Architectural design adaptation of Egyptian residential buildings to accommodate digesters of biogas from food waste

Background

This section introduces a wrap-up of the biogas concept, food waste, digesting processes, and commonly known biogas production key parameters and underscores the most relevant ones that might be useful as design guidelines for architects. Typically, biogas digesters vary in size and scale, depending on their biogas production capacity and unique design elements. While there is no universal prototype design for biogas digesters, except for three main types used in domestic biogas plants¹: – Fixed Dome Digesters, Floating Cover/Drum Digesters, and Balloon/Tube Digesters – while these designs have emerged from diverse individual experiments worldwide, notably in Egypt, there isn’t a standardized biogas plant prototype tailored for different residential building types in the country.

Food waste

Lack of food resources in many places, climate change, depletion of fossil fuels, and increasing energy demand represent major challenges to today’s world. Both the World Bank and the United Nations encourage finding other sources of energy that are sustainable, clean, and renewable. Around one-third of food produced globally is estimated to be lost or wasted, or about 1.3 billion metric tons, resulting in economic losses of approximately 1 trillion US dollars^2,3. However, the global biogas market is rapidly growing. Between 2022 and 2023, the global biogas market grew from 71.59 billion to $78.25 billion US, and it is expected to grow to 102.7 billion US by 2027⁴.

Food waste (FW) generation represents a complex and multifaceted issue that affects the three pillars of sustainability: economic, social, and environmental⁵. Nevertheless, the literature did not explore the potential impacts that FW from food services could have on sustainability pillars. Conventionally, food waste was regarded as the food losses (FL) accrued during the retail and final consumption phases; therefore, its generation is mainly attributable to the behaviors of retailers and consumers⁶. FW contributes to the emissions of greenhouse effect gases in storage, distribution, and transportation operations, landfill disposal because of methane emissions, and other disposal operations like incineration.

The increasing population growth worldwide leads to an increase in the generation of FW in huge amounts. FW consists of vegetable market waste, restaurant food waste, kitchen waste, etc. It is the main element of municipal solid waste (MSW)⁷. A promising renewable energy source is using food waste to produce biogas since it is a cheap and applicable process. Biogas is a smart energy source that is effective for numerous purposes. The production of biogas positively participates in the decentralization of energy generation at the national level.

According to the FAO, food waste (FW) is the elimination of food from the food supply chain (FSC) that is edible but is either discarded or allowed to spoil due to negligence, primarily happening at the household level⁸. Despite the many definitions in the scientific literature for FW, achieving standardization is crucial for studying and understanding this topic⁹. Food waste is the best source for biogas production at a community level. Since it forms a rich nutritive environment for microbes that are needed for the fermentation process, leading to the production of biogas¹⁰. Its composition varies according to its components and types. It is also primarily composed of proteins, carbohydrates, lipids, and traces of inorganic compounds. Sources of FW in the literature, according to the European Commission in 2014, three categories of FW sources were classified based on the different stages of the food supply chain, as follows¹¹. Inevitable food waste: It refers to nutrients lost during the eating phase (fruit pits, vegetable peelings, fruit peels, etc.). Unnecessary food waste: It is waste food that could have been eaten but was disposed of at the eating stage. Food losses: These are food commodities that are lost during their production. Food waste was also divided into five generation sources: agricultural production, agricultural harvest, processing, distribution, and consumption¹², as presented in Fig. 1.

Food waste (FW) classification. Ref. [Author]

Biogas

Biogas is a sustainable gaseous energy source. It is generated from various raw materials like agricultural waste, manure, municipal waste, plant material, sewage, green waste, wastewater, and food waste. This production process involves digestion by organisms or methanogens within a digester (also called biodigester or bioreactor). As a versatile energy source, biogas finds applications as a fuel for heating and cooking. Moreover, it can be utilized in gas engines to transform the gas energy into both electricity and heat^13,14.

Digesting processes

A “Digester” is a system that biologically digests organic waste material with the help of adding specific types of genetically treated micro-organisms to vegetable and fruit waste and other “Agro-food” biomass residuals to generate biogas¹⁵. There are two major types of biological digesters:

Aerobic digesters

Aerobic digestion occurs in the presence of oxygen and produces biogas. Organic material is oxidized, producing biogas and other chemical substances like nitrate, phosphate, and carbon dioxide¹⁶. Aerobic digestion is a fast process that runs at ambient temperature and is much less complex than anaerobic digestion¹⁷. Aerobic digestion: Is simple in construction, requires no cover for its tank, can be easily controlled, does not generate nuisance odors, generates low nutrient concentrations, and eliminates ammoniacal compounds¹⁸.

