Biogas Production from a Solar-Heated Temperature-Controlled Biogas Digester

Abstract

This research paper explores biogas production in an underground temperature-controlled fixed dome digester and compares it with a similar uncontrolled digester. Two underground fixed-dome digesters, one fitted with a solar heating system and a stirrer and the other one with an identical stirrer only, were batch-fed with cow dung slurry collected from the University of Fort Hare farm and mixed with water in a ratio of 1:1. The solar heating system consisted of a solar geyser, pex-al-pex tubing, an electric ball valve, a water circulation pump, an Arduino aided temperature control system, and a heat exchanger located at the centre of the digester. Both the digesters were intermittently stirred for 10 min every 4 h. The digester without a heating system was used as a control. Biogas production in the two digesters was compared to assess the effect of solar heating on biogas production. The total solids, volatile solids, and the chemical oxygen demand of the cow dung used as substrate were determined before and after digestion. These were compared together with the cumulative biogas produced and the methane content for the controlled and uncontrolled digesters. It was observed that the temperature control system kept the slurry temperature in the controlled digester within the required range for 82.76% of the retention period, showing an efficiency of 82.76%. Some maximum temperature gradients of 7.0 °C were observed in both the controlled and uncontrolled digesters, showing that the stirrer speed of 30 rpm was not fast enough to create the needed vortex for a uniform mix in the slurry. It was further observed that the heat from the solar geyser and the ground insulation were sufficient to keep the digester temperature within the required temperature range without any additional heat source even at night. Biogas yield was observed to depend on the pH with a strong coefficient of determination of 0.788 and 0.755 for the controlled and uncontrolled digesters, respectively. The cumulative biogas was 26.77 m³ and 18.05 m³ for controlled and uncontrolled digesters, respectively, which was an increase of 33%. The methane content increased by 14% while carbon dioxide decreased by 10% from the uncontrolled to the controlled scenario. The percentage removal of the TS, VS, and COD was 66.26%, 76.81%, and 74.69%, respectively, compared to 47.01%, 60.37%, and 57.86% for the uncontrolled situation. Thus, the percentage removal of TS, VS, and COD increased by 19.25%, 16.44%, and 16.89%, respectively.

1. Introduction

The main challenge in biogas production lies in controlling and optimizing operating conditions such as temperature, pH, chemical oxygen demand, and carbon-to-nitrogen ratio. Temperature stability is key for the health and efficiency of microorganisms responsible for converting biomass to biogas. These microbes are affected by fluctuations in operating temperature in the digester, and thus, maintaining the temperature in the digester at an optimum level enhances biogas production [1]. Temperature fluctuations should not exceed 2 to 3 °C per day in the mesophilic range [1]. This can be achieved if the plant is well-insulated and has a controllable heating mechanism. The cost of heating is a major setback and many options to alleviate this are available. These include using solar energy, geothermal energy, recovered heat from CHP plants, biogas, and a combination of any of these heating sources. Using electricity and fossil fuels is expensive and they can only be used in municipal waste management where the purpose is not biogas production but waste management.

In recent times, researchers and scientists have explored various technological advancements to optimize the biogas production process. These include, among others, process parameter optimization, co-digestion of different substrates, and pre-treatment. Shrestha et al. [2] co-digested food waste with animal manure, agricultural residue, sewage sludge, and industrial organic waste and realized substantial enhancement in degradability and an increase in biogas yield compared to mono-digestion of food waste. In the research, the low carbon–nitrogen ratio of food waste was enhanced by co-digestion, thereby increasing its biodegradability. In a study carried out by [3], it was observed that co-digesting water hyacinth with biomasses such as animal waste and municipal wastes improves the biogas production rate and increases methane concentration. Research has also shown that pre-treatment of substrates and parameter optimization improve both biogas quality and quantity as well as process stability [4,5,6,7,8].

Temperature is one of the most import process parameters to optimize but it has serious cost implications. Maintaining optimal temperature in a digester has great benefits which include increased biogas yield, high-quality biogas, reduced retention time, reduced operation costs, and improved operation stability of a biogas plant. Research has shown that only insulation is not enough to keep the digester’s temperature constant and that some form of heating is necessary [9]). However, due to the initial costs involved, most household digesters are built without heating sources. Nevertheless, the heating costs can be alleviated to some extent if some cheap heating sources such as solar energy, geothermal heat, and recovered heat from CHP plants are employed.

This paper aims to assess the effect of solar heating on biogas production in temperature-controlled biogas plants and recommend economic ways of controlling digester temperature to enhance biogas yield and quality. The empirical research aims to provide an in-depth analysis and evaluation of the biogas production process from a solar temperature-controlled biogas plant. This paper explores different aspects encompassing this technology, including the significance of biogas as a renewable energy source, the basic principles of biogas production, and the integration of solar heating in temperature-controlled systems.

The paper will provide a comprehensive understanding of biogas production from solar-heated, temperature-controlled biogas plants. By examining various aspects of this technology, including its environmental significance, the fundamentals of biogas production, and the integration of solar heating in temperature-controlled systems, this paper will contribute to the knowledge base of researchers and practitioners in the biogas industry. Moreover, it will serve as a valuable resource for policymakers and decision-makers to promote the adoption of sustainable and efficient biogas production technologies. The findings from this research will provide valuable information for the design and implementation of efficient biogas production using cost-efficient methods, thereby advancing developments in renewable and sustainable energy sources.

The novelty of this research lies in combining solar-heated water from a geyser and the insulation properties of clay soil to maintain mesophilic optimum temperature in an underground fixed dome reactor. Solar heating has been researched extensively on surface biodigesters, including placing digesters in greenhouse enclosures, but little has been achieved in using solar-heated water in heat exchangers in underground digesters. The paper seeks to bridge this knowledge gap by combining the excellent insulation properties of clay soil and cheap energy from the sun to keep slurry temperatures within the required range. Automatic stirring is employed to enhance heat distribution for temperature uniformity in the reactor volume.

2. Literature Review

This section reviews recent studies on temperature control in biogas production and the substrate heating sources. It investigates the advantages and disadvantages of heating sources in biogas production. The section also explores the basics of biogas production, the importance of temperature control in biodigesters, and the applications of biogas as a sustainable energy source.

2.1. Basics of Biogas Production

Biogas is produced from the anaerobic fermentation of organic materials such as animal excreta, kitchen waste, sewage water, crop residues, etc. [9]. It is a mixture of methane (50–70%), carbon dioxide (25–50), hydrogen sulphide (<3%), and water vapour (<10%). Methane is the flammable component of biogas. The quality of biogas is a function of its methane content. High-quality biogas should contain a high percentage of methane.

Biogas is a clean and renewable energy source of which raw materials are readily available. Its production helps to solve two major world problems: increasing renewable energy demand and environmental waste management [10] The by-product residue known as digestate is useful as an organic fertilizer in agriculture, animal bedding, fuel pellets, and building material [11]. Its production involves an enzyme-catalyzed process where biomass is fermented in special airtight vessels called digester plants in an oxygen-free environment [12]. Fermentation in the absence of oxygen is called anaerobic fermentation. Different types of microorganisms are responsible for the fermentation of biomass at different stages of the process. The process has four distinguishable stages: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. During these stages, large biodegradable macro-molecules like carbohydrates, proteins, and oils are broken down into methane and carbon dioxide.

