A SEMINAR PRESENTED BY ACHEBE CHIGOZIE UCHECHUKWU CLINTON UCHENNA. 

DEPARTMENT OF MECHANICAL ENGINEERING  TECHNOLOGY 

FEDERAL POLYTECHNIC NEKEDE, OWERRI IMO STATE. 

CERTIFICATION 

We hereby certify that this construction work was carried out by the  following persons: 

ACHEBE CHIGOZIE  

UCHECHUKWU UCHENNA U J.  

ABSTRACT 

Biogas production from waste could be one better way  addressing the issues of waste management and energy problem  in Nigeria. Biogas produced through the proper waste  management in an anaerobic digestion has a huge potential to  be an alternative source of energy to fossil fuel. In this project,  a 200 liter capacity batch sheet metal biogas plant operated at  mesospheric temperature under 40 day hydraulic retention time,  Fabricated at Mechanical engineering Fabrication center, Federal  Polytechnic Nekede, Owerri, Imo State was used for biogas  production of from agricultural waste (pig dung, cow dung,  poultry dropping) and kitchen waste. 21.25kg of each waste was  mixed with water of same weight at a ratio of 1:1 and charged.  The pressure of the slurry was monitored for a certain period of  time. The sample gags production was passed through the gas  chromatography to determine the percentage composition (mol%  

dry basis) of the biogas content. The result of biogas before  refining were 58.10 mol% dry CH4, 35.9mol% dry CO2 and 0.99  mol% dry H2S, which conformed with literature values of 50-70%  mol dry CH4, 30-40% mol dry CO2 and 0-3% mol dry H2S 58.15%  mol dry N2, 0.02% mol dry O2, 0.05% mol dry NH3, 0.47% mol  dry H2.

Keywords: Biogas Digester, Purification Tank Potassium per Magnate, Lime Water, Flange, Flange Bolt, Digester Stand, Discharge Valve, Discharge Hose, Charging Valve

CHAPTER ONE 

1.0 INTRODUCTION 

Biogas refers to a gas produced by the breakdown of organic matter or biodegradable material such as agricultural waste (Crops, animal dung, plants, grasses etc.), industrial waste, Kitchen waste, sewage in the absence of Oxygen. It is regarded variously as biogas, sludge gas, landfill gas and synthetic gas depending on the source of the substrate for the gas production, and also widely regarded as bio energy and fuel of the future. 

For the past years, developing countries and particularly Nigeria  has experienced increase in level of waste generation,  inadequate power supply due to population explosion, increased  agricultural activities and industrial growth. Consequently there  is intense scrutiny of possible alternative of solid waste  utilization through biogas production using organic residue like  poultry drooping cow dung, and kitchen waste. Government and  industries are constantly on the outlook for technology that will  allow for effective and cost effective waste treatment  (Gurnaswamy et al, 2003) and (Alvarez et al, 2006). A certain  technology that has proved itself worthy of successful treatment  of organic fraction of waste, having advantages of producing  energy, yielding high quality fertilizer and preventing disease  transmission is "Anaerobic Digesters”

The digestion process takes place in an air tight container at room temperature and this produces a product called "Biogas". This biogas composed of 50-70% methane (cooking gas) CH4, 30-40% carbon dioxide (CO) (fire extinguisher), 0-3/% Hydrogen sulphide HS and traces of other gases CO, NH3, N2, H2 and water vapor. The Composition of the biogas produced can vary depending on the Substrates (organic materials) used.  (Okeke, O.R, 2009). 

The biogas when refined of carbon dioxide and Hydrogen Sulphide by passing it through Lime or potassium hydroxide and  activated charcoal or potassium permanganate (KMnO4)  improves its efficiency and thermal content so as to Use for  cooking and generation of power. A biogas system becomes  flammable when its methane content is at least 45%  (Http.design-tutorhtm, 2003). Methane has a heating value 22Mj/m² (15.6mj/kg) (FAO, 1979). For optimum biogas yield in  an anaerobic digester system the necessary factors for proper  growth and effective action of microorganisms on the substrates  must be ensured. In an anaerobic digestion process three  temperature ranges are identified: 0.20°c for psychrophilic  organisms, 20-40°c for mesophilic organisms, 50-60°c  thermophilic organisms (des mes ett al, 2003). 

A temperature range between 32-35°c is more efficient for  stable and continuous production of methane (I.O Itodo et al,  1995).

According to Sambo et al, (1995); temperature has significant effect on biogas production as temperature magnitude in excess of 60°c causes gas production to slow down and eventually stop. 

There are other factors that affect biogas production like PH, Carbon to Nitrogen ratio, Stirring, nature of Substrate etc. The  effluent Of this process is a residue rich in essential inorganic elements needed for healthy plant growth known as bio-fertilizer which when applied to the soil enriches it with no detrimental effects on the environment (Energy Commission 1998). 

1.1 BACKGROUND OF STUDY 

A lot of research has been done on the generation of biogas from animal dung, agricultural waste, few from industrial waste. 

Owing to the fact that most kitchen waste are energy producing substances having the essential nutrients for bacterial growth  and the considering the refuse disposal problem in Nigeria this research work tends to deviate from the normal substrate of biogas production by making maximum use of kitchen waste and animal waste (cow dung and poultry dropping) in other to improve the quantity of biogas produced, cleaning the gas so as to remove carbon dioxide and Hydrogen sulphide in other to improve the thermal content or produce burnable gas. This work also extent its arm to reveal the factors that affect biogas production techniques for enhancing biogas production, types of biogas plant and feeding method of biogas plants. 

1.2 STATEMENT OF PROBLEM

Energy is a key factor for the growth and development of a country. Most developing countries in which Nigeria is one  suffers from energy and waste treatment, management crisis,  these has attributed to depletion of locally available energy  resources, high dependency on fossil fuel and environmental  destruction. 

As more and more waste generated through Kitchen and  agricultural activities are disposed in an uncontrollable manner,  the impact on the environment like pollution, disease transmission global warming, erosion etc. becomes significantly  visible. Poor power supply for domestic and industrial use has  led to the death of so many small scale businesses even  discourage its existence thereby leading to poor economic growth of the nation Nigeria. Biogas technology in Nigeria could  be an importance intervention to the problem of energy supply  waste management both in urban and rural communities since  these waste generated has the potentials fore biogas production.  Biogas technology provides alternative source of energy which is  environmental friendly. Bio-slurry, a by-product of biogas, is a  quality organic fertilizer and conditioner for the soil that has the  potential to replace chemical fertilizer.

