Agro-Industrial Waste Management: The Circular and Bioeconomic Perspective

Abstract

Traditional agricultural production is circular. Virtually no waste is produced. Residues are returned to soil as compost; used as bedding material in livestock husbandry (and returned to soil as compost) or as feed to produce animal protein and manure; utilized as construction materials; or fuel for domestic energy. Circular agricultural production ensures soil conservation, waste reduction, residues reuse, and recycling. The ever rising global population, and demand for food and agro-industrial products, necessitated a transition to linear agricultural production which generates enormous quantities of agricultural residues, agro-industrial, and food wastes. The economic losses, environmental degradation, and health hazards resulting from poor management of excess wastes, and their mitigation have been the subject of research and policy efforts at continental and regional levels. Current waste management models redirect attention to circular agricultural production and bioeconomic approaches aimed at waste reduction, reuse, and recycling. Such approaches view agricultural wastes as raw materials with economic benefits for the farmer, consumer, and investor in varied industrial enterprises (crop and animal production, animal and human health, food, beverage, neutraceutical, pharmaceutical, cosmetics, and material industries). The present review attempts to collate information on global production, and possible valorization of recyclable agro-industrial residues and food wastes.

1. Introduction

Agro-industrial wastes are inedible materials produced as a result of various agricultural and agro-industrial operations. They include wastes from slaughterhouses and meat processing, animal dung or manure, field crop wastes, crop residues, harvest wastes, and wastes from food consumption and processing [1, 2]. The huge diversity characteristic of the agricultural and livestock sectors means that very large and heterogeneous products end up as wastes. There are hence several types of agro-industrial wastes based on material composition and management.

1.1 Types of agro-industrial wastes

Agro-industrial wastes can be divided into three broad categories, namely recyclable and compostable or naturally occurring agricultural and agro-industrial wastes, non-recyclable and non-compostable agricultural and agro-industrial wastes, and hazardous agricultural and agro-industrial wastes. Compostable wastes are recyclable wastes, which can be reused in the farm or recycled in recycling plants. Some such as pruning, straw, leaves, stover, stalk, bagasse, cob, and animal dung or manure are regarded as primary residues because they arise directly from crop and animal production activities while others such as pit, shell, peels, husk, cake, slurry, and slaughterhouse wastes are regarded as secondary wastes because they arise from agro-allied industrial processing [1]. Generally, primary and secondary residues are categorized as least problematic in management. Non-recyclable agro-industrial wastes are wastes that result from farm construction operations, farm mechanization, transport, and livestock protection facilities. They are the most problematic to manage since they are usually bulky and not reused or recycled on-farm. They include plastic sheets and containers, metal containers and equipment, tires, shadings or anti-stone nests, machinery, metal structures for fences or covers, and irrigation facilities. Hazardous agro-industrial wastes are wastes that pose very serious immediate and remote problems if not correctly managed. They include phytosanitary products, chemical containers, acids, fertilizers, waste water, chemical contaminated water, foods, and other materials; medicines, agro-chemicals, and detergents. These wastes are managed following laid down regulations from the appropriate authorities. In this review we focus on the utilization of recyclable (primary and secondary) agro-industrial wastes (agro-industrial residue) (Figure 1) for the production of renewable energy and functional products for household, environmental, industrial, medical, veterinary, and animal production applications.

Figure 1.

Classification of recyclable agro-industrial wastes. Source: Sadh et al. [3].

2. Global availability and estimates of agro-industrial residue biomass

Agriculture-based industries produce vast amounts of recyclable residues and wastes [3]. Food production, transportation, storage, processing, distribution, and consumption yield enormous wastes such as crop residues, food wastes, animal manure, animal wastes, and by-products, and forestry waste biomass. For instance, juice industries produce huge amounts of wastes such as peels, pulp, and drupes [4]; beverage industries produce cocoa, and coffee pulp, pod, and stalk as wastes; processing of cereals, canes, and grains yields husks, bran, bagasse, and molasses; nuts yield shells, cakes, and slurry following oil extraction; palm fruit processing yields large quantities of empty palm fruit bunch, palm fruit fiber, and palm oil sludge; processing of palm kernel yield shells while extraction of palm kernel oil yields palm kernel cake; meat industries produce trimmings, bone, offal, feather, hair or fur, cartilage, and blood [1, 2, 3]. From the growth of crops, various kinds of primary residues and wastes, namely stalk, stover, straw, leaves, stem, bagasse, and cobs are produced [1, 3]. Raising of livestock and poultry in large confined feeding operations produces nearly unmanageable concentrations of dung or manure and slurry in addition to various slaughterhouse wastes [1, 2].

Global agricultural residue potential is rendered in terms of technical, theoretical, economic, and sustainable potentials [5, 6]. The theoretic potential gives the gross quantity of biomass potentially produced [7] while the technical potential is the fraction of the theoretical potential technically recoverable allowing for economic, social, environmental, and political constraints [6, 8]. The economic potential indicates the fraction of the theoretical and technical potential available for purchase as source of revenue to the producer. Globally, approximately 147.2 million metric tons (Mt) of fiber sources are theoretically available while 709.2 and 673.3 Mt of wheat straw and rice straw residues were estimated, respectively, in 1994 [3, 9]. Residues from cereals and sugar cane production account for 80% of the total residue from crops and constitute the most harvestable biomass [1, 10, 11]. Cho et al. [12] estimated global annual rice straw, wheat straw, corn straw, sugarcane bagasse, and rice husk production at 731, 354, 204, 181, 110 Mt, respectively, while wood biomass waste was put at 4.6 Gt yr⁻¹. Wastes from coffee and olive oil industries were estimated at 7.4 and 30 Mt yr⁻¹, respectively. Capanoglu et al. [13] indicated that close to 1.3 billion tons of food (about one-third of food produced) is lost as wastes before or after reaching the consumer.

