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Valorization of sugarcane waste: Prospects of a biorefinery
Valorization of sugarcane waste: Prospects of a biorefinery
3
Ranaprathap Katakojwala*,†, A. Naresh Kumar*,†, Debkumar Chakraborty*, S. Venkata Mohan* *Bioengineering and Environmental Sciences Lab, EEFF Department, CSIR-Indian Institute of Chemical Technology (CSIR-IICT), Hyderabad, India, †Academy of Scientific and Innovative Research (AcSIR), Hyderabad, India
1
Introduction
Sugarcane is one of the most important agricultural crops and the day-to-day population increment, demands greater sugar consumption, therefore increasing the sugarcane cultivation area (Rabelo et al., 2011a,b) (Fig. 1). Sugarcane cultivation requires a tropical or temperate climate, with a minimum of 60 cm (24 in.) of annual moisture. It is one of the most efficient photosynthesizers in the plant kingdom. It is a C4 plant, able to convert up to 1% of incident solar energy into biomass (Whitmarsh and Govindjee, 1995). Sugarcane has also been shown to utilize a wider range of wavelengths of solar radiation within the visible spectrum than most other plants. Physiological adaptations also include sucrose as a principal photosynthetic storage product which facilitates easy translocation of carbohydrates for growth of the plant. As a consequence of such adaptations, sugarcane fixes around four times as much solar energy as most temperate crops, and can consequently yield around 50 t dry matter/ha/year (Paturau, 1989). Globally, Brazil stands in first place followed by India with a production rate of 170.7 and 167 metric tons, respectively (Fig. 2). Sugarcane fields produce mainly two types of biomass: cane trash and bagasse. Cane trash is the field residue remaining after harvesting the cane stalk and bagasse is the milling byproduct that remains after extracting the sugar from the stalk (Rabelo et al., 2011a,b). The potential energy value of these residues has traditionally been ignored by various industries. However, unavoidable rises in fossil fuel prices and depletion of resources have led to viewing the waste as a valuable renewable energy resource (Margeot et al., 2009). In this context, sugarcane molasses is being used as a resource for bioethanol production (Luo et al., 2011). Additionally, during the course of sugarcane processing, various kinds of other wastes are also being produced, such as bagasse, press mud, washwater, and vinasse. Utilization of this waste as a resource facilitates additional benefits to sugarcane industries. In this context, the present chapter discusses the composition of various sugarcane industrial wastes and their significant potential toward various bio-based product synthesis in the biorefinery framework.
Industrial and Municipal Sludge. https://doi.org/10.1016/B978-0-12-815907-1.00003-9 © 2019 Elsevier Inc. All rights reserved.
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Production
Consumption
200
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185
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180
180
175
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170
170
165
2016
2018
2020
2022
2024
2026
Consumption (Mt tn)
Production (Mt tn)
205
2028
Year
Fig 1 Sugar production and consumption profiles. Source: OECD-FAO Agricultural Outlook 2018–2027.
2
Characterization of sugarcane waste (SCW)
The sugarcane industry produces various waste materials at the different processing stages of sugar production (Fig. 3). To utilize this waste as a resource, prior characterization needs to be understood. Therefore, in this section the detailed characterization of sugarcane industrial waste (SCW) is discussed (Table 1). Sugarcane bagasse (SCB), which is an abundant waste fibrous pulp material, results after the juice extraction from sugarcane stalks. The typical chemical composition of SCB is cellulose 40%–45%, hemicellulose 27%–29%, lignin 19%–21%, extractives 3%–4%, and other ashes 2%–4% (Karp et al., 2013). The cellulose, hemicellulose, and lignin fractions of SCB are considered natural polymers. Polysaccharides such as cellulose is composed of branched glucose (C6) units and hemicellulose is composed of hexoses (C6) and pentoses (C5) units, respectively. The most significant character of SCB is its complete biodegradable and compostable nature. Additionally, the sugar content of the SCB (cellulose/hemicellulose) could be extracted and used as a pillar material for various biological and chemical processes. Lignin is a constituent of the cell walls of almost all dry land plant cell walls, the second-most abundant natural polymer in the world, which is a complex polymer of aromatic alcohols. The high organic content of wastewater from the sugarcane industry makes it a good organic resource for biological valorization. In general in the sugarcane industry, wastewater contains (1–4.3 g/L) of COD, but the maximum COD standard for discharge of effluents is 150–250 mg/L (Hampannavar and Shivayogimath, 2010; Rais and Sheoran, 2015). Sugarcane press mud is the residue resulting after the filtration of sugarcane juice. It is soft, spongy, amorphous in nature, and dark brown in color. Press mud is characterized (on dry weight basis) by 9%–14% of wax, 10%–18% protein, 11%–17% cellulose, 15%–27% hemicelluloses, 9%–14% lignin, oil, resin, etc.