1.3 OBJECTIVES OF THE STUDY 

1. To provide a renewable source of energy that is environmental friendly. 

2. To provide a technology that is effective for waste  treatment and management.  

3. To reduce the rate of Carbon dioxide and methane emission  to the atmosphere consequently minimizing the rate of  global warming. 

4. To reduce high depending on fossil fuel for energy. 

5. To encourage afforestation and conservation of natural resources. 

6. To provide bio-fertilizer 

7. To reduce high rate of falling of wood used in the rural areas  for cooking and the hazard of exposing oneself to poisonous  snakes in the forest. 

1.4 SIGNIFICANCE OF STUDY 

Utilization of agricultural and kitchen waste for biogas production  can be justified as follows; 

1. Utilization of agricultural and kitchen waste for biogas production could be useful in solving energy problem of the country.

2. Methane and Carbon dioxide which are main greenhouse gases that causes global warming produced during decomposition of waste will be reduced. 

3. Effective means of waste treatment and management are  provided. 

4. To create awareness to the public of an alternative source  of energy generation that is environmental friendly and cheap, thereby reducing the rate of dependency on fossil  fuel. 

5. The effect of mosquitoes, pathogens and odor from  decayed organic material will drastically reduce from the environment. 

1.5 SCOPE OF STUDY 

The scope of this study is limited to the use of agricultural waste (Cow dung, poultry dropping) and kitchen waste to produce biogas, refining of biogas to reduce the carbon dioxide and hydrogen sulphide content. 

1.6 LIMITATION OF THE STUDY 

In the course of this work, the following problem may be encountered. 

1. Difficulty of handling the waste due to its offensive smell and the belief that it has microorganisms that are harmful to health.

2. Lack of the assistance of engineer with technical knowledge of biogas plant construction. 

3. Unavailability of material on similar seminar topic. 

4. Difficulty in maintaining the necessary for biogas  production. 

5. Plant construction requires extensive care, any crack or a leak at ‘any part of the plant may hinder production of the gas. 

6. Long period of time is taken for a burnable gas to be  produced. 

7. Cost, the materials required for success of this work are expensive.

CHAPTER TWO 

2.0 LITERATURE REVIEW 

2.1 HISTORY OF BIOGAS TECHNOLOGY 

Biogas technology which converts biological waste into energy is considered by many experts to be an excellent tool for improving life, livelihood, and health in the developing countries. About 16milion households worldwide use small-scale biogas digesters according to Renewable 2005. Global Status Report, a study by the World Watch Institute. 

Isolated cases of using biogas technology were documented in China, India, Assyria and Persia beginning from 17th B.C,  however systematic scientific research of biogas started in 18th Century. In 17th Century, Jan Baptista Van Halmount first  determined that flammable gas could evolve from decay organic  matter. In 1821 Avogadro Identified methane (CH4). In 1859  first digestion plant was built in India. India being one of the  countries with many biogas plants have enjoyed so many  benefits of this technology called "Biogas Technology". Since  1970s there have been a big increase in number of biogas plant  in India, presently there are about 3.7 million biogas plants in  operation in India; over 10 million persons use biogas centrally  as fuel. 

In China, a speed up in biogas technology has been achieved since 1970s. According to record, more than 5 million small biogas digesters have been constructed and about 20 million persons use biogas currently as a fuel. Germany is Europe's biggest biogas producer (Eurobserver 2011), it is the market leader in biogas technology according to renewable made in  Germany. In 2010 there were 5,905 biogas plants operating throughout the whole country in which lower Saxony, Bavaria, and the eastern Federal States are the main regions (Renewable made-in-Germany 2011). 

In United Kingdom, there are currently about 60-non-sewage biogas plants most on the farm, but some large facilities exists  of farms which are taking food and consumer Waste (The official Information Portal on AD). The presence of this  technology in these countries has helped to reduce the rate of  pollution, disease transmission, greenhouse emission,  dependency on fossil fuel and encourage waste treatment /  management, afforestation, provision of energy most especially  to farmers in rural area and as well as provision of fertilizer for  healthy and quality agricultural produce. Countries like America Sweden and Some parts of African countries like Rwanda,  Senegal, Burkina Faso, Ethiopia, Tanzania, Uganda, Kenya,  Benin and Cameroon have benefited immensely from this  technology. 

Nigeria is not left behind as such technology could be found in some part of its cities like Sokoto, Benin, Zaria, Ibadan, Lagos etc. The pioneer biogas plants are a 10m3 biogas plant constructed in 1995 by the Sokoto Energy Research Center (SERC) in Zaria an 18m biogas plant constructed in 1996 at Ojokoro Ifelodun piggery farm, Lagos by the Federal Institute of Industrial Research Oshodi (FIRO) Lagos (Zuru et al, 1998). 

The biogas plant in Ibadan that runs on abattoir effluents is one  of the largest plant in Arica providing gas to 5400 families a mouth around a quarter or the cost of queried natural gas (from  non-biodegradable). 

The plant in Benin, installed in Guinness Nigeria PLC operates for  the treatment or brewery effluent. The existence of these plants have helped to improve sanitation, reduce pollution rate,  greenhouse gas emission, provision of energy even bio-fertilizer and encourages afforestation and natural gas conservation. A  result study assessed Nigeria’s biogas potential (Minimum value)  from solid waste and livestock excrements, it revealed that in  1999, Nigeria’s biogas potential represents a total of 1.382 x  109m3 of biogas / year or an annual equivalent of 4.81 million barrels of crude oil (S.J Ojolo et al 2007). 

The raw materials for biogas production include most agricultural and other organic waste. High percentage or Nigerians engages  in agricultural practice like livestock farming, crop production, according to United State Environmental Protection Agency (EPA). 

A single dairy cow produces approximately 120 pounds of wet manure per day by one dairy cow is equal to that of 20-40 people,  considering the fact that one of the biggest challenges facing our  country Nigeria is lack of waste management as approximately  70% of Nigeria's live in areas where no formal waste management system are in place (S.J Ojolo et al, 2007), the use  of cow dung, poultry and kitchen waste will serve as the best for  biomethane production in Nigeria. 