Country-specific estimates of agro-industrial and food waste biomass production and availability are scanty and very patchy since very few countries consistently track (document) residue and waste production and use [1]. Thus, reported statistics are results of modeling studies mostly based on national crop production, crop yield, residue-to-product ratio (RPR), area under cultivation, and moisture content [7]. Reports from a number of such studies estimate on a global level an appropriation (including for energy) of 2.9 billion tons yr⁻¹ (66% of total annual production) [1, 14, 15]. A 2006–2008 estimate of crop residue production from barley, maize, rice, soybean, sugar cane, and wheat gave 3.7 + 1.3 or – 1.0 Pg (billion tons) accounting for ¾ of total production [16]. Regions outstanding include North and South America, Eastern and Southern Asia, with a production of more than 500 Tg (Mt) yr⁻¹ each. South-east Asia and Eastern Europe have estimated value of 200 Tg yr⁻¹ each [16]. Earlier estimates include 3.5–4.0 Pg yr⁻¹ in the 1990s with cereals, sugar crops, and oil crops accounting for 79% of the total [17], 3.4 Pg yr⁻¹ in 1991, and 3.8 Pg yr⁻¹ in 2001 with cereals accounting for 74–75% [10], 4.4 Pg yr⁻¹ in 2000 [14], and 5.4 Pg yr⁻¹ for 1997–2006 using crop-specific harvest indices as estimator instead of RPR [18]. Using agricultural production data from FAO [19], Cooper and Laing [20] quantified crop residues and animal wastes produced on the African continent. The authors indicated that crop residues were mainly from coconut, maize, rice, and sugarcane production. Overall, 639,600 tons of coconut husks and 191,880 tons of coconut shells were estimated. Major producers were Tanzania (140,000 and 42,000 tons, respectively), Ghana (122,000 and 36,600 tons, respectively), and Mozambique (120,000 and 36,000 tons, respectively). Residues from maize were 16,296,301 tons of cobs and 90,602,879 tons of stalks with major production from South Africa (SA) (3,981,199, and 22,134,353 tons, respectively), Egypt (2,405,371 and 13,373,188 tons, respectively), and Nigeria (2,059,759, and 11,451,685 tons, respectively). A total of 22,858,042 tons of sugar cane residues were estimated with SA (6,302,133 tons), Egypt (4,112,925 tons), and Mauritius (1,443,750 tons) leading other countries in the continent. For the USA, the “Billion ton annual supply study” [21] and its update [22] reported an annual crop residue production of 550 Million dry ton matter(Mdt) yr⁻¹; a more recent study reported 518 Mdt yr⁻¹ [23] with 5.6 Mt (1% of total) corn stover appropriated for energy production [16, 23]. The US DOE [24] project estimated that approximately 144 Mt of primary agricultural residues are in use in diverse applications across the United States made up majorly of corn stover and concentrated in the Midwest regions, including the states of Illinois, Indiana, Iowa, Kansas, Minnesota, Missouri, Nebraska, Ohio, and South Dakota. Kim and Dale [25] had reported that corn production yielded roughly 10 Mt of grain ha⁻¹ and approximately the same amount of stover (assuming a 1:1 crop:residue ratio). An estimate of US biomass resources in terms of economic potentials gave 94 Mt at $66 ton⁻¹ coming from barley straw, corn stover, oats straw, sorghum stubble, and wheat straw with an increase between 158 and 180 Mt by 2040 [1, 24]. For Canada, about 48 Mt of dry agricultural crop residue is possible baring such factors as crop yield, cost of production, collection and transport, properties of residues, distance to processing facilities, the profit potentials of targeted use, and the degree of substitutability of other feedstock [1]. Bloomberg [26] had projected the potential supply of agricultural residues (80% grain straw) in the EU to be approximately 170 Mt at an average supply cost of €67 ton⁻¹ while de Wit and Faaij [27] projected approximately 200 Mt at plant gate cost of €51 ton⁻¹. Generally, the cost of crop production, crop yield, cost of residue harvesting, and the supply chain from point of collection to point of processing significantly impact price of residue. For Denmark, Energinet.dk [28] projected 1.0–1.5 Mt yr⁻¹ while annual production of crop residues in Canada over the period 2001–2010 was estimated at 82 Mdt [29]. Ji [30] assessed the potential lignocellulosic biomass or crop residues feedstock in China for biofuel production and found a theoretical amount of 930.8 Mt. In Iran, Alavijeh and Yaghmaei [31] reported 11.33 Mt. Pradhan and Mbowa [32] had stated that availability of reliable feedstock data for biofuel production in SA is a challenge. Barahira et al. [33], however, estimated an annual crop residue production of ~43 Mt in SA with ~32 Mt from grains and ~6 Mt from sugar cane. Other estimates include residue from oil crops (groundnut, soybean, sunflower) (~3 Mt), vegetable crops (potato, tomato, cabbage) (~1 Mt), and other minor crops (~0.8 Mt). Residue from maize ranked first among all crops with ~28 Mt followed by sugar cane, wheat (~3 Mt), sunflower (~2 Mt), and soybean (~1 Mt) [33]. Batridzirai et al. [7] had reported the gross (above ground) crop residue potential from maize and wheat in SA as 14.4 Mt yr⁻¹, but only 6.0 Mt yr⁻¹ can be removed sustainably from the field. Regions in SA with the highest potentials for residue production include Northern Cape, Mpumalanga, and Free-state accounting for 87% of national residue potential. Maize stover is the predominant crop residue accounting for 90% of the current and future total residue potential. In India, about 500–550 Mt of agro-industrial residues are generated per year [34, 35, 36]. Agriculture alone generates 140 Mt yr⁻¹ of biomass [34]. Cereals lead (352 Mt, 70%), fiber (66 Mt, 13%), oil seeds (29 Mt), pulses (13 Mt), and sugar cane (12 Mt, 2%). Of the 70% by cereals, rice has 34%, wheat 22% [37]. For fiber, cotton leads (53 Mt, 11% of crop residues) followed by coconut (12 Mt) [37]. FAO [38] reported an estimated total crop residue production of 95 Mt yr⁻¹ for member states of the Union économique et monétaireouest-africaine (West African Economic and Monetary Union) (Benin, Burkina Faso, Côte d’Ivoire, Guinea-Bissau, Mali, Niger, Senegal and Togo) in 2010 with cereal straw accounting for 80 Mt yr⁻¹ and straw from millet and sorghum ranking highest. Sahelian countries (Niger, Burkina Faso, Mali, and Senegal) had the largest share (96%) of crop residues especially cereal residues. Residues (peels) from tuber crops (yam and cassava) were estimated at 6 Mt yr⁻¹ and came mostly from sub-humid countries (Cote d’ Ivoire and Southern part of Benin). Agro-industrial by-products include cotton cake (2 Mt yr⁻¹ in 2005 but 1.34 Mt yr⁻¹ in 2009), groundnut cake (2.5 Mt yr⁻¹ in 2009), cereal bran (millet: 1.5–1.8 Mt yr⁻¹; sorghum: 1.3 Mt yr⁻¹), and molasses (100,000 tons yr⁻¹) with Cote d’ Ivoire and Senegal accounting for 45 and 15% of the total, respectively. In Nigeria, agro-industrial residues result from crop and animal farming, crop and animal processing, and forestry and timber production [39]. Major crop residues are from cassava, yam, potatoes, fruits and vegetables, plantain, cocoa, coconut, coffee, cowpea, groundnut, maize, millet, rice, sorghum, sugar cane, wheat, soybean, and oil palm [39, 40]. Agba et al. [41] reported that the biomass potential of Nigeria as at 2005 stood at 13 million hectares of fuel wood, 61 Mt yr⁻¹ of animal waste, and 83 Mt yr⁻¹ of crop residues. From 10 crops, Simonyan and Fasina [42] estimated crop residue availability of 145.6 Mt yr⁻¹ while Isola et al. [43] and Okeh et al. [44] reported 227,500 tons day⁻¹ of animal manure. The annual production of agricultural wastes is this high because about 94% and 68% of households are engaged in crop and livestock farming, respectively [45]. In a study that evaluated global production of endocarp tissue from horticultural fruit crops, particularly drupes, as residue for biofuel production, Mendu et al. [4] reported an estimate of 2.4 × 10⁷ tons consistent with FAO’s 3.1 × 10⁷ tons (29% variation). The study reported greatest density of drupe endocarp production in developing countries in South Asia and broad lower-density distribution across Southern and Northern Europe and the Middle East. Isolated productions were mapped to USA, Africa, China, Australia, Central America, and South America. Highest endocarp yield was from coconut and mango (1.31 × 10⁷ and 3.99 × 10⁶ tons, respectively), which together accounted for 72% of total global drupe endocarp production [4]. Smil [17] had indicated that over 60% of global crop residues are produced in low-income countries, and almost 45% of residues come from the tropics. This is despite considerable regional differences in fraction of residue harvested and used (29% in sub-Saharan Africa and 90% in Western Europe) [14].

