Resource Reclamation for Biogas and Other Energy Resources from Household and Agricultural Wastes

Abstract

The chapter’s goal is to highlight how the reclamation of household and agricultural wastes can be used to generate biogas, biochar, and other energy resources. Leftover food, tainted food and vegetables, kitchen greywater, worn-out clothes, textiles and paper are all targets for household waste in this area. Agricultural waste includes both annual and perennial crops. Annual crops are those that complete their life cycle in a year or less and are comparable to bi-annual crops, although bi-annuals can live for up to two years before dying. The majority of vegetable crops are annuals, which can be harvested within two to three months of seeding. Perennials crops are known to last two or more seasons. Wastes from these sources are revalued in various shapes and forms, with the Green Engineering template being used to infuse cost-effectiveness into the process to entice investors. The economic impact of resource reclamation is used to determine the process’s feasibility, while the life cycle analysis looks at the process’s long-term viability. This is in line with the United Nations’ Sustainable Development Goals (SDGs), whose roadmap was created to manage access to and transition to clean renewable energy by 2030, with a target of net zero emissions by 2050.

1. Introduction

The population growth rate is an important factor to consider when examining the past, present, and future resource base for sustainability. The increase in global population, combined with increased agricultural productivity and medical advancements, has resulted in resource consumption exceeding the environment’s carrying capacity [1]. As the human population expands, so does the potential for tremendous, irreversible changes. Increased biodiversity loss, greenhouse gas emissions, worldwide deforestation, stratospheric ozone depletion, acid rain, topsoil loss, and water, food, and forest resource shortages are all signs of severe environmental stress in many parts of the world [2]. This human impact on the environment informs the current biotic and abiotic resource depletion.

Biotic resources are resources that come from the biosphere, which are living or once-living beings and forests, as well as the materials that come from them in the ecosystem [3]. Biotic resources include forests and forest products, crops, birds, wildlife, fish, and other marine life. These resources rejuvenate and duplicate themselves, making them renewable. Fossil fuels like coal, natural gas, petroleum, etc. are biotic resources as well, but they are non-renewable; as non-renewable resources get depleted, human society will increasingly rely on the self-renewing capacity of biotic resources [4]. Abiotic resources, on the other hand, are usually obtained from the lithosphere, atmosphere, and hydrosphere. Examples of abiotic resources are water, air, soil, sunlight, radiation, temperature, atmosphere, humidity, acidity and mineral raw materials [5].

Household waste is defined by Reddy [6] and Viljoen et al. [7] as waste generated by household activities such as cooking, sweeping, cleaning, fuel burning, repairs, and gardening. Old clothing, old furnishings, retired equipment, glass, paper, metal packaging, and old books and newspapers are all examples of used products or materials.

Over the last few years, the reuse of home garbage, harvest, post-harvest, and forest leftovers has gained popularity. This is to close the energy gap that has been formed as a result of rising demand from the rural-urban migration and the general improvement in the human population’s lifestyle. The demand for high-quality, high-value items has put a strain on scientific research and the manufacturing sector of the economy, threatening to deplete fossil resources. Resource reclamation for benefit employs labour and generates income [8].

The strategic approach of the National Waste Management Strategy (NWMS) of South Africa 2020 to waste management adopted the circular economy approach, in which there is no waste. In a circular economy, any material whose value has degraded for a given application becomes a raw material for another process [7]. This chapter highlights the reclamation of waste resources from agricultural harvest and post-harvest operations for energy resources such as biogas and biohydrogen. Also, the flue gas from the production of biochar was harnessed. Household wastes were also harnessed for biogas generation. The chapter also streamlined the economic benefits of waste resources reclamation and the advantages of a circular economy. The life cycle analysis looked at the composition of household and farm wastes, and the volumetric flow characteristics of the waste materials.

2. The characteristics and types of wastes

2.1 The characteristics of wastes

Wastes are often characterized by professionals depending on the sources from which they are created. Household rubbish, hospitals, agricultural waste, industrial waste, mining activities, public spaces, and other sources all contribute to waste production. Wastes are toxic by nature and can harm the environment as well as animal health.

2.1.1 Household waste

Household waste is any waste that is generated from running a domestic facility, accounting for more than two-thirds of the municipal solid waste (MSW) stream [9]. It can include food materials, plastics, cardboard, rubber, metal, paper, wood, fabric, chemicals etc. Hazardous substances in household waste, unlike waste streams from industrial sources, are not strictly regulated under hazardous waste regulations. As a result, household hazardous waste (HHW) is dumped in landfills alongside general household waste (HW). Cleaning products, self-care products, pharmaceuticals, home-care products, automotive maintenance products, electronic equipment, and general maintenance products for machinery are examples of items that are frequently used in places of residence, commercial centres, corporate organizations, and institutions. These products contain substances that, on their own or when combined with others, produce secondary compounds capable of causing severe environmental and public health damage [10].

2.1.2 Healthcare waste

Surgical trash, blood, body parts, medications, wound dressing materials, syringes, and needles are all examples of hospital waste. Hospitals, clinics, veterinary hospitals, and medical laboratories all produce this form of trash. Contamination and illness are common outcomes of hospital waste [4].

2.1.3 Agricultural waste

Agricultural waste is produced by farming, animal husbandry, and market gardens, among other activities. Pesticide containers, expired medications and wormers, extra milk, corn husks, corn cubs, corn silage, rice husks, rice straw, and other agricultural wastes are the most prevalent [1].

2.1.4 Industrial waste

Industries generate a wide range of trash. Petroleum refineries, chemical plants, cement factories, power plants, textile mills, and food processing and beverage facilities are all industrial waste generators. These industries produce a considerable amount of waste, which impairs the environment’s esthetics and may have an influence on the chemistry of the atmosphere [1, 2].