2.2 BIOGAS PRODUCTION 

Biogas is produced by the decomposition of organic matter by  bacteria in the absences of air (Oxygen) that is under anaerobic  condition, Biogas is lighter than air and has ignition temperature of approximately 700oC. The temperature of the flame is 870^oC.  Conversion of organic matter through microbial action has become an alternative means (method) of waste treatment and resource recovery. 

Biogas is produced in several waste treatment process Such as;

1. Agricultural waste 

2. Anaerobic composting 

3. Sanitary land fills 

4. Sewage treatment plants, 

AGRICULTURAL WASTE; Organic waste are produced through agricultural activities weather farming or subsequent food processing like rice and biogas can be produced from Such waste. 

On many farms, plant and animal waste are treated in small 

Anaerobic digesters that produce biogas (S.R Haggarnis et al, 1994).

ANAEROBIC COMPOSTING: Anaerobic composting has stand  to be one of the effective means through which valuable components in organic waste can be recovered, and this has  been making progress throughout the world. Modern anaerobic composting can produce compost of selectively collected organic waste, from domestic, kitchen, garden, food production activities which can produce a useful guaranty of biogas (Okeke O.R, 2009). The biogas can be converted to electricity band is  partially and alternative because their gas tends to be relatively  free of contaminants (Okeke O.R, 2009). 

SANITARY LANDFILLS: All over the world, it remains a major  part of the municipal waste disposal method. The use of landfill for disposing of municipal waste may decline in the future because of the organic content of the wastes, landfills produce per hour as this continues for many years after the closure of  the landfill (S.R Hagarnis et al, 1992). 

Since landfills produce biogas with high methane content, they are collected and burnt either in a flame or utilized as an energy source (Okeke O.R 2009). Collecting landfill biogas and utilizing its potential energy to generate electricity is obviously the desirable alternative (S.R Hagarins et al, 1992). 

SEWAGE TREATMENT PLANT: Most Municipal sewage treatment plants produce large amounts of sludge, the disposal  of which has become a major problem. Sludge can be digested under anaerobic conditions, which makes it possible to recover the potential energy available in its organic content. The biogas produced can then be used to generate heat and electricity (Harganis and Panada, 1993) and Mails and Umesh, 2009). 

In biogas production, three basic facts are outlined. 

1. Most or the important bacteria involved in biogas  production process are anaerobic and stow growing. 

2. The greater degree of metabolic specialization is observed in these anaerobic microorganisms. 

3. Most of the free energy present in the substance is found  in the terminal product methane (B Ganda, et al 1996). Research has shown that one of its serious limitations is the  availability of feedstock followed by defects in construction  and microbial failure (Gadre et al, 1990). 

2.3 PARAMETERS AFFECTING BIOGAS PRODUCTION 

Different parameters affect biogas production as bacteria are sensitive to changes in their environment. 

These parameters are 

i. Temperature 

ii. Nature of substrate 

iii. Available nutrients 

iv. PH level 

v. Carbon / Nitrogen ratio 

vi. Agitation / stirring

vii. Organic / Digester loading rate. 

viii. Hydraulic Retention time 

2.3.1 TEMPERATURE 

The microbial activity is temperature dependent. The  temperature inside the digest has a major effect on the biogas production process. Anaerobic digestion occurs in temperature ranging from 0 to 97°c (Bitton 1999). The microorganisms  responsible for anaerobic digestion are destroyed completely at  temperature below -10°c or above 90°c. 

The process of organic material anaerobic digestion takes place  in three main temperature ranges namely; 

a. Psychrophilic digestion 

b. Mesophilic digestion 

C. Thermophilic digestion 

PSYCHROPHILIC DIGESTION: This occurs at a temperature ranging from 0-20°c under a retention time over 100days. The rate of organic matter conversion into biogas is minimized since the activity of the microorganism is limited due to the low temperature. The consequence is to require a very large retention time, a very large volume of digester and high quantity  of substrates. The action of the digester has been found to  decrease sharply below 16°c (C.W Mgyakma and Akobundu,  2001).

MESOPHILIC DIGESTION: This occurs at a temperature ranging from 20-40'c, having a retention time over 20 days. Majority of methanogens (Microorganisms that form methane from organic matters) belong to the mesophilic. They grow quickly in this temperature range and exhibit high degree of conversion. In practice, this has direct implication in the design of biogas plant as they are the most stable operating plant. The stability and growth conditions in the digester at mesophilic condition makes the process more balanced, more resistant to chemical that inhibits digestion (e.g. Ammonia) and capable of treating efficiently a great variety of different biomass and waste even the most difficult treated. (www.biomassenergy.gr). 

THERMOPHILIC DIGESTION: This occurs at a temperature ranging from 50-60°c and has a retention time over 8 days. 

A small proportion of methanogenic organisms are thermophilic,  meaning that they are attached perfectly to high temperature.  Generally, at this temperature range, the bacteria’s consume the organic substrates with high rate and grow faster. (www.biomassenergy.gr).  

Due to this, the digester operated at thermophilic condition may be constructed in a small dimension while maintaining very high level of biogas. Thermophilic methanogenic bacteria are extremely sensitive to changes in anaerobic digestion to such an extent that even a small change of the operating parameters can impact negatively on their development. For example, a change in temperature greater than 1-2°C has a significant reduction in the amount of produced biogas. The variety of materials that can be processed in anaerobic thermophilic condition is low than that of mesophilic, mainly because of the chemical composition and stronger influence of some digestion inhibitors in the process. 

Production of gas is most rapid between 28°Cand 41°c (mesophilic) or between 49°c and 60°c (Thermophilic) (C.W and  Akobundu 2001). This is due to the fact that high temperature bacteria are much more sensitive to ambient influences. 

2.3.2 NATURE OF SUBSTRATE 

Many substrates are generally used as Feedstock in biogas plant the potential for biogas production varies with feedstock. It has  been reported that the quality of the feedstock in use has a direct  influence on the biogas produced (P.C Mahashewart and P.C Vasuda Van 1981) and (B. Megerson 1980). It is necessary that the organic waste material be easily degraded or digested by the concerned bacteria. It is further observed that the finer the organic waste (2mm size) is, the large the biogas produced. (K.K Meher at al 1990) Comparing the rates of biogas yield from pig dung and cattle dung fed digester, it has been reported that the biogas yield was higher in the former (K.M Mital 1996) and (Http.hear. org 2009). This was attributed to the pressure of Native micro Flora in the dung. 