3. Management of agro-industrial residues: the circular agricultural and bioeconomy perspectives

Traditional (subsistence) agriculture is based on circular sustainability model, which ensures practically nil waste as residues are recycled or used for various purposes including maintenance of soil fertility [46, 47]. Rise in global population, however, necessitated the intensification of agricultural production, linear-agricultural production system, globalization of food distribution, extensive storage, and agro-industrial processing, all of which generate extensive quantities and varieties of agro-industrial and food wastes [47, 48]. Previously considered of little or no economic value, harvest leftovers, crop residues, animal wastes (bones, carcasses, blood, fat, feathers, hair, cartilages, skins, viscera, and dung), and food wastes (household, food service, and retail wastes) are today viewed as valuable resources of significant economic value such that they have become co-products or raw materials from agro-industrial processing, crop, forestry, and animal production [49]. In the traditional circular agricultural model, farmer’s decision on agricultural residue use reflects their needs and preferences [50], and access to and affordability of alternative biomass resources determine the opportunity costs for a farmer or household to sell, use, or replace residues [50]. From time, agricultural waste biomass has fulfilled vital roles including livestock feed, animal bedding material, domestic fuel, construction material, some cash through sales, and maintenance of soil fertility [50, 51, 52]. Today, the huge excess residues and wastes after fulfilling these traditional roles are disposed by burning; a practice viewed by farmers as the most convenient, cheap with regard to time, labor, and finance, and beneficial for control of weeds, crop pests, and diseases (Figure 2) [52, 53, 54]. The traditional agricultural residue management models, however, have poor economic returns to the farmer, and poor soil health conservation and maintenance [52]. In addition, burning of agricultural residues has tremendous negative environmental implications including loss of soil nutrients, soil erosion, and release of climate pollutants including greenhouse gasses [48, 52, 55, 56, 57, 58, 59, 60]. Added to these are considerable adverse human and animal health concerns (Figure 2) [59, 60, 61].

Figure 2.

Impact of crop residue burning (CRB) [52].

Current agricultural residue and waste management models (the circular and bioeconomy models) (Figures 3 and 4) emphasize profitability, sustainability, technical feasibility, and adoption potential. That is, an integrated circular and bioeconomy approach that is sustainable, up scalable, crop- and region-specific, socially inclusive, environmentally sound, and technically robust [52]. The approach further harnesses the synergies existing among alternative options, aims at mitigating climate change, and contributes to achievement of sustainable development goals [52].

Figure 3.

Crop residue management: A paradigm shift from the traditional approach to agricultural bioeconomy and circular economy [52].

Figure 4.

Crop residue management intervention: Bioeconomy and circular economy perspectives [52].

This perspective is hence driven by the urge for better human and animal health and welfare, sustainable agricultural and animal production, the need to decarbonize the agricultural economy, exploitation of emerging opportunities through waste valorization and production of value-added products, to end waste burning and derive optimal economic benefits from agricultural residues and food wastes for improved livelihood, circularity in agricultural production system for example through reuse and recycling of residues, and the need to mitigate climate change (Figure 5) [52, 62].

Figure 5.

Mapping of benefits from bioeconomy-driven crop residue management. CA: conservation agriculture; CSA: climate smart agriculture; C: composting; BF: biofuel; BP: biochar production; BG, biogas; MP: mushroom production [52]. Numbers represent targeted sustainable development goals (SDGs).

The circular and bioeconomy approaches have become critical in the backdrop of declining soil fertility, and productivity, need of food and nutritional security, carbon sustainability, and to mitigate adverse health effects and greenhouse gas emission [52, 62]. Key components of the agricultural circular and bioeconomy waste management interventions are conservation agriculture, in situ crop residue incorporation, biomass energy production, biofuel generation, biochar and activated carbon production, residue composting and biofertilizer production, and substrate for edible fungi cultivation [52, 63, 64, 65, 66], production of materials such as packagings, food coatings, biopesticides, single cell proteins, and animal proteins [13, 67], and extraction of bioactive molecules and functional groups for varied applications [47, 48, 67]. Obviously, the potential availability and sustainability of feedstock supply for particular industry would depend on alternative or competing uses, cost of production and supply, crop yield and revenue from crops to farmers, and scale of crop farming enterprise [1, 11, 68]. Following is a review of application and allocation of agro-industrial residues (crop residues, fruits and vegetable wastes, animal manure, carcasses, and dairy wastes) and food wastes to major competing applications.

3.1 Application of agro-industrial wastes to soil conservation, soil erosion control, and maintenance of soil fertility

Soil conservation and enrichment are among the traditional and modern applications of crop residues and recyclable agro-industrial wastes including animal manure. It is a vital component of conservation agriculture. Traditionally, some quantities of crop residues are left on-farm to consciously or inadvertently prevent or minimize soil erosion, increase soil organic matter, maintain soil organic carbon, improve soil structure, conserve soil moisture, recycle plant nutrients, maintain soil organisms and microbial population, and maintain soil fertility while some proportions are composted or converted to bio-fertilizer for soil enrichment [7, 33, 69]. The circular and bioeconomy models further recommend in situ crop residue incorporation and no- or minimal-till cultivation practices. In China, it is estimated that 15% of annual residues production are converted to fertilizer while 31% are left on-farm [37, 70]. Wang et al. [71] had reported that crop residue use as fertilizer in Henan Province of China is made up of straw left in field, straw returned to field, straw chipping and mulching in field, quick composting in field, straw pile fermentation returned to field, straw-produced organic fertilizer, straw-produced ammoniation, and straw-produced silage. It is reported that in 2009, 15.4 Mt of wheat straw and 9.7 Mt of rice straw were used for fertilizer in China [72]. Also, crop residues accounted for 12–19% of total organic fertilizer resources [73], provided about 25–35% N, P, and K nutrients, and improved nutrient recycling in the soil. Batidzirai et al. [7] reported that in South Africa, out of a gross amount of 16 Mt yr⁻¹ of maize stover, 6.3 Mt are below ground level and serve for soil organic carbon maintenance while 4.2 Mt is required for soil erosion control. Similarly, out of 1.8 Mt of wheat straw, 970,000 tons that are below the ground serve for soil organic carbon maintenance. In a study of the pattern of residue biomass use in cereal-sheep production system of North Africa, Ameur et al. [69] reported that 70.4% of farmers in Tunisia retain below 200 kg of crop residue per hectare (ha⁻¹), 15.1% retain between 200 and 500 kg ha⁻¹, while only 14.5% retain > 500 kg ha⁻¹ as mulch. Baudron et al. [74] had reported that only 3% of farmers in Ethiopian Rift Valley retain more than 1 t ha⁻¹ of crop residues on the farm. A number of studies [50, 69, 74, 75] have highlighted the challenges associated with crop residue allocation to competing applications including soil conservation in conservation agriculture and in crop-livestock integrated systems. Generally, the quantity of crop residue retained for soil conservation in integrated crop-livestock systems depends on scale of farming (acreage), type of crop, stocking density of livestock, share of livestock income, and crop yield, among other drivers of crop residue management [69].

