by the use of chemical reagents to

enhance hydrolysis. The application of chemicals to enhance the first step has

been found to result in a shorter digestiontime and provide a higher methane yield

(RISE-AT, 1998).

Below are examples of Hydrolysis/Liquefaction reactions:

Lipids → Fatty Acids

Polysaccharides → Monosaccharides

Protein → Amino Acids

Nucleic Acids → Purines & Pyrimidines

2.6.2 Acetogenesis

This is the second stage (from figure 2.9), where an acetogenic bacteria, also

known as acid formers, convert the products of the first phase to simple organic

acids, carbon dioxide and hydrogen. The principal acids produced are acetic acid

(CH3COOH), propanoic acid (CH3CH2COOH), butanoic acid

(CH3CH2CH2COOH), and ethanol (C2H5OH). The products formed during

acetogenesis are due to a number of different microbes, e.g., syntrophobacter

wolinii, a propionate decomposer and sytrophomonos wolfei, abutyrate

22

decomposer. Other acid formers are clostridium spp.,

peptococcusanerobus,lactobacillus, and actinomyces (www.biogasworks.com-

Microbes in AD)

An acetogenesis reaction is shown below:

C6H12O6→ 2C2H5OH + 2CO2

2.6.3 Methanogenesis

As seen in figure 2.9 also, the final and also the third stage, methane is produced

by bacteria called methane formers(also known as methanogens) in two ways:

either by means of cleavage of acetic acid molecules to generate carbon dioxide

and methane, or by reduction of carbondioxide with hydrogen. Methane

production is higher from reduction of carbondioxide but limited hydrogen

concentration in digesters results in that the acetatereaction is the primary

producer of methane (Omstead et al, 1980). The methanogenic bacteria include

methanobacterium, methanobacillus, methanococcus and methanosarcina.

Methanogens can also be divided into two groups: acetate and H2/CO2consumers.

Methanosarcina spp. and methanothrix spp. (also, methanosaeta)are considered to

be important in AD both as acetate and H2/CO2 consumers. Themethanogenesis

reactions can be expressed as follows:

CH3COOH → CH4 + CO2

(acetic acid) (methane) (carbon dioxide)

2C2H5OH + CO2→ CH4 + 2CH3COOH

(ethanol)

CO2 + 4H2 → CH4 + 2H2O

(hydrogen) (water)

(www.need.org)

23

Figure 2.9: Anaerobic Digestion Diagram. (Naskeo Environnement, 2009)

2.6.4 General Process Description

Generally the overall anaerobic digestion process can be divided into four stages:

Pretreatment, waste digestion, gas recovery and residue treatment. Most digestion

systems requirepre-treatment of waste to obtain homogeneous feedstock. The

preprocessinginvolves separation of non-digestible materials and shredding. The

waste received byanaerobic digester is usually source separated or mechanically

sorted. The separationensures removal of undesirable or recyclable materials such

as glass, metals, stonesetc. In source separation, recyclables are removed from the

organic wastes at the source. Mechanical separation can be employed if source

24

separation is not available.However, the resultant fraction is then more

contaminated leading to lower compostquality (RISE-AT, 1998).

The waste is shredded before it is fed into the digester. Inside the digester, the

feed is diluted to achieve desired solids content and remainsin the digester for a

designated retention time. For dilution, a varying range of watersources can be

used such as clean water, sewage sludge, or re-circulated liquid fromthe digester

effluent. A heat exchanger is usually required to maintain temperature inthe

digesting vessel (Figure 2.10). The biogas obtained in AD is scrubbed to

obtainpipeline quality gas. In case of residue treatment, the effluent from the

digester is dewatered,and the liquid recycled for use in the dilution of incoming

feed. Thebiosolids are aerobically cured to obtain a compost product.

(www.need.org)

Figure 2.10: The flow diagram of low solids Anaerobic digestion.

(http://www.soton.ac.uk/~sunrise/anaerobicdig.htm#ADsolidwaste)

25

2.6.5 Benefit of Bio-Gas Technology

The following benefits will be obtained from bio-gas technology:

(i) Energy

Bio-gas could be used as a fuel alternative to wood, oil, LPG and electricity.

(ii) Agriculture

The use of sludge from the bio-gas reactor could be used as compost. Organic

nitrogen from waste will be transformed into ammonia nitrogen, a form of

nitrogen which plants can uptake easily.

(iii) Protect environment

Using bio-gas technology on animal waste treatment will reduce risk of infection

from parasite and pathogenic bacteria inherent in the waste. Odor and flies will be

significantly reduced in the area, and water pollution created by the dumping of

waste can also be prevented.

2.7 REVIEW OF BIOGAS REACTOR.

The anaerobic digestion of organic waste materials, such as farm manure, litter,

garbage, and night-soil, accompanied by the recovery of methane for fuel, has

been an important development in rural sanitation during the last few decades.

This development is basically an extension of the anaerobic process for sludge

digestion used in municipal sewage treatment to small digestion-tank installations

on farms. These farm plants comprise of one or more small digesters and a gas-

holder. Manure and other wastes are placed in a tank which is sealed from

atmospheric oxygen, and are permitted to digest anaerobically. The methane gas,

which is produced during the anaerobic decomposition of the carbonaceous

materials, is collected in the gas-holder for use as fuel for cooking, lighting,

26

refrigeration, and heating, and for other domestic or agricultural purposes, such as

providing power for small engines. This method provides for the sanitary

treatment of organic wastes, satisfactory control of fly breeding, efficient and

economical recovery of some of the waste carbon as methane for fuel, and

retention of the humus matter and nutrients for use as fertilizer. Most of the farm

installations have, so far, utilized only animal manure and organic litter; however,

night-soil can be satisfactorily treated together with the other wastes in these

digesters if adequate digestion time is allowed to permit the destruction of the

pathogenic organisms and parasites. Such a practice has many advantages on

farms and in villages where water-carried sewage disposal is not available. The

use of the digestion tank can eliminate the dangerous insanitary practice of

allowing night-soil to be deposited on fields, and in the immediate environment

of homes, without proper treatment. Straw, weed trimmings, or any other type of

cellulose materials may be digested together with the manure and night-soil for

the production of methane.

Digester tanks with gas collection are particularly advantageous in areas which

are short of fuel and where animal dung is burned for cooking. The burning of

dung destroys, with digestion, the valuable nitrogen and other nutrients which

could be used as fertilizer. The nitrogen, phosphorus, potash, and other nutrients

are retained in the tank as humus and liquid while much of the carbon and

hydrogen are evolved as methane, for collection and use as fuel.

The quality of the humus is similar to that obtained from aerobic composting, and

when the liquid is utilized together with the solids as fertilizer; practically all of

the fertilizer nutrients are reclaimed. The evolved gas, which consists

approximately of two-thirds methane and one-third carbon dioxide, will contain

27

4500 to 6000 calories per cubic meter, thus providing a convenient source of heat

at low cost. One cubic meter of the gas at 6000 calories is equivalent to the

following quantities of other fuels: 1,000 liters of alcohol; 0.800 liters of petrol;

0.600 liters of crude oil; 1.500 m3 of commonly manufactured city gas; 1.400 kg

of charcoal; and 2.2 kilowatt-hours of electrical energy.

The gas can be stored in the gas-holder and piped into the house to provide clean

fuel for cooking and lighting. It has a slight barn-yard odor by which any leaks

can be readily detected, and a very low toxicity since it contains very little carbon

monoxide—the toxic constituent of most city gas. It burns with a violet flame

without smoke. Since a considerable amount of CO2 is mixed with the methane,

the risk of fire or explosion is somewhat less than in the case of city gas.

However, every precaution should be taken to avoid obtaining a mixture of

methane and air, except when the methane is burned as an open flame. Mixtures

of 5% - 14% methane in air are explosive when large quantities are ignited.

