Design and Selection Criteria of Biogas Digester

Abstract

Biogas, produced from animal or kitchen waste, is possibly the only energy source that allows farmers to produce their own electricity. There is an added advantage of reducing the contamination and pollution of water, bad odor, and global warming emissions caused due to animal waste. Biogas is generated when bacteria decompose organic waste anaerobically. The biogas is a combination of gases containing about 60–70% CH₄, 20–30% CO₂, and few of other gases like CO, H₂S, NH₃, O₂, H₂, N₂, and water vapor, etc. The gas produced can be used not only for cooking but also for rural electrification where grid connection is not viable. The remaining digested slurry can be used as manure. The idea behind this chapter is to prepare a document which can help in the installation of biogas units and reduce the size of the biogas plant. This communication discusses the various design aspects of the most common types of biogas digesters used today. It takes into account the design aspects of Fixed dome type and Floating drum type. Mathematical equations for measuring the amount of biogas digester volume have been discussed. The selection criterion for biogas digesters has also been discussed.

1 Introduction

The production of biogas takes place by the anaerobic digestion of the bio-degradable materials with the help of bacteria. The biogas is an important source of renewable/non-conventional energy (Martins das Neves et al. 2009). The energy obtained from biogas has some advantages over other energy sources. Optimizing the use of biogas technology can result not only in the production of energy resources and fertilizers production but also other economic and environmental profit counting sanitation, reforestation, and decrease the demand of natural gas and oil (Walekhwa et al. 2009). Like many developing countries of the world, India also faces major problem due to the petroleum products price hike in the international market. Also, the rising demand for energy from urban as well as rural sector implies that newer sources of energy need to be focused on. Not only this, but these newer and alternative sources of energy have to be made available to the common people at a sustainable amount and at an affordable price. For this reason, conversion of agricultural, cattle, and kitchen wastes into biogas will provide a scope for solving some of the mentioned energy crisis. Most of the developing countries face the problem of the dearth of adequate access to cooking gas and electricity for their rural and suburban population (Beall et al. 2002). Conventional stoves generally made of cow dung cause the major producers of greenhouse gases (GHG) (Smith 1994). Generally, there is the formation of CO, methane, and other nitrogen compounds such as nitric oxide and nitrogen dioxide (Ndiema et al. 1998). In developed countries when rising convenience and lessening prices of fossil fuel during 1950–1970, the attractiveness of biogas got down and made less attractive biogas energy (Deublein and Steinhauser 2008). From the 1970s and continuing, there are now 27 million biogas plants in India and China (Bond and Templeton 2011). Production of methane by anaerobic digestion of bio-degradable materials has been experienced since early times of civilization. In the 10 BC, the usage of biogas was for heating tubs in Assyria (Deublein and Steinhauser 2008). In the twentieth century, Louis Pasteur produced 100 L of the gas by fermenting horse dung. The establishment of biogas (methane) plants in these communities is expected to greatly ameliorate these problems and help preserve the environment. Biogas is composed of CH₄ (40–75%) and CO₂ (15–60%) and the rest is other gases in small quantities, i.e., N₂ (0–2%), CO (<0.6%), H₂S (0.005–2%), O₂ (0–1%), NH₃ (<1%), traces of siloxanes (0–0.02%), halogenated hydrocarbons (<0.65%), and other hydrocarbons which have no methane content like aromatic hydrocarbons, alkanes, alkenes, etc. (Bailón Allegue et al. 2012). The waste generated from this has high levels of inorganic elements like nitrogen and phosphorus which are essential for growth of plants and also improves the process, commonly called bio-fertilizer which increases the soil quality and provides no harmful property to the surroundings (Purwono et al. 2013). A biogas plant is an airtight storage place where biogenic wastes when diluted with water are fermented by bacteria in the absence of oxygen (Weiland 2010). Municipal solid waste can be the input raw material of the biogas digester. A detailed study on the anaerobic-based biogas digester was presented (Appels et al. 2008; Hilkiah et al. 2008). The cattle dung digesters can be improved by adding soya sludge/mustard cake. It increases the amount of gas generated and nitrogen and phosphorus content, and further improves the time for capillary suction. The study shows that the performance of digester was enhanced. Methane content in biogas was also risen by adding of soya sludge/mustard cake. Manorial value (N₂ and PO43-) denoted enhancement in the quality of sludge. The addition does not have any negative effect on the digester performance (Satyanarayan et al. 2008, 2010). Simultaneous anaerobic digestion of press water and food products in a biogas digester improves its buffer capacity (Nayono et al. 2010). A performance of the anaerobic process used mixed fruit and vegetable wastes as substrates in a single stage fed-batch anaerobic digester for biogas production has been carried out (Sitorus et al. 2013). Anaerobic digestion of a mix of fruit and vegetable wastes was done in a 200-L digester within 14 weeks at ambient temperature. Chemical analyses based on standard methods were conducted for the initial waste and for the bio-reactor slurry. The biogas was having a methane content of 65% which is the maximum, with the biogas flow of 20–40 ml/min. The biogas plants are taken into consideration, i.e., the family size plants (capacity from 1 to 6 m³), where there are generally three models like KVIC, Janta, and Deenbandhu, where the comparison had taken place, based on cost. However, by comparison, Deenbandhu model was found to be the most economic and useful model (Singh and Sooch 2004). Investments in domestic bio-digester had overall profit and variety of discounts available. It is projected that domestic bio-digester implementation at the national level could be a good source for accessing significant amount in carbon emissions reduction (CER) yearly financing through the Clean Development Mechanism (Laramee and Davis 2013). The plants are generally made of plastic, concrete sometimes steel, and even bricks materials. They could also be in shapes like silos, troughs, basins, or ponds and may be placed underground (pit) or on the surface. Many countries such as India, China, Taiwan, and Nigeria among others have built biogas plants (bio-digesters) based on cow dung. However, the sizes, shapes, constructional materials, etc., vary. Hence there is need to characterize the different bio-digesters with objectives of; to generate energy and provide rich-nutrients manure (EIS et al. 2011).