3.2 Agro-industrial wastes for livestock production (feed and bedding material)

Historically, crop residues were used as animal bedding [11] and as supplemental feed for livestock especially in smallholder mixed agricultural systems. Erenstein [76] had observed that crop residues represent a fundamental resource for crop-livestock integration and intensification, along a broad range of smallholder mixed systems. In tropical and subtropical countries, food wastes serve as food for pet animals and feed for monogastric animals especially pigs. Agricultural biomass is a vital dry season feed resource, providing livestock feed when other resources are scarce. Valbuena et al. [50] stated that in semi-arid and arid areas, crop residue is a vital feed resource for livestock production, and there is increasing demand of agricultural biomass for animal feeding due to low availability of alternative resources [69]. Cooper and Laing [20] stated that the application of crop residue for animal feeding and soil fertility maintenance is very important in maintaining balance and functionality in the rural system. Thus, crop residues play important roles for farmer’s livelihood, in various contexts and under different levels of resource availability [50]. In Ethiopia, and most other developing countries of Africa and Asia, crop residues and food wastes are used primarily for animal feeding [77]. In India, crop residues are enriched with urea and molasses as fodder for livestock [37]. In China, 31% of annual crop residue yield was applied to animal feeding [37, 70]. In Henan Province alone, Wang et al. [71] reported the application of 6.9 Mt of wheat straw, 6.0 Mt of corn stalk, and 2.2 Mt of peanut shell and leaves as forage in 2009 with a projected application of 4 Mt of agricultural residue to forage production in 2016. In Denmark, close to ½ of total collected crop residues are used for animal bedding and animal feed and fodder [1]. An estimated 3.6 Mt of dry crop residue was used for animal bedding in Canada in 2011 aside from the amount applied to supplement forage crops for animal feeding [1, 29]. Krausmann et al. [14] reported that of the 4.4 Tg yr⁻¹ of crop residue, 2.9 Tg (66%) is appropriated for fodder, animal bedding, and energy. The authors reported that 10% of harvested residues were used to feed livestock in Europe and North America while 83% was used in South and Central Asia. Wirsenius et al. [78] reported global appropriation of 41% of total crop residue to food systems (i.e., livestock feeds) while Weiser et al. [79] reported that 24% of harvested cereal straw was used for livestock husbandry. In the EU, Scarlat et al. [80] reported that 1/5th to 1/3rd of harvested crop residues were applied to livestock production while Ericsson et al. [81] estimated 1/3rd. In SA, Batidzirai et al. [7] estimated that 260,000 tons yr⁻¹ of maize stover out of the harvestable 9.7 Mt yr⁻¹ was needed for cattle feed while 70,000 tons out of the harvestable 870,000 tons of wheat straw was applied to livestock bedding. In sub-Saharan Africa with predominantly small holder crop-livestock integration, crop residues are primarily applied to livestock production (fodder, feed, and bedding) such that allocation to other competing applications including soil conservation is problematic [50, 69, 74, 75].