2.1.1. Hydrolysis

Hydrolysis is when a compound reacts with water. The reaction normally takes place in an acidic or basic environment. The acid or base plays the role of a catalyst in this first step of anaerobic fermentation. Water and enzymes degrade insoluble organic compounds like carbohydrates, proteins, and fats into soluble intermediate products including monosaccharides, amino acids, and fatty acids [13]. Fermentative enzymes convert complex insoluble organic compounds into soluble products ready to pass to the next step [14]. Different fermentative bacteria act on different types of insoluble material. Some operate on carbohydrates, others on protein, and still others on fats and oils [15]. The process is brought about by the action of several microbes which include bacteria, anaerobic fungi, aerobic fungi, actinobacteria, and chloroflexi. These microbes secrete enzymes which break down complex macromolecules into simpler soluble forms [13]. After the hydrolysis process, a mixture of simple sugars, volatile acids, H₂, and CO₂ are produced. These products are then used in the next stage of anaerobic digestion known as acidogenesis.

2.1.2. Acidogenesis

During the acidogenesis process, acidogenic bacteria break down intermediate products from the previous stage into small soluble molecules like volatile acids, ketones, and alcohols [15]. This stage is the fastest of the four processes and can result in a high accumulation of VFAs [16]. However, these are quickly degraded into H₂, CO₂, alcohol, and organic acids by the action of acidogenic bacteria.

2.1.3. Acetogenesis

Acetogenic bacteria convert the acidogenesis intermediates, which consist of acetic acid, carbon dioxide, and hydrogen, into acetate. The degradation of volatile acids produces acetate, and on the other hand, carbon dioxide and hydrogen combine to produce more acetate.

Acetogenesis is a process that involves the action of various acetogenic bacteria. Some of these bacteria (hydrogen-producing acetogens) act on the volatile fatty acids to produce acetate, carbon dioxide, and/or hydrogen, and homo-acetogenic bacteria (hydrogen-consuming acetogens) produce acetate and water from carbon dioxide and hydrogen.

2.1.4. Methanogenesis

During the final phase of anaerobic fermentation, methanogenic bacteria act on acetate, formaldehyde, carbon dioxide, and hydrogen to produce CH₄, CO₂, and some traces of H₂S. The process involves several reactions that use intermediate products from earlier phases [17]. Methanogens can be classified as either acetate consumers or H₂/CO₂ consumers. It is worth noting that methanogenic bacteria are affected by both high and low pH and work best in the neutral pH range, which is between 6.5 and 8. Once the fermentation process is complete, the remaining undigested material and any dead bacterial remains form the digestate. The digestate can be separated into liquid and solid, both of which are rich sources of plant nutrients (nitrogen and phosphorus), making them ideal organic fertilizers [18].

It is estimated that in a healthy plant, about 70% of methane comes from acetate degradation. This degradation of acetate is facilitated by acetoclastic methanogens [17]. Acetate decomposes into methane gas and carbon dioxide. The remaining 30% is from the reduction of carbon dioxide through hydrogen and other electron donors. Carbon dioxide reacts with hydrogen gas to produce methane gas and water. The reduction of carbon dioxide through hydrogen is brought about by hydrogenotrophic methanogens [17]. In addition, ethanol reacts with carbon dioxide to produce acetate and methane gas, while methanol degrades into methane and water vapour.

Anaerobic digestion is the natural biochemical process where fermentative microbes decompose organic matter, producing a flammable gas called methane (CH₄) and some carbon dioxide (CO₂), in an oxygen-free environment in closed containers [15]. Anaerobes make use of electron acceptors from sources other than oxygen gas for fermentation. These acceptors can either be the fermenting substrate itself or inorganic oxides from within the substrate. In both cases, the ‘intermediate’ by-products are mainly alcohols, aldehydes, and organic acids, plus carbon dioxide [19]. Specialized methanogens convert acetate, carbon dioxide, and hydrogen to methane, carbon dioxide, and small amounts of H₂S. In any efficient anaerobic system, most of the chemical energy in the substrate is converted to methane by the action of methanogens.

2.2. Types of Biogas Plants

There are four main types of domestic biodigester plants: the fixed dome, the floating drum, the balloon, and the plug flow digester [20]. Biodigesters can be either on the surface or underground depending on their location.

2.2.1. Fixed Dome Digester

The fixed dome digester is composed of three parts: the digester compartment, gas storage dome, and slurry displacement chamber [21]. Although they can be built either above or below ground, they are typically constructed underground to improve insulation and reduce temperature fluctuations in the slurry. However, underground digesters have the disadvantage of taking longer to heat up due to the lack of sunshine. Fixed dome digesters are made of bricks and cement–sand mortar, and skilled labour is required to prevent cracking, which can result in gas leaks [20]. Fixed dome digesters have a fixed volume, so when gas is generated, it applies pressure to the slurry, causing it to displace to the expansion chamber. The inlet pipe is used to feed the digester. The digester is filled up to the lower level of the expansion chamber. The difference in height between the levels of the slurry and the gas storage dome gives the amount of gas produced inside the digester, and the difference between the slurry levels in the digester and the expansion chamber gives the pressure head responsible for the pressure of the gas [22]. If the gas is removed for use, the slurry flows back into the digester from the expansion chamber. The pressure build-up caused by the constant volume of the dome enables the biogas to flow through pipes to the biogas appliances [23]. Figure 1 shows a fixed dome digester.

There are many variations of fixed dome reactors available, including the following:

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The Chinese fixed dome plant: This is the original fixed dome reactor and is the inspiration for other fixed dome models. It is a round-bottom cylinder with a half-spherical top. Millions of these plants have been constructed in China in the past [25].

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Janata model: This was the primary fixed dome plant installed in India and was developed from the Chinese fixed dome plant. This reactor model has been discontinued due to cracks in the gasholder that made it difficult to keep gas tightness [25].

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Deenbandhu: This model replaced the Janata model in India. The model has a half-spherical design that prevents cracks and requires small amounts of building material. It is a better design, with more desirable characteristics than its predecessor [26].

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CAMARTEC model: this model originated in Tanzania in the late 1980s. The model has a hemispherical dome sitting on a rigid circular ring [26].

Advantages of Fixed Dome Digesters

The system has a low initial cost, compact basic design, and long lifespan, requires less land if underground, has no moving and rusting parts, and has a low maintenance cost [24].

Disadvantages

Gas-tight construction requires a high level of technical expertise. In the event of a leakage, repairs can be difficult to perform. Additionally, it is not immediately apparent how much gas is being produced. The gas outlet is also prone to experiencing high-pressure fluctuations. As the system ages, gas leaks may become more frequent [21].

2.2.2. Floating Drum Biogas Digesters

The model is made up of two parts: a digester chamber and a gasholder made of an inverted movable steel drum [27]. The steel drum displaces upwards when the gas is produced and downwards when the gas is removed. The gas holder floats either directly on the substrate or on a water jacket. Some guiding structure prevents the gasholder drum from slanting. The floating drum digester has varying volumes unlike the fixed dome [28]. This gives it the advantage of maintaining constant pressure as the gas flows in the pipes to the appliance. Figure 2 shows a typical floating drum biogas digester.

Advantages of Floating Drum Digesters

Floating drum biogas systems have advantages such as simple operation, visible gas production, constant gas pressure, relatively easy construction, low cost, and low construction complexity [29].

Disadvantages of Floating Drum

However, they also have disadvantages such as high material cost due to an extra steel drum, short lifespan due to corrosion of the steel drum, and high maintenance cost due to regular painting of steel drum [28].

2.2.3. Plug-Flow Biogas Digester Model

This digester model is a low-rate system where the substrate moves like a plug from the inlet to the outlet with no horizontal mixing of the slurry, hence the name. As the influent enters the digester through the inlet, an equal amount of effluent leaves the digester through the outlet. The slurry is thick enough to prevent horizontal mixing and vertical settling of particles. The average length-to-width ratio for this digester is 5:1 [30], which means it is a narrow but long tank.