In this chapter; Sect. 1 deals with the introduction part, Sects. 2 and 3 present the studies about components of a biogas plant, biogas production methodology, under biogas production methodology Hydrolysis, Acidogenesis, Acetogenesis, and Methanogenesis. Sections 4 present the design of biomass digester Sects. 5 and 6 present the effect of working parameters on the performance of biogas digester, under the effect of working parameters on the performance of biogas digester various parameters, required for a successful digester. In Sects. 7–9, the studies about biogas development in some developed countries, biogas development in some developing countries is discussed. A diagrammatic representation of the biogas plant is given in Fig. 1.

Fig. 1

A simple diagram of the biogas and its utilities 1: livestock house, 2: initial tank, 3: waste from agriculture and livestock, 4: mixing tank, 5: bio-reactor, 6: accumulation of the co-generation, 7: tank for keeping the remaining materials after fermentation, 8: fertilizer obtained from the post-fermentation, 9: maize silage, 10: offices, and 11: Electrical grid for energy supply. Igliński et al. (2012)

2 Components of a Biogas Plant

To produce the biogas, an air-tight enclosed chamber is required. This chamber is called a biogas digester. Digestion process takes place anaerobically inside the digester. The major components of bio-digester are: Waste inlet, Digester chamber, Gas collecting chamber, Gas outlet and tube, Slurry outlet. The brief discussion of each component is as follows:

• (a) Waste inlet: The organic solid waste is mixed with water in a 1:1 ratio to achieve a homogenous mixture and poured in through this inlet. For obtaining the right bacteria to digest this waste, a one-time dumping of cow dung is required and the bacteria within is allowed to multiply after which the organic waste can be put in everyday.

• (b) Digester chamber: Anaerobic decomposition takes place and methane gas rises within the chamber while the solid and liquid settle at the bottom.

• (c) Gas collecting chamber: In case of the floating drum type biogas digester, the gas rises into a fiber drum or fiber-coated metal drum placed above the digestion chamber that constantly floats in a small pool of water. As the gas rises into the dome, the dome also rises and exerts pressure on the internal gas and slurry. It is a constant pressure type biogas digester. In case of the fixed dome type biogas digester, the chamber is static, and it is a constant volume type biogas digester. The pressure varies, and care should be taken that pressure build-up should not exceed permissible limit.

• (d) Gas outlet and tube: The pressure from the dome causes the gas to pass through the gas outlet into a tube which is connected to a gas stove placed in a kitchen.

• (e) Slurry outlet: The solid and liquid waste that collects in the digester chamber will rise into the slurry outlet and can be collected with a buck (Tanskanen 2016).

3 Biogas Production Methodology

The pH value of the digester slurry is an important parameter. This should be monitored on weekly basis and should lie between 5.93 and 7.73. Slurry temperature, slurry pressure, and ambient temperature are needed to observed on daily basis for proper functioning of digester. The slurry temperature and ambient temperature should lie between 20 and 45 °C. The slurry pressure should lie between the 0–0.6 bar and the pressure of the outlet biogas is varying 0–0.25 bar (Ezekoye and Okeke 2006). Batch anaerobic digestion experiments using manure from dairy products as feedstocks were experimented at psychrophilic (20–25 °C), mesophilic (37 °C), and thermophilic (52.5 °C) temperatures (Pandey and Soupir 2011). The mesophilic reactors (37–45 °C) produced a less than 30% biogas from the actual rate and less than 23.3% methane, followed by psychrophilic (20–25 °C) which given output of less than 41% biogas and 39.7% less methane (Vanegas and Bartlett 2012). The methane gas obtained is a form of energy which can be utilized and stored for different domestic purposes.

