Alternative energy sources to combat climate change: Biogas production using cost effective material

Table of contents

ABSTRACT

1 Introduction
1.1 Background
1.2 Problem Statement
1.3 Purpose of the Study
1.4 Hypothesis
1.5 Objectives of the Study
1.5.1 General objective
1.5.2 Specific objectives

2 Literature Review
2.1 Fuel Consumption in Ethiopia
2.2 Biomass and Biogas Energy Technologies in Ethiopia
2.3 Theory of Biogas Technology
2.4 Benefits of Low- Cost Plastic Biodigester Technology
2.4.1 Environmental Benefits of Biogas Technology
2.4.2 Social Benefits of Biogas Technology
2.4.3 Economic Advantages of Plastic Biogas Technology
2.4.4 Beneficiaries of Biogas Development
2.5 Input Materials for Bio- Gas Production
2.6 Biogas Production Processes
2.7 Theory of Biogas Burner
2.8 The Slurry after Digestion
2.9 Measurement of Biogas Production
2.10 Designing of Digesters
2.11 Working of Fixed-Dome Biogas Plant
2.12 Selection and Layout of Pipeline and Biogas Accessories
2.13 Transfer of the Plastic Film Biodigester Technology
2.14 Promotion of Fixed and Floating Dome Biogas Plant
2.15 Economic Evaluations of Biogas Plants
2.16 LDPE Geomembrane Plastic
2.17 Theory of Environmental Impact Assessment (EIA)

3 Materials and Methods
3.1. Description of the Study Area
3.1.1 Location
3.1.2 Socio-economic activity
3.1.3 Climate
3.1.4 Land Use
3.1.5 Livestock Population
3.2 Experimental Design and Layout
3.3 Geomembrane Plastic Construction methodology
3.4 Data Collection Procedures
3.4.1. Input to the Digesters
3.4.2 Measurement of Gas Production
3.4.3 Temperature of the Air and Slurry
3.4.4 Total-Solids (DM) Content
3.4.5 The Organic Dry Matter (ODM)
3.4.6 pH of the Fresh Cow Dung and Digested Slurry
3.4.7 Quality of Output Slurry
3.4.8 The Efficiency of Biodigesters
3.4.9 Social Aspect of Biomass and Biogas Technologies
3.4.10 The Economic Viability of a Plastic and Fixed Dome Biogas Plant
3.4.11 The Environmental Impact of the Plastic Biogas Plant
3.5 Statistical Analysis

4 Result and Discussion
4.1 Operation of Plastic Biodigester
4.2 Biogas production
4.3 Temperature of the Air and Slurry
4.4 Characteristics of Bio-digested Slurry (Effluent) and the Influent
4.5 Characteristics of Total-N in the Slurry and Influent
4.6 Characteristics of Organic Matter in the Slurry and Substrate
4.7 Characteristics of pH of Fermented Slurry
4.8 Efficiency of the Biodigesters
4.9 Economic Evaluations
4.9.1 Market Price of Inputs
4.9.2 Market price of outputs
4.9.3 Cost-Benefit Analysis of Biogas Plants
4.10 Social aspect of biogas technology
4.10.1 Income generation through increased crop production
4.10.2 Income Generation through Cost Saving
4.10.3 Perceptions of Habru Woreda People regarding the use of Biomass & Biogas Technology
4.11 Technological aspect of geomembrane plastic biodigester
4.11.1 Sustainability
4.11.2 Simple technology
4.11.3 Demand driven
4.11.4 Replicability
4.12 Technical problems with the geomembrane plastic digester
4.13 Environmental Impact Assessment of the Plastic Biodigester
4.13.1 Reduction of green house gas emissions
4.13.2 Reduction of rate of deforestation

5 Conclusions and Recommendation
5.1 Conclusions
5.2 Recommendations

References

Appendix

Appendix

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Limitations

Appendix

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List of Tables

List of figures

Acronyms

ABSTRACT

The study was conducted in North Wollo, Mersa-Chekorsa village, Ethiopia in 2006/2007, where animal dung for biogas production is available. The overall objective of the study was to introduce economically feasible, technically acceptable and environmentally friendly biogas plant to the farming community and other potential users in Ethiopia. The research was carried on two types of biogas plants of 3m3 capacity (1) geo-membrane plastic (two single and two double layered) biogas plants constructed below and above the ground surface and (2) fixed-dome biogas plant. Each bio-digesters was fed with a mixture of 75Kg of cow-dung and 75Kg pure water at equal volume and proportion. Amount of gas and slurry were measured using calibrated biogas burner and weight balance respectively. The quality of the slurry (i.e. total-N and organic matter content) were analyzed in the laboratory using Kjeldahl and ash method respectively. The bio-digesters were compared after gas has completely produced at the end of 40 days of fermentation with respect to amount of gas and slurry produced, quality of slurry in terms of total-N and organic matter content. Economic analysis of the bio-digesters was carried out using cost-benefit analysis. The social aspect of using biomass and biogas technologies and environmental impact assessment of the new geo-membrane plastic biogas technology was also assessed. The emissions of CO2 and CH4 were computed by measuring the production of biogas in the two models of bio-digester. Fermented slurry contained larger nitrogen content than fresh cow dung in both models of bio-digester. The geo-membrane plastic biogas plant gave higher net benefit than fixed-dome biogas plant. So, from this, investment on geo-membrane plastic bio-digester is economically feasible. Environmental impact assessment of the technology was studied and found that 360.04 m3 of CO2 and 600.06 m3 CH4 was prevented from emitting in to the atmosphere and save 0.562 hectare of forest per year. Generally, it was found that, the geo-membrane cylindrical film bio-digester technology was found cheap and simple way to produce gas in the study area and it was recommended to introduce the technology into the rural areas having even and high temperature which is similar to the study area more preferably to an area having mean daily temperature greater than 20 OC.