3.3 Application of agro-industrial residues and food wastes in renewable energy production

Among the renewable sources of energy (hydropower, solar, geothermal, wave, wind, biomass, and tidal), only biomass can be converted to three different forms of energy carrier: solid, liquid, and gaseous biofuels [82]. The most common biofuels are bioethanol, biodiesel, and biogas. Bioethanol is generated from starch, sugar, and lignocellulosic-rich crops (phase I) and from food wastes and crop residues feedstocks (phase II); biodiesel is produced from oil rich crops, food wastes, and agricultural residue feedstocks; while biogas is produced from carbohydrate-rich biomass including animal wastes and manure [33]. Biofuels are used in different applications such as generation of electricity, heating, and powering of machines [33]. Generation of energy products from agricultural biomass has in addition environmental advantages: biomass source is storable, inexpensive, energy-efficient, and environmentally friendly. Biomass energy production and biofuel generation are major components of the circular and bioeconomic agro-industrial residue and food waste management option [52, 64, 65]. Utilization of agricultural residues and food wastes for large-scale modern bioenergy production is gaining attention in many countries [83, 84]. Countries such as Denmark, United Kingdom, Spain, Sweden, China, and India have developed large bioenergy facilities [85, 86]. Leading countries for bioethanol production are USA and Brazil [33]. Key residues are maize stover, wheat straw, rice straw and husks, and bagasse [21, 87, 88]. Bagasse is the most commonly used residue, but USA is pioneering the use of maize stover while Denmark is focusing on straw. A study by Bentsen et al. [16] estimated the theoretical global potential of primary agricultural residues from cereals and sugar cane available for biofuel production at approximately 3.7 billion tons of dry matter annually. Earlier studies [10, 14, 18] had estimated 2.7–3.5 billion tons yr⁻¹. A more recent study by Panoutsou et al. [89] projected a technical potential of crop and agro-industrial residue availability for biofuel production in the EU28, Western Bulkans, Turkey, and Ukraine as 400,000 tons of dry biomass yr⁻¹ by 2030. In Denmark, Gylling et al. [90] had projected an increase from 1.4 Mt yr⁻¹ to approximately 3.0 Mt yr⁻¹ by 2020. Studies by Larsen et al. [91] and Thomsen et al. [92] estimated that 50% of straw resources in Denmark are collected for various purposes, 45–50% of which is used for energy generation. Ericsson and Nilsson [81] and Scarlat et al. [80] reported that 20–40% of crop residues produced in Denmark was applied to energy generation. Annual straw consumption for domestic heat and power was put at approximately 1.4 Mt (fresh weight), and a significant proportion is channeled to biorefinery (biofuel, bioethanol), chemical, and material production [16, 80, 81]. It is projected that about 155 Mt of agricultural residue (including 60 Mt of manure) would be applied to bioenergy production in the United States by 2030 [11] without increasing the agriculture share of land resources [93, 94]. Regional estimates reported by UCS [11] include Texas (19.8 Mt), California (9.2 Mt), Alkansas (10.3 Mt), and Iowa (31 Mt). The “Billion ton annual supply study” [21] and its update [22] reported appropriation of 5.6 Mt (1% of total) of corn stover for energy production out of an annual crop residue production of 550 Mt [16]. US DOE [24] reported that 111 million dry tonnes (Mdt) and 94 Mdt of crop residue can be collected at farm-gate prices of $60 and $50, respectively; 3/4th of which are corn stover and 1/5th wheat straw. The study projected that by 2030, about 180 Mdt yr⁻¹ residue will be available for biofuel production under the base-line scenario while about 320 Mdt yr⁻¹ will be available under the high-yield scenario with 85% being corn stover. In addition are 20–26 Mdt of processing and other wastes at $40–$60 per dry ton (dt⁻¹). Animal manure was estimated at 30–60 Mdt at farm-gate price of $50–$60 dt⁻¹. IARI [37] reported that of the 700 Mt yr⁻¹ of crop residues produced in China, 19% (~133 Mt) was used for energy generation while a theoretical estimate of 930.8 Mt of lignocellulosic biomass was reported for China by Ji [30]. The report also indicated that 13.5 Mt yr⁻¹ of residues was theoretically available for biofuel production from 19 potential crops. Lignocellulosic biomass availability for bioenergy generation in Iran was reported to be about 11.33 Mt yr⁻¹ by Alavijeh and Yaghmaei [31]. For SA, Batidzirai et al. [7] estimated the sustainable biomass energy potential to range from 400 to 550 PJ excluding energy crops, public grasslands, and roadside grasses. Components include maize and wheat residues (6 Mt yr⁻¹ or 104 PJ energy equivalent, that is, 5.1 Mt yr⁻¹ of maize stover at 94 PJ and 600,000 tons yr⁻¹ of wheat straw at 10 PJ), forestry biomass residue (189 PJ: 1^o = 41 PJ, 2^o = 17 PJ, 3^o = 70 PJ; wood chips = 61 PJ), sugar cane plantation and cane processing or bagasse (19–32 PJ), and organic waste from municipal solid wastes (4.5 Mt at 8 PJ). Barahira et al. [33] reported that 13.5 Mt of crop residues are potentially available for biofuel production in SA. In Nigeria and most other Sub-Saharan Africa, renewable energy production from agro-industrial and food wastes is still in its infancy, and large-scale industrial bioenergy production is yet to be accorded priority. Biomass conversion technologies currently applied at elemental level include physical or mechanical conversion (chipping, grinding, milling, and densification) into solid fuels such as briquette and pellets [39, 95, 96, 97]; thermochemical conversion (combustion, pyrolysis, gasification, and liquefaction) to produce biochar, bio-oil, and gas such as methane [39, 98, 99]; and biochemical conversion (anaerobic digestion, fermentation, and transesterification) to produce biogas, ethanol, and biodiesel [100, 101, 102]. Jakayinfa et al. [39] estimated the theoretical bioenergy potential of agricultural residues and animal wastes for Nigeria at 5.81 EJ (3.64 EJ from agricultural residues and 2.17 EJ from animal wastes) while the technical potential, based on generalized availability factor of 0.3 [103] (range: 0.0–1.0 depending on type of crop residue), was estimated at 1.74 EJ. For Tanzania, total bioenergy potential of crop residues was put at 5714.0 TJ in 2012 with sugarcane and cassava having the highest potential of 2966.4 and 845.0 TJ yr⁻¹, respectively, while a total of 1397.0 TJ yr⁻¹ could be generated from animal waste with cattle having the highest potential of 1,139,074,332 MJ yr⁻¹ followed by goat (181,036,476 MJ yr⁻¹). The least bioenergy potential was for poultry manure (7,859 MJ yr⁻¹) [6].

3.4 Agro-industrial wastes as substrates for edible fungi (mushroom) cultivation

A rapidly expanding valorization of agro-industrial residues is as substrates for edible fungi (mushroom) cultivation (Figure 6). Mushroom cultivation is seen as a major and sustainable component of modern agricultural residue management protocol and circular agricultural systems [105, 106]. Data on global or country-specific allocation of crop residues to mushroom production are scarce in literature. IEA [1] reported that in Canada, about 1.0 Mdt of crop residues were applied to mushroom and horticultural cultivation in 2011. In China, Wang et al. [71] reported that application of agricultural residues to mushroom production has been commercialized. Total crop residue applied to mushroom cultivation in Henan province in 2009 was 2.44 Mt, which accounts for 3% of crop residue generation and 4% of total crop residue utilization in the province. Of these volumes, wheat straw accounted for 0.68 Mt, rice straw, 0.53 Mt [72]. In Luoyang alone, 52 ha and about 200,000 tons of residues were dedicated to mushroom cultivation annually [71, 72]. In India, Raman et al. [107] reported that a total of 39 kinds of agricultural residues from 26 crops provide valuable resources for mushroom cultivation. According to the report, India produces over 620 Mt yr⁻¹ of agricultural residue, a substantial quantity of which could be profitably channeled to mushroom production. In eastern Democratic Republic of Congo, Kaziga et al. [108] reported favorable results of an effort to orientate smallholder farmers to valorize wastes from stable crops through mushroom production.

Figure 6.

Major edible mushrooms on agricultural residue substrates [104].

Global mushroom production has increased tremendously over the past decades from about 0.35 Mt in 1965 to about 3.41 Mt in 2007 [107, 109, 110, 111]. In 2015, global mushroom market was worth USD 35 billion [107] while Royse et al. [104] reported that global mushroom market was worth USD 63 billion in 2013 made up of medicinal mushrooms: 38%, wild mushrooms: 8%, and edible mushrooms: 54%; with an annual projected increase by USD 34–60 billion [105]. In addition to contributing to food and nutrition security, spent mushroom substrate (SMS) represents a vital resource to produce high-quality compost for growth of other fungi [112], improve animal nutrition and health [105, 106, 113, 114], produce materials [115, 116, 117], and extract enzymes for industries [105, 106, 109] and for soil amendment and bioremediation [106, 114, 118, 119]. China is the global leader in mushroom production with 3,918,300 tons yr⁻¹ or 64% of global volume and 85% for oyster mushroom production (Pleurotus spp.) [109, 110].

3.5 Derivation of bioactive molecules from agro-industrial residues and food wastes

Agricultural residue and food waste biomass stream are widely acclaimed as valuable resources for bioactive compounds (molecules) and functional products. Valorization of agro-industrial residues into food and feed ingredients, dietary supplements, novel bio-components, nutraceuticals, and pharmaceuticals is a potential huge industry and a vital component of the bio-economic and circular model of agricultural residue management. Bioactive molecules from agricultural and food residues include phenols, polyphenols, non-starch polysaccharides (cellulose, hemi-cellulose, and lignin), oligosaccharides, carotenoids, soluble fibers, terpenoids, proteins, tocopherols, and phytosterols [47, 48, 120, 121]. Animal wastes and residues (dairy waste, sea food waste, slaughter wastes) have high levels of proteins, lipids, and minerals [48, 122, 123]. Prado et al. [124] had observed that animal and vegetable wastes are low-cost materials for bioactive compounds using suitable processes. The extraction of bioactive compounds can be integrated into bioenergy facilities, thus enhancing revenue potentials [67, 125]. These bioactive molecules or metabolites can impact human and animal health, for instance, as antioxidant, immunomodulatory, anti-hypertensive, anti-inflammatory, cholesterol reducing, antiaging, anti-cancer, and antidiabetic agents [48, 126, 127, 128]; as well as serve as raw materials for the development of functional ingredients useful in food, cosmetics, chemical, and pharmaceutical industries [48, 67, 129, 130, 131, 132]. Figure 7 shows the main classes of bioactive compounds that are derivable from agro-industrial and food waste biomass.