3.1 Hydrolysis/Liquefaction

Hydrolysis is the first step in the process of conversion of the biomass/bio-waste to the biogas. It is also called liquefaction process. In this stage, with the help of the fermentative bacteria, the unsolvable complex organic substance is being converted to the soluble substance. The common unsolvable complex organic substance is cellulose, and common soluble substances are; sugars, amino acids, and fatty acids. In order to enhance the process, some chemical is being used to reduce the digestion time and increase the methane yield (Bansal et al. 2013). The governing chemical reaction of this stage is given below (Arsova 2010):

$$ {\text{C}}_{6} {\text{H}}_{10} {\text{O}}_{4} + 2{\text{H}}_{2} {\text{O}} = {\text{C}}_{6} {\text{H}}_{12} {\text{O}}_{6} + 2{\text{H}}_{2} $$

3.2 Acidogenesis

Acidogenesis process also known as the first process of fermentation. In this process, there is a continuous breakdown of small sub-units from larger unit due to hydrolysis process. This leads to forms varieties of acids of organic nature along with H₂ and CO₂. This part is the generally the fastest part in the digestion process and has high output energy for the microorganisms (Beam 2011). Degradation to methane and carbon occurs with a short or no time lag when the additional substrate is fed to the respective cultures. On the other hand, some time is needed normally for the acclimation of every aromatic compound, nevertheless, when the culture is used to process one aromatic compound and another similar is feed, it may not need any acclimation time (Peris et al. 2011). The acidogenic fermentation resulted in the formation of fermented products of maize silage as the main substrate in a leach bed process was determined by gas and liquid chromatography. The dynamics of bacterial community was monitored by terminal restriction fragment length polymorphism analysis (Sträuber et al. 2012).

The main output of acidogenesis is acetic (lactic, and propionic acids). The pH of the slurry falls as the rate of the generation of these compound increases. In order to accomplish this process, acidogenic bacteria, organic acid, alcohol, and other compounds are required in the biogas digester (Bansal et al. 2013).

3.3 Acetogenesis

Acetogenesis is a process of production of acetate by anaerobic bacteria obtained from a variety of sources of energy and carbon. The species of bacteria that are capable of acetogenesis are collectively termed acetogenesis. Acetogenic bacteria which produces H₂ are capable of producing acetate and H₂ from heavier fatty acids (Merlin Christy et al. 2014). Acetogenic bacteria employing the Wood–Ljungdahl pathway act as biocatalysts in syngas fermentation during the production of biofuels such as ethanol or butanol as well bio-products such as acetate, lactate, butyrate, 2, 3 butanediol, and acetone (Bertsch and Müller 2015). The ability of such processes can be obtained by the global syngas output, which was 70,817 MW thermal in 2010 and is expected to rise to 72% in 2016. Till date, for acetogen is used for the syngas fermentation for industrial purpose and demonstration. The potential for a number of fermentative products obtained is promising. Synthetic biology will now play a major role in constructing a pathway for commercial operations. In such way, a cheap and abundant carbon producing material will mostly replace, processes where it is obtained from crude oil or sugar in the upcoming future (Bengelsdorf et al. 2013).

3.4 Methanogenesis

Methanogenesis process involves the production of methane from the waste material during the final stage. The production of methane is by acetate-degrading methanogen in two ways: either by means of cleavage of acetic acid molecules to generate carbon dioxide and methane or by reduction of carbon dioxide with hydrogen. Methane produced is higher when obtained by reduction of carbon dioxide and is limited when hydrogen is present in digesters resulting in acetate reaction is the primary producer of methane. Methanogens can also be divided into two groups: acetate and H₂/CO₂ consumers. The reactions of methanogenesis are shown below:

$$ \begin{aligned} & {\text{CH}}_{3} {\text{COOH}} \to {\text{CH}}_{4} + {\text{CO}}_{2} \\ & 2{\text{C}}_{2} {\text{H}}_{5} {\text{OH}} + {\text{CO}}_{2} \to {\text{CH}}_{4} + 2{\text{CH}}_{3} {\text{COOH}} \\ & {\text{CO}}_{2} + 4{\text{H}}_{2} \to {\text{CH}}_{4} + 2{\text{H}}_{2} {\text{O}} \\ \end{aligned} $$

Methanogenesis is sensitive to pH, it is in the mildly acidic range (6.6–7.0) (Bansal et al. 2013). Biogas technology among other processes has made their way into emerging sustainable technologies for waste treatment since waste disposal is a major issue in the developing countries. The matter obtained from this process is a material rich in highly useful inorganic elements like nitrogen and phosphorus required for the growth of plants commonly known as the bio-fertilizer, this increases the soil quality without affecting other components in the soil (Ofoefule et al. 2010).

4 Biogas Plants Types

Biogas plants are mainly of three types based on their shapes (Sasse et al. 1988). They are (1) Balloon plants (2) Fixed dome plants (3) Floating drum plants. As represented in Fig. 2.