Key words: - Geo-membrane Plastic bio-digester, fixed-dome bio-digester, biogas production, quality of fermented slurry, environmental impact assessment and economical feasibility

1 Introduction

1.1 Background

Farming is the major rural activity in Ethiopia, i.e. agriculture supplies 51.8% of the gross domestic product and 90% of the export earnings of Ethiopia and 86% of the population is engaged in agriculture (CSA, 1999 cited in Paulos, 2004). Dependency from biomass such as fuel wood, charcoal, dried cow dung cake and crop residue in Ethiopia amounts to 95% and half of the biomass is used for baking injera (Benjamin, 2004). When all forest uses are included, the deforestation rate in Ethiopia is around 1.1% per year (Wikipedia, 2006). According to North Wollo Agricultural and Rural development office (2007), the forest cover of North Wollo and Habru district is 37,183.58 hectare and 1614 hectare respectively.

Fuel is in very short supply in Ethiopia and throughout most of Africa. Where conditions still permit, wood is commonly used as fuel, but in many rural and urban areas, dried cow dung is a major source of fuel for cooking (Benjamin, 2004). Wood burns at 5-8 % efficiency and cow dung at 60% of that of wood (UNESCO, 1982).

According to FAO (2000), the combustion of fossil fuels has caused serious air pollution problems, likewise the excessive consumption of fire wood results in deforestation on a large scale. The deteriorating forest cover in Ethiopia due to deforestation caused the recurrent drought and famine (FAO, 2000). IUCN (1990), estimated that high forests covered 16% of the land area of Ethiopia in the early 1950s, 3.6% in the early 1980s and 2.7% in 1989.It is estimated that these resources are vanishing at an alarming rate, estimated at 150,000 to 200,000 hectares per year (EFAP, 1994).Therefore, deforestation had caused and continuous to cause environmental degradation in the form of land degradation, water resource deterioration and lose of bio-diversity.

Biogas digestion was introduced into developing countries as a low - cost alternative source of energy to partially alleviate the problem of acute energy shortage for households, reduces deforestation and soil erosion, avoids scarcity of firewood, benefits environment globally and provides excellent fertilizer, there by increases crop production (Vandana,2004).

However, few farmers used biogas in practice in Ethiopia (Yacob, 2000). So, to solve the problem of biogas technology dissemination, it is very important to study on alternative biogas plants constructed using a different material and design.

Thus, the economic assessments of the new and previous model digesters consider adequately the costs of traditional alternative sources of fuel, and the benefits of using biogas and output slurry for cooking and as fertilizer. It is also essential to assess the environmental impact of both models of biogas plants by computing the replacement of conventional fuels such as reduction of fuel wood, cow dung and chemical fertilizer with the use of the biodigesters.

1.2 Problem Statement

Fuel wood and charcoal are the primary sources of energy for Ethiopia’s rural population. According to MOA (2000), on average each rural household spends ten hours per week searching for fuel wood. Females & children are engaged to search fire wood for about 5-6 hours journey (MOA, 2000). Consumption of charcoal and fuel wood is a serious factor in deforestation, environmental degradation, air pollution and carbon dioxide emissions (Paulos, 2004). In Ethiopia with the current economic status almost the majority of family spent 20-30% of their monthly income for purchasing fire wood (MOA, 2000).

Given the scarcity of firewood and the prevalent use of dung for cooking, biogas plants would appear to be an appropriate means of reducing the current usage of these renewable energy resources. The biogas technology used almost exclusively today in Ethiopia, i.e. fixed-dome and floating –drum biogas plants became an obstacle to the rapid diffusion of biogas technology, because it takes a relatively long time (3-5 week) to build a plant and high initial cost of investment, shortage of adequate skilled person who can undertake construction & installation of the plant and transportation problem of the prefabricated steel drum from the urban areas to the interior rural regions of Ethiopia (Yacob,2000).

Considering the problem of biogas technology dissemination with the existing biogas plants, the study was conducted on alternative biogas plants constructed using geomembrane plastic in cylindrical shape.

