Biochar from Cassava Waste: A Paradigm Shift from Waste to Wealth

Abstract

Waste is unwanted material left after useful parts have been removed and found to affect our environment and health adversely. Waste from agro-allied industries is massive and claims most land, which would have been used for agricultural purposes when used as a landfill, including other environmental and health issues. This chapter will assess wastes generated during the processing of cassava for variety of products and review their properties when characterized. In the course of characterizing the wastes, which emerged during processing, pre-processing, and post-processing depending on the products, various reports on the physical, chemical, and biological properties of cassava wastes will be presented. The properties of cassava waste when subjected to biochemical and thermochemical processes will be compared with those of conventional raw materials for biochar production. This chapter will showcase the potential of cassava wastes for efficient valorization, especially as adsorbents via biochar. It will be of great significance to engineers, farmers, and manufacturers in their quest to manage cassava wastes for the betterment of our environment and health.

1. Introduction

Manihot esculenta Crants (Manihot utilissima phol), commonly known as cassava, tapioca, mandioca, and manioc, is regarded as the bread of the tropics as it is mainly grown and consumed in the tropical world [1, 2]. Its primary attraction is its tuberous root, which serves as one of the highest yielding starchy staples [3]. The root has been processed into varieties of food, including garri and fufu, among others. Specifically, Abiagom [4] reported that 15% of cassava was consumed as fresh roots; 5% as garri; 10% as starch; and 10% as flour and others. Similarly, other parts of the crop are equally valuable. The stem of the cassava plant is mainly exploited for propagation, while the leaves—found to be nutritious—are equally consumed.

Besides domestic consumption, cassava has recently been processed into many products of high demand, including ethanol, glucose, starch, animal feed, baking flour, pulp, and paper [5, 6]. Its application beyond consumption has increased as cassava is presently regarded as an industrial/cash crop [7]. The economic importance of cassava transcends the tropics as a staple food to a global industrial raw material for the production of myriad products [8]. Despite being exploited in a variety of ways for domestic consumption and industrial products, waste generation is inevitable and this occurs at almost every stage of the production process [9]. However, the benefits of cassava wastes are yet to be assessed as they are generally discarded and disposed of due to their toxicity [6].

The adverse impact of cassava wastes on the environment and health has been one of the challenges confronting the tropics. Acknowledging cassava waste as a major source of pollution in the cassava processing areas, Okunade and Adakalu [10] reported that cassava waste has deleterious effects on the receiving soil and water source as well as the adjourning environment. Research shows that as far as cassava processing is concerned, waste would be inevitably generated [9]. Since human life generally revolves around the activities that result in the production of cassava wastes, it is necessary to examine previous research works on the characterization of cassava waste and they have been chronicled in one piece for accessibility and posterity. Due to the toxicity of the waste [11] and its adverse impact on the environment, wastes generated from cassava processing need to be handled bereft of levity.

This chapter presents the prospect and challenges confronting cassava processing, especially in Nigeria: the world’s largest producer of cassava (Section 2). Waste is inevitable in most industrial processes and cassava processing is not an exemption. The estimated quantity of wastes generated during cassava processing as well as the nature of these wastes is considered next (Section 3). This review is necessary for better management, as the availability of the waste for valorization is paramount. Furthermore, a review on the characteristics of the wastes constituting Section 4 of this chapter showcased the biochemical and physicochemical properties of cassava processing wastes. The concluding section (Section 5) will be on biochar production using cassava waste and will focus on process parameters and the choice of feedstock. This information is a useful guide in our quest to manage cassava wastes for the betterment of our environment and health as well as reveals the potential of these wastes for efficient valorization.

2. Prospect and challenges of cassava processing in Nigeria

This section presents the prospect as well as challenges of processing cassava in Nigeria. Nigeria is the largest producer of cassava in the world for decades, and has a robust cassava industry with prospects and challenges. It is expected that other cassava-producing countries, such as Brazil and Thailand, among others, may have similar challenges. The progress in cassava production in Nigeria is reported next.

2.1 Progress in cassava production in Nigeria

Cassava has been identified as one of the most cost-effective and nutritionally vital native African tubers [12, 13, 14]. Its origin is traced back to South America, and the crop made its trans-Atlantics journey close to the beginning of the slave trade in the sixteenth century into Nigeria [15]. Cassava is a recurrent, vegetative bred shrub, and is cultivated through the plain tropics [16]. It is a dearth resilient crop grown mostly in temperate areas and adds appreciably to the nourishment and livelihood of many countries, including Nigeria. Cassava is one of the major staple foods in Nigeria, and its cultivation is a priority in almost every household, especially in the Southern part of the country [8, 15]. Hence, it is the most extensively farmed crop in Nigeria and it is largely cultivated by small-scale farmers that depend on seasonal rainfall [17].