2.3.3 AVAILABLE NUTRIENTS 

In order to grow, bacteria need more than just a supply of organic Substances as a source of carbon and energy. They also require certain mineral nutrients, In addition to carbon, oxygen and Hydrogen, the generation of biogas requires an adequate supply of Nitrogen, Sulfur, phosphorus, potassium, calcium, magnesium and a number of trace elements such as iron manganese, molybdenum, zinc, cobat tungsten, nickel, selenium etc. (S.J Ojolo et al, 2007) and (Alexander H, 1981). Poultry dropping procedures more biogas with high methane content  due to the high content of nitrogen.  

Table (i) shows the gas produced from different substrate. Table (i) shows the methane yield of animal waste. 

TABLE I  

Biogas produced from different substrate 

(1= FAO, 1997 and 2-Nagamani & Ramasamy No date) 

TABLE II 

Methane yield of animal waste

(Alexander H 1981)  

Higher concentration of any individual substance usually has an 

Inhibitory effect so analysis is recommended to determine  amount of each nutrient of the substrate(s), if any still needs to  be added. 

2.3.4 PH LEVEL 

It has been well established that PH is an important parameter that affect the growth of methane producing bacteria during anaerobic fermentation. Anaerobic digestion will occur best with  a PH range of 6.8 to 8.0 (B Nagamani et al, 998). More acidic or basic mixture has a toxic effect on the methanogenic bacteria thereby causing fermentation to occur at a low speed.  

The introduction of raw material will often lower the PH and  make the mixture more acidic, digestion will stop or slow down  dramatically until the bacteria have absorbed the acids (OKeke  O.R, 2009). A high PH will encourage the production of acidic  carbon dioxide to neutralize the mixture (B. Nagamani et al,  1998) and (A.N Ofoefule and E.0.U Uzodinma 2006) for normal  anaerobic fermentation process, concentration of volatile fatty  acids in terms of acetic acid should exceed 200- 300mgll (Okeke O.R 2009). It was observed that above PH 5.0, the efficiency of  CH4 production was more than 60- 70% (A.U Ofuefule and E.O  Uzodinma 2008). If the PH drops below 5.0 or above 8.0, the fermentation process may be inhibited or even stopped (A.U Ofoefule and E.O Uzodinma 2008). 

2.3.8 CARBON/ NITROGEN RATIO 

All feed material consist of nitrogen (N) and carbon (C) at different ration. Proper ratio carbon and nitrogen is required not  only for the purpose of biogas production from organic waste but  also for optimal yield of methane gas. For high yield of methane  carbon is required for energy and nitrogen is necessary for  building of cell structure of the methanogenic bacteria (G.C Okoli et al 2006), If nitrogen is in excess, ammonia is produced which  makes the slurry alkaline and decrease the growth of bacterial.  Carbon / Nitrogen (C/N) ratio of 20:1 to 30:1 are particularly  favorable for optimal biogas production. 

To obtain this range, it is been stated, the materials with high nitrogen content like pig dung, poultry dung, urine and faces  should be mixed with the substrates having low nitrogen content and high carbon content like saw dust, farm waste, rice husks etc. 

A high C/N ratio will leave carbon still available after the nitrogen has been consumed, starving some of the bacteria of this moisture, but slowing the process. Lower C/N ratio will leave correct ratio of carbon to nitrogen will prevent loss of ether fertilizer quality or methane contact.

2.3.6 AGITATION / STIRRING 

Many substrates and various modes of fermentation require  some sort of substrates agitation or stirring in order to maintain process stability within the digester. The most important  objectives of stirring are:  

∙ Removal of the metabolites produced by the methanogens ∙ (Gas). 

∙ Mixing of fresh substrate and bacteria (inoculation). ∙ Prevention of scum formation and sedimentation. 

∙ Avoidance of pronounced temperature gradients within the digester 

∙ Provision of a uniform bacteria population density. 

∙ Prevention of the formation of dead spaces that would reduce the effective digester volume. 

According to A.S Samdo, stirring is important in digesters to prevent the formation of three layers (A.S Samdo et al 2006).  In the upper part, the scum is formed and in the middle a liquid medium and sludge in the bottom, stirring breaks up these  layers and makes for uniform distribution of feed stock and  seeding bacteria to extend the contact surface of microbes with  feed stocks thus speeding up the digestion rate and gas yield  (Okeke, O.R 2009).

It ensure easy release of CO2 and CH4 from the slurry, in the  case of digesters in which heating element is encomplated.  Stirring helps in distributing the heat uniformly within the  digester. 

Vigorous and continuous stirring is not encouraged as this will  prevent the coming together of the bacteria to react and produce gas (A.S Samdo et al 2006), slow stirring is better adopted. 

2.3.7 ORGANIC LOADING RATE OR DIGESTER LOADING RATE 

Gas production is highly dependent on loading rate. Organic loading rate or digester loading rate is amount of organic  material that can be fed to a digester system at a particular time  that will be suitable for gas production (D.D Schulte et al 1976) Digester loading rate indicates how much organic material per day has to be supplied to the digester or has to be digested. Methane yield was found to increase with a reduction in loading rate, if the loading digester is too high, the PH falls, the plant then remains in the acid phase because there is more feed material than methane bacteria. 

It has been observed that a daily loading rate of 16kg Vs. m3 of digester produced 0.04 - 0.07m of gas 1kg of dung fed (M.R Smith 1980). 

2.3.8 HYDRAULIC RETENTION TIME

This is the time for which fermentation material reside inside the digester. (P.H Smith and R.E Hungate 1958) and (A. A. Van Biran 1979). It also refers to the time water and bacteria remain in the reactor. 

Normally, maximum gas production takes place within the first four weeks and then it triggers off gradually depending on the temperature and climate conditions and the substrates. 

For most animal manure and plant material, the normal  retention time is between 15 -30 days (F Van Kelsen and G.  Letting 2000). Some factors have been identified to affect the  retention time (A.R Webb and F.R Hankes 1985) and (R.S Wolfe  and I.) Higains 1979). If the temperature could be raised,  agitations of the concentrates inside the digester are capable of  reducing the retention time of slurry in the digester. 

