SUSTAINABLE ENERGY EXPANSION TACTICS IN THE SUB-SAHARA BOTSWANA: DESIGN AND DEVELOPMENT OF AN ADVANCED BIOGAS DIGESTER Agarwal A 1, Seretse OM 2 and Molatedi P 3 1,2,3 Department of Mechanical Engineering, FET, University of Botswana, Botswana Abstract- Biogas technology is a bio waste management tool that promotes the recovery and use of biogas as energy by adapting manure management practices to collect biogas. The biogas can be used as a fuel source to generate electricity for on-farm use or for sale to the electrical grid, or for heating or cooling needs. The biologically stabilized bio-products of anaerobic digestion can be used as a crop fertilizers, beddings and as aquaculture supplements. This research was done to solve the problem caused by bio-waste (pollution and global warming problems) and to produce the low cost gas, which will be used as a source of energy. This was achieved by accessing the availability of biowaste then designing and constructing a biogas digester which will use raw waste to produce methane gas. Information on the characteristics of different waste was gathered and calculations were used to find the yield of methane from each type of waste. The type of construction material was also considered as it should suite the types of bio-wastes and the chemical reactions in the digester. The research were done in Gaborone and the surrounding rural areas and it is concluded that a family needs about 10 m 3 of methane for daily family use, which needs about 17 m 3 of digester. Keywords- Bio-waste, Biogas digesters, biodiversity, nightsoil, beef dung, Global warming potential I. INTRODUCTION Biogas typically refers to a gas produced by the biological breakdown of organic matter in the absence of oxygen. Biogas originates from biogenic material and is a type of biofuel. One type of biogas is produced by anaerobic digestion or fermentation of biodegradable materials such as biomass, manure or sewage, municipal waste, and energy crops. This type of biogas comprises primarily methane and carbon dioxide. The other principal type of biogas is wood gas which is created by gasification of wood or other biomass. This type of biogas is comprised primarily of nitrogen, hydrogen, and carbon monoxide, with trace amounts of methane. The gases methane, hydrogen and carbon monoxide can be combusted or oxidized with oxygen. Air contains 21 percent oxygen. This energy release allows biogas to be used as a fuel. Biogas can be used as a low-cost fuel in any country for any heating purpose, such as cooking. It can also be utilized in modern waste management facilities where it can be used to run any type of heat engine, to generate either mechanical or electrical power. Biogas is a renewable fuel and electricity produced from it can be used to attract renewable energy subsidies in some parts of the world [1]. The bio digester is a physical structure, commonly known as the biogas plant. Since various chemical and microbiological reactions take place in the bio digester, it is also known as bio-reactor or anaerobic reactor. The main function of this structure is to provide anaerobic condition within it. As a chamber, it should be air and water tight. It can be made of various construction materials and in different shape and size. Construction of this structure forms a major part of the investment cost [2]. Biogas technology is a bio waste management tool that promotes the recovery and use of biogas as energy by adapting manure management practices to collect biogas. DOI:10.21884/IJMTER.2017.4222.ZGJD1 132
Figure 1: Biogas technology diagram II. BIO-WASTE SCENARIO IN BOTSWANA There is a lot of bio-waste in Botswana. This waste pollutes the air and emits a greenhouse gas (methane) which contributes to climate change and global warming. Exposure of people to biowaste can result in diseases like cholera and bilharziasis. Of the outputs of biogas, the gas is valued for its use as a source of energy and the compost from slurry for its fertilizing properties (soil nutrients). The chemical energy content of biogas can also be transformed into various other forms such as mechanical energy (for running machines), and heat energy (for cooking) and light energy for lighting depending on the need and availability of the technology. Some of the common uses of biogas are: cooking, lighting, refrigeration and running internal combustion engine. III. LITERATURE REVIEW Anaerobic digestion is a multi-parameter controlled process, each individual parameter having control over the process either through its own effect on the system or through interaction with other parameters. The rate of methane production increases as the temperature increases, but there is a distinct break in the rise at about 45 C, as