There are several basic factors to be considered when constructing or purchasing

a digester installation. These are: (1) climate; (2) single or multiple family

installations; (3) amount of wastes available; (4) gas production; (5) location of

digesters; (6) gas requirements and storage. (www.biogasworks.com)

2.8 VARIOUS ANAEROBIC DIGESTION SYSTEMS.

Anaerobic digestion processes can be classified according to the total solids (TS)

content of the slurryin the digester reactor. Low solids systems (LS) contain less

than 10 % TS, medium solids (MS) contain about 15%-20%, and high solids (HS)

processes range from22% to 40% (Tchobanoglous, 1993). Aanerobic digestion

processes can be categorized further on thebasis of number of reactors used, into

28

single-stage and multi-stage. In single stageprocesses, the three stages of

anaerobic process occur in one reactor and areseparated in time (i.e., one stage

after the other) while multi-stage processes make use of two or more reactors that

separate the acetogenesis and methanogenesis stagesin space. Batch reactors are

used where the reactor is loaded with feedstock at thebeginning of the reaction

and products are discharged at the end of a cycle. The othertype of reactor used,

mostly for low solids slurries, is continuous flow where thefeedstock is

continuously charged and discharged.As noted earlier, the anaerobic digetion

systems treat various types of waste-streams and in someplants MSW is mixed

with sewage sludge or other type of waste. These types ofprocesses will be

discussed in more detail later. (www.biogasworks.com)

2.9 IMPORTANT OPERATING FACTORS IN ANAEROBIC DIGESTION

PROCESS

The rate at which the microorganisms grow is of paramount importance in the

anaerobic digestionprocess. The operating parameters of the digester must be

controlled so as toenhance the microbial activity and thus increase the anaerobic

degradation efficiencyof the system. Some of these parameters are discussed in

the following section.

i. Digestion period.

The digestion period is also called the Retention time. The required retention time

for completion of the AD reactions varies with differing technologies, process

temperature, and waste composition. The retention time for wastes treated in

mesophilic digester range from 10 to 40 days. Lower retention times are required

29

in digesters operated in the thermophilc range. A high solids reactor operating in

the thermophilic range has a retention time of 14 days.

(Personal Communication with M. Lakos, May 2001).

Digestion period = Volume of ReactorDailyinput of waste ………………………………..

(2.1)

ii. Waste composition/Volatile Solids (VS)

The wastes treated by Anaerobic Digestion may comprise a biodegradable

organic fraction, a combustible and an inert fraction. The biodegradable organic

fraction includes kitchen scraps, food residue, and grass and tree cuttings. The

combustible fraction includes slowly degrading lignocellulosic organic matter

containing coarser woodpaper, and cardboard. As these lignocellulosic organic

materials do not readily degrade under anaerobic conditions, they are better suited

for waste-to-energy plants.

Finally, the inert fraction contains stones, glass, sand, metal, etc. This fraction

ideally should be removed, recycled or used as land fill. The removal of inert

fraction prior to digestion is important as otherwise it increases digester volume

and wear of equipment. In waste streams high in sewage and manure, the

microbes thrive and hydrolyse the substrate rapidly whereas for the more resistant

waste materials such as wood, digestion is limited.

The volatile solids (VS) in organic wastes are measured as total solids minus the

ashcontent, as obtained by complete combustion of the feed wastes. The volatile

solidscomprise the biodegradable volatile solids (BVS) fraction and the refractory

volatilesolids (RVS). Kayhanian (1995) showed that knowledge of the BVS

30

fraction of MSW helps in better estimation of the biodegradability of waste, of

biogasgeneration, organic loading rate and C/N ratio. Lignin is a complex organic

materialthat is not easily degraded by anaerobic bacteria and constitutes the

refractoryvolatile solids (RVS) in organic MSW. Waste characterized by high VS

and low non-biodegradable matter, or RVS, is best suited to anaerobic digestion

treatment. The compositionof wastes affects both the yield and biogas quality as

well as the compost quality.

iii. pH Level

Anaerobic bacteria, specially the methanogens, are sensitive to the

acidconcentration within the digester and their growth can be inhibited by

acidicconditions. The acid concentration in aqueous systems is expressed by the

pH value,i.e. the concentration of hydrogen ions. At neutral conditions, water

contains a concentration of 10-7 hydrogen ions and has a pH of 7. Acid solutions

have a pH less than 7 while alkaline solutions are at a pH higher than 7. It has

been determined(RISE-AT, 1998) that an optimum pH value for AD lies between

5.5 and 8.5. During digestion, the two processes of acidification and

methanogenesis require different pH levels for optimal process control. The

retention time of digestate affects the pH value and in a batch reactor

acetogenesis occurs at a rapid pace. Acetogenesis can lead to accumulation of

large amounts of organic acids resulting in pH below. Excessive generation of

acid can inhibit methanogens, due to their sensitivity to acid conditions.

Reduction in pH can be controlled by the addition of lime or recycled filtrate

obtained during residue treatment. In fact, the use of recycled filtrate can even

eliminate the lime requirement. As digestion reaches the methanogenesis stage,

31

the concentration of ammonia increases and the pH value can increase to above 8.

Once methane production is estabilized, the pH level stays between 7.2 and 8.2.

iv. Temperature

There are mainly two temperature ranges that provide optimum digestion

conditions

for the production of methane – the mesophilic and thermophilic ranges. The

mesophilic range is between 20°C - 40°C and the optimum temperature is

considered to be 30o-35oC. The thermophilic temperature range is between 50°C-

65°C (RISEAT, 1998). It has been observed that higher temperatures in the

thermophilic range reduce the required retention time. (National Renewable

Energy Laboratory, 1992).

v. Carbon to Nitrogen Ratio (C/N)

The relationship between the amount of carbon and nitrogen present in organic

materials is represented by the C/N ratio. Optimum C/N ratios in anaerobic

digesters are between 20 – 30. A high C/N ratio is an indication of rapid

consumption of nitrogen by methanogens and results in lower gas production. On

the other hand, a lower C/N ratio causes ammonia accumulation and pH values

exceeding 8.5, which is toxic to methanogenic bacteria. Optimum C/N ratios of

the digester materials can be achieved by mixing materials of high and low C/N

ratios, such as organic solid waste mixed with sewage or animal manure.

vi. Total solids content (TS)/OrganicLoading Rate (OLR)

As discussed earlier, Low solids (LS) AD systems contain less than 10 % TS,

medium solids (MS) about 15-20% and high solids (HS) processes range from

22% to 40% (Tchobanoglous, 1993). An increase in TS in the reactor results in

acorresponding decrease in reactor volume.

32

Organic loading rate (OLR) is a measure of the biological conversion capacity of

the AD system. Feeding the system above its sustainable OLR results in low

biogas yield due to accumulation of inhibiting substances such as fatty acids in

the digester slurry (Vandevivere, 1999). In such a case, the feeding rate to the

system must be reduced. OLR is a particularly important control parameter in

continuous systems. Many plants have reported system failures due to

overloading (RISE-AT, 1998).

vii. Mixing

The purpose of mixing in a digester is to blend the fresh material with digestate

containing microbes. Furthermore, mixing prevents scum formation and avoids

temperature gradients within the digester. However excessive mixing can disrupt

the microbes so slow mixing is preferred. The kind of mixing equipment and

amount ofmixing varies with the type of reactor and the solids content in the

digester.

viii. Basic Design

The central part of an anaerobic plant is an enclosed tank known as the digester.

This is an airtight tank filled with the organic waste, and which can be emptied of

digested slurry with some means of catching the produced gas. Design differences

mainly depend on the type of organic waste to be used as raw material, the

temperatures to be used in digestion and the materials available for construction.