Therefore, in order to introduce geomembrane plastic biogas plant, comparisons of gas and slurry yield and economic feasibility analysis with the fixed-dome biodigester should be done and accordingly, the outcome of the study may have some contribution to set remedy to the problem.

1.3 Purpose of the Study

The purpose of the study is to ensure the wide use of biogas plants by the farming community, reducing the workload of women by using low-cost biodigester, to achieve ecological stability, namely burning biogas instead of firewood, and not using cattle dung cake and other agricultural waste directly as fuel, but using it in biogas plants to produce both fuel and fertilizer.

The output of the study provides economically feasible, technically efficient and environmentally sound geomembrane plastic biogas plant.

1.4 Hypothesis

1. Geomembrane plastic biogas plant could provide a cheaper and higher amount of gas and slurry than fixed-dome biogas plant.
2. The negative impact of conventional fuels on the environment can be reduced by replacing their use with biogas plant.

1.5 Objectives of the Study

1.5.1 General objective

The main objective of the present investigation is to introduce economically feasible, technically acceptable and environmentally friendly biogas plant to the farming community and other potential users in Ethiopia.

1.5.2 Specific objectives

1. To measure the quantity of gas, quality and quantity of slurry produced from geomembrane plastic and fixed dome biogas plant.
2. To evaluate the economic feasibility of geomembrane plastic and fixed dome biogas plant.
3. To evaluate the environmental impact of the geomembrane plastic and fixed-dome biogas plants.
4. To document the utilization of biomass and biogas technologies by the surrounding farming community.

2 Literature Review

2.1 Fuel Consumption in Ethiopia

About 95% of the total energy consumption in Ethiopia is provided by wood, dung, charcoal and crop residues. Biomass consumption for fuel in 1980 was about 24 million m3 of firewood (mainly Acacia spp.and Eucalyptus spp), 7 million tones of dung, 170,000 tones of charcoal and 6 million tones of crop residues (FaWCDA, 1982). Half of the biomass was used for baking injera and the use of fossil fuels (petroleum products) account only for 5% of primary energy in Ethiopia, but they cost nearly 50% of export earnings (Benjamin, 2004).

At present more than 90% of the domestic supplies of industrial wood and firewood comes from the natural forests which are the main sources of wood products. Fuel wood accounts for the bulk of the wood used, and is the predominantly preferred domestic fuel in both rural and urban areas. The projected demand for fuel wood based on assumed per capita requirement is on the increase and is expected to be over 100 million m3 by 2020. On the other hand, the projected supply from all sources is expected to be only 9 million m3 which is far below the demand (FaWCDA, 1982). Thus, more efficient ways of firewood utilization need to be investigated and substantial savings in firewood consumption could be achieved by the use of biogas and different cooking methods.

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Figure 1: Ethiopia’s Primary Energy Shares, Benjamin Jargstorf (2004)

2.2 Biomass and Biogas Energy Technologies in Ethiopia

The energy balance of the country as of 1995/96 reveals that total energy consumption in Ethiopia was estimated at 723 peta joules or about 50 million tones of wood equivalent and is characterized by high dependence on biomass fuels. The contribution of wood fuel alone was about 77 percent of the final consumption and agricultural residue and dung accounted for about 16 percent which means that the share of traditional fuels in the national energy consumption was above 90 percent. Modern energy (petroleum fuel and electricity) accounted only for about 5.5% of total energy consumption, the share of petroleum being about 4.8% and that of electricity being 0.7%. About 90 percent of the overall energy consumption of the country is that of households out of which the share of urban households is only 6 %. Rural households almost entirely rely on the traditional fuels where as the share of modern fuels in urban households’ consumption was about 20 percent. So, the extent of dependence on traditional fuels is very high

(Samuel, 2007).

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Figure 2: Fuel Wood Carriers for Fuel Consumption from the Forest, Agent of Deforestation, Benjamin Jargstorf (2004).

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Figure 3: Dry Cattle Dung Cakes used for Fuel in Ethiopia, Benjamin Jargstorf (2004).

Thus, it makes sense to ‘modernize’ biomass it self and convert it from its solid form to more convenient forms such as gases, liquids or electricity. The most promising technology that can convert biomass into such clean, efficient gas is biogas plant.

2.3 Theory of Biogas Technology

Biogas technology refers to the production of a combustible gas (called bio- gas) and a value added fertilizer (Called sludge) by the anaerobic fermentation of organic materials under certain controlled conditions of temperature, pH, HRT, C:N ratio etc. A typical bio-gas plant consists of input unit for feeding the fermentable mixture, a digester where anaerobic fermentation takes place, a gas holder for collecting the bio-gas and to cut off air to the gas outlet pipe and out put unit for removal of fermented slurry (Vandana, 2004). The plant operates on the principle that when dung and other organic materials are fermented in the absence of air, combustible methane gas is produced (Vandana, 2004).