Fig. 2

Fixed dome (top left), floating cover (top right), and balloon type (bottom). Bond et al. (2011)

4.1 Balloon Plants

This digester is made of plastic in the upper region of the digester where the gas is stored. The inlet and the outlet are directly attached to the balloon skin. When the space for holding the gas is full, it works like a fixed dome plant. The fermentation of slurry gets activated due to the movement of the skin of the balloon. This helps in the digestion process. Even different materials for feed such as water hyacinths can be used. The materials of the balloon plant should be UV resistant. Red Mud Plastic material (RMP) is also used in fabrication of this type of digester.

The advantages of the balloon plant are: low cost, can be easily transported from one place to another and easy in construction. Also, it has less complexity of cleaning and maintenance issues. The disadvantages associated with this plant are: has a very small life span and is prone to damage. It is mainly used where it is less prone to damage.

4.2 Fixed Dome Plant

A fixed dome plant consists of a digester enclosed with a fixed, non-movable gas space. The upper part contains the gas of the digester. When gas production is stopped, the slurry is sent into the compensating tank. Gas pressure rises according to the volume of gas stored, hence the digester volume should not be more than 20 m³. The gas pressure becomes low if there is some gas present in the holder.

The advantages of this plant are: it is very cheap and has no moving parts hence there is less chance of oxidation and so the lifespan is very long. It also creates a source of employment. The disadvantages associated with this are: it is full of porosity and cracks, there is a pressure drop of the gas. This plant is usually used where easy availability of supervising members or technicians.

4.3 Floating Drum Plants

Floating drum plants contain a container for holding the moving gas and the digester. The holder stays float due to the fermentation slurry or due to a water jacket. The gas collects in the drum, resulting in the rising of the drum. If gas is taken away, it falls again. The drum is prevented from tilting by a guiding frame.

The advantages of these types of plants are: easy to operate due to their simple design and very few errors in construction. However, the disadvantages are being the high cost of construction of the floating drum, also steels parts have chance of corrosion, hence a shortage of lifespan. It also has a constant maintenance cost due to repeated painting.

Further, the anaerobic digesters again can be classified into (Appels et al. 2008):

• (1) Standard rate (cold digester)—It is a very simple type digester which can be used for a lengthy period of digestion of 30–60 days. It has four stages (a) scum layer (b) a liquid layer (c) a region of digesting solid (d) a region of digested solid.

• (2) Higher rate digester—It is a modified version of the generalized rate digester. In such digester, there is continuous feeding. The sludge is mixed and thickened. All the mixing creates a uniformity reducing the tank volume and casing a stable efficient process. There are heat exchangers included in this type of digester it is because of maintenance of constant temperature.

• (3) Two-stage digester—In this type of digester a secondary digester is attached with the high rate digester. The purpose of the secondary digester is to store the digested solid. Sometimes the secondary and the primary digester both have heat exchangers and are of similar design to serve as a standby digester.

• (4) Mesophilic and Thermophilic digestion—The high rate digesters are operated at a particular range of temperature. It is said to be a mesophilic digester if the temperature is around 30–38 °C. When the temperature is in the range of 50–57 °C, it is known as thermophilic.

5 Effect of Operational Parameters on Biogas Digester

A study on the effect of operational parameters on anaerobic digestion based biogas digester was being studied (Appels et al. 2008). In this study, pH value, temperature of the solid and the hydraulic retention time were considered for observation. Each bacteria function better at certain range of pH such as the methanogenic bacteria functions at a pH range of 6.5–7.2. The fermentative bacteria functions at 4.5–8.2. Acetic and butyric are produced at low pH. However, at high pH value such as 8, the propionic and acetic acid are produced.

Temperature affects the growth rate and the metabolism of the microorganism in the digester. It also affects the production of the acid (propionate and butyrate). The increase in temperature is very useful in the digester as it increases the rate of the reaction, kills pathogens, and increases the solubility of few organic compounds. However, too much increase in temperature can cause the production of ammonia which inhibits the functioning of microorganisms. Solid retention time (SRT) is the average stay time of activated-sludge solids in the system. The reduction in the SRT decreases the reaction. Retention time less than 5 days is not healthy for as proper digestion. For 5–8 days the volatile fatty acid (VFA) concentration is quite high. For 8–10 a proper digestion is obtained. Hence, SRT is a very important factor.

Various design parameters were taken into account during design of municipal solid waste based anaerobic digester (Hilkiah et al. 2008). The first parameter considered was temperature. The temperature had direct impacts on the decomposition rate and the amount of the gas produced. It was seen that higher temperature has a high impact on the digester. However, too much increase in temperature caused instability. Moreover, if the bacteria used were thermophilic then it is very sensitive to small change in temperature. Hence mesophilic bacterium is being proposed to use in practice. Researchers have recommended that a constant temperature should be kept inside the digester for optimum production. After temperature, pH is taken into consideration. The pH value of 6–8 is the best for continuing production. But during the long process, the pH gets unstable hence it was proposed that lime or sodium bicarbonate is to be used for increasing the pH. The carbon–nitrogen (C/N) ratio is another factor which affects the production of biogas. The carbon–nitrogen content should be in the ratio of 30:1 for any material. To increase the decomposition process, moisture should be present hence it plays an important role in the production of the biogas. For municipal solid waste (MSW), material is also an important factor for the biogas production. The MSW provide in the digester should be small in nature. Due to small size, the surface area increases, which increasing the decomposition for MSW because decomposition entirely takes place on the surface of the material. Finally, the last part which is taken into account is the mixing. It creates a uniform concentration, temperature, and other factors such as regaining of any valuable gases lost etc.