2.1.5 Commercial waste

The volume of items purchased and sold, as well as technical improvements in industry and transportation, all contribute to commercial waste [4]. Food, textiles, discarded household and medical supplies, and a range of other objects might be included.

2.1.6 Electronic waste

Discarded old electronic equipment such as televisions, microwaves, vacuum cleaners, and music players are examples of electronic waste sources. E-scrap, or waste electrical and equipment, is another name for it. These wastes are high in cadmium, lead, and mercury, all of which are toxic to persons and the environment [7].

2.1.7 Mining and quarrying wastes

Mine wastes are coarse wastes generated during the mining stages of rock blasting and tunnel preparation, as well as tailings from ore processing. Overburden materials that must be removed and disposed of to get access to ore or precious rock are known as quarrying wastes [10]. Two examples are toxic gases created during blasting and other mining contaminants. The impact of mining waste on the local environment and surroundings is significant.

2.1.8 Demolition and construction wastes

Depending on the project, bricks and masonry, concrete, wood, metal (including plumbing), plaster and drywall, glass and windows, demolition debris, and other demolition and building materials may be employed. Garbage like asphalt, rubble, tile., etc., from huge projects, as well as construction and building materials trash such as packing boxes, concrete debris, plastics, and wood [8].

2.1.9 Radioactive waste

Gamma rays, alpha particles, beta particles, and neutron radiation are all forms of radiation produced by radioactive waste. Radioactive waste is produced by nuclear reactors or atomic explosions, and it is particularly harmful to animals. High-level waste, low-level waste, and transuranic waste are the three categories of radioactive waste. This technology is used in the power generating sector of the economy as well as the radiological unit in hospitals for imaging diagnosis [10, 11].

2.2 Types of waste

Professionals also have characterized waste according to (i) the physical states of materials namely solid, liquid and gas; and (ii) the potential of microbial attack namely biodegradable and nonbiodegradable materials.

2.2.1 Solid wastes

Solid wastes account for the majority of the trash produced by human civilisation. Agricultural wastes, domestic wastes, radioactive wastes, industrial wastes, and biomedical wastes are examples of solid wastes that can be categorized based on their source or nature [1, 4].

2.2.2 Liquid wastes

Liquid wastes are wastes that are formed in a liquid state as a result of industrial production, washing, flushing, or other industrial activities. Liquid waste is also produced in significant amounts by households. Used vegetable oil and kitchen wastewater are two examples [1].

2.2.3 Gaseous wastes

The principal sources of gaseous wastes include internal combustion engines, incinerators, coal-fired power plants, and industrial processes. Depending on their qualities, gaseous wastes might be odiferous or toxic. Smog and acid precipitation develop when they mix with other gases [10].

2.2.4 Biodegradable wastes

These are wastes that originate from the kitchen, such as food scraps, garden trash, and so on. Moist trash, green waste, recyclable waste, food waste, and organic waste are all terms used to describe biodegradable garbage. This may be composted to produce manure, also known as humus. Biodegradable wastes break down over time, depending on the substance and can be destroyed by biotic and abiotic factors such as microorganisms (e.g. bacteria, fungus), temperature, ultraviolet radiation, oxygen, and others. They are digested anaerobically to provide energy in the form of heat, electricity, and fuel [1, 6, 11].

2.2.5 Non-biodegradable wastes

Non-biodegradable wastes are those that are not easily degraded by natural agents or dissolved by them. They stay undamaged for many years and are the primary sources of pollution in the air, water, and soil, as well as illnesses such as cancer [6, 11]. Dry waste refers to non-biodegradable waste. Newspapers, shattered glass shards, and plastics, which are employed in practically every sector, are all good examples. Cans, metals, and agricultural and industrial chemicals are further examples. Dry wastes are recyclable and reusable. Non-biodegradable trash is bad for the environment, thus there’s a rising demand for alternatives. In response, biodegradable polymers (also known as biocomposites) have evolved, although they remain prohibitively costly [12]. Polymers are the backbones of plastic materials, and they are used in an ever-growing number of applications.

3. Household and agricultural waste resources

Household garbage has become one of the most prominent sources of serious impairment to the rural environment due to huge amounts of rubbish discharged and improper disposal. The amount of rubbish created rises in lockstep with the world’s population. Household garbage production will have grown by about 70% per year by 2050, suggesting that waste production will have surpassed population growth by more than twice [1, 4, 7]. Household trash management is a tough task due to the rising volume of rubbish produced throughout the world and the vast variety of different components included in this waste stream. Sorting rubbish at the source is crucial for recycling and the circular economy to thrive [9]. Agricultural waste consists crop remnants, weeds, leaf litter, sawdust, forest detritus, and animal manure. Waste from agro-based industries such as palm oil, rubber, and wood processing factories has increased by several times as a result of increased agric mechanization and automation. Significant quantities of phosphate and nitrogen, as well as biodegradable organic carbon, pesticide residues, and fecal coliform bacteria, are found in agricultural wastes that run straight into surface waters [6].

3.1 Household waste resources

Household wastes can be solid, and liquid. The different categories of household waste are addressed in the following sub-sections:

3.1.1 Household solid waste resources

Many cities in developing countries have a challenge of improper management of solid household waste which is a constituent of municipal solid waste [11, 12]. Improper management is because there is usually a lack of understanding of the waste generation and its composition which leads to municipal authorities being unable to establish and execute efficient management plans [13]. This lack of understanding means that authorities most often use equipment and management plans that are not tailored for some communities/cities [14, 15]. Solid household waste is a broad term for solid waste materials found in a home which can be characterized into different classes such as organics, plastic, paper, glass and ceramics, metals and tins, and other types of wastes. Some of these categories can be sub-divided into more specific products such as food waste, garden waste, magazine, newspaper, office paper, miscellaneous paper all being organic waste [16]. Plastic waste includes bottles, containers, jars and bags while paper waste contains cardboard, packaging material, newspapers. Other waste material includes disposable diapers and sanitation waste [17].