According to Grewal et al.(2000),biogas usually contains 50-65% methane (averaging 60%), 30-40% carbon dioxide (averaging 36%),1-5% of hydrogen,1% nitrogen, 0.1% oxygen, ,0.1% hydrogen sulphide and 0.1% water vapours (H2O).

2.4 Benefits of Low- Cost Plastic Biodigester Technology

Global level: - Using biogas for cooking reduces the need for fuel wood and charcoal. Studies conducted by a Tanzania local energy NGO indicates that every 8 households clear fell one hectare of forestry each year through charcoal consumption alone (SURUDE,2002). When other causes of forestry destruction are added such as fuel wood, agriculture, construction and mining, deforestation rate in Tanzania is estimated at between 300,000 hectares and 400,000 hectares per year. Studies have further shown that each biogas unit is able to reduce scale of deforestation by 37 hectares per year. Since, it also uses cow dung that would otherwise have degraded, further green house gas emissions are avoided. This is realized by adapting to biogas in place of fuel wood and charcoal for cooking & heating (SURUDE, 2002).

National level:- Bio-gas helps to save foreign currency which is spent on kerosene and chemical fertilizers. Researchers have estimated that 5 lakh bio-gas plants will have 750 million liters of kerosene per year and provide 12 million tones of organic manure. Biogas helps in reducing the need for expensive energy distribution in rural areas. Due to inefficient distribution system almost 20 percent of the powers are lost during transmission. Biogas system would help in preventing the denuding of forests in a careless manner by the villagers for fire wood requirements. Today deforestation being a serious threat to environment in large parts of the country, as it is followed by the danger of soil erosion and several other ecological imbalances (Vandana, 2004).

Local level: - Reduced deforestation helps preserve forests and all of the services they provide, such as biodiversity and maintenance of water quality. In addition, the promotion of agro forestry practices in conjunction with livestock helps protect soil fertility, prevent erosion, and reduce the risk of overgrazing problems often associated with cattle (Duong et al., 2002).

Poverty Alleviation:- Biogas production integrated with cattle raising and farming provides a reliable source of cleaner fuel as well as increased in come and employment opportunities. Therefore, increased incorporation of cattle in to farming methods increases employment opportunities there by stimulating rural economy. The production of biogas also produces slurry that is very effective as a fertilizer. Farmers have effectively used it in agro forestry farming. Studies by Sokoine Agricultural University in Tanzania have shown that the use of this fertilizer helps maintain soil quality over time, there improving crop yields (SURUDE, 2002).

Poverty reduction through improved health:- Respiratory diseases and sometimes deaths caused by indoor pollution as a result of prolonged exposure to smoke from fuel wood and charcoal is avoided when biogas is used for cooking (Vandana, 2004). The utilization of biogas freed the house wives from eye- sore, eye and lung diseases. The use of bio- gas as a domestic fuel can be a thrilling experience for a house wife (SURUDE, 2002).

Reduced drudgery: - Women and children do not have to spend as much time looking for firewood. Cooking with biogas is also faster than with firewood. As a result, the drudgery and workload of women is lessened. Cooking by using a biogas cooker is easy and fast, this has two implications. On one hand it has reduced fuel wood collection and pollution laden cooking tasks on the part of women. On the other hand it has increased gender equity by involving men in domestic chores. Projects that provide direct benefits to woman are usually sustainable (SURUDE, 2002).

Table 1: Work Load before and after Biogas Production

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Source: African Development Foundation Assessment Report in Tanzania (2004).

2.4.1 Environmental Benefits of Biogas Technology

Biogas does not contain toxic carbon monoxide so no danger to health and no offensive odour, reduction in pollution as BOD and COD and facial pathogens are considerably reduced and environment improvement in rural area reduces illness and build up people’s health. Besides, in regions where biogas is used to generate electricity, cultural, recreation and spare time study conditions can also be improved. (Duong et al., 2002).

2.4.2 Social Benefits of Biogas Technology

Biogas development brings about social benefits. As the problem of fuel for the farmer's daily use is solved, trees are protected and forests are developed. The protection of trees and increase in vegetation areas can reduce soil erosion and improve ecologic balance. The increase in organic manure can result in using less chemical fertilizer, improving soil and increasing production (UNV, 1983). Therefore, the use of plastic biogas plant saves the time that can be used for wage work, consumption of conventional energy sources for cooking, lighting or cooling and substitution of digested slurry in place of chemical fertilizers and / or financially noticeable increased in crop yields.