Systems intended for the digestion of liquid or suspended solid waste (cow

manure is atypical example of this variety) are mostly filled or emptied using

pumps and pipe work. A simpler version is simply to gravity feed the tank and

allows the digested slurry to overflow the tank. This has the advantage of being

able to consume more solid matter as well, such as chopped vegetable waste,

33

which would block a pump very quickly. This provides extra carbon to the system

and raises the efficiency. Cow manure is very nitrogen rich and is improved by

the addition of vegetable matter.

ix. Compost

When the digestion is complete, the residue slurry, also known as digestate, is

removed, the water content is filtered out and re-circulated to the digester, and

thefilter cake is cured aerobically, usually in compost piles, to form compost. The

compost product is screened for any undesirable materials, (such as glass

shards,plastic pieces etc) and sold as soil amendment. The quality of compost is

dependent on the waste composition. Some countries have prescribed standards

for compost quality. The U.S. Department of Agriculture has set standards for

heavy metals in the compost . These standards are for compost treated by the

aerobic process but may also be applied to AD compost product.

2.10 GAS REQUIREMENT AND STORAGE

The gas may be used for domestic purposes, such as cooking, heating water, food

refrigeration, and lighting. The following are some approximate quantities of gas

for these different uses : domestic cooking, 2.0 m3 per day for a family of five or

six people; water heating, 3m3 per day for 100-litre tank or 0.600 m3 for a tub

bath and 0.35 m3 for a shower bath; domestic food refrigeration, 2.5-3.0 m3 per

day for a family of five or six people; lighting, 0.100-150 m3 per hour per light.

Since the gas is produced continuously, day and night, but is used largely during

the daytime, it is necessary to provide storage facilities so that the gas will not be

34

wasted and will be available when needed. The storage capacity should be

estimated to meet peak demands. For small installations, storage capacity of

about one day’s requirement of gas should be provided. The volume of the gas-

holder should not be less than about 2.0 m3, even for very small installations. The

gas-holder may be circular or square and should be provided with a water seal to

prevent the escape of gas or admission of air. The weight of the floating cover of

the gas-holder provides the gas pressure.

2.11 TYPES OF ANAEROBIC DIGESTION SYSTEMS.

As discussed previously in the methods used to treat MSW anaerobically can be

classified into following categories:

Single Stage

Multi Stage

Batch

These categories can be classified further, based on the total solids (TS) content

of the slurry in the digester reactor. As noted earlier, low solids (LS) contain less

than 10% TS, Medium solids (MS) contain about 15-20% High solids (HS)

processes range from about 22% to 40%. The single stage and the multi stage

systems can befurther categorized as single stage low solids (SSLS), single stage

high solid (SSHS), multi stage low solids (MSLS) and multi stage high solids

(MSHS). The drawback of LS is the large amount of water used, resulting in high

reactor volume and expensive post-treatment technology. The expensive post

treatment is due to de-watering required at the end of the digestion process. HS

systems require a smaller reactor volume per unit of production but this is

counter-balanced by the moreexpensive equipment (pumps, etc.) required.

35

Technically, HS reactors are morerobust and have high organic loading rates.

Most anaerobic digestion plants built in the 80's werepredominantly low solids

but during the last decade the number of high solidsprocesses has increased

appreciably. There is substantial indication from theobtained data that high solids

plants are emerging as winners.

2.11.1 Single Stage Process

Single stage reactors make use of one reactor for both acidogenic phase as well

asmethanogenic phase. These could be LS or HS depending on the total solids

contentin a reactor.

2.11.2 Single Stage Low Solids (SSLS) Process

Single stage low solids processes are attractive because of their simplicity. Also

theyhave been in operation for several decades, for the treatment of sludge from

thetreatment of wastewater. The predominant reactor used is the continuously

stirredtank reactor (CSTR). The CSTR reactor ensures that the digestate

iscontinuously stirred and completely mixed. Feed is introduced in the reactor at

arate proportional to the rate of effluent removed. Generally the retention time is

14-28 days depending on the kind of feed and operating temperature.Some of the

SSLS commercial AD plants are the Wassa process in Finland, theEcoTec in

Germany, and the SOLCON process at the Disney Resort Complex,Florida

(www.soton.ac.uk). The plant examined in more detail is the Wassa processplant

(10%-15 % TS) that was started in 1989 in Waasa, Finland (Figure 2).

Currently there are three Wassa plants ranging from 3000-85000 tons per

annum,some operating at mesophilic and others at thermophilic temperatures.

The retentiontime in the mesophilic process is 20 days as compared to 10 days in

thethermophilic. The feed used in this process is mechanically pre-sorted MSW

36

mixedwith sewage sludge. The organic loading rate (OLR) differs with the type

of waste.The OLR was 9.7 kg/(m3 day) with mechanically sorted organic MSW

and 6 kg/(m3day) with source separated waste. The gas production was in the

range of 170 Nm3 CH4/ton of VS fed and 320 Nm3 CH4/ton of VS fed and 40-

75% reduction of thefeed VS was achieved (Vandevivere, 1999).The advantages

offered by SSLS are operational simplicity and technology that hasbeen

developed for a much longer time than high solids systems. Also, SSLS makesuse

of less expensive equipment for handling slurries. The pre-treatment

involvesremoving of coarse particles and heavy contaminants. These pre-

treatment stepscause a loss of 15 - 25 % VS, with corresponding decrease in

biogas yield. The othertechnical problem is formation of a layer of heavier

fractions at the bottom of thereactor and floating scum at the top, which indicate

non-homogeneity in the reactingmass. The bottom layer can damage the

propellers while the top layer hinderseffective mixing. This requires periodic

removal of the floating scum and of the heavy fractions, thus incurring lower

biogas yield. Another flaw is the shortcircuiting, i.e a fraction of the feed passes

through the reactor at a shorter retentiontime than the average retention time of

the total feed. This lowers the biogas yieldand impairs hygienization of the

wastes.For the solids content to be maintained below 15%, large volumes of

water areadded, resulting in large reactor volumes higher investment costs, and

amount ofenergy needed to heat the reactor. Also, more energy and equipment are

required forde-watering the effluent stream. The high investment costs associated

with dilutionand reactor volume plus the complex pre-treatment step offset the

gains from the lowcost equipment to handle slurry.

37

2.11.3 Single- Stage High Solids (SSHS) Process

The advances of the HS technology were the result of research undertaken in the

80's that established higher biogas yield in undiluted waste. Some of the examples

of SSHS are the DRANCO, Kompogas, and Valorga processes. The DRANCO

and Valorga processes ar described in more detail later in this thesis. All three

processes consist of a single stage thermophilic reactor (mesophilic in some

Valorga plants)with retention time of 14-20 days.In the DRANCO reactor (Figure

2.12 below), the feed is introduced from the top anddigested matter is extracted

from the bottom. There is no mixing apart from thatoccurring due to downward

plug flow of the waste. Part of the extracted matter isreintroduced with the new

feed while the rest is de-watered to produce the compostproduct.The Kompogas

process (Figure 3b) works similarly, except the movement takesplace in plug flow

in a horizontally disposed cylindrical reactor. Mixing isaccomplished by the use

of an agitator. The process maintains the solidsconcentration at about 23% TS. At

solids content lower than 23%, the heavyfraction such as sand and glass can sink

and accumulate at the bottom; higher TSconcentrations impede the flow of

materials.(Vandevivere, 1999).

Figure 2.12: The DRANCO reactor (A) and Kompogas Reactor (B)

38

(Vandevivere, P. et al, 1999).

The design of the Valorga process is unique. The reactor is a vertical

cylindricalreactor divided by a partial vertical wall in the center (Figure 2.13).

The feed entersthrough an inlet near the bottom of the reactor and slowly moves

around the verticalplate until it is discharged through an outlet that is located

diametrically opposite tothe inlet. Re-circulated biogas is injected through a

network of injectors at thebottom of the reactor and the rising bubble result in

pneumatic mixing of the slurry.The injectors require regular maintenance, as they

are prone to clogging.