this temperature favors neither the mesophilic nor the thermophilic bacteria. However, no definite relation other than increasing rate of gas production (within certain limits) can be established. Below 10 C gas production decreases drastically; therefore operation below this level is not recommended due to the limited amount of gas production (among other technical problems). During the winter period heating of biogas digesters may be necessary, so that growth of anaerobic bacteria, especially the methanogens, is possible. The heating of a digester can be accomplished by heating the influent feeding materials (e.g. with the biogas produced) and feeding it to the digester or by recirculating hot water through pipe coils installed inside the digester. Figure 2: Effect of temperature on gas production The operational range of ph in anaerobic digesters should be between 6.6 and 7.6, with the optimum range being 7-7.2. In general, the feeding of the digester should be stopped to allow the methanogens to utilize the accumulated volatile fatty acids and at their own pace. When the optimal gas production rates are re-established the normal loading of the digester can be resumed. In addition, the ph of the digester needs to be adjusted to neutrality by the addition of lime or other basic materials. If the alkalinity of the digester slurry is maintained within the range 2,500-5,000 mg/l, a good buffering capacity is normally obtained in the digester [3]. To guarantee normal biogas production it is important to mix the raw materials in accordance with a proper C/N ratio. Bacteria use up C 25-30 times faster than they use N. Therefore, at this ratio of C/N (25-30/1) the digester is expected to operate at the optimal level of gas production. However, C/N ratio is considered to be the @IJMTER-2017, All rights Reserved 133
essential factor. As human nightsoil, animal manures, and sewage sludge have C/N ratios lower than the optimum values, they may be mixed with other agricultural residues that have high C/N ratios. Examples of these residues are wheat straw, rice straw, water hyacinth, and duckweeds, all of which are usually biodegradable, and can be made more so by physically reducing their size (e.g. shredding) or by precomposting. However, problems can arise with these agricultural residues because they float to the top, thereby forming a hard layer of scum on the slurry surface inside the digester. Another term Loadings can be expressed as organic loading (kg COD or volatile solids (VS) / m 3 - day) and hydraulic loading or retention time (HRT). A high organic loading will normally result in excessive volatile fatty acid production in the digester (sour condition) with a consequent decrease in ph, and will adversely affect the methanogenic bacteria. A low organic loading will not provide a sufficient quantity of biogas for other uses, and will make the digester unnecessarily large. To increase the process performance or achieve higher organic loading rates, the dispersed-growth digesters can have part of their slurry recycled back to the digesters in order to retain more active biomass and increase the solids retention time [3]. For anaerobic digestion of organic wastes such as human excreta, animal manure, and other agricultural residues, accumulation of volatile fatty acids,, and un-dissociated ammonia is commonly associated with digester failure. IV. ASSESSMENT OF THE AVAILABILITY OF WASTE In designing facilities for the handling, treatment, and disposal or reuse of these wastes, knowledge of their nature and characteristics is essential for proper sizing and selection of a suitable process. Gaborone City Council Treatment plant receives the capacity of wastewater ranging between 45 000 and 60 000 m 3 / day. This wastewater is a combination of excreta, flushing water, and other gray water or sullage, and is much diluted depending on the per capita water uses. The volume of water used ranges from a daily mean consumption per person of a few litters to about 25 L for rural consumers without tap connections or standpipes. Consumption is 30-90 L for those with a single tap in the household, and 30-300 L for those with multiple taps in the house. The strength of a wastewater depends mainly on the degree of water dilution, which can be categorized as strong, medium, or weak [3]. Rural areas like Mmaphashalala were also visited and some raw materials like night soil, cow dung, beef manure, chicken manure and pig manure were found there. It was found that an average of 5 kg of nightsoil is produced by each family. The amount and composition of animal wastes (feces and urine) excreted per unit of time also vary widely. They depend on various factors such as the total live weight of the animal (TLW), animal species, animal size and age, feed and water intake, climate, and management practices, etc. For design of facilities for animal waste collection and treatment, measurements and samples should be taken at the farm site or (if the farm is not built) at similar sites. Young animals excrete more waste per unit of TLW than mature animals. Gaborone landfill was also visited. A lot of waste gets damped at the landfill every day. Waste there includes clinical waste, hazardous waste paints, chemicals, papers, plastics, tins and bottles. V. DESIGN AND CONSTRUCTION OF THE DIGESTER An ideal plant should be as low-cost as possible (in terms of the production cost per unit volume of biogas) both to the user as well as to the society. At present, with subsidy, the cost of a plant to the society is higher than to an individual user. The design should be simple not only for construction but also for operation and maintenance. This is an important consideration especially in a country where the rate of literacy is low and the availability of skilled human resource is scarce. Use of easily available local materials should be emphasized in the construction of a biogas plant. This is an important consideration, particularly in a country where transportation system is not yet adequately developed. A plant of short life could also be cost effective but such a plant may not be reconstructed once its useful life ends. Especially in situation where people are yet to be motivated for the adoption of this technology and the necessary skill and materials are not readily available, it is necessary to construct plants that are more durable although this may require a higher initial investment. The design should be compatible with the type of inputs that would be used [2]. Figure @IJMTER-2017, All rights Reserved 134
shows the sketch of the design to be made. The design satisfies the design factors described above since it is economic, simple, durable, and suitable for the type of inputs (semi- continuous feeding). Figure 3 sketch design of a biogas digester In figure 3 above, A represents the biodigester tank where the water and raw waste mixture is digested by the bacteria and it should be (0.3 meters 0.3 meters 0.3 meters) 0.027 m 3. B represents the space for biogas collection and it should also be (0.3 meters 0.3 meters 0.3 meters) 0.027 m 3. The entrance (inlet) tube should enter the tank near the bottom and the exit (outlet) tube should enter the tank just beneath the fluid level. The mixture should have a uniform consistency to facilitate optimal digestion throughout the tank. It should be noticed that the fluid level comes right up to the rim of the exit tube. This parity is important, as every day that you put in a certain volume of mixture, the exit tube, in theory, will discard the same volume to be used for fertilizer. This happens because when the mixture is added, the liquid pressure at point C increases due to the gauge pressure above C and becomes larger than the pressure at D. Gauge pressure, = where h = height C-E The biogas then escapes through the PVC tubing that extends above the middle of the plastic. Through this tubing the biogas is transported to the kitchen (or other applications) to be burned for cooking. The material used is polythene because it has a good resistant to many solvents, acids, chemicals and corrosion. In general it is tough and flexible. It was chosen because there is acid formation due to chemical reactions in the digester. Figure 4: Picture of the constructed digester VI. PRODUCTION OF BIOGAS AND EVOLUTION The rate of biogas yield per unit weight of organic wastes can vary widely depending on the characteristics of raw waste and environmental conditions in the digesters as stated under literature review. Among all the three hydraulic retention times (HRT) studied, 20, 30 and 70 days, biogas production is highest from the digesters operated at an HRT of 30 days. The normal range of volatile solids loading = 1 4 kg VS / (m 3 -day) VI.I Characteristics of night soil: Total volatile solids (TVS), % total solids = 86 TVS required / day = volume of digester volatile solids loading rate = 0.027 m 3 3 kg VS / (m 3 -day) = 0.081 kg / day Volume of raw waste to be added / day = @IJMTER-2017, All rights Reserved 135