Advantages of Plug-Digesters

Plug-flow digesters have advantages such as low cost, easy transportation, and easy maintenance and are less subject to climatic variations. They do not require mechanical mixing and are constructed with low sophistication.

Disadvantages of Plug-Flow Digesters

Plug-flow digesters have a short lifespan, low gas pressure, negative environmental impact, limited job creation, and high susceptibility to damage.

2.2.4. Balloon Digesters

Another digester model in common use in South Africa is the flexible balloon digester. It is composed of a narrow and long tank made up of heat-sealed plastic or rubber. It is five times longer than it is wide [30]. The digester is normally buried underground, making an incline of 2–5°, with horizontal positioning to allow the slurry to move by gravity [31]. The inlet and the outlet are maintained above the ground at opposite ends of the digester. As the influent is fed into the digester, the effluent is pushed towards the outlet of the digester tank. The gas produced collects at the top of the slurry in the balloon. The inclined positioning of the balloon digester allows for longitudinal separation of the acidogenesis step and the methanogenesis step. This makes the digester a two-phase system [31]. Figure 3 shows a typical balloon digester.

Advantages of Balloon Digesters

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Standardized prefabrication at low cost.

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Easy to transport.

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Easy to maintain.

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No corrosion.

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UV-stabilized and better chemical resistance.

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Quick and easy installation.

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Customized sizes and shapes available.

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Recommended and used by many biogas manufacturers.

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Low construction sophistication [32].

Disadvantages of Balloon Digesters

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Relatively low useful lifespan since it is susceptible to mechanical damage (if plastic).

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Low gas pressure.

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No creation of local employment.

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Not easy to repair.

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Scum cannot be removed during operation [32].

2.3. Biogas Composition, Characteristics, and Uses

The main product of anaerobic digestion of organic matter, biogas, has several energy applications. It can be used for cooking and space heating, lighting, and electricity generation. Biogas can also be upgraded to bio-methane and used in the production of compressed natural gas (CNG) and liquefied natural gas (LNG) [33]. These have similar energy ratings as fossil-fuel natural gas and can be used as transport fuels.

The biogas mixture is composed of mainly methane (55–80%), carbon dioxide (20–30%), and trace amounts of hydrogen sulphide and water vapour [34]. The proportions of these components depend on process parameters like the nature of feedstock, the organic loading rate, the retention period, and the temperature [35]. Biogas has varying energy content, ranging from 16 to 28 MJ per cubic metre, and can be upgraded to biomethane. Biomethane has an average lower heating value (LHV) of 36 MJ per cubic metre [36].

It is important to note that biogas is 20% less dense than air and has explosion limits of 6 to 12% of biogas in the air. Biogas’ ignition temperature ranges from 650 to 750 degrees Celsius, and it has a calorific value of 4740 to 7500 Kcal/Nm³ [37]. Biogas is colourless and odourless, and it burns with a blue smokeless flame, like that of petroleum vapour [9,38]. Additionally, according to [9], the heat from biogas is higher by a factor of 3.5 compared to the heat from burning firewood.

The uses of methane, the flammable fraction of biogas, range from simple heating and lighting to electric power generation and transport fuel [39,40].

Biogas is a flammable mixture of gasses with methane as the main component [38]. Biogas is a smokeless renewable source of energy, emitting much less carbon dioxide and soot when burned than fossil fuels. It produces 96.16% less carbon per kWh than natural gas [41]. This is due to its short carbon chain and low carbon-to-hydrogen ratio. Biogas is mainly used for cooking, lighting, space heating, and other heat requirements due to its smoke-free characteristics. It is made from cheap and abundant raw materials and has several uses such as space heating, power generation, and transport fuel and as a raw material for producing hydrogen, biofuel, and carbon dioxide. Additionally, the residue from its production can be used as a bio-fertilizer. Biogas is also useful in waste management as it uses solid waste as a raw material, providing clean energy while reducing waste.

Benefits of Using Biodigesters as a Sustainable Energy Source

The process of producing biogas from biomass in biodigesters is sustainable and environmentally friendly. Although methane, the main component of the biogas mixture, is a greenhouse gas just like carbon dioxide, when produced in a biodigester the gas is captured. When burnt in the air, it produces 96.16% less carbon than natural gas [41]; thus, the carbon footprint is far less, thereby reducing its effect on climate change. Biodigesters reduce methane and black carbon emissions while producing renewable energy. This demonstrates the sustainable nature of biogas production. Biogas digesters mitigate methane emissions better than in landfills and manure lagoons, where methane is released into the atmosphere and causes global warming. Methane has a 34-times greater impact on climate change than carbon dioxide [42,43]. Apart from its eco-friendly attribute, biogas reduces water and soil pollution, thereby enhancing health by reducing waterborne diseases. It is legitimate to say that biogas improves water quality and leads to improvements in sanitation and hygiene. The digestate from biogas production can be used as an organic fertilizer, thereby reducing the use of chemical fertilizers which are harmful to humans and animals. Biogas plants can generate income and decrease the cost of waste management in rural areas. Additionally, biogas reactors can create employment in rural communities, thereby supporting economic development, in addition to reducing energy poverty in areas without a national electricity grid.

2.4. Factors Influencing Biogas Production in Digester Plants

Several factors influence the production of biogas in digesters [44]. A favourable environment for the microorganisms thriving in the digester needs to be created [45]. A conducive microbial environment in a digester can be achieved if recommended optimum process parameters like temperature, hydraulic retention time (HRT), pH, carbon-to-nitrogen ratio (C/N), volatile solids (VSs), total solids (TSs), substrate loading rate, stirring mode, type of substrate, and particle size are maintained [44]. Fluctuations in any of these operating parameters can slow down or paralyze biogas production.

2.4.1. pH of the Substrate

The favourable pH range is around the neutral range (pH 7) for a good balance between acid-forming and methane-forming microbes [46]. Optimum substrate degradation in an anaerobic digester can be obtained within a pH range of 5.5 to 8.5 [47]. The acidogenesis and acetogenesis processes increase the acidity of the slurry, while the methanogenesis process increases the alkalinity of the slurry because of ammonia accumulation [48]. While the acid-forming bacteria can thrive in an acidic environment, the methane-forming bacteria are paralyzed when the pH is lower than 6.2 [49]. For a good balance between acidogenesis and methanogenesis processes, the pH thus must be maintained at the neutral value, which is pH 7 [49]. If the pH goes too high, it must be lowered by adding lime or recycled filtrate from the residue treatment [50].

2.4.2. Particle Size

The surface area of the total solid available for microbial activities must be increased by grinding the biomass to smaller particle sizes [44]. Breaking large substrate particles into finer particles ensures uniformity of the slurry and effective digestion. A reduction in the particle size of biomass increases the surface area of the solid substrate exposed to the fermentative bacteria to act, thereby increasing the reaction rate. According to a study conducted by [51], reducing the particle size of wheat straw to 0.5 cm and 0.2 cm increased the methane content by 54% and 83%, respectively.

2.4.3. Carbon–Nitrogen Ratio

For optimum biogas production, the ratio of carbon to nitrogen must be between 20:1 and 30:1 [49]. Carbon is the source of biogas, while nitrogen is necessary for the development and multiplication of fermentative microbes. Too high a C/N ratio can paralyze biogas production. It is important to keep the ratio in check for maximum gas production.