2.4. BIOGAS PLANT TYPES 

2.4.1 FIXED - DOME PLANT 

A fixed- Dome plant consist of an enclosed digester with a fixed, non - moveable gas holder which sits on top of the digester. The waste manure, during human excrement) is fed into the digester. After that the mathanogenic bacteria "digest" the waste and produces gas and slurry (digested waste). When gas production commences, the slurry is displaced into the compensating tank while the gas is captured in the gas holder. The more the gas is produced the higher the level of the slurry outlet will be. The  level of slurry in the digester depends on the loading rate, gas production and consumption. (Urmila B et al, 2008). Gas  pressure increases with the volume of gas stored and the height  difference between the slurry level in the digester and the slurry  level in the compensating tank. (http://www.cd3cwd.com). 

Therefore the volume of the digester should not exceed 20m3. If  there is little gas in the holder the gas pressure is low. During gas production, slurry is pushed back sideways, displaced into  the compensating tank. When gas is consumed, slurry enters  back into the digester from the compensating, tank. As a result  of these movements, a certain degree of mixing is obtained of  slurry of different ages. Therefore this design approaches a  mixed digester reactor (Stalin, 2007). 

ADVANTAGES 

1. The fixed - doom plant is relatively inexpensive. 

2. It has no moving parts, no rusting steel parts, hence has a long life span up to 20 years (Giz, 1999). 

3. The underground construction saves space and protects the digester from temperature changes. 

4. The construction provides opportunities for skilled local employment. 

5. DISADVANTAGES 

1. The frequent problems with the gas - tightness of the brickwork gas holder (a small crack in the upper brickwork can cause heavy losses of biogas).

2. The gas pressure fluctuates substantially depending on the volume of the stored gas. 

3. Fixed - Doom plants can be recommended only where construction can be supervised by experienced biogas technicians. 

2.4.2 FLOATING – DRUM PLANT 

The operation of floating - drum plant is not different from a  fixed - doom plant. Floating - drum plant consist of an  underground digester and a movable steel drum, the gasholder  where the produced gas is collected. The steel gas holder floats  either directly on the fermentation slurry or in a water jacket of  its own. As gas is produced and collected in the gas holder,  pressure increases and the steel drum rises, if gas is drawn off  the digester it falls again. The gas drum is prevented form titling  by a guide frame. The slurry is pushed out of the digester after  digestion. 

ADVANTAGES 

1. The operation of the plant is easy to understand and operate. 2. The volume of gas stored is directly visible. 

3. Gas pressure is constant due to the weight of the drum.

DISADVANTAGES

1. High cost of construction since steel drum is relatively expensive, many steel parts liable to corrosion, resulting in short  life span (up to 15 years, in tropical coastal region about five  years for the drum), regular maintenance cost due to painting. 

2. Steel drum can get struck. 

In spite of these disadvantages, floating drum plants around always to be recommended in cases of doubt. Water jacket  plants are universally applicable and especially easy maintain.  The drum won't stick, even if the substance has high solids  content. Floating - drums made of glass- fiber reinforce plastic  and high density polyethylene has been used successfully, but  the construction cost is higher than with steel. Floating - drums  made of wire - mesh-reinforce- concrete are liable to hairline  cracking and are intrinsically porous. 

2.4.3 BALLOON PLANTS/ BAG DIGESTER 

A balloon plant or also referred to as bag digester consist of a plastic or rubber digester bag. The gas is stored and collected in the upper part while the slurry (manure) in the low part. The  inlet and the outlet are attached directly to the plastic skin of the balloon. The gas pressure is achieved through the elasticity of  the balloon and by added weights e.g. stones placed on the  balloon. When the gas is full, the plant works like a fixed doom  plant i.e. the balloon is not inflated, it is not very elastic. The  fermentation slurry is agitated slightly by the movement of the  balloon skin. The balloon material must be U-V resistant. 

Materials which have been used successfully include RMP (Red  Mud Plastic). 

ADVANTAGES 

1. Balloon plant has low cost of construction 

2. Transportation is easy 

3. It maintains high digester temperature 

4. Low construction sophistication, uncomplicated cleaning, emptying and maintenance. 

DISADVANTAGES 

1. It has a short life span. According to GIZ (unknown date) and Daxiong 1990, the effective life span of the bag is limited to 3 - 5 years. 

2. Susceptible to physical damage 

3. Hard to repair 

4. High quality plastic is needed. 

2.5 FEEDING METHODS OF DIGESTER 

Three different forms of feeding methods can be distinguished. 1. Batch feeding for batch plants. 

2. Continuous feeding for continuous plants 

3. Semi - batch feeding for semi - batch plants.

2.5.1 BATCH FEEDING (MOSTLY SOLIDS) 

This type of feeding is used mostly for solid vegetable waste, mixture of dung and vegetable waste. Batch plants are filled completely and then emptied completely after a fixed retention time. Depending on the waste material and operating temperature a batch digesters which will start producing Biogas after two to four weeks will slowly increase in production. Then drop off after three or four months. Batch digesters are best operated in groups so that at least one is always producing  

useful quantities of biogas. 

2.5.2 CONTINUOUS FEEDING (MOSTLY LIQUIDS) 

This type of feeding is mostly for fluid and homogeneous substrate continuous plants are filled and emptied regularly, normally, dally. Continuous plants are Suitable for rural households as the necessary work fits well into the daily routine. In a continuous feeding system, it is essential to ensure that the digester Is large enough to contain all the material that will be fed through in a whole digestion cycle. 

2.5.3 SEMI CONTINUOUS FEEDING (SOLIDS AND LIQUIDS) 

Biogas plants can be operated on a semi - continuous basis when a sold and a liquid waste are used as a feedstock. The feeding is done at interval depending on the waste material. Example when the straw and dung are to be digested together, the slowly digested straw- type material is fed in about twice a year as a batch load. The dung is added and removed regularly.

2.6 REFINING OF BIOGAS 

As earlier noted, biogas produced constitute from anaerobic digestion of waste, constitute of other gases. Incombustible carbon dioxide reduces flame calorific value and flame velocity  of biogas. The content of carbon dioxide which varies as a  function conditions prevailing in a digester and digester feed  composition, introduces constrains on the efficient operation of  appliances, such as burner. It is necessary where possible to  remove the gas from biogas before storage or use. 

The refining f biogas can be achieved by direct reaction of carbon  dioxide with solutions of alkalis where it is absorbed. Despite the  low concentration of hydrogen sulphide, its presence in biogas is  very undesirable. Hydrogen sulphide is a toxic gas with an  unpleasant odor, similar to rotten eggs, forming sulphuric acid in combination with water vapor in biogas. The surphuric acid is  corrosive and therefore reduces the life of the metallic (copper,  steel, and lead) pipe, gasholder and metallic accessories. To  prevent this effect, biogas must be refined of hydrogen sulphide. 