2.4.3 Economic Advantages of Plastic Biogas Technology

Production and utilization of biogas are beneficial in many ways. They have both direct and indirect economic benefits. The direct economic benefit of biogas as a fuel, in place of firewood and coal, is a reduction in fuel expenses. Compared with direct burning of stalks; biogas produced from biomass fermentation increases the quality of organic manure which can be sold to production teams, increasing the direct benefit to farmers. Biogas production also has many indirect benefits, which sometimes play a very important role in biogas development. For instance, crop stalks, when no longer burned, may be used as animal fodder, increasing the income from animal husbandry, while still providing raw material for biogas production. Farmers can use the time saved from fire wood collection for additional production, and there by increases their income; fermentation effluent can be used as fodder to raise fish, mushrooms and earth worms, and a proteins fodder for poultry (GTZ, 1989). Therefore, from the use of plastic biogas technology, national energy savings, primarily in the form of wood and charcoal, with the later being valued at market prices or at the cost of reforestation, reduced use of chemical fertilizers, so organic crop yield could be obtained and additionally, foreign currency may be saved due to reduced import of energy and chemical fertilizers could be acquired as an advantage.

2.4.4 Beneficiaries of Biogas Development

Rural farming families benefit most from biogas. Women benefit especially, since biogas reduces their workload, improves their access to income through the sale of milk, and reduce health problems (Lekulel et al., 2002).

2.5 Input Materials for Bio- Gas Production

The following organic matter rich feed stocks are found feasible for their use as input materials for bio-gas production (Vandana, 2004).

Animal Wastes

Cattle dung, urine, goat and poultry droppings, litter, house wastes, fish wastes, leather wastes, sericulture wastes, elephant dung, piggery wastes, etc.

Human wastes

Faces, urine and other wastes emanating from human occupations.

Agricultural wastes

Aquatic and terrestrial weeds, crop residues, spoiled fodder, tobacco wastes, oil cakes, fruit and vegetable processing wastes, cotton and textile wastes, spent coffee and tea wastes.

Waste of aquatic origin

Marine plants, twigs, water hyacinth and water weeds.

Industrial wastes Sugar factory, tannery and paper wastes are industrial wastes that can be used as input material for biogas production. When the cattle dung is used as a feed stock, the biogas plant is to be filled with homogeneous slurry made from a fresh dung and water in a ratio of 1:1 according to the quantity of input design calculation (Table 2).

Table 2: Quantity of cattle dung Required for Feeding of different Sizes of Bio-gas units

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Source: Training course of IREP, organized by planning commission and Gandigram Rural Institute (2004).

Table 3: Potential gas Production from Different Feedstock

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Source: Training course of IREP, organized by planning commission and Gandigram Rural Institute (2004).

2.6 Biogas Production Processes

Biogas production is a relatively slow process happening over a period of several days. The principle reaction taking place in anaerobic digester is consecutive but simultaneous. The different phases of the process are solubilization (hydrolyzing phase), acid generation (non-methanogenic phase) and methane generation (methanogenic phase) (Santra, 2001).

The first step involves the solubilization of complex organic materials constituting the digestor food stock. They are composed of carbohydrates, fats, protein, nitrogen compounds, salts and debris. In the second stage, the bacteria reduce the soluble organic material from the first step to soluble simple organic acid. In the third step methane bacteria reduce organic acid primarily acetic acid and certain other oxidized compounds to methane and carbon dioxide (Grewal et al., 2000).

2.7 Theory of Biogas Burner

Biogas can be used as a cooking fuel and in any gas- burning appliances that requires low- pressure gas (such as lamps, stoves, refrigerators etc). Biogas is a lean gas that can, in principle, be used like any other fuel gas for household and industrial purposes, the main prerequisite being the availability of specially designed biogas burners or modified consumer appliances. The heart of any gas appliance is the burner. In most cases, atmospheric- type burners operating on premixed air/gas fuel are considered preferable. This type of burner was used in this study.

A biogas burner (stove) used for cooking purposes, consists of nozzle, an air inlet, mixing chamber and fire sieve element. The nozzle is a hollow tube made of glass, metal, plastic or bamboo. As biogas passes through the nozzle, air is allowed to be drawn in to the mixing chamber. For obtaining desired flame temperature, nozzle adjustment is done by trial & error. Combustibility of gas is maximum when flame is blue (Grewal et al., 2000).

2.8 The Slurry after Digestion

The residue from biogas has been used traditionally as a soil conditioner or fertilizer because the process produces chemical forms of minerals that are more soluble than the organic forms. Residue is also used as a feed supplement (Vandana, 2004).Anaerobic digestion modifies the properties of the waste, such as: Carbon, hydrogen and oxygen are transformed in to CO2, CH4 and H2O, fermentation reduces the C/N ratio increasing the fertilizing effect, the nitrogen appears mineralized and thus suitable for the plant, well - digested slurry is practically odor less and does not attract flies and anaerobic digestion deactivates pathogens and worm ova.

Compared to the source of material, digested slurry has a finer, more homogeneous structure, which makes it easier to spread. There is an increase in the ammonia content in the digested manure, which means the amount of nitrogen available for the plants is bigger in the degassed manure (Vandana, 2004).

The organic content of the digested slurry improves the soils texture, stabilizes its humic content, and intensifies its water holding capacity (Uli et al., 1989; Nielson et al., 2002; Anderson and Sorensen, 2001).