2.4.4. Hydraulic Retention Time (HRT)

The HRT is the time the slurry is kept in the digester actively producing biogas [52,53]. It is the period needed for the complete digestion of nutrients in the slurry [54]. The HRT depends on the slurry temperature and substrate composition [53,55]. In the mesophilic range, the HRT can be between 10 and 40 days, while in the thermophilic range, it can be as low as 14 days [52].

2.4.5. Volatile Solids (VSs)

The VSs are defined as the biodegradable fraction of the total solids in each substrate. They are a measure of the biodegradability of the substrate and help in estimating how much biogas can be produced, since biogas comes from the degradable fraction of the total solids. Total volatile solids also indicate the effectiveness of the fermentation process [56]. A substrate characterized by high VSs produces burnable biogas faster than a substrate with low VSs. Cow dung, pig, and goat manure are known to have high VS content and are suitable substrates for most biodigesters [57]. The literature reports that the quantity of methane produced increases with the increasing percentage of volatile solids in substrates if other factors such as the C/N ratio are kept at the optimum levels [56].

2.4.6. Organic Loading Rate (OLR)

The OLR is the amount of substrate fed into the reactor per unit of time. It is governed by the digester volume so it can be expressed as the mass of VSs per digester volume per day. Typically, 0.5–3 kg/m³/day is recommended [58]. Over-feeding the digester lowers biogas production because of the accumulation of fatty acids (FAs). The accumulation of fatty acids decreases the pH, leading to digester failure. Therefore, it is necessary to have a consistent loading rate for high biogas yields.

2.4.7. Stirring Frequency

Regular stirring of the slurry in the reactor is required to mix the new substrate with the effluent containing active bacteria and to free the produced biogas trapped in the substrate [59]. Stirring the substrate also improves the availability of the slurry to the digesting microbes. Mixing keeps solid materials from settling at the bottom of the digester and maintains an even temperature distribution throughout the volume of the slurry [60]. Stirring also ensures an even distribution of substrate concentration, thereby enhancing stable fermentation. Uniform pH conditions in the digester are achieved by sufficient stirring. Maintaining regular stirring of the slurry reduces temperature gradients in the volume of the slurry. Nevertheless, continuous strong stirring can inhibit bacterial activities, thereby decreasing biogas yield [60]. Therefore, the agitation must be at low speeds and be regular for increased biogas production [61].

2.4.8. Type of Substrate

The amount and quality of biogas produced in anaerobic fermentation are influenced greatly by the type of substrate used. This is because of the different physio-chemical characteristics of different substrates. Methane content also depends on other parameters like operating temperature and pH [57]. Co-digestion of different substrates improves both the amount and quality of biogas [62]. Providing the substrate with microorganisms, creating a favourable microclimate for the microbes, increases the degree of substrate degradability and quantity, as well as the flammability of the biogas. The presence of some traces of metals like nickel, cobalt, and iron enhances bacterial multiplication and increases enzymatic activities, thereby boosting methane yield [63]. Providing easily degradable substrates like volatile fatty acids (VFAs) improves the general biodigester performance because of the increased action of VFA degraders.

2.4.9. Total Solids (TSs)

The TSs content of a given amount of biomass is the fraction that remains after the complete removal of water from the substrate [64,65]. The substrate is heated at a temperature of 105 °C for one day then it is weighed to measure the TSs. A substrate with high TSs (22–40%) has the potential to release a higher yield of methane than a substrate with low TSs content (<10%) [65,66].

2.4.10. Temperature

Temperature is the environmental factor that influences biogas production the most [67]. Generally, temperature affects the chemical reaction rate; thus, the rate of biochemical reactions producing biogas in the digester will increase with temperature. Temperature dictates the type of microbes that are operational in a biodigester that is either mesophilic (20–40 °C), thermophilic (42–57 °C), or psychrophilic (<20 °C) [67]). The VFA consumers are very much affected by temperature variations in the digester [68].

The quantity and quality of biogas are influenced significantly by the temperature of the surroundings of the digester plant. If the plant is not well insulated, there will be significant heat exchange between the plant and its environment, thereby increasing temperature fluctuations. A stable operating temperature ensures high biogas yield and methane content [44]. The rate of yield increases as the temperature increases up to 38 °C in the mesophilic range. Research has shown that mesophilic plants have better process stability than thermophilic plants if cow dung, pig manure, or chicken manure is used as a substrate [61]. Unheated and uninsulated digesters are paralyzed when the average slurry temperature is below 15 °C [69].

2.5. Importance of Temperature Control in Biogas Production

Temperature is a fundamental environmental parameter influencing biogas production in biodigesters [44]. An increasing temperature results in increased biogas yield, higher methane content, and better pathogen killing [47]. Babaei et al. [70], discovered that biogas production increases linearly with temperature up to 44 °C in the mesophilic range. The benefits of maintaining a stable temperature in a digester cannot be over-emphasized [71]. The methanogenic bacteria responsible for methane production are sensitive to temperature fluctuations [9].

Temperature is known to affect the rate of chemical reactions in general and in biogas production. A favourable temperature in the digester causes a boom in the multiplication and health of anaerobic microbes responsible for methane production, thereby increasing the fermentation process and reducing the hydraulic retention time.

Much research has been carried out to maintain a constant favourable temperature in biogas plants. Insulation only has been proven to be inadequate to maintain an optimum operating temperature in biogas plants, making it imperative to heat the substrate in biogas plants. The cost of heating is a prohibiting factor. Numerous researchers have experimented with various cost-effective methods for heating biogas substrates.

2.6. Challenges and Limitations in BioDigester Heating

The main challenge in digester heating is the cost involved. Some researchers have employed solar energy for substrate heating. This is a promising way of digester heating since the cost is reduced significantly compared to other ways of digester heating such as using electric currents, fossil fuels, and biogas. Mahmudul et al., (2021) [72] used a solar-assisted biodigester to maintain a constant temperature in a digester and reported that the solar energy system efficiently raised the digester temperature and maintained it at optimum, thereby improving both the quality and quantity of biogas. Additionally, they observed an improvement in digester performance and a reduction in pollution as compared to conventional digesters. Using recovered heat from digester effluents is another possible cheaper alternative, though solar heating is more environmentally friendly [72]. Solar substrate heating depends on the geographical location of the plant. In temperate zones, insolation is not sufficient for this purpose. It can work satisfactorily in the tropics where solar radiation is abundant. Another challenge is the seasonal variations in solar radiation, which will warrant some non-solar heating backups in cold seasons and sometimes at night. The major advantage of solar digester heating is that it produces less pollution and significantly reduces heating costs.

Solar water heaters have been employed in conjunction with heat exchangers in digesters recently to lessen heating expenses [73]. The solar heater needs to be properly sized for the type of digester and heat exchanger. The sizing of the heating system is a function of several factors including the amount of solar energy received, the heat requirements, and the type of technology used [74,75]. The amount of solar energy received per square metre depends on the location and season of the year [76,77]. Evacuated tube collectors are currently the most appropriate technology available if temperatures are less than 100 °C [78]. Evacuated tubes work perfectly with both direct and diffuse radiation and are efficient with high absorber temperatures [76]. In addition, evacuated tubes work more efficiently than their flat plate counterparts in cold and cloudy weather [79]. Although evacuated tubes cannot operate with high pressures and cannot tolerate thermal shocks during operation, they perform much better thermally than flat plate collectors [80]. Figure 4 shows the construction of an evacuated tube solar collector. It is composed of two borosilicate glass tubes.

The number of collector tubes is determined by the heat load the collector should overcome and the volume of water to be heated, as well as the insolation level [81]. In this project, the solar heater is used to heat the biodigester substrate from a temperature of 20 °C to an operating temperature of 35 °C. In South Africa, the average solar radiation (insolation) ranges between 4.5 and 6.5 kWh per square metre per day [82].