Other effective methods of purifying biogas are by:

1) Water washing 

2) Pressure swing absorption 

3) Selexol absorption 

4) Amine gas treating. The most prevalent method of all is water washing where high pressure gas flows into a column where the CO2H2S and other trace elements are scrubbed by cascading water running counter flow to the gas. This arrangement could deliver 98% CH4 with manufacturers guaranteeing maximum 2% CH4 loss in the system. 

CHAPTER THREE 

3.0 RESEARCH METHODOLOGY 

3.1 DESIGN CONSIDERATION 

The genera consideration in designing this biogas plant is to produce system that can be used for waste treatment and biogas  production for cooking. A system that will be efficient in to use,  cheap to construct, maintain, safe and easily operated, easy to  assemble and disassemble even mobile. Factors that affect the system were properly considered also. For adequate  construction and better working standard, proper selection of adequate construction process and protective measures were employed. 

3.2 MATERIAL SELECTION 

The selection of proper material for engineering purpose is one  of the most difficult problems facing the designer. The best  material is one which serves the de sired objective at minimum  costs as different materials Such as metal, plastic, concrete are  used for biogas plant construction.

In selecting desired material, the following factors should be considered 

a. Availability of the materials 

b. Suitability of the materials for the working conditions in service. 

c. The costs of the material and possible maintenance. 

Material selected for a particular design should be expected to  have some mechanical properties as well as thermal and  chemical properties. For mechanical properties, strength is  considered first, as the design is meant to function under various  load distribution. A score chart developed to select the material  is shown below comparing three selected materials.

Score chart: for any desirable property, each material type was  scored of 3marks (very good) -2marks (good) and 1 mark (fair). 

From the chart above, metal (mild steel) was selected as the  proper material to suit the design. Plastic materials fittings were  introduced in the design due to some reasons such as cost  availability and chemical resistance Brass fitting material were  used at some points. 

Materials used in construction of biogas plant.  

TABLE IV 

3.3 DESCRIPTION OF PLANT PARTS

1. HOPPER: The hopper is a funnel shaped metal with four sides. It serves as feed inlet unit where the prepared feed stock (Slurry) is been feed into the plant. This unit is designed in a way to accommodate a reasonable amount of feed. 

2. INLET PIPE WITH VALVE: It is a 2 inches cylindrical shaped plastic pipe having 2inches plastic ball valve fitted to it. The inlet pipe with valve is located in between the hopper and the tank.  It serves as the unit through which the fed feed s tock flows from the hopper to the tank. The attached valve is locked after feeding to prevent escape of gas during digestion. 

3. DIGESTER TANK: The digester tank is the main frame of the plant. It is a 3mm thick metal tank in cylindrical form having a truncated cone designed metal form at the top and bottom. The digester tank comprises of the digestion unit and gas holding unit. Shaped metal of 3mm thickness mounted at the side of the tank. 

4. GAS COLLECTION CHAMBER: It is a closed cylindrical storage before the refining or cleaning. It is where' biogases  generated from the tank are moved for storage before the  refining or cleaning. 

5. IMPELLER TRIRRER: It is a 20mm round metal shaft having flat rectangular metal pieces attached to it an angle. The stirrer provides proper mixing of the slurry in the tank for effective bacteria action, scum breaking and proper digestion of slurry for proper gas yield.

6. STANDS: The stands are made of metal of 3.5 thick. They provide support and stability to the plant. 

7. OUTLET PIPE WITH VALVE: 2inch cylindrical plastic pipe having 2inches plastic ball vale and 2inches plastic elbow fitted  to it. The effluent (digested feed) flows out of the digester tank through this channel after digestion. The out pipe is fitted to the bottom of the tank. 

8. GAS VALVES: The gas valves are of ½ inches and ¼ inch. One mounted on the tank and the other at the end of a gas hose. The values provide proper regulation of outflow of the gas. These valves are made of brass. 

9. GAS HOSE: The as hose is made of plastic. It provides flow  of gas from one chamber to another and final distribution. Deferent sizes of pipes are used at different points. 

10. NON-RETURN VALVE: The non-return valve is mounted to  the collection chambers, it is made of metal. Its function is to prevent the flow back of gas from the collection chambers to the digestion tank when there is pressure difference. 

11. HANDLE: The handle made of metal assist in removing of lifting of the upper section replacement or maintenance  operation is to be carried out. It also provides mobility of the  plant. 

12. PRESSURE GAUGE: Pressure gauge is mounted on the digester tank and gas collection chamber. These pressure  gauges are used to measure gas pressure in the tank and gas collection chamber and to determine if these chambers are gas  leak free. The pressure gauge on the tank is used to determine  when there is low or non-production of gas in the tank. 

13. FLANGE: Flange is one of the integral parts of the plant. It  is round mild steel welded at the meeting ends of the cut sections of the tank. It provides access to the internal parts of the plant for easy maintenance, inspection repair and replacement of  parts. 

3.4 DESIGN CALCULATION 

3.4.1 SCALING OF BOGAS PLANT 

The general steps followed to calculate the 200liters capacity plant is given below 

Step 1: calculation of the plant volume dimension specifications Upper cone truncated part 

Small circle diameter 220mm radius (r2) = 110mm Big circle diameter 590mm radius (r2) = 25mm 

Height 53mm 

Lower cone truncated 

Small circle diameter 50mm, radius (r2) = 25mm Big circle diameter 590mm, radius (r1) = 295mm Height 206mm 

Cylindrical part

Diameter 590mm, radius ®= 295 

Height 630mm 

Formula tor volume of the outlined shaped volume of truncated cone 

½ ∏ (r2 + (r1x r2) + r22) h 

Volume of cylinder 

∏r2 h 

Computation of values 

Upper truncated cone 

1/3∏ (295)2 + (295 x110) + (1102) 53mm3 

=2324491.667∏ mm3 

Lower truncated Cone 

1/3 ∏ (295)2 + (295 x25) + (252) 206mm3 

=6525050 ∏mm3 

Cylindrical part 

∏ (295)2 630 

=5482570∏mm3 

Total volume VT 

= volume of upper truncated cone + volume of lower truncated Cone + volume of cylinder