Figure 2.13:The Valorga Digester

(The Anaerobic Digestion and the Valorga Process, Jan 1999.)

The high solids content in HS systems requires different handling, mixing and

pretreatment than those used in the LS processes. The equipment needed to

handle andtransport high solids slurries is more robust and expensive than that of

the LS,comprising of conveyor belts, screws, and powerful pumps. On the other

39

hand, thepre-treatment is less cumbersome than for LS systems. The HS systems

can handleimpurities such as stones, glass or wood that need not be removed as in

SSLS.Contrary to the complete mixing prevailing in SSLS, the SSHS are plug-

flowreactors hence require no mechanical device within the reactor (De Baere,

1999).

The economic differences between the SSLS and SSHS are small.SSHS

processes exhibit higher OLRs, as compared to SSLS; for example, OLRvalues of

15 kg VS/m3 per day are reported for the DRANCO plant in Brecht,Belgium,

where, whereas in the Waasa Process the OLR is 6 kg VS/m3 per day. Thebiogas

yield is usually high in SSHS as heavy fractions or the scum layer is notremoved

during the digestion.There are pronounced differences between SSHS and SSLS

reactors, in terms ofenvironmental impacts. The LS process consumes one m3 of

fresh water per tonMSW treated whereas the water use in HS is one tenth of that

(Nolan- ITU, May1999). Consequently, the volume of wastewater to be

discharged is several-fold lessfor HS reactors.

2.11.4 Multi-Stage Process

The introduction of multi-stage AD processes was intended to improve digestion

byhaving separate reactors for the different stages of AD, thus providing

flexibility tooptimize each of these reactions. Typically, two reactors are used, the

first forhydrolysis/liquefaction-acetogenesis and the second for methanogenesis.

In the firstreactor, the reaction rate is limited by the rate of hydrolysis of

cellulose; in the second bythe rate of microbial growth. The two-reactor process

allows to increase the rate ofhydrolysis by using microaerophilic conditions (i.e.,

where a small amount of oxygen issupplied in an anaerobic zone) or other means.

40

For methanogenesis, the optimum growthrate of microbes is achieved by

designing the reactor to provide a longer biomassretention time with high cell

densities or attached growth (also known as “fixed filmreaction”, where the

microbes responsible for conversion of the organic matter areattached to an inert

medium such as rock, or plastic materials in the reactor). Animportant

requirement to be met in such reactors is removal of the suspended particlesafter

the hydrolysis stage. Multi-stage processes are also classified as multi-stage

lowsolids(MMLS) and multi-stage high-solids (MMHS). There is a lot of

similarity, in terms of solids content, pre-treatment steps, handling of waste,

requirement of water etc., between SSLS and MMLS as well as SSHS and

MMHS processes.

2.11.5 Multi-Stage Low Solids Process

Some of the MSLS facilities are the Pacques process (Netherlands), the BTA

process (Germany, Canada) and the Biocomp (Germany) process as shown on

figure 2.14. The Pacques process uses two reactors at mesophilic temperature.

Initially, The feed consisted of fruit and vegetable waste but recently source-

separated MSW is also being processed. The first reactor where hydrolysis occurs

has solids content 10 %. Mixing is achieved by means of gas injection. The

digestate from the first reactor is de-watered, and the liquid is fed to an Upflow

Anaerobic Sludge Blanket reactor where methanogenesis occurs. The fraction of

the digestate from the hydrolysis reactor is re-circulated with the incoming feed to

the first reactor for inoculation. The remaining fraction is sent for compost

production. In the BTA process (Figure 2.14 ) the solid content is maintained at

10% and the reactors are operated at mesophilic temperatures. This process is

41

described in detail in the case study section. It is very similar to the Pacques

process except that the methanogenic reactor is designed with attached growth

(“fixed film reaction”) to ensure biomass retention. The effluent from the

hydrolysis reactor is de-watered and the liquor is fed to the methanogenic reactor.

This reactor receives only the liquid fraction from hydrolysis reactor to avoid

clogging of the attached growth. At times, in order to maintain the pH within the

hydrolysis reactor in the range of 6-7, the process water from the methanogenic

reactor is pumped to the hydrolysis reactor. The multi-stage low solids processes

are plagued with similar problems to those of the SSLS reactors, such as short-

circuiting, foaming, formation of layers of different densities, expensive pre-

treatment. In addition, the MSLS processes are technically more complex and

thus require a higher capital investment. (www.soton.ac.uk)

Figure 2.14: The flow diagram of BTA Process

42

(www.canadacomposting.com).

2.11.6 Multi -Stage High-Solids Process

The Biopercolat process is a multi-stage high-solids process but is somewhat

similar to th Pacques process (MSLS) in that it consists of a

liquefaction/hydrolysis reactor followed by a methanogenic Upflow Anaerobic

Blanket Sludge reactor (UASB) with attached growth. However hydrolysis is

carried out under high solids and microaerophilic conditions (where limited

amount of oxygen is supplied in anaerobic zone). The aeration in the first stage

and the attached growth reaction in the second provide for complete digestion at

retention time of only seven days.

The advocates of multi-stage processes cite the advantages of high OLR for all

types of multi-stage systems, such as 10kg VS/(m3.d) and 15kg VS/(m3.d) for the

BTA(MSLS) and Biopercolat processes (MSHS), respectively. This is due to

higher biomass retention with attached biofilm, which increases the resistance of

methanogens to high ammonium concentrations (Vandevivere, 1999). The

biological stability thus achieved offers potential for increased OLR. However,

high OLR does not result in high biogas yield. The lower biogas yield observed in

practice is due to removal of solids that contain some biodegradable matter, after

the short hydrolysis period before feeding the methanogenic reactor. In recent

years, the single-stage systems have also achieved high OLRS thus canceling this

advantage of multi-stage systems. According to De Baere 1999, commercial

applications of multi-stage systems amount to only 10 % of the current treatment

capacity, as will be discussed later, under current trends of AD systems.

43

2.12 BATCH REACTORS

Batch reactors are loaded with feedstock, subjected to reaction, and then are

discharged an loaded with a new batch. The batch systems may appear as in-

vessel landfills but in fact achieve much higher reaction rates and 50- to 100%

higher biogas yields than landfills for two reasons. First, the continuous re-

circulation of the leachate and second, they are operated at higher temperatures

than landfills. There are three types of batch systems - single stage batchsystem

sequential batch system and an Upflow Anaerobic Sludge Blanket reactor.

(Vandevivere, 1999)

Figure 2.15:Types of Batch Reactors (Vandevivere, et al (1999)

The single-stage batch system involves re-circulating the leachate to the top of the

same reactor An example of such a system is the Biocel process in Lelystad, The

Netherlands that was started in 1997 and treats 35,000 tons/y of source-sorted

biowaste. The system operates at mesophilic temperatures and consists of

fourteen concrete reactors each of 480m3 capacity. The waste fed to these

unstirred reactors is pre-mixed with inoculums. The leachates are collected in

44

chambers under the reactors and recycled to the top of each reactor. The waste is

kept within the reactor for over40 days, until biogas production stops. The Biogas

plant produces on the average 70kg biogas/ton of source-sorted biowaste which is

40 % less than from a single stage low-solids digester treating similar wastes

(Vandevivere, 1999).

The sequential batch process comprises two or more reactors. The leachate from

the first reactor, containing a high level of organic acids, is re-circulated to the

second reactor where methanogenesis occurs. The leachate of the methanogenic

reactor, containing little or no acid, is combined with pH buffering agents and re-

circulated to the first reactor. This guarantees inoculation between the two

reactors.