Digester substrate heating sources other than solar heating include using electrical coils, geothermal energy sources, heat pumping, coal-fired boiler heating, and a combination of any of the above. Heat recovered from combined heat and power (CHP) plants is another alternative. Currently, research is mainly focused on solar heating because of its cost-effectiveness and environmental friendliness [83].

2.7. Techniques of Enhancing Biogas Production

Research has shown that there are several ways to enhance biogas production, including pre-treatment of the substrate, co-digestion of different substrates, and process parameter optimization.

2.7.1. Pre-Treatment

Pre-treatment is a way of preparing organic matter for digestion by microorganisms. It is meant to enhance the efficiency of methane production [84]. Pre-treatment can be physical (mechanical or thermal), chemical, or biological [85]. Mechanical pre-treatment is the reduction in biomass particle size to expose a greater surface area of the substrate to the microbes for degradation [86]. It is like chewing food for digestive enzymes to work on it. Increasing the surface area available for microbes to attack the substrate generally increases the rate of reaction. Mechanical treatment is mainly used in agricultural anaerobic digestion [84]. A reduction in the particle size of the substrate improves the biodegradability of the substrate. On the other hand, thermal pretreatment uses thermal energy to break the cell wall of the target compound, making it available for microbial attack and therefore increasing the degradation process. Many different methods can be used to supply the thermal energy needed. Saha et al. [7] reported an increase in methane yield of 10.8 times from municipal sludge after one hour of thermal treatment at 90 °C. Generally, thermal pretreatment improves the solubilization of the substrate.

Chemical pretreatment involves the use of organic or inorganic compounds such as alkali and acids to denature the structure of biomass. The chemical interactions between these added compounds and the biomass break the intra- and inter-polymer bonds. This process makes the biomass vulnerable to microbial degradation. Chemical pretreatment was noticed to increase the cellulose content, COD, glucose, and COD to nitrogen ratio in a study conducted by [87]. Kaur [51] realized an improvement in methane concentration after treating sugar bagasse with sodium hydroxide and calcium hydroxide as compared to untreated bagasse.

Biological pretreatment involves mixing the organic compound with fungi or enzymes [85,86]. Research has confirmed that biological treatments increase lignocellulose hydrolysis, thereby enhancing biogas production [88]. Many researchers reported improvements in gas production after biological treatment of different lignocellulose biomasses [4,6,89,90].

2.7.2. Co-Digestion of Different Substrates

Co-digestion is the process of simultaneously digesting two or more substrates in one anaerobic digester to settle the drawbacks of mono-digestion [91]. Different substrates have different chemical and physical properties which can enhance or inhibit biogas production. Chemical composition and process parameters such as pH and temperature have a strong influence on the biodegradability of substrates. The core purpose of co-digestion is to utilize positive properties in different substrates to enhance biogas production (both in quantity and quality) from less-yielding or hard-to-digest biomass. The improvement in biogas yield is due to the positive interactions of favourable characteristics in the co-digested substrates and the reduction in negative influences and inhibitory compounds. Co-digestion improves nutrient balance, increases the volatile solid load, and creates a favourable environment for microbes in the system. A substrate rich in nitrogen can be co-digested with a substrate rich in carbon such that the C/N ratio is improved, leading to high biogas and methane yield. A carbohydrate-rich substrate is usually co-digested with a fatty substrate [92]. Many researchers point to the fact that both biogas quantity and quality are improved by co-digestion [3,63,93].

2.7.3. Process Parameter Optimization

Research has shown that biogas production is at its maximum under certain operating parameter values, for example, a pH value between 6.5 and 8.5 and a temperature of 35 to 37 °C in mesophilic systems and 50 to 55 °C in thermophilic systems [94]. Process parameter optimization thus involves creating a favourable environment in an anaerobic digestion system to maximize biogas production. In a convenient environment, the health of microorganisms is enhanced, thus making the digestive system stable and the digestion rate fast. Many researchers have investigated different process parameter optimization and observed improvements in the quality and quantity of biogas produced [5,8,94,95,96,97].

3. Materials and Methodology

Two underground fixed dome digesters, one fitted with a solar heating system and a stirrer and the other one with an identical stirrer only, were batch-fed with cow dung slurry collected from the University of Fort Hare farm and mixed with water in a ratio of 1:1. The solar heating system consisted of a solar geyser, pex-al-pex tubing, an electric ball valve, a water circulation pump, an Arduino aided temperature control system, and a heat exchanger located at the centre of the digester. The heat exchanger was fabricated from 22 mm copper tubes of 400 W/(m·K) thermal conductivity and a maximum flow rate of 0.6 L per second. Hot water from a solar geyser was allowed to flow into the exchange through an electric ball valve which could close or open in response to the set digester temperature. Both the digesters were intermittently stirred for 10 min every 4 h. The choice of stirring for 10 min for 4 h was based on recommendations from the literature, which reported higher biogas production rates at intermittent stirring modes. Chol et al. [60] reported a higher biogas production when they stirred the slurry for 3 min at an interval of 6 h as compared to 12 h and 2 h. Zhang et al. [59] reported that intermittent stirring had a higher biogas production and COD removal rate than continuous stirring when they studied the performance of dry anaerobic digestion (DAD). The main purpose of stirring in this paper was to uniformly distribute heat to ensure temperature uniformity in the digester and stable microbial activity. It was found that 10 min was reasonably long enough to distribute heat and short enough to allow sufficient contact between the microorganisms and the substrate. Additionally, 4 h was short enough to not bake the slurry. A digester without a heating system was used as a control. Biogas production in the two digesters was compared to evaluate the effect of solar heating on biogas production. The total solids, volatile solids, and the chemical oxygen demand of the cow dung used as a substrate were determined before and after digestion. These were compared together with the cumulative biogas produced and methane content for the controlled and uncontrolled digesters.

3.1. Deployment of Temperature Sensors

Three temperature sensors were deployed in each of the two digesters (the experimental one and the control) at corresponding positions to measure slurry temperature at different positions. DS18B20 temperature sensors, supplied by Mantec Electricals in Johannesburg, South Africa, were used to measure temperature [98]. The three temperature sensors were secured at the centre of the heat exchanger along the diameter of the digester, one at the centre and the other two at 0.3 m from either wall of the digester. Figure 5 shows the schematic positions of the temperature sensors inside the digesters. Two more temperature sensors were placed at the heat exchanger’s inlet and outlet to measure the hot water’s temperature entering and leaving the heat exchanger. One more temperature sensor was used to measure ambient temperature outside the digesters. All the sensors were directed out of the digesters by connecting cables inside a conduit and connected to an Arduino mega data logging shield.

An Arduino mega 2560 data logging shield, supplied by Mantec Eletricals in Johannesburg South Africa, was fitted with a 32 gig SD card and a Real-Time Clock (RTC). The temperature values, date, and time were recorded and stored on the SD card and were transferred to a laptop for further analysis and processing at the end of the experiments.

The DS18B20 sensor is an immersion waterproof thermometer that passes information over a 1-wire bus. Thus, it needs only one data line and ground to border with a microprocessor. The advantage of this type of sensor lies in that it can be directly powered from the data line, thereby removing the need to power it externally. Each DS18B20 sensor has its own distinct identity, and thus, many DS18B20 sensors located over a large area can function independently on the 1-wire bus and be controlled by one microprocessor. This sensor was selected for this reason and its compatibility with Arduino Mega 2560, since the temperature from different points in the digester was measured. The DS18B20 sensor has a temperature range of −55 °C to +125 °C and an accuracy of ±0.5 °C from −10 °C to 85 °C. The probe also has a stainless-steel waterproof housing which is 6 mm in diameter and a length of 35 mm. It is connected using three wires, red for VCC, black for ground, and yellow for data. The temperature response of the DS18B20 sensor is 300 s. The temperature control system has an Arduino-aided on/off controller, and thus, some overshooting could occur when reaching the setpoint temperature.