Solid household waste composition and quantities produced are influenced by socio-economic dynamics such as family size, income, car ownership, age, education etc. [18]. This evidence has been shown in studies where overall waste generation and generation of individual components of waste streams have differed between the less and more prosperous sectors of a city [19, 20]. Research has also shown that lower-middle-class communities generate waste with a high potential of recyclability [21]. Solid household waste has been identified as a huge contributor (82%) of the total solid waste compared to waste from commercial, institutions and industrial locations [22]. Different strategies have been developed for resolving the challenges of waste including solid household waste. It is well known for example that plastic and its related materials, glass and ceramics are non-degradable, however can be recycled into new products instead of being thrown into dumping sites as they have incredibly negative impacts on the environment [23, 24].

The organic part of solid household waste (about 68%) is biodegradable and therefore presents a great opportunity to be further used as a resource. This organic-rich waste is a good medium for microbial growth, consequently, it can be used to produce energy (in the form of biogas) which is an excellent provision for positive contribution to the environmental, energy and economic needs [25, 26]. Energy derived from household waste becomes very significant since sufficient energy access is one of the crucial factors of improvement in any country in the world. In this way of economic development, the fight against poverty, education and adequate healthcare is facilitated [27]. Biogas is a result of a four-step (hydrolysis, acidogenesis, acetogenesis and methanogenesis) microbially aided anaerobic digestion process with approximately 50–70% methane, 30–45% carbon dioxide and some trivial amount of other trace elements [28, 29]. Biogas is produced from organic substrates and therefore the resulting waste is rich in nutrients that are used as bio fertilizer [30].

Liu et al. [31] have produced hydrogen and methane from solid household waste using a two-stage fermentation process. In another study, solid household waste consisting of 80.4% organic matter has been used to produce biogas through an anaerobic batch reactor [32].

3.1.2 Household liquid waste resources

Liquid products are used in common rooms of a household such as a kitchen, bathroom, garages as well as basements. These products have the potential of causing serious environmental and health problems both during their time of use as well as after they have been discarded [33]. Often the consumer of these products is not aware of how to properly dispose of them after usage. The need for identifying appropriate ways of discarding liquid household waste has been realized when serious health problems and damage to areas of disposal started to manifest [34].

Liquid household waste incorporates any liquid waste from places such as the kitchen (cooking oil, dish detergents, floor cleaning products, microwave/oven cleaners, furniture and metal polishes, drain cleaners, etc.), bathroom (health and beauty products, disinfectants, basin and tub detergents and toilet bowl cleaners), laundry room (bleaches, fabric softeners, detergents, spot removers, etc.) as well as the garages and/basements (paints, pesticides and herbicides, lawn and garden care products, fuel, oils, glues and adhesives, etc.) [34, 35]. Most of these are made from hazardous chemicals although they can be paid for over the counter by any person from supermarkets, automotive centres and hardware stores.

The negative impacts to surface water, groundwater and the soil caused by improper disposal of liquid household waste have brought about the need to look for solutions to the challenge of waste management [36]. Consequently, research has been done across the globe and solutions are slowly being realized and embraced. These include strict prevention, reduction at source, treatment of liquid waste before disposal, recycling the waste into other useful products, valorisation of waste as a form of meeting other demands (energy demands) in societies [37, 38]. For example, liquid waste is used to produce biogas, electrical energy and heat through the following processes; anaerobic fermentation, pyrolysis, biothermal composting, hydrothermal destruction,etc. [39, 40]. The realization that household liquid waste is a renewable energy source is the beginning of solving socio-economic issues because the whole technology employs people in the production of both the technology gear as well as energy thus addressing environmental issues while benefiting the economy [41].

Kitchen wastewater has been used as a substrate for the production of biogas by Kumar et al. [42] where the Up Flow—Anaerobic Sludge Blanket (USAB) reactor was employed. Another study that employed kitchen wastewater co-digested it with several other substrates such as water hyacinth, cow manure and sewage sludge for biogas production which had 60–65% methane, 14–18% carbon dioxide as well as 20–21% other gases [43]. Domestic liquid waste has been used as a constituent of the municipal liquid waste for the production of electricity in sufficient volumes to lessen the electrical load of the water treatment plant while producing surplus power to feed into the grid [44].

3.2 Agricultural waste resources

Modern agriculture depends primarily on annual crops, which are crops that can be harvested within two to three months of seeding. Annual crops live their whole life cycle in a year or less. These crops are typically classified as summer crops (warm-season) and winter crops (cool-season crops). Warm-season crops develop faster during warmer times of the year and are typically seed and fruit crops. Cool-season crops develop faster during cooler times of the year and are typically root, leaf, flower bud, and stem crops. Examples of annual crops include onions, tomatoes, popcorn, carrots, peas, kale, and corn [45]. These crops have a lower water requirement and tend to generate more crops produced per drop of water [46]. Bi-annual crops are comparable to annual crops, but they can live up to two years before dying [45]. The first year of bi-annuals results in a short stem and leaves which eventually bloom in the second year.

A more sustainable alternative to annual crops that have been advocated for is perennial crops. Examples of perennial crops include sugarcane [47], coconuts, pineapple, peppermint, spearmint [48], apples, and peaches or apricot [49]. These crops last two or more seasons and are planted once and harvested every year. They lower the chances of soil erosion and limit losses of water and nutrient due to the greater root mass nature of perennials. Perennial crops are preferred for both the quality of the product harvested and their total production [49]. Shifting to perennial crops may enhance many ecosystems services but this will come at a cost as perennial crops have higher water requirements. Furthermore, lower yields that are more stable than those of annual crops can be expected [46].