2.9 Measurement of Biogas Production

Total biogas production varies depending on the organic materials digested, the digester loading rate, the environmental conditions in the digester, design of digester, materials used for digester construction. Not all of the bio- gas energy is available for use. Energy is required to heat and mix the digester, pump the effluent, and perhaps compress the gas. Biogas production and biogas yield are the most widely used parameters to control the anaerobic fermentation process at full scale biogas plants. Moreover, pH value and the composition of the effluent biogas are also measured. Individual evaluations of the data obtained from these measurements are vital to perform a proper control of the biogas plant. The available information on biogas production, biogas composition and composition of biomass input are used for control, improve and regulate biogas plants in the future (Grewal et al., 2000).

2.10 Designing of Digesters

Designing a properly sized digester to obtain the maximum biogas production per unit of reactor volume is important in maintaining low capital construction costs. The digester should be sized to achieve desired performance goals in both winter and summer. Design goals could be maximal gas production with minimal capital investment, achieving pollution control and reduction of pathogens, or simply the production of a reasonable amount of gas with a minimum of operational attention. The uses of the slurry after the digestion process is a critical consideration since the main income to the plant can come from that material. The differences in uses also determine the digestion retention time. Criteria must be established, prior to design, since not all goals can necessarily be achieved with a single design (Hao et al., 1980; Hong et al., 1979; Umana, 1982).

2.11 Working of Fixed-Dome Biogas Plant

When the cattle dung is used as feed stock, the biogas plant is to be filled with homogeneous slurry made from a fresh dung and water in a ratio of 1:1 up to the level of the second step in the outlet chamber. As the gas generates and accumulates in the empty portion of dome of the biogas plant, it presses down the slurry of the digester and displaces it into the outlet chamber. The slurry level in the digester falls, where as in the outlet chamber, it starts rising with the formation of gas. This fall and rise continues till the level in the digester reaches the upper end of the outlet opening, and at this stage, the slurry level in the outlet chamber will be at the slurry outlet. When the gas is used, the slurry which was earlier displaced out of the digester and stored in the outlet chamber begins to return into the digester. The difference in levels of slurry in digester and the outlet chamber exerts pressure on the gas which makes it flow through the gas outlet pipe to the points of gas utilization (Grewal et al., 2000).

2.12 Selection and Layout of Pipeline and Biogas Accessories

It is often seen that the size of the pipeline for conveying the gas is not selected properly. This results in lower efficiency or higher cost to the plant owner. In designing the gas distribution system, the parameter that needs to be controlled is the gas pressure. When the gas flows in a pipe, there is a loss in its pressure due to friction. A properly designed pipeline is one which does not cause a pressure drop of more than 2-3 cm of water column under any circumstances.PVC or HDP pipe can also be used for carrying biogas instead of steel pipe. The efficiency of these pipes to carry biogas is more than that of steel pipe as it is very smooth. The cost of these pipes is also less as compared to that of steel pipe. Due to these reasons, normally HDP or PVC pipes are being used for carrying biogas.PVC pipes were used in this study due to the above reason (Biogas support programme, 1994).

According to Grewal (2000), the following points should be kept in mind while selecting and laying the pipelines:

1. The pipes and fittings to be used for laying gas distribution system must be of best quality. Additional emphasis should be given for the selection of valves to be fitted.
2. All steel pipes should be coated with protective paints to avoid corrosion.
3. As far as possible only bends (not elbows) should be used for 900turns in pipelines. This reduces pressure drop.
4. Only gate valves, plug valves or ball valves should be used for the gas pipeline to minimize pressure loss during flow through the valves.
5. For connecting burners with gas pipelines, use of transparent polyethylene tubes should be avoided and as far as possible only neoprene rubber tubes should be used.
6. The over ground pipe should be laid at minimum slope of 1:100 (i.e. one cm in one meter).
7. The over ground pipes should be laid along the walls and should not be hanging free. It could be hooked with clamps at every 2m and should not sag at any point. A continuous slope is essential.

2.13 Transfer of the Plastic Film Biodigester Technology

More than 40 provinces in Vietnam have participated in the transfer of the plastic film biodigester system. The technology was also introduced to Cambodia, Lao, the Philippines and Thailand. More than 15,000 units have been set up in Vietnam during the past ten years; with numbers installed annually showing a consistent increase (Duong et al., 2002).In Ethiopia construction and use of the plastic film biodigesters are yet in an infant stage(Yacob, 2000). So, more efforts are warranted to promote this technology in the country.

2.14 Promotion of Fixed and Floating Dome Biogas Plant

The national programme of India in collaboration with the khadi village industries commission in Bombay has technicians who provide on the spot training and help to get rid of bottlenecks. Constraints were also emphasized, e.g. climatic considerations, lack of accessories locally available etc. A 3m3 plant costs US $750, which is outside the reach of many rural people. Other major constraints are the cost of the installation, operation, and maintenance of biogas plants. Lack of technical experts and servicing personnel constitute a serious drawback. Various problems of digester construction and use of materials were described. An Indian design used steel for the digester, which of course, is very expensive, a Chinese design using earth materials has been tried, which is cheap but allows leakage of effluent, and a Tanzanian design built with oil drams has proved not to be durable(Gunnerson and Stuckey ,1986).