The third type of batch process is the hybrid batch-UASB process, which is very

similar to the multi-stage process with two reactors. The first reactor is simple

batch reactor but the second methanogenic reactor is an up flow anaerobic sludge

blanket (UASB) reactor. Batch processes offer the advantages of being

technically simple, inexpensive and robust. However, they require a large land

footprint as compared to single-stage HS reactors since they are much shorter and

their OLR two-fold less (Vandevivere, 1999). Other disadvantages are settling of

material to the bottom thus inhibiting digestion and the risk of explosion while

unloading the reactor.

2.13 ECONOMICS OF BIOGAS

Biogas technology is a complete system in itself with its set objectives (cost

effective production of energy and soil nutrients), factors such as microbes, plant

design, construction materials climate, chemical and microbial characteristics of

45

inputs, and the inter-relationships among these factors. Brief discussions on each

of these factors or subsystems are presented in this section.

2.13.1 Economic.

An ideal plant should be as low-cost as possible (in terms of the production cost

per unit volume of biogas) both to the user as well as to the society. At present,

with subsidy, the cost of a plant to the society is higher than to an individual user.

2.13.2 Simple design

The design should be simple not only for construction but also for operation and

maintenance. This is an important consideration especially in areas where the rate

of literacy is low and the availability of skilled human resource is scarce.

2.13.3 Utilization of local materials.

Use of easily available local materials should be emphasized in the construction

of a biogas plant. This is an important consideration, particularly in areas where

transportation system is not yet adequately developed.

2.13.4 Durability.

Construction of a biogas plant requires certain degree of specialized skill which

may not be easily available. A plant of short life could also be cost effective but

such a plant may not be reconstructed once its useful life ends. Especially in

situation where people are yet to be motivated for the adoption of this technology

and the necessary skill and materials are not readily available, it is necessary to

construct plants that are more durable although this may require a higher initial

investment.

46

2.13.5 Suitable for the type of inputs.

The design should be compatible with the type of inputs that would be used. If

plant materials such as rice straw, maize straw or similar agricultural wastes are

to be used, then the batch feeding design or discontinuous system should reused -

instead of a design for continuous or semi-continuous feeding.

2.13.6 Frequency of Using Inputs and Outputs.

Selection of a particular design and size of its various components also depend

on how frequently the user can feed the system and utilize the gas.

Inputs and their Characteristics and any biodegradable organic material can be

used as inputs for processing inside the bio-digester. However, for economic and

technical reasons, some materials are more preferred as inputs than others. If the

inputs are costly or have to be purchased, then the economic benefits of outputs

such as gas and slurry will become low. Also, if easily available biodegradable

wastes are used as inputs, then the benefits could be of two folds: (a) economic

value of biogas and its slurry; and (b) environmental cost avoided in dealing with

the biodegradable waste in some other ways such as disposal in landfill.

2.14 SAFETY CONSIDERATION

Like water, electricity, automobiles and most of life biogas is not completely

safe. Some of it effects include:

i. Asphyxiation

Biogas consists mainly of CH4 and CO2, with low levels of H2S and other gases.

Each of these components has its own problems, as well as displacing oxygen.

CH4 – Lighter than air and will collect in roof spaces, explodes when in mixed

with air.

47

CO2 – Heavier than air and will collect in sumps; slightly elevated levels affect

respiration rate, higher levels displace O2 as well.

H2S – Has a rotten egg smell, destroys olfactory tissues and lungs, and become

odourless as the level increase to dangerous and fatal. The preventive measures

include; adequate ventilation, suitable precautions and adequate protective

equipment will minimize the dangers associated with biogas.

ii. Diseases

Anaerobic digestion relays in a mixed population of bacteria of largely unknown

origin, but often including animal wastes, to carry out waste treatment process,

car should be taken to avoid contact with the digester content and to wash

thoroughly after working around the digester. This also helps to minimize the

spread of odours which may accompany the digestion process.

iii. Fire/Explosion

Methane forms explosive mixtures with air, the lower limit being 5% methane

and the upper limit being 15% methane. Biogas mixtures containing more than

50% methane are combustible while lower percentages may support, or fuel,

combustion. Therefore, no naked flames should be used in the vicinity of a

digester and electrical equipment must be of suitable quality, normally explosion

proof. If conducting a flame test, take a small sample well away from the main

digester or incorporate a flame trap in the supply line.

As biogas displaces O2 level restricting respiration, so any digester are needs to

be well ventilated to minimize the risks of fire/explosion and asphyxiation

(www.adelaide.edu.au/biogas/safety/).

48

CHAPTER THREE

3.0 MATERIALS AND EQUIPMENT

3.1 MATERIALS

The following are the materials were used during the cause of this

research work;

3.1.1 Waste and Water

Waste and water are the materials needed for the experimental process. Also,

water is also used to wash any other equipment that is needed to be washed

before use.

3.1.2 Cattle dung

This is the feed material to the floating drum bio-digester that is mixed with water

and some other needful materials, which undergoes fermentation to produce the

biogas needed.

3.1.3 Single Super Phosphate

This served as source of acid which was added to the slurry during mixing,

decreasing the pH of the slurry, and thereby increases the acidity of the slurry.

3.1.4 Ash

Ash consists majorly of the element known as potassium. In solution in easily

reacts with water to form a basic or alkaline solution (KOH). It serves as the

source of base to the slurry during mixture.

3.2 EQUIPMENT

3.2.1 Weigh Balance; Model PIC 45284

This equipment was used to measure the weigh the waste, water and any other

material that needs to be weighed for the purpose of the laboratory work.

49

3.2.2 Aluminium Sheet

The aluminium sheet was the material used for the fabrication of the bio-

digester.

3.2.3 Thermometer

This equipment/instrument was used to measure the temperature of the

surrounding (ambient) and that of the water in the bio-digester.

3.2.4 Hose

This is a slender rubber tube (about 5-10mm i.e. internal to external diameter)

which was used as a channel of collecting the gas from the digester.

3.2.5 Mortar and Pestle

This was used in reducing the size of the waste in order to increase its surface

area for better digestion of the slurry by the bacteria.

3.2.6 pH Meter JENWAY model 3150

This was used to determine the PH of the waste both before and after digestion.

Figure 3.1 a bio-digester Figure 3.2 Burner

50

Figure 3.3: a pH meter Figure 3.4: a Weighing balance

Figure 3.5: Slurry before digestion Figure 3.6: Slurry after digestion

51

CHAPTER FOUR

4.0 METHODOLOGY

4.1 MATERIAL SELECTION

The material selection for this research work was done considering the feed

material, type of gas to be produced, convenience and availability. Aluminium

was used as the material of fabrication.

4.2 SELECTED BIO-DIGESTER

From all the types of digesters described in the literature survey, with respect to

the scope of this research work, the digester selected for the production of biogas

using Cow-Dung is the ‘Floating drum digester’.

4.2.1 Criterion for selecting Floating drums digester

The following factors are considered for selecting the Floating drums digester in

the production of biogas using cow dung:

i. The design is simple to understand, operate and maintain.

ii. They provide gas at a constant pressure, and the stored gas-volume is

immediately recognizable by the position of the drum.

iii. In terms of production cost per unit volume of biogas, both to user and to

society, the designing and fabricating of the Floating drum digesters is

cost effective.

iv. It is durable, depending on the type of material used in fabrication and

how it is done.

52

4.3 DESIGN, FABRICATION AND ASSEMBLY OF THE BIODIGESTER

The bio-digester was designed and then fabrication was done by a well

experienced fabricator in Samaru. These joints where sealed using a special gum

and then checked again using water and allowed to dry. After the sealing, the

digester was coupled in this manner; the Reactor (layer 2) was put into the water

seal (layer 1) and Gas collector (layer 3) served as the cover of which a tube was

connected to it, leading to the burner. For this fabrication, the Gas collector was

calibrated to observe the changes of volume of gas produced.