Presently, cassava has been changed from a low-yielding dearth spare crop for consumption to a high-producing cash crop, with its many different uses in livestock feeds, source of raw materials for agro-industry, and beyond [18, 19]. Over half a billion people around the world depend on cassava as a major food source. It is the third largest source of calories after rice and corn [5, 6, 20]. The crop is known to thrive well on any soil and its ability to grow well in poor soils and withstand drought makes it an ideal crop to cultivate in places where other crops struggle [21, 22].

Fortunately, Nigerian soil is fertile and the crop thrives very well making Nigeria the world’s largest producer of cassava for about a decade. The country went from harvesting 36 million tons of cassava in 2003 to 53 million in 2013, a 47% increase [15]. Growth was driven by a substantial increase in yields—which jumped by 44% over this period as Nigeria overtakes Brazil as the top producer of cassava. This increase is attributed to several interventions and initiatives, including the Institute of Tropical Agriculture (IITA), the presidential initiative on cassava, etc., by the Nigerian governments at all levels. It was reported that through these programs large hectares of land were dedicated to cassava cultivation in order to boost the production and exports of processed cassava products [23, 24]. With 59.5 million tons of cassava produced in 2019 (Figure 1), the country maintained its top spot in global cassava production since it outpaced Brazil in total production output in 1991 [26, 27].

Figure 1.

Top cassava-producing countries (in million tons). Data are adapted from PwC [25].

The plant as a whole is useful as its roots and leaves are consumed, while the stems are mainly exploited for propagation. About 70% of Nigeria’s cassava is processed into garri—a granular flour that is used mainly to make porridges and fufu—a type of mash [22]. Specifically, the roots are processed by several methods to form products that are used in diverse ways according to local preferences [28, 29]. However, when left unprocessed, the cassava roots perish quickly, often spoiling within 48 hours [30]. Processing offers not just the ability to produce higher-value, exportable cassava-derived items like garri and essential items, such as glucose, starch, and flour, but also to preserve the root (Figure 2).

Figure 2.

Potentials of cassava root. Data are adapted from PwC [25].

Meanwhile, Thailand, the second-largest producer of cassava, considered the crop more of a cash crop than a staple food with the vast majority of the root being processed and exported [31, 32, 33]. Currently, Thailand is the world’s leading country in the exportation of cassava-based products (Figure 3). It is reported that in Thailand, about 90% of all cassava is processed and exported [22]. Thailand accounted for approximately 76% of global trade and 84% of its cassava exports were sent to China, where the products are largely used for ethanol production [34]. Specifically, over 6.4 million tons of cassava-based products were exported from Thailand accounting for about 50% of the total volume of exported products worldwide generating huge revenue for the country [22, 27].

Figure 3.

Top cassava exporting countries (in million tons). Data are adapted from PwC [25].

2.2 Challenges of cassava processing in Nigeria

In Nigeria, about 80% of the cassava is processed into garri for local consumption and exportation [22]. Specifically, processing cassava into garri is relatively simple—it requires only that cassava roots are peeled, grated, and sieved, and then placed in a porous bag from which excess water can be squeezed out (Figure 4). The resulting dry flour can be stored for several months [29]. Approximately 70% of cassava processing occurs at small- and medium-size centers near villages.

Figure 4.

Cassava processing facility in Nigeria.

In 2012, it was reported that there were 75,000 total small- and medium-processing centers that employed roughly 3 million people—most of which were small-scale farmers, and generated less than 5 tons of high-quality cassava flour per day [25]. However, medium- and large-scale processors struggle to stay afloat due to high transportation costs, mainly due to the poor condition of rural Nigerian roads [16]. The challenge of limited access to cassava processing facilities has not only hindered efficient large-scale processors, but also extends to storage facilities as unreliable transportation compounds the problem presented by the crop’s perishability. Consequently, post-harvest losses for cassava are high as estimated by Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ) and found to be more than $600 million annually in Nigeria alone [22, 35].

The challenges confronting Nigeria’s cassava processing capacity are responsible for the country’s poor role in international cassava trade as large-quantity cassava needs to be processed in order to be exported. Thailand’s position as the global leader in cassava processing and exportation is a clear indication that Nigeria—as the largest producer—is not doing well in the area of processing. Nigeria’s emergence in the international cassava market coupled with achieving self-sufficiency in cassava-based products, such as starch, glucose, and flour production, is not out of reach. The country has already proven success in improving raw cassava production through its increase in yields and needs to extend these successes throughout the value chain. The economic potential of cassava is huge as seen in Figure 5.

Figure 5.

Current demand for cassava-based products in Nigeria. Data are adapted from PwC [25].

Specifically, the country has the huge economic potential to generate USD 427.3 million from domestic value addition and derive an income of USD 2.98 billion in the exportation of cassava-based products [25, 36]. According to PwC [25], the local addition to cassava via local manufacturing and processing could potentially unlock about USD 16 million as revenue for Nigeria.