3.2. Measurement of Biga’s Volume and Composition

The biogas production rate was measured using a gas flow metre with a digital display. The flow metre measures and displays the volume of the biogas produced in m³. The gas produced was measured and tabulated daily at 18:00 h for both scenarios. The daily biogas produced was then added together for the 30-day retention period to obtain the cumulative biogas. Figure 6 shows a photograph of the flow metre used.

A Bosean biogas analyzer (model K-600) was used to determine the methane, hydrogen sulphide, and carbon dioxide concentrations. The analyzer can detect the concentration of methane, carbon dioxide, and hydrogen sulphide at the same time. The analyzer has the following technical parameters: the gas sampling method is pumping, a working temperature from −20 °C to 50 °C, a power of 3.7 v, a 3600 mA rechargeable lithium battery, a charging time of 4–6 h, a 2.4-inch coloured LCD display, and an operating time entailing working continuously for at least 10 h.

Table 1. Technical specifications for biogas measurements.

Gas	Range	Accuracy	Resolution
Methane	1–100% vol	±5% (F.S)	1% vol
Carbon dioxide	1–100% vol	±5% (F.S)	1% vol
Hydrogen sulphide	0–500 ppm	±5% (F.S)	1 ppm

shows the technical specifications for biogas measuring applications as given by the supplier. The gas analyzer is connected to the gas tape of the digester, and once the gas tape is opened, the analyzer pumps the biogas through the gas tube. The concentrations by volume of the methane and carbon dioxide are then displayed on the LCD. The hydrogen sulphide reading is also displayed in parts per million (ppm).

The biogas analyzer uses the infrared optical principle to detect the concentrations of CH₄ and CO₂ by using the wavelength characteristics of the gasses. The electrochemical principle is employed for H₂S detection. Knowledge of the concentration of methane gas in the produced biogas determines the flammability of the biogas. A high methane concentration means high-quality biogas. Figure 7 shows the Bosean biogas Analyzer used to detect the gasses’ concentrations in biogas.

The concentrations of methane, carbon dioxide, and hydrogen sulphide were measured and recorded daily at 1800 h. The relative concentrations of these gasses indicated the changes in the quality of biogas produced during the entire retention period. Initially, the carbon dioxide concentration was higher than the methane concentration, but from day 6, the methane concentration became higher. The methane concentration continued to increase from day 6 in both digesters, reached a maximum, and then decreased as the digestion process proceeded.

3.3. Temperature Control and Automation

The temperature control and automation system were programmed to calculate the average of the three digester temperatures and produce an output based on the comparison between the set temperature (35 °C) and the average digester temperature. The temperature control circuit would compare the average digester temperature with the set temperature and give a command to either close or open the electric ball valve, to stop or allow hot water to pass through the exchanger. The temperature control circuit was set to give a high output if the measured temperature was less than 35 °C and a low output if the measured temperature was greater than 35 °C. A high output fed to the electric ball valve opens the valve and allows hot water to flow, thereby raising the temperature of the slurry. A low output fed to the electric ball valve will cause it to close and stop the flow of hot water, thereby allowing the digester temperature to decrease. Thus, the digester temperature was maintained at ±35 °C. One relay was used to switch the electric ball valve circuit and the water circulation pump circuit. The water circulation pump was utilized to recirculate heat-depleted water back to the geyser for reheating. Thus, a high output from the temperature control circuit could start the pumping and open the electric ball valve simultaneously. The stirring was programmed to occur once every four hours and last for 10 min. A separate relay was used to switch the stirring circuits. Figure 8 depicts a schematic diagram of the control and automation circuit, consisting of an Arduino Mega 2560 with a data logging module and connected temperature sensors.

3.4. Experimental Setup

The experimental setup is shown in the photograph in Figure 9. The geyser was used to heat the water to a temperature of 80 °C. The solar panel was used to charge the lithium-ion battery, which was powering the electric ball valve, the circulation pump, the stirring system, and the Arduino board.

Figure 10 shows the connections inside the power and control housing where the electric ball valve, the circulation pump, the battery, and the controlling circuit are connected.

3.5. Evaluation of the Performance of the Automated Digester

The performance of the heated biodigester was evaluated by comparing it with the control digester and the existing literature. The methane content and quantity of biogas produced in the two scenarios were measured and compared. The effectiveness of the temperature control system was evaluated based on how well it kept the slurry temperature at the set temperature of 35 °C. The number of days the temperature was out of the required temperature range (35 ± 0.5 °C) was determined, expressed as a percentage, and subtracted from 100 to give the system efficiency in percentage. The performance was also evaluated based on the methane concentration and the quantity of biogas produced, compared to those in the control digester. It was also assessed based on the percentages of VSs and COD removals in the two experimental setups.

The slurry temperature was analyzed to see whether it remained at the pre-set value of 35 °C or not. The produced biogas was analyzed in terms of quantity and methane concentration. A comparison of the quality and quantity of the biogas produced per day in the temperature-controlled digester and that produced from a digester without a temperature control facility was made. Graphs of biogas produced against retention time were plotted and compared. Also, cumulative gas was plotted against retention time and analyzed. In addition, the percentage removals of TSs, VSs, and COD were plotted and compared for the two scenarios.

4. Results and Discussion

The results of the investigation are discussed in this section.

4.1. Efficiency of the Temperature Control System

The efficiency of the temperature control system was assessed by comparing the number of days the temperature was out of range with the number of days the temperature was in the required range. The following formula was used:

Efficiency = 100 − (number of days the temperature was out of range)/total number of days × 100

It was found that the temperature control system was 82.76% efficient. Since the digester was underground and the soil in which it was buried was clay soil, the temperature stability can be attributed to the excellent insulating properties of the clay soil. The temperature was maintained stable in the required range (35 ± 0.5 °C) even at night solely by the heat from hot water and the insulation of the clay soil. The insulation properties of clay soil depend on the moisture content and thus are affected by seasonal changes. Repeating the experiments in all the seasons is necessary to come up with conclusive results.

Figure 11 shows the variations in the slurry temperature and ambient temperature for the retention period. It took two days for the slurry temperature to rise from 22.5 °C to 35 °C for the heated digester. This result points to the fact that the temperature in the digester was increasing at a rate of 0.52 °C per hour. The 100 L solar heater provided the heat needed to raise the slurry temperature to operational temperature and maintained the temperature at plus or minus the optimum for two days. The daily average temperature for the unheated digester followed the variations in ambient temperature, showing some weak correlations between the two variables. Thus, though the ground provided some good insulation to the digesters, it was not 100% efficient. This points to the fact that insulation alone cannot achieve digester temperature stability. The temperature of the heated digester slurry was ±35 °C for the rest of the retention period, showing the ability of the temperature control system to keep the slurry temperature at the mesophilic optimum temperature. Li et al. [99] obtained a similar temperature stability throughout the year using solar energy in cold and dry areas of China. In their research, one batch was maintained at 26 °C (psychrophilic), another one at 37 °C (mesophilic), and the third one at 52 °C (thermophilic) by using a hybrid heating system of solar energy and electricity. Solar energy was the main heating source. In the present research, only solar energy was used and there was no need for extra heating sources. This could be due to the benefits of good insulation of the clay soil and that solar radiation is abundant in the tropics in summer when the experiments were performed.