Figure 7.

The main classes of bioactive compounds [133].

Traditional and novel approaches for extraction of bioactive molecules from organic agricultural and food wastes (Figure 8) include (a)solvent extraction (characterized by low processing cost and ease of operation) [120], (b) microbial fermentation (solid-state fermentation) [48, 70, 134, 135, 136, 137], (c) supercritical fluid extraction (SFE) [13, 120, 138, 139], (d) subcritical water extraction (SCWE) (characterized by shorter extraction time, lower solvent cost, high quality product, and eco-compatibility) [13, 120, 138, 140, 141], (e) enzyme assisted extraction (EAE) (uses water as solvent, and employs enzymes such as α-amylase, proteases, chitinase, tanase, cellulase, β-glucosidase, xylanase, β-glucanase, and pectinase, which help to degrade cell wall structure and depolymerize plant cell wall polysaccharides, facilitating the release of linked compounds) [13, 48, 120, 142, 143, 144], (f) ultra-sound assisted extraction (UAE) [13, 120, 134, 138, 145, 146, 147], pulse electric field-assisted extraction [13], and (g) micro-wave assisted extraction (MAE) (characterized by shorter extraction time, higher extraction rate, lesser solvent requirement, and lower cost) [13, 120, 134, 147, 148, 149]. Generally, the choice of applied extraction method and recovery rate depends on residue or waste type and bioactive compound of interest [13, 150, 151]. To enhance the efficiency of conventional extraction methods, deep eutectic solvents (DESs) and natural deep eutectic solvents (NADESs) have been suggested to replace organic solvents for extraction of bioactive compounds from agricultural waste biomass materials [13, 152, 153]. Also, the application of encapsulation and nanoemulsion to enhance the stability, bioavailability, and accessibility of derived bioactive compounds is being studied [13, 154, 155].

Figure 8.

A schematic representation of different techniques for extraction of bioactive compounds from food wastes and their health benefits [120].

Khaksar et al. [47] suggested the integration of metabolomics approaches into extraction of bioactive compounds from organic agricultural wastes to gain deeper understanding of the metabolic profile of agro-industrial wastes and enhance their value. Metabolomics approaches include liquid chromatography-mass spectrometry (LC-MS), gas chromatography-mass spectrometry (GC-MS), nuclear magnetic resonance spectroscopy (NMR), and high-performance liquid chromatography (HPLC). Metabolomics had enabled the discovery of over 10,000 different phenolic structures with diverse natures including phenolic acids, flavonoids, and tannins [47, 156].

3.6 Agro-industrial residues as resources for production of materials

Agricultural residues can also be valorized as substrates for production of insects as sources of fat and proteins [67]. Arthropods such as flies and maggots can grow on a variety of organic matter including animal manure, vegetable, and fruit wastes. These arthropods can be converted to animal flour of higher protein content and incorporated as sources of protein and fat in animal feed (livestock, aquaculture, pet) [67]. Larval digestion (bioconversion or biotransformation) of plant and animal materials yield high-value products such as organic fertilizers [67]. In addition, high-value products such as chitin, antibiotics, and peptides with bio-stimulant activity can be extracted for use in animal health and nutrition [67]. Other useful biocompounds derivable from agricultural residues include biopesticides [157, 158], pectin as edible food coating [159, 160], natural aromas [67, 161], quercetin, and flavonols [67, 162]. The replacement of non-recyclable raw materials for production of functional components in industries has become essential as a waste management strategy and in line with circular economic principles. Consequently, valorization of agricultural waste biomass and food wastes to functional materials is being promoted. Growth or culture media, packaging materials, biochar, biopolymers, bioplastics, single cell proteins (from microbial biomass), enzymes, organic acids, biofertilizers, and compost are materials derivable from agricultural residues [13, 67, 134, 163, 164, 165, 166, 167]. Biopolymers and bioplastics are highly biodegradable, biofunctional, biostable, and biocompatible and have wide applications in cosmetics, pharmaceutical, chemical, food, and beverage industries [13, 168].

3.7 Valorization of agro-industrial residues through activated charcoal production

Activated biochar or activated carbon or activated charcoal (AC) is a carbonaceous material of high surface area, and large pore volume, widely employed for adsorption of pollutants such as toxins, poisons, metals, and chemicals. Activated carbon production from agricultural residues and food wastes is viewed as a major valorization product with considerable and sustainable potential for application in various fields including bioremediation, water treatment, soil amendment, soil fertility, enhancement of soil biophysical and chemical characteristics, carbon sequestration, medical health, animal health, animal production, horticulture, and climate change mitigation [52, 169, 170]. Biochar production from agro-industrial and food wastes is particularly important in developing countries including Nigeria owing to its simplicity and wide applications. Adding activated carbon to soils could lock away carbon for centuries owing to slow microbial decomposition [171, 172, 173]. Minasny et al. [174] submitted that recycling stable crop residues as biochar substantially contribute to carbon sequestration in soils. Biochar addition to soil was shown to increase crop yield by about 25% on average [172, 175] attributed to enhanced cation exchange capacity, soil aggregation, and hydraulic conductivity [176]. In industrial waste water treatment, AC is widely employed for reduction of pollutants such as heavy metals and toxic chemicals (e.g., phenols and their derivatives) [177]. The surface chemistry of activated charcoal confers on it the ability to adsorb many gases, aqueous liquid, chemicals, and poisons [177, 178, 179]. Several studies showed that activated charcoal is harmless even when it is accidentally consumed, inhaled, or comes in contact with the skin. When mixed with water and swallowed to counteract poisoning, activated charcoal adsorbs the poison or drug, inactivating it and then carries it inert through the entire length of the digestive tract out of the body [180]. Majewska [181] reported a 3.5–5.0% increase in body weight and higher carcass and organ weights compared with control in broiler chickens on 3% dietary inclusion level of hard wood charcoal. The author attributed the results to the detoxifying effects of charcoal, lowered surface tension of the intestinal digesta, and enhanced liver function with respect to fat digestion. Jiya [182] supplemented activated charcoal at 0.5% in broiler feeds and noted increased relative organ weights and reduced cholesterol level in the carcass attributed to efficient mineral uptake and nutrient utilization. Drunna et al. [183] reported improved growth rate and reduced flatulence, fly population, and litter odor at varied inclusion levels of wood charcoal in feed of broiler chickens. Dim et al. [184] observed improved daily weight gain and feed conversion ratio in broiler chickens fed 6% dietary charcoal inclusion compared with other groups. Linhoss et al. [185] reported positive effects of biochar in litter amendment in broiler production while Schmidt et al. [186] reported that biochar has the potential to improve animal health, feed efficiency, and livestock housing climate; reduce nutrient losses and greenhouse gas emissions; increase soil organic matter content, and thus soil fertility. Zhang et al. [187] observed that biochar remediates organic pollutants by hydrogen binding, surface complexation, electrostatic attractions, and acid-base interactions; and heavy metals in soils by precipitation, surface complexation, chemical reduction, cation exchange, and electrostatic attraction. Biochar can improve cation exchange capacity, neutralize acidic soils, and enhance soil fertility [134, 188, 189]. It enhances organic solid waste decomposition by enhancing microbial population. It removes pollutants such as antibacterial drugs from water and wastewater [190, 191].