3.2.1 Farm produce waste

Farm waste is classified under the agricultural waste stream. Agricultural waste refers to the residues produced from growing and processing crops. They are the non-products of production [50]. Agricultural waste includes natural (biodegradable) and non-natural wastes (inorganic) which are produced from agricultural activities such as horticulture, dairy farming, livestock breeding, seed germination, nursery plots, market gardens, grazing land, and forestry. This waste comes as either liquids, solids, and slurries, or sludge. Agricultural waste can be classified according to the activity undertaken [51]. For example, crop production and harvest, sugar processing, fruit and vegetable processing, animal production, rice production, dairy product processing, and coconut production. Each of these activities generates its unique wastes.

The global agricultural waste production has been estimated to be approximately 998 million tonnes yearly. This estimate is likely to increase if farming systems are intensified in developing countries. The total solid wastes produced in any farm includes up to 80% of organic waste and the generation of manure can amount to 5.27/kg/day/1000 kg live weight, on a wet weight basis [50]. Agricultural waste tends to pose serious problems to the environment and humans due to it being toxic, especially the waste that includes pesticides, insecticides, etc. It also has a high pollution potential over extended periods, threatening surface water, underground water, and soil resources [51].

Agricultural waste can be used in various applications. These include fertilizer application (employing animal manures), anaerobic digestion (generating methane gas from manures), pyrolysis (generating bio-oil, char, and gas), animal feed, adsorbents to eliminate heavy metals (used in the adsorption process), and direct combustion [50]. The utilization of waste must either happen rapidly or the waste must be stored under controlled conditions to avoid spoilage of the residues.

3.2.2 Post-harvest wastes

In a world that is ever-growing in population which results in an increasing demand for food, postharvest waste is a critical phenomenon worth addressing. At the forefront of the bio-economy sector are plans to minimize the quantity of waste produced, advance the inescapable waste produced as a resource, and attain noteworthy levels of safe disposal and recycling [52]. Postharvest waste is defined as the amount of food wasted or lost throughout the food chain, after harvesting till consumption. Roughly a third of the food produced on a global scale for human consumption gets wasted or lost, which is about 1.3 billion tonnes of the harvest lost annually [53, 54]. Postharvest waste is intentional. Generated products can be rejected and discarded by growers, distributors, retailers, and consumers if they fail to meet established preferences [55].

Post-harvest wastes are a major contributor to agricultural biomass loss. This is true for the whole world but postharvest losses and waste are more prevalent in developing countries because of low levels of technology, poor infrastructure, low investment in the food production systems, and poor temperature management [56]. Postharvest waste is estimated to be approximately 60% depending on the production region, the season, and the crop [57]. In Brazil, for example, the postharvest losses and waste of vegetables and fruits is approximately 30% [53]. It can lead to soil fertility challenges, especially where agriculture is predominant. Soil fertility problems occur as a result of inadequate amounts of residues which through direct application, as manure, or as compost find their way back to the land [52]. Examples of agricultural residues include straw from wheat, stover, cobs and corn from maize, husks and shells from coconuts, and stalks from cotton.

Stages of an entire postharvest system include harvesting, threshing, drying, storing, processing, and use. Harvesting operations account for 5–8% losses, storage 15–20% and transport 10–12% [58]. The key sources of postharvest waste in economically developing and economically developed nations are uncontrolled handling, substandard planning of the amount to purchase, and defective packaging [53].

In a study, [52] assessed the residues that are generated on a farm at the time of harvest and also considered the by-products that are generated when crops are processed, for example, sugarcane bagasse produced from sugarcane. The investigation found the total post-harvest losses to be about 92 Mton/year, where 32 Mton/year is attributed to sugarcane losses, 16 Mton/year to wheat losses, and 9 Mton/year to rice.

Studies have pointed out that the reduction of postharvest waste can contribute to increasing the availability of food in the food system, thereby reducing food insecurity, improving farmers’ income, bettering nutrition, and reducing the wasting of critical resources such as water, land, energy [57]. Additionally, postharvest waste can be used to generate valuable products such as bioenergy and biochar when technologies such as gasification and anaerobic digestion are employed.

4. Energy resource reclamation

Paper, cardboard, food waste, grass clippings, leaves, wood, and leather goods are examples of biogenic (plant or animal-based) materials. Non-biogenic combustible materials include plastics and other petroleum-based synthetic materials, as well as non-combustible materials like glass and metals. Many nations employ waste-to-energy plants to harness the energy contained in solid waste. Waste-to-energy facilities are widely used in various European nations and Japan, owing to a scarcity of open landfill areas in such countries. Solid waste is often burnt in waste-to-energy facilities, which use the heat from the fire to produce steam, which is then used to generate electricity or heat buildings. Figure 1 shows the worldwide composition of solid waste from homes, towns, and farms as follows: paper (18%), plastics (12%), organic materials (43%), glass (5%), metal (4%), rubber, leather, and textile (9%), and miscellaneous materials (9%). (9%) [58].

Figure 1.

Typical composition of solid waste from households, municipalities and farms.

Different types of biofuels may be recovered and purified from organic waste fractions for usage at home or in the workplace. The energy content of garbage determines the quantity of energy that can be retrieved (calorific value). Figure 2 shows the average energy content of solid waste from houses, towns, and farms, with paper having a potential of 16 MJ/kg, plastics 35 MJ/kg, organics 4 MJ/kg, glass and metal 0 MJ/kg, and other 11 MJ/kg [59].

Figure 2.

The average energy content of home/municipal/farm solid waste.