Anaerobic digesters

Anaerobic Digestion (AD) is a microbiological process that degrades organic waste without oxygen¹⁹. AD is an environmentally friendly technique extensively employed to address organic waste problems while producing renewable energy and achieving sustainable development. Studies and applications implementations of AD have grown notable in recent decades^20,21, attributable to many reasons, particularly its suitability in treating any biodegradable residue, such as food waste or municipal waste, along with the need for the production of renewable energy and other waste disposal options that are divergent from landfilling²².

The composition and moisture content of FW makes it perfectly suitable for AD²³. Generally, proteins and carbohydrates are rapidly transformed into a common biogas production. In contrast, lipids present slower biodegradability development; however, they offer better quality and level of biogas²⁰. Consequently, comprehending the mechanisms of the AD process, key parameters, and all involved reactions is crucial.

Commonly known biogas production key parameters

Chemical parameters

In general, the biogas composition is primarily methane (CH) and carbon dioxide (\(\:{\text{C}\text{O}}_{2}\)) and may have small amounts of hydrogen sulfide (\(\:{\text{H}}_{2}\text{S}\)), moisture, and siloxanes. The high amounts of FW might result from deficient storage facilities, packaging, infrastructure, and insufficient market facilities²⁴. FW contains a high content of nitrogen, potassium, carbon, phosphorus, and other elements²⁵. The C/N ratio is a significant parameter, measuring FW’s potential for energy recovery²⁶. FW composition: It contains 15–25% proteins, 13–30% lipids, and 41–62% degradable carbohydrates, with a high VS fraction of approximately 85% ± 5%, 74–90% high moisture content, and 5.1 ± 0.7 mean acid PH²⁷.

The initial part of FW generation (livestock and agriculture) mainly consists of non-edible substances that have been separated from the feedstocks. The composition of food waste generated during the final stage of the food supply chain (markets) differs from that of earlier stages, where additional material fractions are included, like paper, glass, plastics, metals, etc., which originate from packaging^28,29.

The composition of FW consumer’s produce closely depends on diverse cooking and eating habits. Due to these reasons, numerous researchers have characterized FW, each delineating its components and characteristics based on its geographical location, local products, social behaviors, origin, habits, etc³⁰. Biogas produced from Anaerobic Digestion usually consists of Methane. \(\:\text{C}{\text{H}}_{4}\), Ammonia \(\:{\text{N}\text{H}}_{3}\), Water \(\:{\text{H}}_{2}\text{O}\), Oxygen O, Nitrogen N, Carbon dioxide \(\:{\text{C}\text{O}}_{2}\), Hydrogen Sulphide \(\:{\text{H}}_{2}\text{S}\), Hydrogen \(\:{\text{H}}_{2}\), And other trace contaminants³¹.

The AD process serves as a way of recycling, recovering energy, and utilizing it industrially as fertilizer, as well as a system that reduces landfill disposal, either partially by eliminating digestate or reusing it completely^22,32. During the AD process, a series of metabolic reactions occur that follow the conversion of organic matter (OM) into methane and carbon dioxide, as well as inorganic nutrients and compost¹⁹.

As a result of this, organic matter is decomposed by anaerobic microorganisms under anaerobic conditions, resulting in an energy-dense biogas; this reaction happens simultaneously through four major intricate biochemical stages, namely, hydrolysis, acidogenesis, acetogenesis, and methanogenesis³³, as shown in Fig. 2.

Stages of the AD Process. Ref. [Author]

Physical parameters

The following is a summary of the physical parameters of biogas, specifically for its components, methane (CH₄) and carbon dioxide (CO₂)³⁴:

Methane (CH₄):

• Molecular Weight: The molecular weight of methane is 16.04 g/mol, reflecting its chemical structure and atomic composition.

• Proportion: Methane comprises 0.554 of the biogas mixture, making it the primary constituent of biogas.

• Boiling Point: The boiling point of methane is -164 °C, which means it is typically in gaseous form at standard temperatures.

• Freezing Point: The freezing point of methane is -182.5 °C, indicating that it remains in a gaseous state under normal conditions.

• Density: The density of methane is 0.66 kg/m2, which is lower than that of air, contributing to its tendency to rise in the atmosphere.

• Dangerous Temperature: The dangerous temperature threshold for methane is 64.5 °C, above which it can pose significant risks of combustion or explosion.

• Dangerous Pressure: Methane reaches a dangerous pressure point at 45.8 kg/cm2, indicating the pressure level at which it may become hazardous.

• Specific Heat Capacity: The specific heat capacity of methane at 1 kg/cm2 is 6.962 × 10^-4/kg °C, reflecting the energy required to change the temperature of methane.

• Rate Cv/Cp: The ratio of specific heat at constant volume (Cv) to specific heat at constant pressure (Cp) for methane is 1.037, important for understanding its thermal behavior.