Traditionally, AC was produced from coal, lignite, petroleum residue, and hard wood biomass (fossil-related resources) [177, 178]. These materials are costly, exhaustible, and unfriendly to the ecosystem [177, 192]. Today, recyclable agro-industrial residues and food wastes are promoted as viable and sustainable alternatives being renewable, readily available, inexpensive, environmentally friendly, and an additional income to growers, farmers, and vendors [67, 177, 193]. Generally, the use of agricultural wastes such as corn cob, groundnut shell, poultry litter, rice husk, palm kernel shell, and coconut shell in the production of value-added products is gaining momentum [67, 173, 178, 194]. Activated carbon can hence be produced from all plant parts, animal manure, bones, fruit and tuber peels, husks, corn cob, stalk, straw, shell, and fruit stone [52, 67, 178]. These materials are broadly classified as woody materials composed of cellulose, hemicellulose, and lignin, and non-woody biomass composed of cellulose, hemicellulose, lignin, lipids, proteins, sugars, water, hydrocarbons, starch, and many other functional groups such as carbonyl, carboxylic, chromene, ethers, lactone, phenol, pyrone, and quinone groups, which contribute to the physicochemical properties and activity of the final product [178, 195]. Generally, choice of feedstock for AC production is informed by high carbon content, low inorganic matter, high density, and high content of volatile matter, ready availability, and low cost, low rate of degradation during storage, and high AC yield upon pyrolysis [173, 178, 195]. Studies on AC production from bioresources include tomato waste [196], corn cob [197], corn stalk [198], groundnut shell [199], palm kernel shell [200, 201], coconut shell [202, 203], chestnut oak shell [204], peanut shell [147, 205, 206, 207, 208], rice husk and straw [209, 210], apple waste [211], grape stalk [212], coffee grounds [213], palm oil mill residue [214, 215], oil palm empty fruit bunch [216, 217], bio-waste mixtures [218], food waste [177], poultry litter [203, 219, 220], pig dung [203, 221], dairy cattle carcass [222], cow dung [203, 223], and chicken feather [224].

The characteristics, properties, and performance of any AC sample would depend on the type of feedstock, temperature, and resident time of pyrolysis (carbonization) [134, 177, 178, 218], the activation process adopted: physical, chemical, or physicochemical [177, 218, 225], as well as the content of inorganic elements and other functional groups [177, 218]. These factors influence the internal pore structure of the AC, which determines its adsorbent capacity [177, 218]. Pores are classified as micropores (< 2nm), mesopores (2–50 nm), and macropores (> 50 nm) [177, 218, 226]. Figure 9 is a schematic representation of pore structure in activated carbon. The higher the internal pore structure, the more the surface area and adsorbent capacity of the AC [177, 178].

Figure 9.

Pore structures in activated carbon (AC) [227].

3.7.1 Production process of activated carbon

In principle, AC production involves physical or chemical treatment [178]. Physical treatment includes carbonization of feedstock in the absence of oxygen and in the presence of an inert gas (e.g., argon, Ar) followed by activation of the charcoal (biochar or carbon) using an oxidizing agent (steam, CO₂, or their mixture). Chemical treatment involves carbonization of feedstock to which had been added an activating agent (a strong dehydrating and oxidizing agent such as H₃PO₄, ZnCl₂, KOH, NaOH, and H₂SO₄ under nitrogen atmosphere. Thus, two principal steps: carbonization and activation, are involved in AC production [177, 178, 218, 228]. Equipment for AC production varies in sophistication depending on the degree of mechanization.

Carbonization aims to decompose the feedstock, eliminate non-carbon species (volatile matter, non-carbon elements, namely nitrogen, oxygen, hydrogen, sulfur; aromatics, etc.) and deposit biochar (charcoal or carbon) having essentially a fixed carbon content [178] and substantial pore structure. Reported pyrolysis (carbonization) temperature and resident time vary widely probably on account of differences in feedstock material, level of sophistication of equipment and automation, as well as other environmental conditions. Generally recommended temperature range is 400–900°C under inert atmosphere [75, 229, 230]. Budi et al. [231], however, reported pyrolysis of coconut shell in a kiln at 75–150°C for 6 h in argon atmosphere. Yasin and Pravinkumar [218] pyrolyzed wood pieces at 400–500°C, coconut shell at 320–400°C, and saw dust at 200–300°C. Yu et al. [177] using a vertical tube furnace pyrolyzed a blend of food wastes at 275–525°C for 30–120 min.

Activation is a key step in AC production, which aims to enhance the critical performance parameters of AC such as pore structure, pore volume, porosity, surface area, fixed carbon, and mineral contents. Activation can be by physical or chemical treatments. Physical activation involves treating biochar with an oxidizing gas or a combination of oxidizing gases such as oxygen, carbondioxide, and steam at high temperatures commonly in the range 500–1000°C [178]. Budi et al. [228] reported physical activation of biochar derived from coconut shell in argon gas furnace at 532, 700, and 868°C for 10–120 min. The activating gas opens previously formed pores blocked by tar or pyroligneous liquids formed during carbonization, creates new pores, and widens existing ones by removal of reactive carbon species, and other volatile contents of the biochar [177, 178]. The reaction(s) during physical activation could involve the below [178]:

In chemical activation, strong dehydrating and oxidizing chemicals are employed. These include alkali: potassium hydroxide (KOH) [177, 178], potassium carbonate (K₂CO₃) [153, 177, 178, 232], sodium hydroxide (NaOH) [177, 178, 233], and sodium carbonate (Na₂CO₃) [178]; alkali earth metal: aluminum chloride (AlCl₃) [178], and zinc chloride (ZnCl₂) [177, 178, 230, 231, 233, 234, 235, 236, 237, 238, 239]; and acid: phosphoric acid (H₃PO₄) [177, 178, 218], and sulfuric acid (H₂SO₄) [177, 178, 236]. The raw material for biochar production is usually impregnated with the appropriate activating agent prior to carbonization (pyrolysis). The activating agent acts as a dehydrating and/or oxidizing agent, promotes decomposition of the feedstock, inhibits deposition of tar and volatile contents [218] such as pyroligneous liquids, and enhances activated charcoal yield, carbon content, porosity, and surface area. At the end of the process, the resulting activated carbon is washed in acid or alkali depending on the activating agent used in order to remove remaining activating agent lodged in the pore structure [218]. An alternative procedure is to immerse biochar resulting from pyrolysis in the choice activating agent for a time duration of 24 h [231] followed by washing and drying. Esmar Budi et al, [231] reported the application of double activation, which involved chemical activation followed by physical activation using a horizontal furnace at 400°C in argon (Ar) gas (200 kg m⁻³) environment for I h. Whereas activated charcoal produced by physical activation is neater and requires no further washing that from chemical activation has higher surface area and pore volume. Chemical activation also requires lower temperature commonly of range 450–600°C [177]. Among the chemical activators, ZnCl₂ and H₃PO₄ are the most applied in the industry [177]. The atmosphere for chemical activation is either inert gas (e.g., Ar) or air [177].