According to Yacob (2000), experience of biogas technology in rural Ethiopia is very limited. Thus the best solution to this problem is to promote the wide use of plastic biodigesters which are economically feasible, technically simple, technologically efficient and environmentally comfortable.

2.15 Economic Evaluations of Biogas Plants

The financial viability of biogas plants depends on whether out put in the form of gas and slurry can substitute for fuels, fertilizers or feeds which were previously purchased with money. If so the resulting cash savings can be used to repay the capital and maintenance costs and the plant has a good chance of being financially viable. However, if the out put does not generate a cash inflow, or reduce cash out flow, the plants lose financial viability (Hao et al., 1980; Hong et al., 1979; Umana, 1982).

2.16 LDPE Geomembrane Plastic

Low density polyethylene (LDPE) Geo- membrane is made of high quality LDPE and physical and chemical parameters are excellent. It is named for its high quality, flexibility and width. The product is widely used for the water proofing, such as reservoir, Channel, airport, garden and construction (Wang, 2004).

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Figure 4: Geomembrane Plastic used for Biodigester Construction

Prior to liner installation all surfaces to be lined shall be smooth, free of all foreign and organic material, sharp objects or debris of any kind, the ground shall provide a firm, unyielding foundation with no sharp changes or abrupt breaks in grade. The installer, on a daily basis, shall approve the surface on which the geomembrane will be installed (Wang, 2004).

The technical parameter of the geomembrane plastic welding machine is as follows,

- Temperature Scope: O- 450 Oc
- Speed Scope : O.5- 4m/min
- Supply voltage: 220V consumption : 440W
- Heat- Seal strength: 85 % master material (tensile strength at cutting direction).
- Applied material: PE (LDPE, HDPE, PVC and compound geomembrane).
- Thickness of material: 0.3-1.2 mm.
(Wang, 2004)

According to Wang (2004), the operation rule for the hot wedge welding machine is as follows

- Put the operate shaft in the Position “off” then put the plug fit in to the socket for an electrical connection.
- The over lapped width of geo- membrane should be between 80-100 mm the edge of the sheets should be cut straight and cleaned.
- Turn on the switch of speed controlling and put it in the position of “3" or so turn on the switch of temperature controlling and adjust the temperature up to 250 Oc or so. When the red light bright, you can put the sheets between the belts (the left sheets under the hot wedge, the right sheets above the hot wedge), push the shaft in the position. “On”, it can work now.
- When hot wedge welder propels at the end of the sheets, you must push the operate shaft back to the position ", to avoid damage the belt.
- Attention! if the surface of the pond or reserve make it propel at a uniform speed.

(Wang, 2004)

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Figure 5: Geomembrane Plastic Welding Machine,(photo shoot by Author)

2.17 Theory of Environmental Impact Assessment (EIA)

Environmental impact assessment (EIA) is a process or an instrument used to forecast and consider both positive and negative environmental and social consequences of a proposed development project to their implementation (EPLAUA, 2007).

The overall goals and objectives of EIA are to promote environmentally sound and sustainable livelihood and development in the country i.e. to bring ecological, economical and social sustainability in the way of development (EPLAUA, 2007).

3 Materials and Methods

3.1. Description of the Study Area

3.1.1 Location

The research was conducted in Mersa-Chekorsa village located in Habru district, North Wollo administrative zone of the Amhara region, Ethiopia. The region has a total area of 170, 752 km2 and a population of 14.5 million (CSA census, 1994 cited in Ayen Mulu, 2004). Mersa is located at 490 kilo meters North of Addis Ababa and 90 kilo meters North of Dessie town and the main all weather road from Addis Ababa crosses the town. Mersa is located at latitude of 11035’N, longitude of 39038’E and an elevation of 1557 m.a.s.l. The Woreda land coverage is 47210 hectare and the average altitude ranges from1200 to 2350 m.a.s.l.

North Wollo

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Ethiopia Habru

Figure 6: Map of the Study Area

3.1.2 Socio-economic activity

In 2000, the population of the Woreda was 16,209 of which 8043 were female (HARDO, 2006), and 90 % of the economic activity of the Woreda depends on mixed farming and 5.5 % on arable farming. The rest 1%, 1.5%, and 2% depend on trade, daily labour and cottage, respectively (Bech and Wavern, 2002).

3.1.3 Climate

Habru Woreda is classified under moist warm climatic zone (Bech and Wavern, 2002).The mean annual rainfall is 1090mm.The mean minimum and maximum daily temperature ranges from 13.27 OC to 28.96 OC with an average of 21.12 OC (Appendix 10). So, the temperature is mesophilic which favors the basic requirement to increase biogas production proportionately.