• Burning Heat: The burning heat of methane is 55,403 J/kg, indicating the amount of energy released during its combustion.

• Fire Propagation Rate: The rate of fire propagation in methane when mixed with air is 0.0581 (mass), highlighting its flammability and potential risk in confined spaces.

Carbon Dioxide (CO₂):

• Molecular Weight: The molecular weight of carbon dioxide is 44.01 g/mol due to its carbon and oxygen composition.

• Proportion: CO₂ represents 1.52 of the biogas mixture, a significant but secondary component compared to methane.

• Boiling Point: The boiling point of carbon dioxide is -78.5 °C, which means it sublimes from a solid to a gas under standard conditions.

• Freezing Point: CO₂ freezes at -56.6 °C, changing from a gaseous state to a solid form.

• Density: Carbon dioxide is 1.82 kg/m2, making it heavier than air, which causes it to settle in low-lying areas when released.

• Dangerous Temperature: The dangerous temperature for CO2 is 48.9 °C, above which the gas may cause health risks, including suffocation in high concentrations.

• Dangerous Pressure: The dangerous pressure for CO2 is 73 kg/cm2, beyond which the gas may become hazardous due to the increased risk of liquefaction and the potential for explosion under confined conditions.

• Rate Cv/Cp: The Cv/Cp ratio for CO₂ is 1.303, which is slightly higher than methane, indicating a different thermal expansion behavior.

In summary, the physical properties of methane and carbon dioxide, including their molecular weight, boiling and freezing points, density, and dangerous conditions, are essential for understanding their behavior in biogas production, storage, and utilization. Methane is the primary combustible component, while carbon dioxide, although inert, plays a significant role in the overall composition of biogas.

Safety parameters

The most probable hazard scenarios for a biogas plant failure involve either the leakage of gas and residual water generated during the fermentation process within the plant or the occurrence of both leakages simultaneously. Gas leaks typically result from inadequately installed gas valves and pipes connected to the fermentation tank. On the other hand, water leakage may occur due to insufficient insulation or the use of inappropriate materials lining the internal walls of the biogas plant, potentially causing chemical reactions with the internal wall materials and resulting in erosion and, consequently, the formation of holes in the walls of the plant^35,36,37. Underscoring the most relevant biogas production key parameters that might be useful as design guidelines for architects. Based on examining the essential Biogas Production Key Parameters outlined above, architects should focus primarily on understanding the physical and safety aspects of biogas when tailoring biogas plant designs for domestic and residential settings.

Green/sustainable/environmental architecture

The concept of green architecture holds significant sway within contemporary architectural discourse, propelled by a worldwide push for sustainable community development. However, a pervasive conflation exists in the literature between sustainable, environmental, and green architecture, often interchangeably used to describe structures with comparable characteristics. Despite this ambiguity, green architecture potentially possesses a distinct advantage over the other two terms, represented by the fact that sustainability and environmental impact are included under the umbrella of green architecture and not the opposite.

Green buildings are pivotal in mitigating adverse impacts on the natural environment by conserving water, energy, and other resources, incorporating renewable energy sources and eco-friendly materials, recycling waste, and minimizing emissions. They can yield net positive effects, such as generating energy or fostering biodiversity. Among sectors contributing significantly to greenhouse gas emissions, the building industry stands out for its capacity to effect substantial reductions.

Adopting green building practices, which leads to these performance advantages, also translates into economic gains for various stakeholders. Developers reap rewards from increased property values resulting from efficient resource utilization and constructing durable, high-performing structures. Such buildings attract business owners and occupants due to their environmental friendliness, enhanced comfort, heightened efficiency, reduced waste, and lower operational expenses—factors that positively influence occupancy rates³⁸.

Architecture of biogas digesters

Generally, biogas digesters come in various sizes and scales, depending on the volume of biogas they can produce and the specific components of their design. However, according to existing literature, the three common domestic biogas plants are Fixed Dome Digesters, Floating Cover/Drum Digesters, and Balloon/Tube Digesters¹.

Fixed dome digesters

A “Fixed-dome biogas plant”, Fig. 3, consists of a sealed dome-shaped digester housing an immovable, rigid gasholder, a feedstock inlet, and a displacement pit known as the “Compensation tank”. Gas generated in the digester is stored in the upper section of the reactor. As gas production increases, the pressure inside the digester rises, pushing the digestate into the compensation tank through a closed outlet gas valve. When the gas valve opens for utilization, the pressure drops, allowing a proportional amount of slurry to flow back from the compensation tank into the digester. This design results in continuous gas pressure variation based on production and use. Typically constructed underground, fixed-dome plants are shielded from low temperatures, with surrounding soil counteracting internal pressure (Usually 0.1–0.15 bar)³⁹.