Realizing the incredible amount of untapped potential that lies waiting in this sector, Nigerian government has waded in to address the challenges confronting cassava processing. In an attempt to support the cassava processing industry, the government launched the Cassava Transformation Agenda in 2011 [22]. The initiative is working to expand the cassava value chain and Nigeria is making millions from cassava production and export. It was reported that Nigeria exported 509 tons of cassava products, half of which went to China, the world’s top importer of the product in 2019 [8, 36]. Several medium- and large-scale cassava processing facilities are set up on a daily basis and processing clusters are domiciled in every community as seen in the Ojapata processing cluster, Kogi state, and Nigeria [8, 37].

Although the commercialization of cassava processing and subsequent exportation and expansion of its value chain generate huge revenue and job opportunities for Nigerians, this development is welcomed with mixed feelings due to poor waste management (Figure 6) and its adverse impact on the environment [38]. The cassava processing environment (Figure 4) is heavily polluted in all ramifications (air, water, and land) and the impact of environmental pollution attributed to cassava wastes is significant, especially with commercial processing of the cassava.

Figure 6.

Cassava processing waste (cassava peel).

3. Cassava processing and quantity of wastes generated

Processing cassava into varieties of products inevitably result in the generation of wastes. Basically, according to Ubalua [37] and Zhang et al. [38], these wastes are categorized into (i) peels prior to crushing, (ii) sieved fibrous residue after crushing, (iii) bagasse and settling starch, and (iv) wastewater effluent. In view of this, cassava processing facilities continuously generate waste as the demands for cassava-based products soar. Chunk amount of cassava processing wastes and residues generated has been reported to be one of the major environmental threats, especially in rural regions of developing countries [39, 40]. These regions are mainly dominated by small-scale processing facilities managed mainly by rural dwellers bereft of standardized waste treatments and disposal strategies. The informed operators of these facilities considered the cost of treatment and disposal of these wastes a huge financial burden.

Zhang et al. [38] estimated that the processing of fresh cassava roots generates liquid waste between 8.85 and 10.62 MT per MT of fresh cassava processed, containing approximately 1% total solids (TS). In the case of dry cassava processed, the authors reported that between 0.93 and 1.12 MT of wet cassava bagasse and peels are produced per metric ton of dry cassava processed.

A breakdown of wastes generated from fresh cassava root during the production of high-quality cassava flour (HQCF), starch, garri, and fufu are presented in Figures 7–10 respectively. Specifically, during the production of 150–200 kg HQCF from 1 MT of fresh cassava roots, 550–700 kg of wastes was generated constituting peels, fibrous waste, sifting juice, and wastewater (Figure 7).

Figure 7.

Flow sheet of high-quality cassava flour production process. Data are adapted from Sanni and Jaji [7]; FAO [41].

Figure 8.

Flow sheet of the starch production process and waste generated. Data are adapted from FAO [41].

Figure 9.

Flow sheet of garri production process and waste generated. Data are adapted from Sanni and Jaji [7]; FAO [41].

Figure 10.

Flow sheet of fufu production process and waste generated. Data are adapted from Sanni and Jaji [7]; FAO [41].

A recent report stated that for every ton of cassava processed, 10–15% constituting 125 kg/tons are lost in form of wet peels, which are poorly utilized, dumped as waste, or burnt [42, 43]. These methods of disposing cassava wastes though easy and cheap are not economically viable in terms of lands claimed and environmental-friendly due to pollution.

Starch is one of the cassava-based products with high demand in several industries, including laundry, and the flow sheet for its production process is presented in Figure 8. FAO [41] reported that 1 MT of fresh cassava roots when processed can produce between 180 and 200 kg starch with about 680 kg of waste generated.

Currently, garri (cassava flake) is the most preferred cassava-based product widely recognized as a staple food in the tropics. Figure 9 presents the flow sheet of garri production process. However, it is reported that of the 200–240 kg of garri produced from 1 MT of fresh cassava roots, and 500–600 kg of waste was generated [7, 41].

Fufu is another cassava-based product known mostly in southern Nigeria. In terms of consumer preference, fufu was the most preferred in the early 1970s with more than 60% of cassava exploited for its production [21]. However, by the early 1980s, the consumption of fufu had declined to 14% of all cassava eaten, while consumption of garri rose to 65% according to a national consumption survey by the Federal Office of Statistics (FOS) [44]. It is considered that the consumer preference for fufu had reduced due to its inherent undesirable characteristics of poor odor, short shelf life, and tedious preparation [27, 29]. The flow sheet for the production of fufu is presented in Figure 10. It has been found that 1 MT of fresh cassava roots produced between 280 and 300 kg of fufu generating between 80 and 130 kg of waste [41].

4. Characteristics of cassava wastes

The properties of wastes generated from cassava processing constituting the peels prior to crushing, the sieved fibrous residue after crushing, the bagasse and settling starch, and the wastewater effluent are presented in this section. The physicochemical properties of the wastes are critical as they reveal the way in which the wastes interact with other substances physically and chemically when discharged. Section 4.1 presented the physicochemical characteristics of these wastes. The thermochemical properties of the wastes are presented in Section 4.2. The biochemical properties of the wastes are equally useful as the wastes when discharged are expected to interact with the fauna and flora content of the medium as well as the environment. This is discussed in Section 4.3.