= 0.000475 m 3 / day Volume of dilution water required to be added to the influent mixture / day = Volume of waste to be added / day - Volume of raw waste to be added / day = 0.0009 m 3 / day - 0.000475 m 3 / day = 0.000425 m 3 / day VI.II Characteristics of pig dung: Total volatile solids (TVS), % total solids = 85 TVS required / day = volume of digester volatile solids loading rate = 0.027 m 3 3 kg VS / (m 3 -day) = 0.081 kg / day Volume of methane produced / day = TVS required / day methane production rate = 0.081 kg / day 1.02 m 3 / kg TVS added = 0.0826 m 3 / day Volume of dilution water required to be added to the influent mixture / day = Volume of waste to be added / day - Volume of raw waste to be added / day = 0.0009 m 3 / day - 0.000423 m 3 / day = 0.000477 m 3 / day VI.III Characteristics of Beef dung: Total volatile solids (TVS), % total solids = 85 = The volume of the digester = Volume of methane produced / day = TVS required / day methane production rate = 0.081 kg / day 1.11 m 3 / kg TVS added = 0.0899 m 3 / day Volume of dilution water required to be added to the influent mixture / day = Volume of waste to be added / day - Volume of raw waste to be added / day = 0.0009 m 3 / day - 0.000620 m 3 / day = 0.00028 m 3 / day Table 1: Characteristics of nightsoil and rice straw nightsoil Rice straw Organic carbon (C), % total solids 48 43 Total nitrogen (N), % total solids 4.5 0.9 Total volatile solids(tvs), % total 86 77 solids Percentage moisture 82 14 An optimal condition for anaerobic digestion should have a C / N = 25:1 in the influent feed: = 0.645 / 0.205 = 3.15 TVS required / day = 10 / 0.3 =33.33 kg, or 0.86 + 0.77 = 33.33 From previous = 10.16 kg / day = 32 kg / day Wet weight of nightsoil required = 10.16 / 0.18 = 56.44 kg / day Wet weight of rice straw required = 32 / 0.86 = 37.22 kg / day Choose a volatile solids loading rate = 2 kg VS / (m 3 -day) The volume of digester required = 33.33 / 2 = 17 m 3 To provide additional space for gas storage the digester should be approximately 22-25 m 3. Waste volume to be added / day = 17 / 30 = 0. 57 m 3 / day Volume of raw waste to be added / day = (56.44 / 1100) + (37.22 / 0.1) = 0.424 m 3 Volume of dilution water required to be added to the influent mixture / day = (0. 57 0.424) m 3 / day = 0.146 m 3 / day VI.IV TESTING Beef manure was added into the digester and the methane produced. @IJMTER-2017, All rights Reserved 136
Table 2: Total solids added and volume of methane Volume of digester ( m 3 ) Total solids added (kg / day) Volume of methane produced (m 3 / day) Night soil 0.027 0.0942 0.0308 Pig dung 0.027 0.0953 0.0826 Beef dung 0.027 0.0953 0.0899 Table 3: Total solids added and volume of methane Volume of digester ( m 3 ) Total solids added (kg / day) Volume of methane produced (m 3 / day) Night soil 8.766 30.58 10 Pig dung 3.269 11.54 10 Beef dung 3.003 10.60 10 The quantity of methane (10 m 3 / day) is enough to be used by a family and can be used for a lot of applications like cooking, boiling water, refrigerator and driving engines for production of electricity. Figure 5: Total solids of nightsoil added and methane produced against volume of digester Series 1: Total solids added (kg / day), Series 2: Volume of methane produced / day (m 3 ) VII. CONCLUSION The volumes of the designed digester waste tank and the gas collector are 0.027 m 3 each. Since the digester volume is fixed, the volume of methane produced by this digester depends on the raw materials methane production rate and volatile solids loading rate. The volume of waste to be added per day depends on raw materials Hydraulic retention time while the volume of raw waste depends on its bulk density. The raw material that will be used most is beef dung because its yields more methane than other raw wastes. As stated earlier, it can be concluded that a family needs about 10 m 3 of methane for daily family use, which needs about 17 m 3 of digester. The amount of biogas produced will be used for cooking, gas lighting, refrigerator, incubator and other applications. The use of raw waste to produce methane gas will reduce the exploitation of wood resources as main source of energy as it leads to desertification, loss of biodiversity and environmental damage. It is estimated that on complete combustion 1 m 3 of biogas is sufficient to - Run a 1 horsepower engine for 4 hour, Provide 2.5 kw-h of electricity, Provide heat for cooking three meals a day for ten people, Provide 12 hours of light equivalent to a 60-W bulb, Run a refrigerator of 1 m 3 capacity for 2 hour, Run an incubator of 1 m 3 capacity for 1 hour. This research can be replicated and large scale digesters can be made so that a lot of methane gas can be produced as useful energy. The use of (small scale) biogas digesters is simple and cheap and it can be afforded by an average person. This will help them save money they spend on buying electricity and cooking gas. Companies and institutions can also use the raw waste they have to produce methane gas. At BMC for example they are producing a lot of methane gas from cow dung and they use it for boiling water. @IJMTER-2017, All rights Reserved 137
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