A serious monitoring of the biogas digester was done (Ward et al. 2011). The various concentrations of the gases such as H₂, CO₂, H₂S, CH₄ and N₂ were monitored. Figure 3 shows the results of monitoring of various gases of the digester.

Fig. 3

Results of monitoring of the gases using μ-GC as a function of time. Ward et al. (2011)

Figure 4 illustrates a variation of CH₄ and H₂S concentration in μ-GC and MIMS for 33 days of operational time. Figure 5 shows the variation of dimethyl sulfide and methanethiol measured in MIMS during the operational time.

Fig. 4

Concentration of CH₄ and H₂S using μ-GC and MIMS for 33 days. Ward et al. (2011)

Fig. 5

Variation of dimethyl sulfide and methanethiol measured in MIMS during the operational time. Ward et al. (2011)

A review on household-based biogas plant was being presented by authors (Rajendran et al. 2012). The authors took an account of few parameters which affect the biogas production. The materials used in the digester plant had an important part in the production of the biogas. Previously biogas digesters were made of stone and bricks but now with technological advancements, PVC and polyethylene are used which are light in weight and economic. The next important parameter to be considered is the temperature. High temperatures are useful for increase the production, but it is not applicable for all cases as with increasing temperature there is a decrease in production in some cases. The substrate selection primarily depends on the type of digesters used and its raw materials. The kitchen waste is a useful substrate as it contains a high amount of fat which increase the productivity of the biogas. Solid concentration and the material of the feedstock are also important parameters. The increase in solid concentration decreases the biogas production. The feedstock material C/N ratio and digestion time together affects the biogas production.

6 Productivity Enhancement of Biogas Digester

A study on the effects of the parameters and the substrates of a biogas digester in presence of sulfate-reducing bacteria was conducted (Moestedt et al. 2013). Study revealed that the temperature and the sulfate concentration significantly affected the productivity when there are any changes due to any of these two parameters.

An enhancement of biogas production using solid substrates is carried out using different technique (Sreekrishnan et al. 2004). The different techniques used for biogas production are:

• (i) Use of additional materials to enhance the process of digestion.

• (ii) Reusing of the slurry obtained and its filtrate.

• (iii) Varying the working parameters such as the temperature, size of the particles, etc.

• (iv) Use of films or biofilters.

Increase the production of the biogas, by adding various additives of the biological and chemical composition. At first, green biomass was used where the production rate increased in the range of 18–40%. On using the microbial strains, the production rate increased about 8.4–44%. The maximum production was obtained for inorganic additives which was around 54%. The recycling of slurry filtrate takes place. The slurry shows a 10% rise in obtained biogas. Any change in working parameters had drastic changes in the productivity of the biogas. For films and biofilters, various modifications are being made till now and the latest results show a 17% rise in the obtained biogas.

A study on the optimization of the biogas for anaerobic digestion was conducted in Zimbabwe (Jingura and Matengaifa 2009). The following techniques were adopted for the improvement of the biogas production.

• (a) Anaerobic digestion of MSW—As MSW is produced in large quantities in Zimbabwe hence it is useful material.

• (b) Anaerobic digestion of sewage sludge and wastewater—In Zimbabwe, the gases produced in the sewage plants are mostly lost. Hence in order to reduce the loss and to increase the power generation, this product should be used for the biogas production. This will lead to increase in overall production of the biogas in the country.

• (c) Co-digestion—Co-digestions helps in the places where an adequate amount of resources are not available for the biogas production.

• (d) Centralized anaerobic digestion—This is also a useful technology for improving the biogas production by decreasing the raw materials and enhancing the process.

7 Enhancement of Biogas Production

A test was conducted for a semi-continuously mixed tubular digester (Bouallagui et al. 2003). Fruits and vegetable waste was used for the preparation of biogas. There was a reduction in performance due to the change in concentration of the feed from 8–10%. The final conversion of the fruit and vegetable waste was 75% and methane content of 64% in the produced biogas.

An investigation of the digestion process from products based on fat, oil, and grease were conducted (Li et al. 2015). A two-stage thermophilic semi-continuous flow co-digestion system was used. Among the digesters, one of the two-stage co-digestion was made, using a pretreatment based on thermo-chemical process having pH 10 at 55 °C. Another two-stage digestion have no pretreatment process. Each of the two systems had a hydraulic retention time of 24 days. The result showed that the process involved pretreatment had a better yield than the second one which did not include any pretreatment.