4.1 Physicochemical characteristics of cassava wastes

These are the intrinsic physical and chemical characteristics of cassava wastes. These include appearance, boiling point, density, toxicity, volatility, water solubility, and flammability,. In the case of cassava wastes, several studies have been conducted by various researchers, and the outcome of some of the works is presented in this section. Zhang et al. [38] reported that cassava starch wastes (wastewater and solid waste) are weakly acidic liquids with high nitrogen and phosphorus contents of about 1300 and 780 mg/L, respectively. In addition, this category of cassava waste contains between 9.6 and 37.5 g/L of total carbohydrates and 2.3 total proteins. Table 1 presents the physicochemical properties of cassava starch wastes as well as their composition.

Parameters/properties	Values (G/L)
pH	3.6–6.2
Total solid (TS)	4.5–38.2
Volatile solid (VS)	3.4–33.0
Total chemical oxygen demand (TCOD)	8.0–66.2
Soluble chemical oxygen demand (SCOD)	14.2–345
Biochemical oxygen demand (BOD)	—
Total carbohydrate	9.6–37.5
Solid carbohydrate	—
Oil and grease	0.6
Total protein	2.3
Total nitrogen	0.1–1.3
Total phosphorous	0.07–0.78

Table 1.

The physiochemical characteristics of cassava wastes.

Data are adapted from Zhang et al. [38].

In the case of cassava bagasse, Zhang et al. [38] further reported that a typical solid residue of cassava processing contains between 40.1 and 75.1% starch (dry weight) and between 14.9 and 50.6% fiber. Table 2 shows the composition of the cassava residue.

Composition	% by dry weight
Starch	40.1–75.1
Crude fiber	14.9–50.6
Cellulose	4.1–11.4
Hemicellulose	4.2–8.3
Lignin	1.2
Crude fat (lipids)	0.5–1.1
Crude protein	0.3–1.6
Total ash	0.7–11.9
Total solid	—
Volatile solid	—
Total nitrogen	—

Table 2.

Cassava residue composition % by dry weight.

Data are adapted from Zhang et al. [38].

Wastewater is inevitably generated during cassava processing either as a byproduct of the initial production process or arises when the cassava tubers are indiscriminately discharged to a nearby water body. Okunade and Adekalu [10] reported on the organic components of cassava wastewater (Table 3) as they found that cassava wastewater, which is five times denser than water contains alcohols, acids, and others (3-penten-2-ol, 1-butanol, 3-hexanol, octadecanoic acid, oleic acid, n-hexadecanoic acid, acetoin, dibutyl phthalate, squalene, and bis (2-ethylhexyl) phthalate). The rust-removing properties of the wastewater from metallic substances, such as nails, are attributed to the presence of these organic compounds [10].

Component	Concentration in ppb
3-penten-2-ol	276.007
1-butanol	259.561
3-hexanol	95.897
Octadecanoic acid	495.085
Oleic acid	135.546
n-hexadecanoic acid	71.417
Acetoin	362.956
Dibutyl phthalate	140.801
Squalene	76.9188
Bis (2-ethylhexyl) phthalate	73.686

Table 3.

Concentration of organic compounds in cassava wastewater.

Data are adapted from Young and Markmanuel [35].

In the related development, Aripin et al. [45] have recognized that without proper waste management, and the organic wastes like cassava peels could result in an increased amount of solid waste dumped into landfills. This eventually leads to less soil available for agricultural purposes as most of these soils are rendered infertile, in an attempt to utilize these organic wastes as pulp for paper-making industries and to promote the concept of “from waste to wealth and recyclable material.” The authors exploited Kurscher-Hoffner and Chlorite methods to determine the chemical properties of the wastes in accordance with the relevant Technical Association of the Pulp and Paper Industry (TAPPI) test. It was found that the cassava waste was rich in holocellulose, cellulose, hemicellulose, lignin, and ash content with 1% of sodium hydroxide and hot water solubility (Table 4).

Constituents	W/W oven-dried materials
Holocellulose	66.0
Cellulose	37.9
Hemicellulose	37.0
Lignin	7.5
1% NaOH	27.5
Hot water	7.6
Ash	4.5

Table 4.

Chemical composition of oven-dried cassava peels.

Data are adapted from Aripin et al. [45].

In order to determine the suitability of cassava peel as an alternative fiber resource in pulp and paper making, its properties were compared to other published literature, especially from wood sources. Aripin et al. [45] reported that the amount of holocellulose contents in cassava peels (66%) is within the limit suitability to produce paper although it is the least when compared with that of the wood (70–80.5%) and canola straw (77.5%). Similarly, the lignin content (7.52%) is the lowest than those of all wood species (19.9–26.22%). However, the morphological properties of the cassava peel are promising as the authors went further to subject the peels to scanning electron microscopy (SEM) under different magnifications (Figures 11 and 12).