Most biogenic and non-biogenic components may be found in household, municipal, and farm solid waste (HMFSW) used to produce power. Newsprint, paper, cartons/packaging, textiles, wood, food waste, yard trimmings, and leather are biogenic components, while rubber, PET, HDPE, PVC, LDPE/LLDPE, PP, PS, and other (plastic) and metals are non-biogenic [60]. The biogenic fraction of solid waste declines when consumers reuse or recover more biogenic waste (such as paper, packaging, food waste, and yard trimmings) while discarding more non-biogenic trash (such as plastics and metals). Because non-biogenic material has a larger heat content than biogenic material, as shown in Figure 2, the average heat content of HMFSW as a whole is rising, making it a more efficient fuel for generating power [59].

Because of a rise in the consumption (and discarding) of non-biogenic materials, as well as enhanced recovery of biogenic materials before they reach the waste stream as discards, the biogenic proportion of HMFSW continues to decline due to more recycling activities. As a result, as the consumption of plastics rises, renewable energy provided by solid garbage decreases, and biogenic waste is more collected and/or repurposed [61].

The technologies that emphasize the biochemical processes leading to the production of biogas in Figure 3 are discussed in depth in this chapter. The anaerobic generation of biogas, which is a combination of methane and carbon dioxide, was described in detail by Angelidaki et al. [62]. They found three major physiological groups of microorganisms that drive the bio-methanation process: (1) primary fermenting bacteria, (2) anaerobic oxidizing bacteria, and (3) methanogenic archaea, a phylogenetically varied group of strictly anaerobic Euryarchaeota whose energy metabolism is limited to the production of methane from carbon dioxide and hydrogen, formate, methanol, methylamines, and/or acetate.

Figure 3.

The technologies that drive the conversion of waste to energy.

4.1 Bio-hydrogen production

Bio-hydrogen can be produced through different thermochemical, electrochemical and biological processes. By 2020, steam reforming of natural gas, partial oxidation of methane, and coal gasification had produced around 95% of hydrogen from fossil fuels [63]. Other techniques of hydrogen synthesis include biomass gasification, methane pyrolysis with no CO₂ emissions, and water electrolysis. The later processes, such as methane pyrolysis and water electrolysis, may be carried out using any form of electricity, including solar power [64].

Anaerobic and photosynthetic bacteria can produce bio-hydrogen from carbohydrate-rich and non-toxic basic materials. Hydrogen is obtained as a by-product during the conversion of organic wastes into organic acids, which are subsequently utilized to generate methane under anaerobic conditions [63, 64]. The availability, affordability, carbohydrate content, and biodegradability of waste materials utilized in bio-hydrogen generation are the most important factors to consider. Simple sugars like glucose, sucrose, and lactose are easily biodegradable and are excellent hydrogen generation substrates.

Ginkel investigated hydrogen synthesis from industrial effluents from confectioners, apple and potato processors, as well as greywater. From potato processing wastewater, the maximum production yield was 0.21 L H₂/g COD [65, 66].

Bio-hydrogen may be produced in a variety of techniques, including dark fermentation (DF), microbial fuel cells (MFC), and microbial electrolysis cells (MEC). DF and MFC are more effective for bio-hydrogen synthesis from carbohydrate-rich effluents than photo-process (CRE). To enhance the optimum bio-hydrogen generation, either hydrothermal preparation of the inoculum or a high dilution rate can be utilized to minimize the activity of inhibiting bacteria and lower the pH of the medium. For an optimum bio-hydrogen generation, a mixed microbial culture is more trustworthy than a pure microbial source because pure cultures take more care to maintain, but mixed cultures include a wider range of bacteria for the biological conversion of organic materials into useful products. Some of the technologies that produce bio-hydrogen are given in Table 1.

Waste feedstock	Waste source	Resource recovered	Technology used	Reference
Cheese process effluent	Agro-based industry	Hydrogen	Thermophylic DF	[63]
Sugar mill effluent	Agro-based industry	Bio-electricity	MFC	[63, 65]
Bagasse hydrolysate	Agro-based industry	Bio-hydrogen	DF	[63, 67]
Brewery wastewater	Agro-based industry	Bio-hydrogen	DF	[63, 67, 68]
Olive mill effluent	Agro-based industry	Bio-hydrogen	DF	[63, 69, 70]
Food waste	Household	Bio-hydrogen	Bio-processor	[69]
Greywater	Household	Bio-hydrogen	AD, DF	[70]
Blackwater	Household	Bio-hydrogen	AD, DF	[69, 70]

Table 1.

Technologies that produce bio-hydrogen from waste resources.

Among the various renewable energy sources, bio-hydrogen is gaining a lot of traction as it has very high efficiency of conversion to usable power with less pollutant generation. During fermentation, bacteria release enzymes that hydrolyse biopolymers, resulting in depolymerization of lipids, proteins, nucleic acids, and carbohydrates to intermediate soluble monomers such as fatty acids, glycerol, amino acids, purines, pyrimidines, Mono sugars, and others, which are then converted to short-chain fatty acids, alcohols, hydrogen, and carbon dioxide. Table 1 shows that different technologies use different fermentation methodologies to transform organic substrates into hydrogen in the absence or presence of light, such as dark fermentation and photo-fermentation. Bio-hydrogen is a valuable and potential source of energy [71].

4.2 Biogas

Agricultural leftovers, such as manure and straw, are among the many potential substrates for AD [72]. However, because of their high percentage of lignocellulose, which is difficult to decompose due to its complicated structure, with cellulose fibers securely connected to hemicellulose and lignin, their utility for biogas production is still limited. As a result, lignocellulosic materials have a sluggish decomposition rate and a poor biogas output [72, 73].

4.2.1 Production of biogas from agricultural waste

Around half of the world’s habitable land is dedicated to agriculture [74] and is the largest ecosystem managed by humans [75]. Due to continual development and intensification in response to the important dietary needs of the growing populace and bioenergy demand, agriculture has become the most anthropic activity with the largest impact on the environment, especially in the developing countries (ES) [76].