4.4 PREPARATION OF SAMPLES

The cow dung being the raw material for the preparation of the sample for the

production of biogas was obtained from the Abattoir at Zango here in Zaria. It

was initially wet, but after transporting it to the school, it was dried by spreading

it in an open air and at ambient temperature. It took about four days for the

substrate (Cow dung) to get dried. After drying the substrate, it was taken to the

department of chemistry to be pounded, in order words, reducing the dried

substrate to smaller size, therefore increasing the surface area of the dried

substrate for better solubility between it and the solvent (water) and also better

digestion by the bacteria. Before mixing the substrate with water and loading it

into the digester, nylons and other foreign materials were carefully picked. The

concentration of substrate from literature (between the ranges of 5-10) was

assumed to be 7.5%.

Volume of reactor is 34380cm3/1000 = 34.38L

7.5% of VR = 0.075×34380= 2578.5 =equivalent value of the weight of substrate.

Therefore, the calculated weight of dry cow dung = 2578.5g.

53

In order to have a favourable pH range for the anaerobic digestion of the slurry,

Single super phosphate (SSP) which is acting as the acid, is added in the ratio of

10:1 = 257.85g. Also, ash which is acting as the base/alkali is also added in the

ratio of 2:1, with respect to the weight of SSP. Therefore, the weight of ash

= 257.85

2

= 128.925g.

The ratio of the cow dung to water mixture is 1:10. Therefore, the weight of tap

water mixed with the cow dung is = 2578.5×10 = 25785g.

The mixed slurry was then poured into the digester tank and sealed properly to

ensure air-tightness. The experiment was subjected to a retention period of 14-

40days.

The temperature of the slurry in digester was observed daily through the

thermometer. The ambient temperature, that is, the temperature of the

surrounding was also measured daily. The pH of the fresh Cow dung as well as

the slurry was determined using the pH Meter JENWAY model 3150 at the

Department of Biochemistry A.B.U, Zaria.

The percentage composition of the Cow dung (Carbon and Nitrogen) was

carried out at I.A.R (Institute for Agricultural Research) A.B.U, Zaria using

Walkley Black method and Kjedhal method.

4.5 COST EVALUATION FOR THE DESIGN AND FABRICATION OF

THE BIO-DIGESTER

The cost of every material, equipment and all other expenses made during the

research work was recorded and the total cost per unit volume of the bio-digester

was also calculated.

54

4.6 TEST RUN OF THE COW DUNG

The cow dung was test ran for a period of 95 days, to observe the retention time

(Digestion period) and also the days of gas production. The gas obtained was

also burned to test its combustibility.

55

CHAPTER FIVE

5.0 RESULTS AND DISCUSSION OF RESULTS

5.1 RESULTS FROM THE MATERIAL SELECTION

The material selection was done considering the feed material, type of gas to be

produced, convenience and availability. Aluminium was used as the material of

fabrication. The reason why Aluminum was selected as the material is because of

its ability to resist corrosion, which is caused by gases such as H2S, which when

mixed with water can cause acid corrosion. Using other materials such as steel

will combine with water and cause the metal to rust, therefore, reducing the

efficiency of the outcome of the research work and also becomes a liability, as it

causes more spending in trying to control it. And so this makes Aluminum the

choicest material for the fabrication of the bio-digester.

5.1.1 Cost Evaluation for the Design and Fabrication of the Bio-Digester

The cost evaluation for the design and fabrication of the bio-digesters is shown on

table 5.2, including the total cost per unit volume of digester.

Table5.2: Cost Evaluation for the Design and Fabrication of Bio-Digester

S/N Description Of Items Quantity Unit Price(N)

Amount(N)

1 Fabrication of Bio-digester(aluminium sheets, fabrication and small size hose)

1 - 6,500

2 Burner 1 500 500

3 Thermometer 1 300 300

4 About 400g of Inoculums (Rumen content)

2 40 80

56

5 Bags of Cow dung 3 60 180

6 Polyethene Bag 6 30 180

7 Detergent 1 20 20

8 Superglue 4 50 200

9 Potti 1 50 50

10 Transportation ------ ------ 500

11 Pair of mortar and pestle 1 800 800

12 miscellaneous ------ ------ 2,500

TOTAL 11,810

VOLUME ……….. ……….. 31,863.57cm3

TOTALCOST /UNIT VOLUME ……….. ……… 3.70643×10-1N/cm3

(Note: N= Naira, Nigerian currency)

5.2 RESULTS FOR THE DESIGN OF THE BIO-DIGESTER

The whole idea behind the bio- digester is the production of biogas with waste

using anaerobic digestion process. Some factors were considered in the design of

the bio-digester including the material selection, and its availability, pressure of

the gas and the volume. The digester is a series of cylinders which include the

water seal and the Gas collector that are coupled together.

The volume of the digester was estimated and calculated. The shape of the

digester is cylindrical. Therefore, the volume was used to determine the size of

the digester. The estimated volume is; V = 31863.57cm3

From the formula of a cylinder given as: V =π R2 H , Where; R = D2 = 13cm;

(Where D=Diameter, R=Radius of the Cylinder)

57

H = 60cm = Height of the Cylinder. The design of the floating drum bio-

digester was done with the aid of AUTOCAD, having all layers placed

accordingly, that is; layer 2 (Reactor), placed in the layer 1(Water seal), and then

layer 3 (Gas collector) placed on layer 2. This is illustrated in figure 5.1.

LEBEL

First layerCylinder 1

Second layerCylinder 2Third LayerCylinder 3

NOTE: All dimensions are in centimetres (cm)

58

Figure 5.1: AUTOCAD design of the Bio-digester with all layers coupled

together

5.3 RESULTS OF FABRICATION OF THE BIO-DIGESTER

In order to account accurately for the volumes of gas produced and any other

calculations needed in this research work, it is vital that the sketched dimensions

of all the layers of the bio-digester was done. The Table 5.1 shows the dimensions

of all the components of the Bio-digester used for various calculations and also

given to the fabricator for fabrication.

Table 5.1: shows the design specification and sketches that was used for the

calculation and fabrication.

Specification Sketches Calculation

HR =60cm

DR =26cm

Layer 2 (Reactor)

60

26

Volume of digester is given as; V=π R2 H

Where; R =13cm,

H = 60cm.

V=π∗(13)2∗60

V=31863.57cm3

Hw =68cm

D w =28cm

Layer 1

(Water seal)

68

28

H c =63cm

Dc =27cm

Layer 3

(Gas collector)

63

27

The following figures are the pictorial fabricated layers of the Bio-Digesters.

59

Figure 5.2a: Layer 1 (Water Seal) Figure 5.2b: Layer 2 (Reactor)

Figure 5.2c: Layer 3 (Gas Collector) Figure 5.2d: Bio-Digester (Layer1,

Layer2 and Layer 3 all together)

60

5.4 RESULTS FROM THE TEST RUN OF COW DUNG IN THE

DIGESTER

The cumulative volumes of biogas produced from day 1 of mixing the cow dung

slurry, to through the digestion period and the biogas production period, are all

illustrated on figure 5.3

1 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86 910

20000

40000

60000

80000

100000

120000

Barchart representation of the Cummulative volume of Biogas and Days (from Day 1 to Day 95)

Series2Series1

Days

Cum

mul

ative

vol

ume

of B

ioga

s

Figure 5.3: a bar chart, representing the cumulative volumes of biogas produced

from day 1 to day 95.