VT= 2324491.667 + 6525050 + 54825750) ∏mm3 VT= 200041828.5mm3 

Note, by conversion 

1x 109mm3 = 1m3 

VT= 0.200042 m² 6dp 

By capacity conversion 

1m = 1000liters 

0.200042 m³ = 200.042 liters 

= 200 liters 

Step 2 

Computation of volume of gas holder Vgh from experience, the gas holder volume ranges from 10-25% of the total plant volume 

Taking 15% 

Volume of gas holder Vgh 

Vgh = 15% VT 

0.03m3= 30liters 

Steps 3 

Active slurry Volume  

V/ active slurry = Vtotal -Vgas holder 

(0.2-0.3) m3 x 1 1000 = 170 liters

Step 4 

Hydraulic retention time in days = 40 

Calculating the volume of the gas collection chamber Circle diameter = 65mm, r=32.5mm 

Height 285mm 

∏2h = (∏ (32.5)2 285) mm3 

=30103125∏mm3 

1 ×109 mm3 = 1m3 

= 0.945 liter 

= 1 liter Gas collection chamber capacity 

3.5 PLANT DESIGN | CONSTRUCTION PROCESS

The steps involved in design and construction of the biogas plant Are as follows: 

1. Measuring operation 

2. Marking out operation 

3. cutting and trimming operation 

4. Folding operation 

5. Welding operation 

6. Surface finishing operation 

7. Assembling operation

8. Pressure Testing Operation 

9. Painting 

1. MEASURING OPERATION: Measuring out from the  parent material the desired dimensions for the design like;  length, width, diameter from the selected materials was  done by the use of measuring tape, try square and venire  caliper. 

2. MARKING OUT OPERATION: This is the second stage;  the measured parts were marked out by the use of scriber. 

3. CUTTING AND BORING OPERATION: After measuring  and marking operation, cutting follows. This operation was  done by the use of cutting machine for cutting of sheet  material, angle iron and shaft. Cutting of plastic pipe was  done by the use of hack saw. Boring of holes of different  sizes was done by the Use of drilling machine. The rough  edges of the cutout materials were trimmed with a grinding  machine to achieve smooth surface edge. 

4. FOLDING OPERATION: Rolling machine was introduced  at this stage. The cut sheet materials were rolled or folded  to desired diameter to suite the design. 

5. WELDING OPERATION: The type of welding operations employed in the construction works are welding. Arc  welding machine and mild steel electrodes were used to provide a permanent joint of the required metal parts. The  welding was continuous so as to achieve an airtight joint. 

6. SURFACE FINISHING OPERATION: Grinding machine having a grinding wheel was used to smoothen the rough  welded parts. 

7. ASSEMBLING OPERATION: Coupling together all the required components to bring out the desired structure of  the biogas plant was done by different mechanical  assembling materials. Assembling of sections with Flange,  the gas collection chamber, the shaft and bearing was done  by the use of bolt and nut. Non-threaded PVC pipes were  assembled by the use of adhesive bond, threaded body  parts where screwed into their respective positions. While  gas rubber pipes fitted to their respective places where held  tight with clip. 

8. PRESSURE TESTING OPERATION: After assembling all  the parts, a pressure testing machine was used to carry out pressure testing operation. This was done to dictate points  or leakages for proper sealing. 

9. PAINTING OPERATION: This is the last operation carried out. It is a surface finishing operation, anti-rust paints were used both on inside of the plant and outside for rust prevention and beautification of the design. The inner part was painted before assembling.

CHAPTER FOUR  

4.0 RESULT OF DESIGN CALCULATION AND SPECIFICATION  TABLE V  

4.1 PERFORMANCE TEST  

4.1.1 Material Collection  

200 liter type, sheet metal biogas plant fabricated at mechanical  engineering fabrication center, Federal Polytechnic Nekede,  Owerri, Imo state. Pig dung, cow dung, poultry droppings and  kitchen waste were the four waste used for this project work. Pig dung was collected freshly from co-operative  farm, Ezeakiri Naze Owerri, Imo State. Dung was collected from Egbu Slaughter house Owerri, Imo State, while kitchen waste  (bio-degradable) was collected from restaurants along the  school road. The kitchen waste was reduced in size, and stored  in a black sealed polythene bag. 

4.1.2 LOADING OF PLANT AND TESTING 

21.25kg of each waste type was measured and mixed with water of the same weight at a ratio of 1:1 in a mixing container ad stirred properly for some minutes to ensure homogeneity. The slurry was charged into the digester tank. The entire valves were closed to ensure air tightness. The slurry charged was stirred twice a day. The biogas was trapped in a balloon on the day a flammable gas was observed before and after refining. For analytical purpose. The refining solutions are potassium permanganate (KmnO4) for absorbing of hydrogen sulphide (Hs and lime water (Ca (OH) 2 (aq) for CO2 absorption. Later the gas hose was connected to a burner. The plant was operated at mesophilic temperature under hydraulic retention time of  40days with the slurry having initial PH level of 7.34. The  pressure reading of the tank from the day the slurry was charged  to the day refining took place was taken. 

The time for a burnable gas to be produced was obtained

4.1 PERFORMANCE TEST  

4.1.1 Material Collection  

200 liter type, sheet metal biogas plant fabricated at mechanical  engineering fabrication center, Federal Polytechnic Nekede, Owerri, Imo state. Pig dung, cow dung, poultry droppings and  kitchen waste were the four waste used for this.

4.1.3 Result and Discussion  

4.1.3.1 Result  

Operating temperature = Mesophilic 

PH level of slurry = 7.34 

Pressure reading of gas in the tank before refining  TABLE VI  

TABLE SHOWING GAS ANALYSIS RESULT  

TABLE VII 

The eight day, a fall occurred, leakage was detected, gas  pressure triggered and remained constant from the twelfth day. Increase was observed from the thirteenth to fourteenth day of refining. Gas was tested unrefined on the ninth day was  observed, this indicated the presence of methane. From the  table Vii, it was observed that the percentage of H2S CO2 was  largely reduced after refining. This improved thermal content of  the gas. Few days of constant pressure reading, the volume of  gas that escapes from the leaking points is compensated by the  volume of gas produced. Chemical reaction showing the refining  process and combustion of methane.  