Figure 11.

Surface morphology (SEM) of cassava peel at ×27. Data are adapted from Aripin et al. [45].

Figure 12.

Surface morphology (SEM) of cassava peel ×300. Data are adapted from Aripin et al. [45].

Aripin et al. [45] observed that under different levels of magnifications (27 and 300), there exist differences in fiber morphology of the peels (Figures 11 and 12). The surface morphology of cassava peels depicted in Figure 11 shows that it is dominated by a low (micro) pore size structure, while that of Figure 12 differs. The differences in fiber morphology of the peel indicate variation in the major character of the fiber’s physical structure. This variation had been reported to be attributed to differences in the physical properties of the cassava peels [46]. The porous structure of the peel can be exploited for several industrial applications.

4.2 Thermochemical characteristics of cassava wastes

The thermochemical properties of agricultural wastes have been a subject of research interest recently. One of these was reported by Pattiya [47] on cassava wastes. The study includes proximate, ultimate, structural, inorganic matter, heating value, and thermogravimetric analyses. Cassava waste was found as shown in Table 5 to have high volatile contents (78–80%, dry basis) and contains 51% carbon, 7% hydrogen, 41% oxygen, 0.7–1.3% nitrogen, and <0.1% sulfur. Structural analysis reveals that cassava residues are composed of about 36% cellulose, 44% hemicellulose, and 24% lignin. The main inorganic elements found are potassium, phosphorus, and calcium. The lower heating values (LHV) of the biomass are approximately 18 MJ kg⁻¹.

Composition	Value (% dry basis)
Volatile organic contents	78.0–80.0
Carbon	51.0
Hydrogen	7.0
Oxygen	41.0
Nitrogen	0.7–1.3
Sulphur	<0.1
Cellulose	36.0
Hemicellulose	44.0
Lignin	24.0
Heating Values	18 MJ kg^−1

Table 5.

Proximate, ultimate, structural, inorganic matter, heating value, and thermo-gravimetric analyses of cassava wastes.

Data are adapted from Pattiya [47].

Similarly, Aro et al. [48] carried out a similar study on cassava tuber wastes (CTW) produced by a cassava starch-processing factory in the Ondo State of Nigeria. They investigated the properties of five different types of CTW wastes: cassava starch residues (CSR) or pomace cassava peels (CAP), cassava effluent (CAE), cassava stumps (CAS), and cassava whey (CAW). The proximate composition of samples collected in respect of these five types of wastes (Tables 6–8) showed that moisture was the highest in CAW (96.7%) and the lowest in CAS (64.1%). Crude fiber was highest in CAP (29.6%) but was not detected in the whey (CAW). The CAS had the highest content of fat (5.35%), while it was not detected in CAW. Protein was the highest in CAP (4.20%) and lowest in CSR (1.12%). Ash content was the highest in CAP (7.47%) and lowest in CAW (1.88%). The nitrogen-free extractives (NFE) were the highest in CAW (95.7%) and lowest in CAP (55.5%).

Parameters	CAP	CAE	CAW	CSR	CAS
Dry matter	17.9	8.63	3.34	15.8	35.9
Crude protein	4.20	2.92	2.46	1.12	1.71
Crude fiber	29.6	6.69	ND	19.3	12.9
Fat	3.26	1.75	ND	2.37	5.35
Ash	7.47	3.16	1.88	2.84	3.39
Moisture	82.1	91.4	96.7	84.2	64.1
N.F.E.*	55.5	85.5	95.7	74.4	72.3

Table 6.

Proximate composition (g/100g D.M.) of different types of fresh cassava tuber wastes (CTW) collected from the factory.

*

N.F.E. = nitrogen free extractives, CAP = cassava peels, CAE = cassava effluent, CAW = cassava whey, CSR = cassava starch residues, CAS = cassava stumps, ND = not detected, and D.M. = dry matter. Data are adapted from Aro et al. [48].

Parameters	CAP	CAE	CAW	CSR	CAS
Potassium	269	67.7	42.7	138	117
Calcium	122	50.2	5.17	60.0	15.2
Magnesium	236	33.2	22.7	129	117
Iron	14.7	16.2	2.21	5.66	2.46
Manganese	0.43	ND	ND	ND	ND
Copper	0.24	0.13	ND	10.8	0.19
Molybdenum	ND	ND	ND	ND	ND
Cobalt	ND	ND	ND	ND	ND
Arsenic	ND	ND	ND	ND	ND
Selenium	1.16	0.42	ND	1.27	0.46
Sodium	261	24.0	7.54	93.4	46.9
Phosphorus	3233	2517	251	2251	1663

Table 7.

Mineral composition (mg/kg D.M.) of fresh cassava tuber wastes (CTWs) collected from the factory site.