Biogas productivity of cassava peels mixed with various types of livestock waste was compared (Adelekan and Bamgboye 2009). The livestock waste were poultry, piggery, cattle waste according to the input of 1:1, 2:1, 3:1, 4:1 by mass in anaerobic digesters. There was a significant influence of the mixing of the waste from the livestock with the tapioca peels. The obtained value had increased to 13.7, 12.3, 10.4, 9.0 L/kg-TS on mixing with poultry waste for the ratio of 1:1, 2:1, 3:1, 4:1. The yield on piggery waste had increased to 35.0, 26.5, 17.1, 9.3 L/kg-TS for the ratio of 1:1, 2:1, 3:1, 4:1. The yield on mixing cattle waste had increased to 21.3, 19.5, 15.8, 11.2 L/kg-TS for the ratio of 1:1, 2:1, 3:1, 4:1. Hence, the results show that the mixing the livestock waste in the ratio of 1:1 produced the maximum yield.

An experiment was conducted to increase biogas productivity by using Brassica compestris (mustard oil cake) in cattle dung digesters (Satyanarayan et al. 2008). The mustard oil cake was added to the digester in different ratios. The result showed a 12.2–13.08% increase which 30% mustard cake. Hence, there is a 63.44% increase in comparison to only cattle dung. The biogas production increased 13.38, 25.27, 39.16, 52.26, 63.44% with 10, 15, 20, 25, 30% of mustard cake respectively.

Neves et al. have developed the methods to increase the production of methane from industrial waste composed of 100% barley (2006). In the first process, the barley waste was treated with alkaline solution resulting in water formation pretreatment before activated-sludge co-digestion process. The methane production was 224 L. The second process involved co-digestion with 40% barley waste and 60% kitchen waste. The result methane obtained was 363 L.

An enhancement process of digestion in anaerobic process of waste activated-sludge (WAS) using bio-cremation process was examined (Jih-Gaw et al. 1997). The WAS pretreated with NaOH and its examination occurred. There were 4 reactors A, B, C, D. A was given untreated WAS and total solid (1% TS). The other 3 reactors B, C and D were given WAS which was pretreated but of different percentage of TS with 20 meq/l NaOH and WAS (1% TS), 40 meq/l NaOH and WAS (1% TS), 20 meq/l NaOH with WAS (2% TS). The production of the B, C, and D reactors increased to 33, 30 and 163%.

8 Development of Biogas in Developed Countries

The development of biogas was mostly seen in the northern European countries like Sweden, Denmark, Germany, Austria, and Switzerland (Plöchl and Heiermann 2006). Initially, the idea of biogas plant was to clear the odor of, livestock waste and a supply of electricity to the farms. Later, the main idea of the biogas plant was to generate electricity. In Europe, the digester was made of concrete and steel. The digesters have insulation and heating system to control the temperature. Propellers are installed for the stirring of the materials.

The technology used was for wet anaerobic digestion, but recent developments have made in for dry anaerobic digestion. Also, the feedstock used in biogas plants was another factor. In countries like Germany and Austria, agricultural biogas plants only used agricultural waste as feedstock in the biogas plants. Some the materials of the feedstock are maize, sugar beet, barley, etc. Each crop has a different amount of yield for biogas.

The setting of biogas plants in countries like Denmark started during early 1970 (Raven and Gregersen 2007). The development and the usage of biogas had started during the 1970s. The first type of biogas plants was at farm scale levels. These plants were not constructed on technical knowledge hence the plants failed. This lead to another development which was the centralized biogas plants took place in the 1980s. After 1980 there was no new type of plants until the mid of 1990 when the centralized plants and the farm scale plants were improved in technology.

A case study conducted on the Dutch biogas plants (Geels and Raven 2006). The case study shows that previous biogas plants had a single usage but after 1990 the biogas plants were constructed with the idea of multiple usages such as (1) increasing the agricultural sustainability by reducing artificial fertilizers. (2) decrease the amount of methane emitted into the atmosphere. (3) good source of alternative energy. Many improvements were made during the mid of the 1990s. The learning are:

• Co-digestion can increase biogas production.

• New purification system can remove the hydrogen sulfide.

• Process manure was more homogenous and can be used for fertilizer application.

• Process manure was much more feasible for crops hence reduction in the artificial fertilizer.

• The manure which was processed had less affecting germs or pathogens and weed.

• The accumulation and usage of the methane reduced the methane quantity in the air reducing greenhouse effect.

Finally, two changes in government policy made the way for developing new projects for the next 10 years.

The countries like Ukraine and Poland technical aspects of biogas plants were studied (Chasnyk et al. 2015). The beginning of biogas plant in Poland was in 1928. Since then there were many technological changes. The latest biogas plants from 2011 to 2015 produced methane under mesophilic conditions. In 2014 the biogas plants produced 222, 856, 466-m³ of biogas, 59.6 MW electric power and 61.26 MW of thermal power. In Ukraine, it all started in 1933. Now there exist ten landfill plants, six agricultural plants, and three plants of sewage treatment.