While agricultural landscapes have a lot of potential for reaching renewable energy objectives and supporting local economies, bioenergies are frequently seen as a contentious solution for long-term development due to the rivalry for agricultural land. In recent years, a lot of effort has gone into resolving such food-energy conflicts. A promising source of renewable energy is the use of residual biomass for energy production. It has gained significant economic and environmental importance in recent decades, and it has the potential to close material and energy cycles, protect the environment, recover resources, and reduce the impact and quantity of wastage [77].

Biogas is a versatile biofuel that can be produced from a variety of feedstocks [78]. The anaerobic digestion (AD) process facilitates the transformation of biomass into energy and digestate using biogas technology. The energy generated by (AD) has been utilized to generate heat, electricity, and biomethane, the digestate has also been used as a biofertilizer to restore soil nutrient levels and so boost feedstock productivity [79]. Biogas will account for 25% of all bioenergy in Europe (shortly) due to its many benefits for energy supply, security, and economic benefits [80].

Many agricultural residues have the potential to be valuable resources if they are managed properly. Stalks, straw, leaves, roots, husks, seed shells, and farm and animal farming waste make up the raw material base. These sources of biomass have a diverse set of properties. The most noteworthy difference is between dry residues (such as straw) and those that are more suited to thermo-chemical conversion routes such as combustion, gasification, and pyrolysis, while wet residues (such as animal slurries) which are more suited to biological conversion routes, like biogas production as depicted inFigure 4 [80].

Figure 4.

Classification of agricultural residues.

Various studies into the South African wine industry have looked at grape pomace as a potential biogas feedstock, notably in the Western Cape, which has a concentrated wine sector [77]. The study found that because the wine and grape industry is reliant on seasonal production, the use of grape pomace for energy generation is not feasible for a sole. However, the study found that 1 tonne of grape pomace could produce approximately 230 m³ of biogas and 828 kWh of renewable electricity. According to the study, communal digesters serving neighboring wineries would increase their viability as a long-term remedy to winery waste [81].

4.2.2 Production of biogas from food waste

A report published by the Food and Agriculture Organization of the United Nations (FAO) in 2019, Globally, over 33% of human food is wasted, equivalent to approximately 1.3 billion tonnes each year. Food waste per capita in West Asia and North Africa amounts to 6–11 kg per annum, compared to 95–115 kg in Western countries. Food waste occurs throughout the food supply chain, including agricultural processing, sorting, storage, transportation, distribution, selling, preparation, cooking, and serving [82]. Food waste costs the world economy over $ 750 billion (US) annually [83].

Compared to other technologies such as incineration and landfilling, AD of food wastes has a lower environmental impact [82, 84]. As a result, multiple efforts to enhance biogas production from food waste have been made in recent years [85]. Despite the high potential for valorising waste food into biogas in nearly any city on the planet, not many industrial-scale plants, especially in industrialized countries, have been put into service [82].

4.3 Biochar

Crop residues, non-commercial wood and wood waste, manure, solid waste, non-food energy crops, construction scraps, yard trimmings, methane digester residues, or grasses are used in the production of sustainable biochar. Biomass for biofuels or biochar must be surplus that is, more than what should be left on-site to maintain forest and agricultural cropland health [86].

Biochar is created when biomass is pyrolyzed or gasified. These are thermal conversion methods that involve superheating and thermally converting biomass at high temperatures (350–700°C) in a specially designed furnace that captures all of the emissions produced [87].

Biochar is just one of the many valuable bioenergy and bioproducts produced during pyrolysis. Volatile gases (methane, carbon monoxide, and other combustible gases), hydrocarbons, and the majority of the oxygen in the biomass are burned or driven off, resulting in carbon-enriched biochar. All of the emissions (also known as air pollution and greenhouse gases) produced by burning biomass are captured and condensed into liquid fuels such as bio-oil, industrial chemicals, or syngas (synthetic gas). These products can be containerized for sale, stored for future use at the manufacturing facility, or used on-site as part of the energy production process.

4.3.1 Production of biochar from agricultural waste

Biochar is a unique carbonaceous porous material generated by pyrolysis or thermochemical conversion of biomass with little or no oxygen [88]. Due to its distinct characteristics such as large surface area, porous structure, oxygenated functional groups, and cation exchange capacity, biochar has recently attracted increased attention in several engineering applications [89].

Biochar can be made from a variety of different feedstocks, wood chips and pellets, tree bark, crop residues (corn stover, nutshells and rice hulls), and other feedstocks that are used commercially, internationally and in research studies. Organic wastes such as grain, sugarcane bagasse, chicken and dairy manure, and sewage sludge have been studied as potential feedstocks [90, 91, 92, 93, 94, 95, 96].

As a result, BC has a huge diversity of composition. Xie et al. [97] compiled a list of biochar conversion technologies, detailing product yields and operating conditions, finding that the biochar yield ranged from 15 to 35% with long residence periods of up to 4 h at a moderate temperature of not more than 500°C, and the bio-oil yield ranged from 30 to 50%. More bio-oil (50–70%) was discovered with a shorter residence time (up to 2 s). The thermochemical processes of pyrolysis and carbonization are used to convert biomass into biofuels and other bioenergy products. Biochar is produced by pyrolysis, thermochemically converts biomass in the absence of oxygen at a temperature greater than 400°C. The main components of biochar are carbon (C), hydrogen (H), oxygen (O), nitrogen (N), sulfur (S), and ash. Pyrolysis is divided into three categories: slow, intermediate, and rapid. Kung et al. [98] found a slow pyrolysis process produced more biochar than other processes. Steiner et al. [99] used a top-lit updraft gasifier to make biochar from rice husk and discovered farmers produce biochar in the field with a 15–33% efficiency. Each year, biochar made from on-farm crop residues can contribute 6.3–11.8% of the total production area [100]. Carbonization (a slow pyrolysis process) produces biochar as a by-product and has been around for thousands of years. Slow pyrolysis is a technique for heating biomass to a low temperature (400°C) in the absence of oxygen over a long period [99].