Data are adapted from Aro et al. [48].

Parameters	CAP	CAE	CAW	CSR	CAS
Cyanide, mg/kg	32.9	54.4	61.1	15.5	34.8
Phytate, mg/kg	8238	4264	3947	15930	9276
Oxalate, mg/kg	330	674	520	270	610
Tannins, %	3.90	2.16	0.98	270	3.44
Saponin, %	0.06	0.08	0.02	2.53	0.15
Total alkaloids, %	0.48	0.40	0.16	0.10	0.52

Table 8.

Antinutrient composition of cassava wastes (dry matter basis).

Data are adapted from Aro et al. [48].

The proximate analysis of cassava waste conducted by Janz and Uluwaduge [49] is presented in Table 9, while that of Obadina et al. [50] on cassava peel is presented in Table 10.

Parameters	Values (g)
Moisture	59.4
Total carbohydrate	38.1
Protein	0.7
Lipid	0.2

Table 9.

Proximate analysis of cassava waste on the basis of 100 g.

Data are adapted from Janz and Uluwaduge [49].

Composition	Value (% dry basis)
Carbohydrate	42.6
Protein	1.6
Ether extract	12.1
Total ash	5.0
Crude fiber	22.5

Table 10.

Proximate analysis of cassava peel.

Data are adapted from Obadina et al. [50].

4.3 Biochemical characteristics of cassava wastes

Apart from the nutritional value of cassava, the chemical oxygen demand (COD), biochemical oxygen demand (BOD), and cyanide content are the parameters of interest because of their effect on the flora and fauna as well as the environment. Based on cyanide contents, cassava can be classified as sweet and bitter and it wastes are often laden with suspended solids, high COD, and BOD making them toxic. Some of the works carried out in this direction show that one liter of cassava wastewater has 23.9 g of COD, 23.1 g of volatile solids (VS), and 22.9 g of total solids (TS) [51, 52].

In the case of nutritional value, some of the works presented earlier (vide supra) indicated the presence of carbohydrates, proteins, and other nutritional components that support life. Dresden [11] conducted research on the nutritional profile of cassava waste and reported its nutritional composition as presented in Table 11.

Nutritional composition	Value
Calories	330 kcal
Protein	2.8 g
Carbohydrate	78.4 g
Fiber	3.7 g
Calcium	33.0 mg
Magnesium	43.0 mg
Potassium	558.0 mg
Vitamin C	42.4 mg
Thiamine	0.087 mg
Riboflavin	0.048 mg
Niacin	0.854 mg

Table 11.

The nutritional profile of cassava waste.

Data are adapted from Dresden [11].

Glanpracha et al. [53] reported on the cyanide content of cassava waste as presented in Table 12. The cyanide content includes hydrocyanic acid (HCN) and cyanogenic glucoside (linamarin).

Properties/parameters	Values
Cyanide (cyanohydrins), hydrocyanic acid (HCN), and cyanogenic glucoside (linamarin)	45–154 mg
Fibers	—
Protein	—
C/N	17.64 ± 3.1
Total nitrogen (% dry weight)	0.3 ± 0.1
Total organic carbon (% dry weight)	52.1 ± 3.2
Volatile solids (% wet weight)	19.6 ± 0.8
Total solids (% wet weight)	20.9 ± 0.9
Moisture content (% wet weight)	78.9 ± 0.7

Table 12.

Biochemical composition of cassava waste.

Data adapted from Glanpracha et al. [53].

Mégnanou et al. [54] conducted a study on the physicochemical and biochemical characteristics of nine varieties of cassava roots (V4, V23, V60, V61, V62, V63, V64, V65, and V66). The mean values of their physicochemical and biochemical characteristics are presented in Table 13.

Parameter	Mean ± SD
Cyanide (mg/100 g)	10.15 ± 4.36
Dry matter (g/100 g)	33.70 ± 5.85
Moisture (g/100 g)	66.31 ± 5.83
Carbohydrate (g/100 g)	30.53 ± 6.16
Starch (g/100 g)	12.40 ± 8.357
Energy value (kcal/100 g)	142.48 ± 22.60
Reducing sugar (g/100 g)	1.09 ± 0.48
Ash (g/100 g)	0.44 ± 0.18
Acidity (meq/100)	82.19 ± 21.33
Total sugar (g/100 g)	16.75 ± 6.08
Fat (g/100 g)	1.55 ± 1.11
Ph	5.95 ± 0.12
Carotenoid (mg/100 g)	0.11 ± 0.06
Vitamin A (mg/100 g)	17.89 ± 15.42
Protein (mg/100 g)	1.61 ± 0.70
Vitamin C (g/100 g)	6.89 ± 2.17

Table 13.

Physicochemical and biochemical characteristics of nine cassava varieties.

Data adapted from Mégnanou et al. [54].