9 Development of Biogas in Developing Countries

In the developing countries, three types of biogas digesters are used (Plöchl and Heiermann 2006). They are Chinese fixed dome, Indian floating drum, and the tube digesters. The floating digesters are manufactured using steel and materials of concrete. The Chinese digester is made of local materials. The Tube digesters are made of polyethylene foils. The inlet and the outlet are made of porcelain. Even though their design is different, but all are based on the same idea. The feedstock enters through an inlet after mixing with the digester.

The tube shape digester, floating over digester, Chinese dome digester have many advantages such as: inexpensive, locally available, can be handled easily and less moving parts hence less prone to failure. China and India have a massive impact on domestic biogas technology (Bond and Templeton 2011). As per surveys, the working biogas plant in India are 40 to 81%. The floating drum plants have better working than the fixed dome biogas plants. The reasons for massive developments in the biogas technology were because of government support providing free servicing, maintenance and provids subsidies. Also, many other factors were involved as: an abundance of cattle manure and other materials for feedstock for the biogas plant. As per the survey in Bangladesh, only 3% biogas plants were properly functioning. 76% of the plants were defective and 21% were not functions. In rural China, only 19% of the biogas potential is used (Chen et al. 2010). In China, Luo Guo Rui type biogas digester was used. This is a hydraulic type biogas digester made of concrete and brick. After 2000 commercial household biogas digester came into action. These were made of glass reinforced plastic. These have several advantages over the previously biogas digesters such as low maintenance cost, less time in construction and a long period of operation. As per survey in Sri Lanka there survey in 1986 and 1996, only 303 and 369 plants were functioning out of 5000 (de Alwis 2002). This shows that there is not much development in the biogas area. In Pakistan, the reports show that presently there are 5357 biogas plants throughout the country. The estimated production biogas production is 12–16 million m³ per day (Mirza et al. 2008). In Nepal, fixed dome digesters are used (Gautam et al. 2009). It was a modification of the Indian and the Chinese model. As per the survey, Nepal uses only 9% of its biogas potential (Akinbami et al. 2001).

10 Biogas Development in the Underdeveloped Countries

In the Sub-Saharan countries, very less development in the biogas technology. The reason for such low development is due to the lack of basic research and awareness (Mshandete and Parawira 2009).

Nigeria does not have much developments in the biogas gas technology, but the design and research activities were conducted. For instance, a plastic bio-digester plant was designed, constructed and tested (Ezekoye and Okeke 2006).

Various digester were designed based on different type of waste to be treated (Hilkiah et al. 2008). A covered lagoon digester is used for the treatment of liquid manures. The semi-solid digester was used for partly solid manure. Plug flow digesters are used for materials which have solid contents of 11–13%. Also, high solid digesters are being implemented for non-fluid and solid materials.

In Sudan, the biogas technology is very much used (Mshandete and Parawira 2009). It is because of Sudan is widely dependent on agriculture. Mainly anaerobic digesters are used for the biogas production. A suggestion was made to use water hyacinth as a feedstock material. In Tanzania, due to government involvement, there is a usages of biogas technology. In 1993 polythene tubular digesters were used which is continued till now. Since there is lack of research in this area there is no such development in the biogas production.

Another study of biogas use in Nigeria was conducted (Akinbami et al. 2001). The country has very less production of biogas due to the poor financial condition of the country. If proper steps are not taken then biogas will be another fuel for high-income groups. Proper planning is needed for entering biogas in the Nigerian energy market.

11 A Case Study on Biogas Digester

A horizontal continuous based digester was used for conducting experiments of 12″ ID PVC pipe having length 4, 5, 6 m (Budhijanto et al. 2012). Mathematical model was developed to obtain the rate of production.

11.1 Mechanism

The first stage of making biogas is the conversion of organic matter into carboxylic acids. After this, the carboxylic acids get converted into CH₄ or into CO. However, proper carbon dioxide and methane ratio are required. The digesters are designed to increase the amount of CH₄ and decrease the amount of CO₂. Now the organic matter, CH₄ and CO₂ are obtained at a rate of k₁, k₂, and k₃ respectively. These rates are entirely dependent on the concentration of the nutrients, temperature, pH, etc. Optimization of these properties leads to an increase in the production of methane. The mathematical equations used for the simulation are as follows:

The following equation is the mass balance equation for the digester:

$$ Q.O_{Y} - Q.O_{Y + \Delta Y} - \mu_{0} C\frac{1}{{Z_{\text{c/o}} }}\Delta V_{0} - p_{\text{r}} C\frac{1}{{Z_{\text{p/o}} }}\Delta V_{0} $$

(1)

Here, Q = amount of feed flow by volume (m³/day), O = organic content in the feed expressed as volatile solids (kg/m³), μ₀ = specific rate of growth of bacteria (1/day), C = amount of microorganism of the basis of mass per unit volume (kg/m³), $Z_{\text{c/o}}$ = the amount of cell obtained per unit mass of bio-degradable materials consumed (kg cell/kg mass), V₀ = digester volume (m³), $p_{\text{r}}$ = Is a constant denoting the amount of products formed (1/day), $Z_{\text{p/o}}$ = product obtained per unit mass of bio-degradable materials consumed (kg obtained/kg mass).