4.3.2 Energy recovery from the production of biochar

Two aspects of biochar production’s energy recovery have been identified. The first is that the energy value of the steam, gas, and oil by-products of biochar production can be recovered, resulting in a secondary revenue stream and a reduction in greenhouse gas (GHG) emissions [100, 101]. The volatiles in the feedstock burned during pyrolysis, releasing energy as heat, which can be used to generate steam or for combustion in electricity generation plants [102]. Bio-oils can be refined into transportation fuels or burned to provide energy for heating if adequate quantities are available [103]. The syngas and bio-oils can be used to generate steam, which can then be used to power turbines in centralized power plants [103]. However, the ability to use bio-oil is limited by the size of the operation and the volume of oils produced.

Secondly, biochar can be burned directly as a carbon-neutral or low-carbon energy source when. Worldwide, 41 million tonnes of char are produced annually for cooking and industrial purposes [104].

Biochar production and use consume less energy than burning wood for cooking or heating directly [105]. Because it expands the feedstock base to include crop residues and other by-products of agricultural-related activities. These feedstocks are already being used to meet a large portion of the world’s household energy needs [106]. When opposed to conventional cooking fuels (e.g. paraffin) which pose indoor fire risks and health problems linked with poor indoor air quality due to its combustion, using char for energy provides several social benefits [107].

4.4 Charcoal

In 2018, coal combustion accounted for about 38% of global electricity production [108]. According to the International Energy Agency (IEA), the world’s recoverable coal reserves are roughly 888.9 billion tonnes, with the majority of them situated in China, Australia, India, Russia, South Africa, and The United States of America [108]. The IEA’s energy supply statistics for coal in 2003 and 2017 were 2,619,947 kilo tonnes and 3,789,934 kilo tonnes, respectively. Annual global combustion was estimated to be around 2.5 billion tons [109].

Charcoal is a carbon-rich solid that is derived from biomass in the same way. Charcoal is typically used for heating or cooking and is associated with barbecuing. The temperature at which charcoal and biochar are produced is a significant difference. Charcoal is produced at temperatures ranging from 400 to 1000°C, whereas biochar is produced at temperatures ranging from 600 to 1000°C. When biochar is made at lower temperatures, volatiles (smokiness) are left behind, which has been shown to limit plant growth [105].

The temperature affects porosity as well; the higher the temperature, the greater the porosity. This means that charcoal is not as good as biochar at retaining water and nutrients. Microbes have less surface area when there are fewer pores. As a result, using crushed up charcoal instead of biochar will not be as beneficial to your plants. Because charcoal is made at a lower temperature, it produces a less stable form of carbon, which means it does not provide the long-term carbon sequestration properties associated with biochar [99].

Carbon, which occurs naturally in wood, forms a crystalline structure at higher temperatures, according to research. That is, charcoal has a shorter lifespan in the soil than biochar, which can last hundreds, if not thousands, of years. As a result, it is less effective in the soil and less beneficial to the environment. Biochar made at higher temperatures performs better and sequesters more carbon [87]. The potential of extracting biogas from waste resources is shown in Table 2.

Waste feedstock	Source of waste	Energy resource	Technology	Biogas potential (m^3/kg dry mass)	Reference
Food waste	Household	Biogas	AD	0.027–0.312	[72, 84, 87]
Animal dung	Agro-waste	Biogas	AD	0.012–8.25	[76, 78]
Bagasse	Agro-based waste	Biogas	AD	0.182	[72, 76, 79]
Blackwater	Household	Biogas	AD	0.052–0.232	[82, 83, 87]
Greywater	Household	Biogas	AD	0.035–0.145	[82, 83, 87]
Silage	Agro-waste	Biogas	AD	0.213–0.458	[72, 77, 79]
Husks	Agro-based waste	Biogas	AD	0.013–0.	[77, 79]
Slaughterhouse residue	Agro-based waste	Biogas	AD	0.315–0.812	[76, 87]
Straw	Agro-waste	Biogas	AD	0.161–0.214	[72, 83, 85]
Municipal waste	Household	Biogas	AD	0.035–0.268	[76, 84, 87]

Table 2.

The potential of energy resource extraction from household and agricultural wastes.

5. Economic impact of resource reclamation

Population growth resulted in modest increases in per capita income at the macro level. Economic activity in any of the world’s poorest countries, on the other hand, has stalled. This economic downturn coincided with significant (and, in some cases,) population growth, resulting in stagnant or decreasing per capita incomes [1]. Agricultural economics applies economic ideas to agriculture without taking into account the profession’s economic, social, and environmental concerns. It’s important to remember that agricultural economics encompasses a far larger spectrum of food and fiber-related activities than just farming. The agriculture sector accounts for around 12–15% of the nation’s production when considered in this light [110].

The circular economy notion [111] is a valuable method of comprehending the waste management hierarchy’s implementation in terms of its contribution to the green economy and other energy power recovery (EPR) initiatives. A circular economy is characterized as “closing the loop” between resource extraction and waste disposal throughout the economic cycle by applying waste avoidance, reuse, repair, recycling, and recovery strategies to minimize waste output and demand for virgin resources as production inputs. An economy that is meant to be restorative and regenerative to maintain the greatest utility and value of goods, components, and materials [112].

Recycling efforts account for a significant portion of waste reclamation. In a circular economy, the interchange of products and services is boosted. As the activities in the sector increase, the economic sectors of interest are impacted. This affects the income of all active participants in the economy. Cans and bottles may be recycled by consumers, shipping cardboard and unsold food can be recycled by businesses, and scrap materials can be recycled by manufacturers. Thousands of recycling brokers and processors exchange source-separated and aggregated materials, as well as treat waste to provide feedstock for manufacturers to employ as product inputs. This shows that waste recovery initiatives have a substantial global economic impact.