In a similar development, Izah et al. [55] investigated the heavy metal content of cassava mill effluents (cassava processing wastes) collected from a cassava processing mill at Ndemili in Ndokwa west local government area of delta state, Nigeria. It was found that the effluent contains 1.46 mg/l of copper, which is comparable to the value 1.83 mg/l reported by Orhue et al. [56], 1.91 mg/l reported by Adejumo and Ola [57], and lower than the value of 2.50 mg/l as reported by Patrick et al. [58] as well as 2.60 mg/l by Olorunfemi and Lolodi [59] and higher than the value of 0.00 mg/l reported by Omomowo et al. [60]. Another heavy metal present was zinc with a concentration of 4.35 mg/l. This is comparable to the value of 4.1 mg/l reported by Patrick et al. [58] and lower than the value of 5.90 mg/l reported by Olorunfemi and Lolodi [59] and higher than the value of 1.07 mg/l reported by Orhue et al. [56] as well as 0.00 mg/l as reported by Adejumo and Ola [57].

Manganese was equally found with a concentration of 4.64 mg/l, which is lower than the value of 0.71 mg/l reported by Adejumo and Ola [57], as well as 0.00 mg/l by Omomowo et al. [60] and lower than the value of 7.10 mg/l reported by Olorunfemi and Lolodi [59]. About 28.27 mg/l of iron was reported to be found in the effluent, which is far higher than the value of 2.35 mg/l reported by Adejumo and Ola [57], as well as 2.30 mg/l reported by Omomowo et al. [61] and 2.00 mg/l by Orhue et al. [56] and lower than the value of 30.9 mg/l reported by Olorunfemi and Lolodi [59]. The study further revealed the presence of 0.18 mg/l of chromium, which was lower than the value of 1.14 mg/l reported by Olorunfemi and Lolodi [59].

Generally, the arbitrary variation (with no trend) in the heavy metal concentration (Table 14) could be attributed to the age of the cassava prior to processing, activities leading to individual heavy metals disposition in the plantation where the cassava was cultivated, and possible leaching of metals from the processing equipment.

Heavy metals	Izah et al. [55]	Omomowo et al. [60]	Orhue et al. [56]	Adejumo and Ola [57]	Patrick et al. [58]	Olorunfemi and Lolodi [59]
Cu, mg/l	1.460 ± 0.460	0.00	1.83	1.91	2.50	2.60
Zn, mg/l	4.353 ± 0.365	NA	1.07	0.0	4.10	5.90
Mn, mg/l	4.637 ± 0.195	0.0	NA	0.71	NA	7.10
Fe, mg/l	28.270 ± 1.130	2.30	2.00	2.35	NA	30.9
Cr, mg/l	0.180 ± 0.020	NA	NA	NA	NA	NA
Ni, mg/l	1.810 ± 0.110	NA	NA	NA	NA	NA

Table 14.

Heavy metal contents of various effluents from cassava processing mill.

NA: not available.

From the foregoing, it is obvious that a huge quantity of waste from cassava processing would unavoidably be generated irrespective of the cassava-based products of interest. The waste constituents are not all toxic as researchers found that in it (the waste) contains 11% of the crop energy coupled with valuable mineral nutrients [61, 62, 63, 64]. These valuable contents of the waste can be exploited to boost the economic potential of cassava processing. Thus, the characteristics of cassava wastes as reviewed are within the range, presented in Table 15.

Parameters	Range of value
C/N	17.64–30.0
Total solid (TS)	4.5–38.2 mg
Volatile solid (VS)	3.4–33.0 mg
pH	3.6–6.2
Total chemical oxygen demand (TCOD)	8.0–66.2
Soluble chemical oxygen demand (SCOD)	14.2–345
Total carbohydrate	9.6–37.5 mg
Cellulose	36.0–43.2 mg
Hemicellulose	44.0–64.4 mg
Lignin	24.0–46.2 mg
Cyanide	45–154 mg

Table 15.

Parameters of interest.

5. Biochar from cassava wastes: parameters and choice of feedstock

Characterizations of cassava wastes are necessary for efficient valorization. Through characterization, it is obvious that embedded in the wastes are fractions of the crop energy and minerals. The huge quantity of waste generated during cassava processing translates to a huge quantity of energy and minerals that need to be recovered. The cassava wastes can be converted into biogas (energy recovery) as well as digestate filtrate and residue for biofertilizer, bio-oil, and biochar (mineral recovery) [65]. Several studies have been carried out on bio-oil production using cassava wastes but much has not been reported on biochar from cassava wastes [66, 67]. A brief description of biochar properties would be beneficial to identify its applications.

Biochar is a carbon-rich product obtained when biomass (cassava waste) is heated in a closed system with restricted oxygen. Structurally, it is similar to charcoal but with different properties. However, biochar has a high surface area (highly porous) and negative surface charge, and charge density [68]. Due to its superlative adsorption properties, biochar has been extensively used as an adsorbent [69]. Biochar as adsorbent finds application in removing “emerging contaminants” from flue gas and wastewater. It is equally applied to soil to improve soil properties as biochar can hold nutrients and become more stable than most fertilizer or other organic matter in soil [69].