These plants are recommended only when experienced biogas technicians are available for construction, ensuring a gas-tight structure. Despite their modest initial cost and long operational lifespan (Approximately 15–20 years), fixed-dome plants may develop porosity and cracking over time, leading to gas leaks. While special sealants can address porosity, cracking often causes irreparable leaks. The fluctuating gas pressure in fixed-dome digesters may complicate gas utilization. Various designs exist, including the “Chinese fixed-dome plant,” the “Indian Dee Bandhu”, and the “Camartec model from Tanzania”. Fixed-dome digesters come in different sizes, typically ranging from 6 to 16 m³, with consistent design elements across all variations.

The general concept of the “Fixed Dome Digester”. Ref.:³⁹.

Floating cover/drum digesters

The “Floating drum digester “, Fig. 4, is typically constructed using concrete and steel, whereas the fixed dome digester is commonly built with locally available materials such as bricks and stones. In both types of digesters, only the cover of the floating drum digester is situated above ground, while the remaining components are housed underground, with a retention time of 20–30 days.

Floating drum digesters feature a steel cover that floats on the slurry, moving vertically to accommodate constant biogas pressure. Additional weight can be added to the top of the cover to increase gas pressure. Conversely, gas volume remains relatively constant in fixed dome digesters while pressure fluctuates. Despite these differences, the operational principles of both designs are similar. Feedstock is introduced directly or after mixing in a pit through an inlet pipe into the digester tank. Biogas produced is collected above the slurry and exits the tank through a gas pipe connected to the top of the digester. The digested slurry is expelled through an outlet pipe into either an outlet pit or a displacement tank.

Depending on the configuration, the digester tank may have one or two compartments. In developing countries, the system lacks proper mixing and operates without temperature control. Additionally, there is no provision for removing settled inert materials, gradually reducing the digester’s effective volume over time. While the absence of moving parts and simple construction make this digester easy to operate and maintain, the high installation cost and requirement for skilled craftsmen limit the widespread adoption of this technology in developing countries^40,41,42.

The general concept of the “Floating Cover Digester”. Ref.:⁴⁰.

Balloon/tube digesters

A “Balloon digester”, Fig. 5, comprises a plastic or rubber bag to hold the digester contents. The upper portion is a gas storage space, with the inlet and outlet directly attached to the balloon’s surface. The gas exits the balloon due to internal pressure buildup, which can be augmented by placing weights on it. The movement of the balloon’s surface aids in agitating the fermenting slurry, enhancing digestion. Balloon digesters offer versatility, accommodating even complex feed materials like water hyacinths.

UV resistance is essential for balloon materials, with successful options including (Red Mud Plastic – RMP), Trevira, and butyl. These materials are heat-sealed to form a single unit’s digester and gas holder. However, exceeding the balloon’s pressure limit may damage its surface, necessitating safety valves.

For higher gas pressures, a gas pump may be required. Given the exposure to weather and UV radiation, preference is given to specially stabilized and reinforced plastic or synthetic caoutchouc. Despite these considerations, the useful lifespan of balloon digesters typically ranges from 2 to 5 years^43,44.

The general concept of the “Balloon or Tube Digester”. Ref.:⁴³.

Other domestic digester types

“Portable home biogas systems”, Fig. 6, are compact, small-scale digesters constructed from either metallic or polyethylene tanks, accompanied by complementary equipment such as gauges, inlets, and outlets. In India, China, and several European nations, numerous companies supply these digesters to the market in various sizes that are customized for installation in domestic environments^45,46,47.

An example of a “Portable biogas domestic digester”. Ref.:⁴⁷.

Masonry digesters are not necessary for stable soil (E.g., laterite). To prevent seepage, it is sufficient to line the pit with a thin layer of cement (wire mesh fixed to the pit wall and plastered). The pit’s edge is reinforced with a ring of masonry that also serves as anchorage for the gasholder. The gas holder can be made of metal or plastic sheeting. If plastic sheeting is used, it must be attached to a quadratic wooden frame extending into the slurry and anchored in place to counter its buoyancy.

The requisite gas pressure is achieved by placing weights on the gas holder. An overflow point in the peripheral wall serves as the slurry outlet. The Ferro-cement construction type can be applied as a self-supporting shell or an earth-pit lining. The vessel is usually cylindrical. Very small plants (Volume under 6 m3) can be prefabricated. As in the case of a fixed-dome plant, the ferro-cement gasholder requires special sealing measures (proven reliability with cemented-on aluminum foil).

Research significance

This research investigates the correlation between sustainable architectural design and biogas generation. Its goal is to identify guidelines for integrating architectural layouts with biogas facilities and evaluate current prototypes suitable for implementation in Egyptian households. Additionally, the study discusses architectural limitations and suggests layouts for biogas facilities in various residential structures in Egypt.