During the course of the project work, three regimes of the biogas production

were observed from the results obtained. This is as a result of notable changes of

the rate of biogas production. They are:

i. the Digestion period (Retention time), from day 1 to day 37,

61

This is the period of anaerobic digestion (fermentation) of the cow dung slurry,

consisting of the three distinct stages of reaction, which are the:

hydrolysis

Acetogenesis, and

Methanogenesis

Although this is the most vital regime of biogas production of which the success

of the biogas production, no biogas was produced. On the 35th day, more

ruminant content (Inoculums) was added to the cow dung slurry to increase the

anaerobic bacteria, to hasten the digestion of the cow dung slurry, thereby

releasing the production of the biogas. This is illustrated by Figure 5.4

0 5 10 15 20 25 30 35 400

0.10.20.30.40.50.60.70.80.9

1

Rate of Biogas production during the Retention time/Digestion Period from Day 1 to Day 37

Series2Linear (Series2)

Days (Retention Time)

Cum

mul

ative

Vol

umes

of B

ioga

s

Figure 5.4: shows that there was no biogas produced during the digestion

period/Retention time.

ii. Rate of Biogas production from day 38 to day 55, and

On the 38th day, there was a notable change in height from the gas collector of

7cm, equivalent to the calculated volume of 4011cm3 of biogas. The cumulative

volume of the biogas from that day was daily taken and recorded. The range of

62

height during this period was between 5 and 7cm. Also, the average rate of biogas

production obtained illustrated by Figure 5.5, was 2863.8cm3/day =

119.325cm3/hour = 1.98875cm3/min.

35 40 45 50 55 600

10000

20000

30000

40000

50000

60000

f(x) = 2863.8173374613 x − 104231.00619195R² = 0.999669769461322

Rate of Biogas production from the 38th Day to the 55th day.

Series2Linear (Series2)

Days

Cum

ulati

ve V

olum

es o

f Bio

gas

Figure 5.5: shows the rate of biogas production from the 38th day to the 55th day,

with respect to the cumulative volumes of Biogas produced and the days.

iii. Rate of biogas production from day 56 to day 95.

From day 56, there was a change (decrease) in the rate of biogas production from

2863.8cm3/day to 1273.1cm3/day. The range of height observed from the gas

collector during this period was between 2 and 3cm. This is shown by figure 5.6.

63

50 55 60 65 70 75 80 85 90 95 1000

20000

40000

60000

80000

100000

120000

f(x) = 1273.14230769231 x − 13242.8192307692R² = 0.995483175708674

Rate of Biogas production from the 56th day to the 95th day.

Series2Linear (Series2)

Days

Cum

ulati

ve v

olum

es o

f Bio

gas

Figure 5.6: shows the rate of biogas production from the 56th day to the 95th day.

5.4.1 Burning Test

After the 38th day, a burning test was carried out several times to ascertain the

combustibility of the gas that has been produced. It was observed that, the gas

produced from the digester and stored in the gas collector was combustible and it

burnt with a blue flame. This is illustrated by the following figures 5.6

Figure 5.7 a: Blue flame from Biogas Figure 5.7 b: match, burner and flame

64

Figure 5.7c: Flames illustrating burning Figure 5.7d: Research student in the

Test, from the Biogas produced. Laboratory with the Bio-digester

65

CHAPTER SIX

6.0 CONCLUSION

From this research project, the following are the conclusions:

i. A floating drum bio-digester was designed and fabricated.

ii. The fabricated bio-digester was test ran using the cow dung as the feed

material.

iii. The digestion of the slurry was done anaerobically (in the absence of air),

for a period of 95 days.

iv. Biogas was produced in the floating Drum Digester with a cumulative

volume of 106432cm3, and was tested to be combustible, using a burner,

with a theoretical calculated equivalent heating value is about 1974.084

BTUm3 .

66

CHAPTER SEVEN

7.0 RECOMMENDATIONS

During the course of this research project, from challenges encountered and

observations noted, the following are my recommendations:

i.The continuous method of the floating drum instead of the batch method should be

applied in subsequent biogas production research project.

ii. Also, other types of digesters such as the bay digesters, fixed drum et al could

also be good platforms of biogas research work.

iii. More research should be carried out on the methane productivity of other waste

types such as chicken droppings, grasses, pig dung et al, so that their operating

conditions can also be optimized.

iv. The biogas project work could also be done not just in a small scale as it has

always been carried out, but in group since its expenses will definitely be high.

67

LIST OF REFERENCES

1. Adelekan B.A., Assessing Nigeria’s Agricultural Biomass Potential as a

Supplementary Energy Resource through Adoption of Biogas Technology.

Nigeria Journal of Renewable Energy. Sokoto Energy Research Center, Vol.10,

Nos. 1&2, Pp. 145-150, 2002.

2. Chynoweth, D.P., C.E. Turick, J.M. Owens, D.E. Jerger and M.W. Peck (1993).

Biochemical methane potential of biomass and waste feedstocks. Biomass and

Bioenergy, 5:95-111.

3. Cornejo, C. and Wilkie, A.C., “Greenhouse gas emissions and biogas potential

from livestock in Ecuador”. Energy for Sustainable Development, 2010.

4. D. Bourn, Wint W., Blench and Wolly E. “Basic information on biogas”, March,

2011.

5. Ezekoye, V.A and Okeke C.E “Design, construction, and performance evaluation

of plastic bio-digester and the storage of biogas.” Pacific journal of science and

Technology. pg 176-184, 2006.

6. Florian N., .Electricity from wood through the combination of gasification and

solid oxide fuel cell. A PhD Thesis, Swiss Federal Institute of Technology

Zurich, 2008.

68

7. Lagrange B. “Bio-methane 2: Principles - Techniques Utilisation.” EDISUD, La

Calade France, 1979.

8. Lincoln, E.P., Wilkie, A.C. and French, B.T. “Cyano-bacterial process for

renovating dairy wastewater. Biomass and Bio-energy”, 1996.

9. Lettinga, G. and van Haandel, A.C. Anaerobic digestion for energy production

and environmental protection. In: Renewable Energy Sources for Fuels and

Electricity. T.B. Johansson et al. (Eds.), Island Press, Washington DC. p.817-839,

1993.

10. Marchaim U., “Biogas Processes for Sustainable Development”, FOA

Agricultural Services Bulletin 95, 1992.

11. Song Y.C, Kwon J.H., “Mesophilic and Thermophilic Temperature Co-Phase

Anaerobic Digestion Compared with Single Stage Mesophilic and Thermophilic

Digestion of Sewage Sludge”. p. 1653-1662. 2004.

12. Smith, W.H., Wilkie, A.C. and Smith, P.H. “Methane from biomass and waste - a

program review”. TIDE (Teri Information Digest on Energy), 2(1):1-20 (1992)

13. State Energy Conservation Office (Texas) “Biomass Energy: Manure for

Fuel.”State Energy Conservation Office (Texas).State of Texas, 2009.

69

14. Tower P., Lombard, “New Landfill Gas Treatment Technology Dramatically

Lowers Energy Production Costs” Applied Filter Technology. (Retrieved, March

2012)

15. Wilkie, A.C., Anaerobic digestion of flushed dairy manure. In: Proceedings -

Anaerobic Digester Technology Applications in Animal Agriculture - A National

Summit. p. 350-354. Water Environment Federation, Alexandria, Virginia, 2003.