4.2. Performance Evaluation of the Temperature-Controlled Digester

The performance evaluation of the temperature-controlled digester involved comparing the quantity and quality of gas produced with that of the control digester. Additionally, the assessment included comparing the reductions in percentage VSs, percentage TSs, and percentage COD between the two experimental conditions.

4.3. Comparison of Volume and Quality of Gas Produced

The burnable biogas was first observed after 6 days of batch-feeding the two digesters. The biogas production rate increased gradually in both experimental conditions to a maximum. The highest rate of 1.72 m³ per day in the temperature-controlled digester was reached on day 22 and for the uncontrolled digester; the maximum rate was reached on day 21 and was 1.21 m³, an increase of 42% from the uncontrolled scenario. Since both digesters were agitated at the same rate, the difference in biogas production rate can be attributed to the controlled heating of the digester slurry. The theoretical biogas production rate was 1.92 m³ per day, giving a difference of 0.2 m³ between the theoretical rate and the experimental rate, showing that the temperature-controlled system was very efficient. It had 90% of the theoretical value as compared to 63% for the uncontrolled situation. Figure 12 shows the graphs of biogas produced during the retention period for both the temperature-controlled and uncontrolled digesters.

From day 8, the rate of biogas production from the temperature-controlled digester increased significantly. This can be explained by the fact that a conducive microclimate (for the operation of methanogens) was created in the temperature-controlled plant. The microbes could convert volatile acids to methane at a higher rate due to temperature stability in the digester. The production rate in the uncontrolled digester was much slower because of temperature fluctuations, which affected the digestion efficiency and the health of the methanogens. The average temperature in the uncontrolled digester was 25 °C while the controlled digester temperature was maintained at ±35 °C. Al-Zoubi et al. [100], observed a similar trend in biogas production rate when they used a temperature-controlled system to assess the rate of biogas production from poultry manure at three different temperatures. Using 2 L rectors, they observed a maximum biogas production rate of 2500 mL per day at 37 °C and 1000 mL per day at 22 °C. In addition, they reported a higher cumulative biogas volume at 37 °C than at 22 °C. The result points to the importance of temperature optimization. Thus, temperature-controlled biodigester systems provide a higher biogas yield than uncontrolled systems when the digester temperature is stabilized. The gradual drop in biogas production rate from day 24 can be attributed to the depletion of digestible volatile solids in the reactors since they were batch-fed.

A graph of the cumulative volume of biogas against the retention period is presented in Figure 13. The cumulative biogas produced in the 30-day retention time was 26.77 m³ for the temperature-controlled digester and 18.05 m³ for the uncontrolled digester. The cumulative biogas produced increased by 33% from the uncontrolled digester to the controlled digester. Basumatary et al. [101] observed a biogas yield of 21.72% higher compared to an uncontrolled digester under mesophilic conditions using cow dung as a substrate. The digester temperature was maintained at 35 ± 2 °C with a pH at 6.5. In the research, the mixing ratio was 3:2; meanwhile, in the present work, it was 1:1. This result shows the effectiveness of maintaining a stable optimum temperature in the biodigester. Avinash & Mishra [102] reported a similar percentage increase when they investigated the effects of moisture sources in the anaerobic digestion of municipal solid waste (MSW). They observed 30% higher biogas when the moisture content was increased from 50% to 100% of full capacity (FC).

4.4. Quality of Produced Biogas

The methane content for the controlled digester ranged from 52% to 77% during the 30-day retention period, and for the uncontrolled digester, it ranged from 50% to 63%. Similar results were reported by [59], who observed a maximum methane concentration of 62.8% at 35 °C for cow dung during a 20-day retention time. Chen et al. [103] achieved the highest methane content of 64.3% at an operating temperature of 35 °C. Using cow abdominal waste as a substrate in an underground fixed dome digester with a 30-day hydraulic retention time, Oji Achuka et al. [23] reported a maximum methane content of 68.39%. Their underground digester was not temperature-controlled, which could explain the difference between their observations and those of the present study. This result shows that the process temperature at 35 °C in an intermittently stirred underground fixed dome digester improves both the methane content and amount of biogas. The methane content increased by 14% and the carbon dioxide content decreased by 10% compared to the uncontrolled scenario. This improvement can be attributed to temperature and process stability. Figure 14 compares the biogas composition between temperature-controlled digestion and uncontrolled digestion.

4.5. TS, VS, and COD Removal

The efficiency of the controlled digester was further assessed by attending to the percentage reduction in the total solids, volatile solids, and chemical oxygen demand. One of the functions of anaerobic digesters is to reduce the VSs and COD in the influent so that the efficiency of an anaerobic system can be evaluated by looking at the extent of the percentage reduction in the aforementioned parameters.

Table 2. TSs, VSs, and COD before and after digestion for controlled digestion.

Parameter	Before	After	% Removal
% TS	19.59	6.61	66.26
% VS	15.61	4.11	76.81
COD (mg/L)	45.687	11.562	74.69

shows the percentage of TSs, VSs, and COD before and after the digestion period for the controlled digester.

The percentage reduction for TSs, VSs, and COD was 66.26%; 76.81%, and 74.69%, respectively. The percentage reductions were calculated using standard equations reported in the literature [102,104]. In a healthy plant, the percentage of VSs removal should be above 50%, and for this research, the percentage is much higher than the recommended minimum. Figure 15 shows a graph of the percentage reductions for the three parameters analyzed for the temperature-controlled digester. Abdel Daiem et al. [105] reported similar reduction percentages when they investigated the effect of co-digestion of waste-activated sludge and wheat straw using two-dimensional mathematical models and artificial neural networks.

Table 3. TSs, VSs, and COD before and after digestion for the uncontrolled digester.

Parameter	Before	After	% Removal
% TS	19.59	15.61	47.01
% VS	45.69	6.83	60.37
COD (mg/L)	10.38	19.25	57.86

shows the TSs, VSs, and COD percentages before and after digestion of cow dung for the uncontrolled system. Figure 16 shows the percentage removals for the unheated digester’s TSs, VSs, and COD.

The percentage reductions in the TSs, VSs, and COD, in the unheated digester were 47.01, 60.37, and 57.87, respectively. Compared to the heated digester, the percentages decreased by 19.25%, 16.44, and 16.83%, respectively. Thus, maintaining an optimum constant temperature in a bioreactor fed with cow dung significantly improves the percentage removals of TSs, VSs, and COD, thereby improving the efficiency of the digestion process and biogas production rate.

4.6. Variation in pH in the Digesters During the 30-Day Retention Time

The pH of the slurry is one of the factors which affects the rate of methane production. Acidic conditions inhibit methane production since methanogens thrive in slightly alkaline environments. The optimum pH for methane production is between pH 6.5 and pH 8.5.