The area has a bimodal type of rainfall. The first rainfall season is the ‘Belg’ (March to May) and ‘Kiremt (July to September). The topography of the area is some what flat. Therefore, the area was very suitable to undertake the proposed research.

3.1.4 Land Use

The land use pattern of Habru Woreda is summarized in Table 3.1. Annual crop production, sparse vegetation and grazing land constitute about 44.9 %, 27.16% and 7.18 % of the land mass of the Woreda respectively. Only 1614 hectare is covered by forest (772 hectare natural and 772 hectare planted) and 11207 hectare is covered by shrub and bushes. The forests are owned and managed by government and cooperatives (HARDO, 2006).

Table 4: Land use in Habru Woreda

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Source: HARDO (2006).

3.1.5 Livestock Population

Ethiopia has the largest livestock population in Africa, totaling some 30.6m tropical livestock units ( TLU ).Cattle are the most important, numbering some 27m head, followed by sheep(24 m),goats (18 m ),equines (7m) and camels (1m).There are also 53 million poultry in the country (Badege,2007).

According to the Ethiopian Agricultural Sample Enumeration (2002), the total cattle population in Amhara region and North Wollo Administrative Zone is 10,512,777 and 910,763 respectively.

3.2 Experimental Design and Layout

Four biodigesters were made from single & double layered cylindrical geomembrane plastic film, constructed below the ground on a trench excavated at a dimension of 7m*1.5m*0.5m and above the ground surface on a concrete block wall platform filled with stone, sand, aggregate and selected fill constructed at a dimension of 7m*1.5m*(0.75m and 0.5m) with a slope of 20.The main purpose of constructing the plastic biodigester below and above the ground surface is to compare the amount of gas production in both conditions and to come to the real conclusion that in which location of installation the quantity of gas production will be higher with respect to the amount of heat acquired by the biodigesters. Besides, the plastic biodigester was constructed by using single and double layer geomembrane plastic to enable to compare and proof their resistance against biogas pressure and select the best out of which. The biodigester has internal dimensions of 6.2 meter long and 1.33 m in diameter, and to provide a liquid volume in the proportion of 75% of bio digester volume. One Chinese model fixed-dome biogas plant constructed by the college was also another treatment used for comparing the geomembrane plastic biodigester. The capacities of the digesters were 3m3 . The experiment was carried out in Mersa ATVET College, Ethiopia from Nov 20, 2006 to July, 2007.The treatments of the experiment were:

- PSA : plastic biodigester , single layered and constructed above ground.

- PDA : " " double " " " " "

- PSU : " " Single " " " under ground.

- PDA : " " double " " " " "

- FDU : Fixed dome biodigester constructed underground.

The biodigesters were fed with cattle manure and water at equal proportions. The manure was collected from Mersa Agricultural TVET College dairy farm and the nearby private cattle shed. A constant loading rate (10.36 kg manure DM per 1 m3 digester liquid volume) was applied to all five bio digesters (Table 5). The fresh manure was previously mixed with enough water so as to ensure a retention time of 40 days.

Table 5: Technological Parameters of the Experimental Biodigesters.

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Figure 7: Layout of the Experimental Site

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Figure 8: Field Installation of the Plastic Biodigester after Feeding (Photo taken by the author)

3.3 Geomembrane Plastic Construction methodology

The plastic biodigester was constructed with the use of materials like geomembrane plastic, PVC pipes, GI pipes, gate valves, reducers, GI caps, sockets, nipples, neoprene rubber hose and biogas stoves with the help of electrical geomembrane welding machine from Ambasel trading company and CM-43 adhesive with other mixtures from one technician working in North Wollo,GARDO.

Three rolls of geomembrane plastic material, was bought from North Wollo, Mersa town. Each rolls of plastic has a dimension of 13 meter width and 13.5 meter length.

The plastics were taken and placed carefully to the work shop of Mersa agricultural T.V.E.T College after the floor has been cleaned. The plastics were cut at a dimension of 7meter length and 4.50meter width. Then, it was welded with the help of electrical plastic welding machine across the length and the circular part of the cylinder was fitted with the help of CM-43 adhesive and with other chemical mixtures. Few number of small openings found in the plastic were closed with the help of vehicle inner tube and the CM-43 chemical mixtures. In this manner two double layered and two single layered cylindrical geomembrane plastic biodigester were made.

The cylindrical geomembrane plastic biodigester has a length of 6.2 meter and 1.33 meter in diameter. It has an inlet for entry of input materials, gas outlet for exit of produced gas and slurry outlet for disposal of fermented slurry. The digester and gas holder was made as one unit in a cylindrical shape from black 5 mm thick geomembrane plastic.

The digester costs EB 1056.15 and takes about 3 hours to assemble. The components and prices are shown in Table 10.For proper functioning; the digester requires the excreta from five cows dropping 15 Kg of waste on the average per day. The digester also requires an adequate water supply, ideally operating on one parts of water for one part cattle dung.

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