16. Wilkie, A.C., Smith, P.H. and Bordeaux, F.M. An economical bio-reactor for

evaluating biogas potential of particulate biomass. Bio-resource Technology,

92(1):103-109, 2004.

www.claverton-energy.com (Accessed march, 2012)

www.afdc.energy.gov (Accessed march, 2012)

www.ashdenawards.org (Accessed march, 2012)

www.clarke-energy.co.uk (Accessed march, 2012)

www.sgc.se/dokument/Evaluation/pdf (Accessed march, 2012)

www.ecotippingpoints .org (Accessed march, 2012)

www.alfagy.com (Accessed march, 2012)

www.nnfcc.co.uk/publications/nnfcc-renewable-fuels-and-energyfactsheet-

anaerobic-digestion (Accessed march, 2012)

www.wikipedia.org (Accessed march, 2012)

www.biomassenergycenter.org.uk (Accessed march, 2012)

www.wiki.answers.com. (Accessed March, 2012)

www.library.thinkquest.org (Accessed march, 2012)

70

www.fao.org/sd/Egdirect/Egre0022.html (Accessed February, 2012)

www.habmigern2003.Info/biogas/biofuels.html (Accessed March, 2012)

www.environment.nationalgeographic.com (Accessed March, 2012)

www.alternative-energy-news.info (Accessed march, 2012)

www.biofuelguide.net (Accessed March, 2012)

www.rsc.org (Accessed march, 2012)

(www.fastonline.org/CD3WD40/BIOGSHTM/EN/APPLDEV/DESIGN/

DIGESTYPES.HTML). (Accessed June, 2012)

71

APPENDIX

DETERMINATION OF TOTAL NITROGEN

(Regular Macro-kjedahl Method)

Apparatus:

Macro-kjedahl digestion-distillation apparatus/Aluminum block digester.

Macro-kjedahl Flask, 500ml or Tubes.

Volumetric Flasks, 100ml.

Reagents:

Concentrated sulphuric acid.

Catalyst- 500gm sodium sulphate (Na2 SO4) or Potassium Sulphate (K2 SO4).

50gm of anhydrous Copper Sulphate. 0.5gm of Selenium powder (mix this

different reagents and ground to powder).

Boric Acid (2%) - Dissolve 20gm of Boric Acid in 1000cm3 volumetric flask.

40% NaOH- Weigh out 400gm of NaOH and dissolve in 800cm3 distilled

water .Leave to cool before making to mark in 1 liter volumetric flask.

0.01M HCL- Measure 0.86ml of concentrated HCL and make it up in a 1000cm3

volumetric flask.

Mix Indicators: Weigh out 0.2gm of methyl blue into a 100cm3volumetric flask

and also add 0.4gm of methyl red into another 100cm3volumetric flask. Dissolve

both indicators with ethanol. Mix content of both flask in a 200cm3volumetric

flask and mix thoroughly.

OR

Methylred-Bromocresol green mixed indicator: Dissolve 0.5gm of bromocresol

green and 0.1gm of methylred in 100cm3of 95% ethanol and adjust PH to 4.50

with 0.1M of NaOH or HCL.

72

Procedure:

Weigh out 2gm dried cow dung into kjedahl flask or digestion tube.

Add 20ml of distilled water swirl the flask for some minutes and allow it to stand

for 30minutes.

Add about 30gm of catalyst and 20ml of H 2 SO4.

Heat continuously on a low heat. When the water has been removed and frothing

has ceased, increase the heat until the digest clears. Then boil the mixture for 5

hours. Regulate the heating during this boiling so that the H 2 SO4 condenses

about half way up the neck of the flask or tube.

Allow the flask to cool and slowly with shaking, add some distilled water.

Carefully transfer the digest into another clean flask. Retain all particles in the

original digestion flask because, the particle can cause severe bumping during

distillation. Wash the residue with 50ml of distilled water for about four times

and transfer each portion into the same flask. Make to mark with distilled water.

Add 10ml of H 3 BO3 into a 500ml Erlenmeyer flask which is then placed under

the condenser of the distillation apparatus. The end of the condenser should be at

about 4cm above the surface of the H 3 BO3 solution.

Keep condenser cool (below 30℃) by allowing sufficient water to flow through

and regulate heat to minimize frothing and prevent suck back.

Collect about 50ml distillate and then stop distillation.

TitrateNH 3 liberated with standard HCL orH 2 SO4. Add 2- 3 drops of indicator.

The colour change at the end point is from green to pink.

Calculate the %N Content in the cow dung thus

Run a blank similar but without cow dung sample.

73

% N = (0.014*VD*N*100*TV)

(Weight of Cow dung*AD)

Where VD= Volume of digest

N= Normality of acid

TV= Titer value

AD= Aliquot of digest.

ORGANIC CARBON DETERMINATION

(Walkley- Black Method)

Apparatus:

Burette; 50ml 0r 25ml

Conical flask;250ml or 500ml

Measuring Cylinders

Pipette

Reagents:

Potassium dichromate (K2 Cr2 O7); 1N- dissolve 49.04gm of K2 Cr2 O7 in distilled

water and dilute to 1 liter.

Concentrated Sulphuric acid (H 2 SO4) if chloride is present in the sample, add (

Ag2 SO4) to the acid at the rate of 15gm per liter.

O- Phosphoric acid (H 3 PO4), concentrated.

O- Phenanthroline – ferrous complex 0.025M (ferrous). When the indicator is not

available it can be prepared by dissolving 14.85gm of O- Phenanthroline

monohydrate and 6.95gm of Fe SO4 .7H 2O in water and dilute to 1 liter.

Barium diphenylamine sulphate (0.16%). This can be used in place of

74

O- Phenanthroline- ferrous complexes (0.25M). Dissolve 7.425gm of O-

Phenanthroline and 3.475gm of ferrous sulphate in distilled water and dilute to

500ml.

Ferrous sulphate (0.5N)- dissolve 140gm of Fe SO4 .7 H 2O in water ;add 15ml

concentrated H 2 SO4 , cool and dilute to 1 liter. Standardize this reagent daily or

each time before determining organic carbon by titrating against 10ml 1N

K 2 Cr2 O7.

Procedure

Take a representative sample and grind to pass through a 0.5mm sieve.

Weigh out 1.00gm cow dung and place in the 250ml flask. Use 2.00gm cow dung

if organic carbon is suspected to less than 1%.

Pipette 10ml 1N K2 Cr2 O7 accurately into each flask and swirl gently to disperse

the cow dung.

Rapidly add 20ml concentrated H 2 SO4 from a measuring cylinder. Immediately

swirl flask gently until cow dung and reagent are mixed, then swirl more

vigorously for one minute. Rotate flask again and allow to stand on a sheet of

asbestos for about 30minutes. End point is easier to observe when suspension is

cool.

Add 100ml of distilled water after standing for 30minutes and let cool again.

Filter suspension if cloudy.

Add 5ml of O- Phosphoric acid (H 3 PO4) to sharpen the end point (optional).

Add 3- 4 drops indicator and titrate with 0.5N ferrous sulphate solution on a

white background. Note; As the end point is approached, the solution takes on a

greenish cast and then changes to dark green. At this point add the ferrous

75

sulphate solution until the color changes sharply from blue to red (maroon color)

in reflected light against a white background.

Make the blank determination in the same way but without cow dung sample to

standardize the dichromate.

Calculate the result according to the following formula:

% organic carbon in cow dung = (meqK2 Cr2 O7 – meq FeSO4)*0.003*100*f

(Air-dry basis) Weight of air- dry cow dung

(Where f=Correction factor = 1.33)

Meq=Normality of solution*ml of solution used

OR

% organic carbon in cow dung = (Blank titer – actual titer) * 0.3* m * f

(Air-dry basis) Weight of air- dry cow dung

Where f= Correction factor= 1.33

m=Concentration of Fe SO4 .

% organic matter (OM) may be calculated thus:

% OM= %OC*1.729

% organic carbon can also be expressed on oven dry basis after correction for

moisture content in air- dry cow dung.

RATES OF BIOGAS PRODUCTION

The cumulative gas as at the 83rd Day was 92,680cm3 (92,680,000mm3=

92.680L= 0.09268m3). Also, the rate of gas production was observed and

calculated to have changed (decreased) from 2863.8cm3day

76

= 119.325cm3hour ×

1 hour60 mins= 1.98875

cm3min (between day 38 and55), to

1273.1 cm3day ×

1day24 hours = 53.046

cm3hour (between the 56th Day and the 95th day).

77