6. Life cycle analysis (LCA) of waste resource reclamation

The propensity of life is based on the creation of new cells and the elimination of old or expired ones. Another way of looking at life is as a process of ingesting nutrients-based materials and expelling waste. As a result, waste generation is a normal occurrence. Man’s effort to bring rationality to waste generation and disposal procedures is known as waste management. LCA is defined by the Society of Environmental Toxicology and Chemistry (SETAC) [113] as “an objective process for evaluating the environmental burdens associated with a product, process, or activity, by identifying and quantifying energy and materials used, waste released to the environment, and evaluating and implementing opportunities to effect environmental improvements.” LCA is a methodology for analyzing the environmental implications of a product, process, or service from the raw material production through the final disposal of wastes.

6.1 Composition of household and farm wastes

The composition of HMFSW from households, municipalities, and farms is influenced by a variety of factors including cultural traditions, lifestyles, eating preferences, climate, and income. Many diverse sources of solid waste were found by Yadav and Samadder [114] in families, municipalities, and crop farms. Family units, hostels, governmental and private organizations, and commercial centres all produce waste. Waste is created on the farm during harvest and post-harvest operations. Solid wastes can be categorized into biogenic solid waste (BSW) and non-biogenic solid waste (nBSW) based on their origins. Location, socioeconomic position, habits, environmental awareness, and other variables all influence the frequency of one over the other [113]. The biogenic constituent of solid waste includes paper, packaging/cartons, wood, textiles, food leftovers and waste, yard trimmings, leather, and others; while the nBSW include rubber, polyethene tetrafluoride (PET), high-density polyethene (HDPE), polyvinyl chloride (PVC), low-density polyethene (LDPE), polypropylene (PP), polystyrene (PS), and other plastics. The biogenic component supports biogas production after some customized pretreatment steps.

The Life Cycle Analysis (LCA) process is most commonly utilized as a support tool in the strategic planning and decision-making process for Waste-to-Energy projects [113]. However, the Waste-to-Energy systems’ inputs and outputs differ from one project to another; in particular, the waste composition and cost are highly dependent on the project’s location. The WtE plant design and waste composition can have a considerable impact on efficiency and emissions.

6.2 Flow characterization of materials

Any country’s waste management industry is under growing pressure to improve its environmental performance. Solid waste management (SWM) is essentially a local responsibility in most countries [115]. Low- and middle-income nations confront hurdles in terms of sustainable waste management strategies compared to higher-income countries due to a lack of resources and ability in local governments, as well as the ineffective execution of specialized regulations. As a result, nations with greater incomes are leading the way in creating sustainable waste management systems. Source reduction is the most prevalent waste management method in the sustainable waste management ladder.

In a stated mapping strategy, the material flow analysis (MFA) technique is utilized to characterize or quantify the efficiency of waste collection and disposal. Due to their complexity and volume, the system is divided into four subsystems to reflect the management of major waste streams: residual trash, commingled materials, source segregated dry recyclables, and source segregated food and garden wastes. The collected primary waste streams are the system’s import flows, while secondary products and emissions are the system’s export flows [116].

The use of integrated material flow analysis (MFA) and life cycle analysis (LCA) to make decisions in SWM systems is becoming increasingly popular. By acting as a great tool for assessing and controlling flows of wastes, secondary products, and residues, MFA on the levels of commodities aids in understanding the functioning of processes and the connections between processes in waste management [117]. LCA assesses the environmental advantages and disadvantages of waste management solutions. LCA examines system performance and allows for alternative comparisons as well as the identification of potential system improvements.

7. Conclusion

The increase in human population and the rural-urban drift has continuously placed a strain on fossil energy resources. Waste piles have increased across the globe with limited land space for landfills. Solid waste from the household, municipality and farm have an inherent energy content that could be harnessed to bridge the energy gap that has continued to get wider due to the increasing demand. The biogenic component of the solid waste is sorted for customized biochemical processes thereby accessing energy resources that could be used in homes or private and public institutions. Anaerobic digestion produces methane and carbon dioxide. However, the system could be tailored to produce hydrogen, which is an energy resource of value. The circular economy is necessarily employed to cub the waste piles and to enhance environmental sustainability.

One of the methods to ensure equilibrium in the energy economy is converting waste resources into value materials. Bioethanol, biogas, biohydrogen are some of the energy resources that can be extracted from the BSW; and because BSW is constantly produced from their sources, these energy resources are described as renewable. The residue that is left after extracting the energy resources can be turned into biochar, which is a resource that could be used to amend the soil for agricultural production.

Acknowledgments

The authors appreciate the support of the Durban University of Technology for funding this research focus area in Water and Wastewater. This work is also supported by the Green Engineering and Sustainability Group. The authors would like to acknowledge enabling atmosphere provided to harness the scientific information compiled and presented in this book chapter.

Conflict of interest

The authors declare that there is no conflict of interest.

Nomenclature

AD	anaerobic digestion
BSW	biogenic solid waste
EPR	energy and power recovery
FAO	Food and Agriculture Organization
GHG	greenhouse gas
HDPE	high-density polyethene
HMFSW	household, municipal and farm slid waste
IWWT	Institute of Water and Wastewater Technology
LCA	life cycle analysis
LDPE	low-density polyethene
LLDPE	linear low density polyethelene
MFA	material flow analysis
nBSW	nonbiogenic solid waste
PET	polyethylene terephthalate
PP	polypropylene
PS	polystyrene
SETAC	Society of Environmental Toxicology and Chemistry
SWM	solid waste management
USAB	up-flow anaerobic sludge blanket