Equation (1) can be written as below

$$ \frac{{{\text{d}}O}}{{{\text{d}}y}} = - \frac{{k_{1} O^{n} }}{{M_{\text{s}} }} $$

(2)

Equation (2) is an empirical equation, does not take into the consideration of microorganisms’ population during the process and the mechanisms behind the conversion of the organic matter. Here, k₁ is a constant denoting the rate constants of O conversion to new cells and biogas (1/day), $M_{\text{s}}$ is the rate of flow of mass at a linear rate of digester materials (m/s), and n is a constant denoting the order of a reaction of O consumption (dimensionless).

The expression for the amount of CO₂ and CH₄ formation is being represented by Eqs. (3) and (4).

$$ \frac{{{\text{dCH}}_{4} }}{{{\text{d}}t}} = k_{2} {\text{N}}^{\text{c1}} $$

(3)

$$ \frac{{{\text{dCO}}_{2} }}{{{\text{d}}t}} = k_{3} {\text{N}}^{\text{c2}} $$

(4)

Here, Eqs. (3) and (4) $\frac{{{\text{dCO}}_{2} }}{{{\text{d}}t}}$ is the rate of CO₂ formation (kmol/day). $\frac{{{\text{dCH}}_{4} }}{{{\text{d}}t}}$ is the rate of CH₄ (kmol/day). N is the volatile fatty acid concentrations. The c1 and c2 are the value of the constant representing the reactions of empirical order.

Equation (5) represents the selectivity

$$ \frac{{k_{2} }}{{k_{3} }} $$

(5)

11.2 Experiment

The experiments were conducted on six digesters. The inlet was fed with slurry consisting of cattle manure mixed with water. It filled 80% of the digester by volume. The surface of the mixture did not have any holes to avoid the contact of air getting into the digester. The gas valve is open after this and was kept for 20 days which was the intermediate period since there was no input or output of the gas. A regular measurement was done for taking in account of the volume of the gas produced. A sample of the gas was taken to find the amount of methane present. At first, the gas obtained was mainly composed of carbon dioxide. The feed concentrations were varied for every 2 months. Again, after one month, after changing the feed concentration the methane content was obtained. The data obtained from the experiments had fitted in the mathematical model.

11.3 Results and Discussions

The results showed that the digester consumed good amount of organic matter. From the experimental data and the mathematical model, a simulation of the digester was conducted. The simulation results revealed that the amount of CH₄ and CO₂ obtained was less. 50% of the organic matters were digested initially. The biogas production was dependent on the various types of microorganism present in the manure. Hence, an increase the amount of manure would increase the microorganisms which increase in the production. Another simulation result showed that even with the increase in the feed there would be very fewer changes in the production rate. Hence, the alternative pathway was taken which was increasing the remaining time and also increasing the stability of digester. There was 30% increase in the CH₄ compared to the previous case. It was noticed that drastic decrease in the residence time after three times recycling the organic matter.

The rate constants k₁ is need to increase for increasing biogas production. However, the slight increase in k₁ will not affect the biogas production rate. The increase in k₁ can be done by increasing the amount of organic matter, but, too much increase in the organic matter would result in a drawback in a continuous digester. Therefore, to increase the production rate of a biogas digester a combination of increasing the rate constant and a method of recycling is to be done.

12 Overall Conclusion

Global increase in energy demand is leading to efficient management of energy. The continue rise in the LPG price and other fuel made it necessary substitute for cooking and other domestic purposes. In developing countries, implementation of biogas digester plants as a renewable source of energy has great potential. An inclusive review of the various designs, details of structure, and operational philosophy of the wide variety of biogas digesters designs have been described. Two major groups of biogas digester can be identified, viz., fixed dome and floating drum. The introduction of low-cost plastic drum type biogas digesters has made it very attractive for domestic application. The retention period is low and gas yield is also higher than the conventional types. Its portability is an added advantage. Some digester easy to construct and user-friendly that can be suitably employed at small as well as the large scale at rural domestic purposes. Scientifically designed biogas digesters are found to be more effective and controllable than the ordinary installation. It makes rural population independent from the dependency over fossil fuel/electricity. Therefore, biogas digester is agreed to be suitable for remote rural village cooking lighting and other applications in most developing countries. Considerable research and field level design of the biogas digester has been done in all over the world. Mathematical equations for fixed and floating drum types of biogas digester are developed to design the optimal digester. As per observations, the size of biogas digester increases and the production cost decreases. The aim in the future is to decrease the production cost to a very affordable level. Hence, it is important to design, development, high-tech, and low-cost biogas digester.