The pH was 7.44 on day 1 and it decreased during the first 7 days to the lowest value of 6.4 on day 7 for the heated digester. On the other hand, the pH took 9 days to reach a minimum value of 6.22 for the unheated digester. The initial decrease in pH for both scenarios can be attributed to the accumulation of volatile fat acids (VFAs) from the acidogenesis stage and the fact that the methanogens that convert VFAs to methane were still very low in the digester. During the acidogenesis stage, there are more acid-forming bacteria than acid-consuming bacteria. The high concentration of volatile fatty acids lowers the pH. The lag in the unheated digester may be explained by low microbial activities in the digester due to lower temperatures. From day 8, the pH started increasing gradually until day 13. The gradual increase in pH can be explained by the increase in the population of methanogens, causing a rapid conversion of VFAs, thereby increasing the pH. From day 14, the pH reached near stabilization at around 7.6 to 7.9 up to day 28 for the heated situation. For the unheated digester, the pH stabilized at around 6.5. There was a sudden drop in pH from day 28 in both digesters. This can be explained by the depletion in the volatile acids caused by a high concentration of methanogens in the reactor. The stabilization of the pH from day 14 can be due to the balance between the acid-forming bacteria and the acid-removing bacteria, and it points to the system stability in terms of the bacterial activities. Similar trends in the variation in pH with retention time have been reported in the literature [94,95,102]. Zainudeen et al. [106] observed the same trend when using cow dung in mono-digestion. The pH dropped from 7.7 to 6.2 in the first three days and then oscillated between 6.4 and 6.5 for the rest of the retention period. The stabilization of the pH between 6.4 and 6.5 can be due to the bacterial balance between the VFA consumers and producers in the reactor.

The pH values in this investigation were between 6.2 and 7.69, which fall in the optimum range for biogas production. A graphical illustration of the pH variation during the retention period is shown in Figure 17.

4.7. Effect of pH on the Volume of Biogas Produced

The conditions in the biodigester have a strong bearing on the health of the microorganisms, the stability of the plant, and the quantity of biogas produced [46]. One of the process parameters that dictate the conditions of the biodigester is pH, the acidity or alkalinity of the slurry. Microbes thrive well within a pH range of 6.5 to 8.5 but the optimum is 7 [94].

The relationship between the volume of the biogas produced and the pH during the retention period is shown in Figure 18 for both the temperature-controlled and uncontrolled digesters.

The graphs shown in Figure 18 show a positive correlation between the volume of biogas produced and the pH of the slurry in the digester for both the temperature-controlled and the uncontrolled digesters. The R² values for the temperature-controlled and the uncontrolled digesters were 0.7884 and 0.755, respectively, showing a strong correlation between the variables concerned. Bahira et al., (2018) [50] reported a higher biogas production when the pH was slightly in the alkaline range compared to the neutral and acidic ranges. Boontian et al. [107] observed a similar trend when using heat treatment on cassava pulp wastewater at different pH levels. They reported a higher cumulative biogas production in the alkaline range than in the neutral and acidic ranges. In the present research, the pH stabilized around 7.7 for heated digesters and 6.5 for unheated digesters, which is slightly in the alkaline range.

4.8. Potential Effects of Seasonal Variations in Biogas Production

The experiments were carried out in summer. South Africa has four seasons; summer autumn, winter, and spring, of which summer is the hottest and winter is the coldest season. Average temperatures range from 15 °C to 36 °C and from −2 °C to 26 °C in summer and winter, respectively, according to the Climate Change Knowledge Portal (1991–2020).

Table 4. Average seasonal mean temperature for South Africa 1991–2020 (Adapted from the Climate Change Knowledge Portal).

Season	Summer (December, January, February)	Autom (March, April, May)	Winter (June, July, August)	Spring (September, October, November)
Seasonal mean Temperature (°C)	23.29	18.4	12.37	18.97

shows the observed average seasonal mean temperatures in South Africa from 1991 to 2020.

On the other hand, insolation in summer is much more than in winter [108]. These seasonal variations have the potential to affect biogas production significantly from season to season. The average seasonal variation in temperature between summer and winter is 10.92 °C, which is about 47 percentage points. It is therefore imperative that more research be carried out in other seasons for good generalization.

4.9. Cost Implications of Temperature-Controlled Digester

The major reason why most household digesters are built without heating mechanisms and temperature control systems is the extra cost of heating and controlling. Scaling a simple digester to a temperature-controlled system will attract some costs, though there is an unequaled benefit in the quantity and quality of biogas. The cost can be reduced depending on the method of heating involved. If solar energy or recovered heat are used, the cost is significantly reduced. The cost to buy the heat exchanger, the solar heater, the solar panel, and the battery for powering the control system are some of the extra initial costs. Based on the results from this research, it has been observed that the cumulative biogas in the 30 days increased by 33%, meaning that for the same amount of substrate, you obtain more biogas. The methane concentration also increased by 22%. Thus, even though the initial cost might be higher, the payback period can be shorter. Using the energy equivalence that 1 m³ of biogas is equivalent to 6 kWh of caloric energy, the difference between temperature-controlled and uncontrolled translates to 52.32 kWh of extra energy. So, though the scaling attracts some extra initial costs, the benefits are worth the risk and the payback is amazing.

5. Conclusions and Recommendations

The section gives conclusions and recommendations for future studies based on the observations and findings from this study.

5.1. Conclusions

The automatic temperature control system managed to maintain the slurry temperature at 35 ± 0.5 °C for most of the cases (82.76%). Thus, the efficiency of the temperature control system was 82.76%. Some maximum temperature gradients of 7.0 °C per metre were observed in both digesters, pointing to the fact that the angular speed of the stirrer blades (30 rpm) might not have been fast enough to create the necessary vortex to uniformly mix the slurry to ensure even temperature distribution. This observation compromised the efficiency of the temperature control system to some extent. A speed of 100 rpm would be recommended for further research. It was further observed that the heat from the solar geyser and the ground insulation were sufficient to keep the digester temperature within the required range; no other heat source was necessary.

The pH range for the whole retention period was between 6.4 and 7.69, which falls in the optimum range for biogas production (6.5 to 8.5). The regression of biogas yield and retention time showed a greater R² value for the temperature-controlled digester than the uncontrolled digester. The cumulative biogas production for the temperature-controlled digester was 26.77 m³, while the uncontrolled digester had a cumulative value of 18.05 m³, an increase of 33%. The methane concentration increased by 14%, while the carbon dioxide concentration decreased by 10%, respectively, from the uncontrolled digester to the controlled digester. The percentage removal of the TSs, VSs, and COD was 66.26%, 76.81%, and 74.69%, respectively, compared to 47.01%, 60.37%, and 57.86% for the uncontrolled situation. Thus, the percentage removal of TSs, VSs, and COD increased by 19.25%, 16.44%, and 16.89%, respectively. Maintaining the optimum mesophilic temperature in an underground fixed dome digester fed with cow dung as a substrate increases biogas yield, methane content, VSs, and COD percentage removals, as well as the overall efficiency of the digestion process. Clay soil proved to have good insulation properties.

5.2. Recommendations

This research was carried out during summer due to the available time. It is recommended that the investigation be repeated in the other seasons of the year (winter, autumn, and spring) to investigate the influence of seasonal variations for easy generalization. It is recommended to deploy additional temperature sensors in the digester to create a temperature distribution map, accounting for temperature gradients and causes. For further research, the stirring speed should be increased to 100 rpm to create a sufficient vortex in the digester for temperature uniformity in the volume of the slurry.

The biogas production process has several processes that occur simultaneously and are significantly affected by inhibitors and fluctuations in operating process parameters. For this reason, systematic monitoring and control is a necessity. Identifying inhibitors and possible instabilities in the plant early and taking corrective measures saves the plant from a complete crash [109]. Human error in the monitoring process can be costly. To benefit from this technology the most, automation of the process is the way to go. It is advisable to consider automating the loading rate so that the reactor can load itself in semi and continuous systems, ensuring a constant biogas supply. Not only the loading rate but all measurements of temperature, pressure, gas produced, and composition can be automated so that human involvement in the process becomes minimal.

This research focused on parameter optimization only; it is recommended for future research to combine two or more ways of enhancing biogas yield for excellent efficiency, for example, parameter optimization and co-digestion, pre-treatment and co-digestion, etc.