Equation for CO2 absorption

(OH) 2 (aq) + CO2 (g) CaCO3(s) + H20 (l) 

Equation for H2S absorption 

mnO4 (aq) + 3H2SO4 (aq) + 5H2S (g) K2SO4 (aq) + mmSO4 (aq) + 8H2O (l) + 5S(s) 

Biogas burns in oxygen to give C02, water and energy Content  in ethane is released 

H4 (g) + 2O2 (g) CO2 (g) + 2H2O (g) + Energy 

2. BILL OF QUANTITY  

TABLE VIII

MISCELLANEOUS COST  

This includes the cost of transportation in buying material  for the construction, transportation of waste and going to the  workshop. 

Total miscellaneous cost = ₦6,800 

LABOUR Cost = ₦18,000 

Research cost = ₦45,000

Total cost of the project = total material cost + miscellaneous  cost + labor cost + research cost = ₦174,310. 

CHAPTER FIVE 

5.0 MAINTENANCE OPERATION 

The maintenance of biogas plant comprises all work necessary  to guarantee trouble free operation and a long working life of the plant. Repair reacts to break down of the biogas system. All doubtful measurements have to be verified. Often, one symptom has variety of possible reasons. Maintenance service should be carried out by a trained technique. 

The following maintenance should be carried out; 

1. Check the plant in respect of corrosion and if necessary  new protective coating material. 

2. Lubricate movable parts 

3. Regular gas leakage check should adopt. All parts of the plant including pipe fittings should be checked of leakage with a special leakage detector. 

4. Leaking points should be properly sealed. 

5. In case of strong slurry odor, attributed by sub optimal fermenting condition. Substrate in take should be reduced and PH level should be corrected with adequate means. 

5.1 SAFETY OF BIOGAS PLANT

Construction and operation of biogas is related to a number of important safety issues. Taking proper precaution and safety measure have the aim of avoiding the risks and hazardous situation and contributes to ensure a safe operation of the plant. 

The safety measures are: 

1. Proper personal protective equipment should be used during collection and preparation of waste. 

2. before initiating any repair on the gasoline, gas supply should be cut-off 

3. Plant should be operated at proper temperature under standard hydraulic retention time and PH, since they have direct influence on the sanitation efficiency of anaerobic digestion process. 

4. Naked light should be off the plant area. 

5. Gas mask should be worn during operation and repair of plant. 

6. Clear warnings must be placed on the respective parts of the pant and the operating personnel must be trained. 

7. Fire extinguisher should be provided in plant area. Construction safety 

1. 1 All the necessary safety equipment like cover all, hand glove, face mask etc. should be provided during construction.

2. Proper care should be taken during cutting, welding and handling of the machines. 

3. Adequate material selection should be done to avoid explosion of the plant. 

5.2 CONCLUSION 

The result of this project has shown that there lie some beneficial potential in waste materials that imposes threat to life and environment through production of flammable gas from pig dung cow dung, poultry dropping and kitchen waste when they are subjected to anaerobic digestion. These wastes are always available in our environment and can be used as a source of fuel if managed properly. This project further revealed that the thermal quality of biogas produced is imposed by H2S and CO2 reduction when refined and so can serve as a substitute for petroleum based cooking gas. 

5.3 RECOMMENDATIONS 

Improvements are always the norm in scientific projects like this and as such, in order to make the system better and to operate efficiently, we recommend the following: 

1. Effort should be made at designing a biogas plant that will operate on continuous feeding. 

2. 2 Point for thermometer fitting should be provided in  subsequent plant design for temperature check. 

3. proper rubber seal should be employed where necessary

4. Visual point should be incorporated on the design. 

5. Fund should be made available by our educational system  to enable students properly finance their project. Finance  is a major factor in the realization of any project. 

1. Biogas Digester 

2. Purification Tank Potassium per Magnate 3. Lime Water 

4. Flange 

5. Flange Bolt 

6. Digester Stand 

7. Discharge Valve 

8. Discharge Hose 

9. Charging Valve 

10. Charging Funnel/ Hopper 

11. Stirrer Bearing 

12. Starrer 

13. Pressure Gauge 

14. Collection Valve 

15. Tank Valve 

16. Gas Collection Hose 

17. Gas Collection Chamber 

18. Pure Gas Collection Tank 

REFERENCE 

1. Alvearz R; Villcas. Liden G (2006) Biogas production from  ulama and cow manure at high altitude. Biomass bio energy 30:66 75 

2. AOAC (1990): official methods of Analysis: Association of Analytical Chemists. 14" Edition, Washington UJSA, 22209. 

3. Sambo A.S, B. garba, and B.G Dansheh (1995) "Effect of  some operating parameter on biogas production rate"  Renewable energy, Vol. 6, no 3 pp. 343 - 344, view at  sscopousl. 

4. Demas TZD, Stams, A.J.M, Reith, J.H, Zeeman G, (2003) Methane production by anaerobic digestion of waste water  and solid wastes in : Reith, J.H Wryfels, R.H. Barten, H.  (Eds). Bio-methane and Bio hydrogen. Status and  perspective or biological methane and hydrogen foundation.  Petten, The Netherlands, pp 58 - 102.

5. Balch E.E (1979), microbiology Rev pp 43, 260 

6. Energy commission of Nigeria (1998); rural renewable  energy needs and five supply technologies. Pp 40- 42. 

7. "European Biogas Barometer" Eurobserver. Retrieved 7 November 2011. 

8. FAO (1979), China: Azolla Propagation and Small Biogas Technology Agricultural Service Bulletin No 41 FAO ROME. 

9. Guruswamy T., Kannan N., Kumar V., (2003). Design. Development and evaluation of biogas using selected  biomaterials as feed stock UE.J., 84:65 

10. Beaker H.A (1936). Appi, Mikrobiol. Pp 7 420 

11. Http:Flle://A: Design - Tutor.htm, 2003. Waste  digester Design, University of Florida Civil Engineering P.3. 

12. Indiani M.A, Laura R.D 1971. Increased production of  biogas from cow dung by adding other agricultural waste  materials .J. Sci. Food Agric, 164 - 167. 

13. Itodo I.0, C.E Onuh and B.B Ogar (1995). Effect of  various total solid concentration of cattle waste of Biogas  yield. Nigeria Journal of Renewable Energy vol. 13 pp 36- 39. 

14. Meher K.K, D.R Renade, and R.V Gardre (1990) factors affecting biogas production reds ind, Nepal Pp 115 - 117.

15. Singh, R., Jain M.K., Tauro P, 1983 Pre - digestion to improve production of biogas from cattle wastes P 167 - 174.