A number of studies have evaluated the physicochemical properties of AC derived from a wide range of recyclable biological waste materials under varied pyrolysis conditions, activation agents, and time duration. Budi et al. [228] studied the effect of activation temperature (532, 700, and 868°C), and resident activation time (10, 60, and 120 min) under argon gas pressure (6.59, 15, and 23.4 kg/cm²) on pore structure and carbon content of coconut-shell-derived biochar and reported decreases in pore size but increases in pore volume and uniform pore distribution with increasing activation temperature, resident activation time, and gas pressure. Resident activation time alone did not influence pore size and pore distribution. The authors attributed the increase in pore volume to formation of new pores especially micro pores due to release of more volatile components from the biochar, as well as decreased pore size. Increase in pore volume increases the surface area of the activated carbon sample. The authors also reported increase in carbon content, which was attributed to the increase in pore numbers with increase in activation temperature, which caused the release of more volatile components. Esmar Budi et al, [231] evaluated the effects of chemical and physical activation on coconut-shell-derived charcoal. Chemical activation was performed with KOH (30, 40, 50, and 60%), NaOH (1, 2, 4, 7, and 11%), HCl (2, 4, and 6%), and H₃PO₄ (2, 4, and 6%) for 24 h while physical activation was with steam at 400°C in argon gas environment (200 kg m⁻³) for 1 h. There were increases in pore size; pore number initially increased but decreased as chemical concentration increased. Carbon content decreased with increasing chemical concentration. The decrease in pore number was linked to excessive widening of pre-formed pores, coalesce of pores, and collapse of carbon structure due to excessive chemical attack. The authors also noted an increase in total surface area, which was attributed to formation of new micro pores, and widening of existing pores. Yasin and Pravinkumar [218] investigated the effects of pyrolysis parameters and chemical (phosphoric acid, H₃PO₄) activation on properties of charcoal derived from some bio-waste materials. Biochar yield was higher with increasing pyrolysis temperature and resident time, and yield varied with type of starting material with coconut shell (56.66–63.33%) > saw dust (50.00–56.66%) > wood piece (46.66–53.33%%) at all temperatures and time duration. The result was attributed to higher loss of volatile components in the feedstock and higher oxidization of reactive carbon species at the optimal pyrolysis temperature and time duration. Activated charcoal adsorbent capacity was positively correlated with activation temperature and biochar from coconut shell had higher values than that from other materials (1.95–2.67 vs 1.73–1.97 vs 1.45–1.92 mg g⁻¹ for coconut shell vs saw dust vs wood piece). Bulk density did not vary clearly with activation temperature, but values were highest for wood piece (0.75–0.78 gm/cm³) compared with coconut shell (0.56–0.58 gm/cm³) and saw dust (0.18–0.21 gm/cm³). Moisture content followed a similar trend as bulk density being highest in wood piece (4.35–5.25%) compared with coconut shell (2.21–3.10%) and saw dust (1.07–1.25%). Volatile matter was highest in saw-dust-activated charcoal (12.60–18.10%) compared with wood piece and coconut shell (11.25–12.35 and 7.30–7.40%, respectively) while ash content was highest in biochar from wood piece (6.95–7.85%) compared with saw dust (5.95–6.15%) and coconut shell biochar (1.50–1.75%). Biochar from coconut shell had the highest content of fixed carbon (88.10–88.88%) compared with saw dust (74.53–80.18%) and wood piece (74.55–77.45%).High bulk density, moisture, and volatile matter are undesirable properties of activated charcoal. A good activated carbon should be highly porous (high total pore volume), of high fixed carbon content and surface area, which enables high adsorption capacity [177]. In a study that evaluated the physicochemical properties and adsorption capacity of biochar and physically (steam) activated biochar derived from a complex of edible food waste feedstock, Yu et al. [177] observed increased biochar yield with increasing pyrolysis temperature and longer pyrolysis resident time. Carbon content also increased with pyrolysis temperature, but this became stable after 60 min pyrolysis duration. Activated carbon yield reduced with higher activation temperature and longer activation time. Before activation, biochar surface area was 10 m² g⁻¹, micro pore volume was 0.004 cm³ g⁻¹, and total pore volume was 0.016 cm³ g⁻¹. These indices ranged between 288 and 745 m² g⁻¹, 0.072 and 0.196 cm³ g⁻¹, and 0.160 and 0.792 cm³ g⁻¹, respectively, after activation. The activation temperature and time duration for the highest activated carbon surface area (745 m² g⁻¹), micro pore volume (0.196 cm³ g⁻¹), and total pore volume (0.792 cm³ g⁻¹) were 950°C and 1 h, 850°C and 3h, and 950°C and 5 h, respectively. Thus, surface area and total pore volume increased with activation temperature while micro pore volume reached maximum value at 850°C. Biochar and activated carbon produced at 750°C had very small volume of meso and macro pores but higher activation temperatures (850 and 950°C) produced enlarged pores yielding more meso and macro pores. Longer activation time reduced surface area and micro pore volume but increased total pore volume at 950°C. Larger pores were obtained with increased activation time, thereby reducing number of micro pores. Because larger pores have relatively smaller surface area, converting micro pores to meso and macro pores will reduce activated carbon surface area. The authors reported that carbon content was 48.8% in the feedstock, 71.9% in derived biochar (pyrolysis temperature, 525°C for 2 h), and 68.8% after activation at 750°C for 3 h, 62.6% at 850°C for 3 h, 40.9% at 950°C for 3 h, 48.0% at 950°C for 1 h, and 33.2% at 950 for 5 h clearly showing reduced fixed carbon with increasing activation temperature and activation duration. Nitrogen (N), H, and S content followed a similar trend as fixed carbon. Sodium (Na), Ca, and P increased with activation temperature and time duration and were generally higher in activated sample than in biochar and feedstock. It does appear, hence, that increased activation temperature and time duration preserved the mineral content of biochar.

4. Conclusion and future perspective

With rising global human and animal population, demand for food, and food production will continue to increase leading to increases in waste generation and negative environmental challenges. Sustainable agricultural production and agro-industrial processing, environmental, human, animal, and climate health depend substantially on effective waste management. Circular agricultural production and bioeconomic agro-industrial waste management models are key to achieving the vision to considerably reduce waste generation, reuse wastes, and recycle wastes. Continued intellectual brainstorming and research will enable the arrival at the goal of turning wastes to wealth and an agricultural production system that produces nil wastes.