Biochar (pyrochar) is a solid product from the pyrolysis process. Of all the thermochemical conversion processes (combustion, incineration, etc.), pyrolysis offers a great opportunity of transforming wastes into wealth. Varieties of biomass, including cassava wastes, are exploited as feedstock to produce valuable gas, liquid, or solid products, including biochar. Research has shown that pyrolysis is relatively environmentally friendly when compared with its counterparts as it produces low emissions [70]. During the production process, it was reported that biochar is able to scrub carbon dioxide, nitrous oxides, and sulfur dioxide from the flue gas constituting greenhouse gases (GHG) [68]. These gases (GHG) contribute immensely to global warming leading to climate change with heat waves, flooding, and typhoons [71, 72, 73]. Scrubbing GHG during biochar production can be harnessed as a potential tool to slow global warming [74].

Pyrolysis process is versatile—fast or slow depending on residence time—and can be optimized to enhance the production of desired product. Fast pyrolysis with residence time in seconds generates more liquid products (bio-oil), while slow pyrolysis with residence time in hours favors more solid products (biochar) [75, 76, 77]. In addition, the properties of biochar produced are varied by the pyrolysis parameters and choice of feedstock.

Besides residence time, temperature and heating rates equally influence the yield and composition of the pyrolysis products. The temperature and heating rates are two of the pyrolysis parameters that affect the yield and composition of the pyrolysis [70, 78]. Noor et al. [79] reported that temperature has a more significant influence than the heating rate during biochar production. Although the nature of feedstock determines the fixed carbon content in the biochar produced, it was found that a higher pyrolysis temperature increased more fixed carbon in the biochar than a higher heating rate [79, 80]. Thus, the effect of production parameters is significant. However, the choice of feedstock on the quality of biochar is equally important.

We extensively reviewed the complementary role of thermochemical conversion process after subjecting the wastes to a biochemical (anaerobic) process [81]. Anaerobic digestate of cassava waste is a valuable pyrolysis feedstock for biochar production due to its high volatile matter content, low ash, and sulfur content [81]. Consequently, thermally treated digestate is more suitable than any other materials subjected to pyrolysis for biochar production. Meanwhile, cassava plantation residues: cassava stem and cassava rhizome have been exploited for biochar production [79, 80]. However, the biochar produced from cassava wastes contains a high percentage of fixed carbon, which is about five to eight times higher than that from cassava plantation residues.

Cassava irrespective of its components can be exploited for biochar production via pyrolysis. Three categories of cassava waste can be exploited as pyrolysis feedstock for biochar production (cassava plantation residues, cassava processing waste, and digestate from anaerobic digestion of cassava processing waste). The quality of biochar produced depends on the process parameters and choice of feedstock. Slow pyrolysis when optimized produces high-quality biochar suitable for several applications [79, 80]. Nevertheless, digestate from anaerobic digestion of cassava waste is the most preferred pyrolysis feedstock. This is followed by cassava processing wastes and the least is the cassava plantation residues, especially the stem, this is subject to further investigations. Besides the production of high-quality biochar, the digestate from cassava processing waste has been effectively exploited for biogas and biofertilizer production.

6. Conclusion

This review has established that waste generation during cassava processing is inevitable irrespective of the cassava-based products. In the course of characterizing the wastes, which emerged during processing, pre-processing, and post-processing depending on the products, various researchers reported that the physical, chemical, and biological properties are within the range as paired: carbon/nitrogen (17.64–30.0), total solid (4.5–38.2 mg), volatile solid (3.4–33.0 mg), pH (3.6–6.2), total chemical oxygen demand (8.0–66.2), soluble chemical oxygen demand (14.2–34.5), total carbohydrate (9.6–37.5 mg), cellulose (36.0–43.2 mg), hemicellulose (44.0–64.4 mg), lignin (24.0–46.2 mg), and cyanide (45–154 mg).

It can be confirmed that the cassava waste through toxicity contains a valuable component of interest. This was proven as its energy content is about 11% of the crop energy. This can be harnessed and added to our energy mix. The microporous structure of cassava residue, especially the peel is equally promising for adsorbent formulation. This review pinpoints the potential of these wastes for biochar production. The quality of biochar produced depends on the process parameters and choice of feedstock.

Three categories of cassava waste can be exploited as pyrolysis feedstock for biochar production (cassava plantation residues, cassava processing waste, and digestate from anaerobic digestion of cassava processing waste). Digestate from anaerobic digestion of cassava waste is the most preferred pyrolysis feedstock. Slow pyrolysis when optimized produces high-quality biochar suitable for several applications. Anaerobic digestion of cassava processing wastes generates much more than high-quality biochar. It is an effective waste to wealth strategy.

Conflicts of interest

The authors declare that they have no conflicts of interest regarding the publication of this paper.

Data availability

No data were used to support this study.