Valorization of sugarcane waste: Prospects of a biorefinery
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Fig 2 Global contribution in sugarcane production with respect to country. Source: FAO report.
Sugarcane
Washing
Milling
Waste water
Bagasse
Filtration
Sugar
Press mud Molasses
Alcohol
Distillation
Fermentation
Vinasse
Fig 3 Various processing methods for the sugarcane industry and the corresponding waste generation.
50
Table 1 Physical properties and chemical composition of sugarcane bagasse, press mud, molasses, and vinasse S. no.
Bagasse
Press mud
Molasses
Vinasse
References
1 2 3 4
pH Total solid (%) Total VS (%) C.O.D.
4.5–5.5 59 97.6 NA
4.95 47.87 34.8 284.5(g/kg)
4.8 5.3 78.6 62.3 (g/L)
5
B.O.D. (5 d, 27°C) Total nitrogen (%) Cellulose (%) Hemi cellulose (%) Lignin (%) Protein (%) Sugars (g/kg) Fiber (%) N NH4 + (g kg1) Phosphorous (%) Potassium (%) Crude wax (%) Ash
NA
222.0 (g/kg)
5–5.5 46 48.1 70–100 (g/ L) 40–40 (g/L)
Espan˜a-Gamboa et al. (2011), Gupta et al. (2011), Radjaram and Saravanane (2011a,b), Saleh-e-In et al. (2012), Janke et al. (2015), Janke et al. (2016), Talha et al. (2016), Sundaranayagi et al. (2017); Gonza´lez et al. (2017), Ansari and Gaikar (2014), and Kumar and Thankamani (2016)
1.86
1.68
<0.2
0.6
40–45 27–29
11.4 27.1
– –
– –
19–21 1.4 NA 35 –
9.3 3.08 22.5 15–30 –
– 1.25 49.9% – 200–215
– 0.4 31.6 – 5.6 103
0.345 2.1 <1 2%–4%
2.5–3.0 0.30–1.80 5–14 19.2%
3 2.89 – 10.25%
0.1 2.06 – 21.4%
6 7 8 9 10 11 12 13 14 15 16 17
5.05 mg/L
Industrial and Municipal Sludge
Parameters
Valorization of sugarcane waste: Prospects of a biorefinery
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(Gonza´lez et al., 2017). Press mud also contains a sizable quantity of macro- and micronutrients, besides 20%–25% of organic carbon. The X-ray fluorescent spectrometric analysis of the elemental composition carried out by Saleh-e-In et al. (2012) showed the presence of calcium (Ca), magnesium (Mg), silicon (Si), phosphorus (P), sulphur (S), aluminium (Al), and potassium (K) in press mud. The chemical composition may vary based on the cane variety, soil condition, nutrients applied in the field, the process of clarification adopted, and other environmental factors. The disposal of press mud is one of the major issues in the sugarcane industries because higher amounts of press mud are left in piles and their drainage causes the pollution of nearby water bodies. However, due to its high organic content, press mud can be used as a viable resource for secondary product synthesis using chemical and biological processes. Molasses is the syrup remaining after the crystallization of sugar from cane or beet juice. It is separated from the sugar crystals by the repeated centrifugation process. There are several grades of molasses and the final extraction yield is known as black strap molasses, which is heavy, viscous, and dark colored. The chemical composition of black strap molasses is primarily composed of total soluble solids (83.2%), followed by total sugars (49.9%), ash (10.25%), calcium (54%), sodium (54%), and potassium (2.89%) (pH 5.6–6.0) (Abubaker et al., 2012). Molasses is widely used as animal feed and for the synthesis of various bio-based product synthesis such as ethanol, vinegar, glycerin, inositol, lactic acid, succinic acid, glutamic acid, aconitic acid, kojic acid, and single cell proteins (SCP). Additionally, vinasse is a liquid fraction discharged during the refinement and distillation of ethanol, which is 20-fold higher than the ethanol produced. Vinasse is sulfur-rich in nature with a low pH (3.8–4.0), dark in color and an odorous effluent. The typical chemical composition of vinasse is like BOD (5046 mg/L), potassium (2056 mg/L), sodium (50.2 mg/L), sulfate (710 mg/L), calcium (719 mg/L), magnesium (237 mg/L), phosphorus (190 mg/L), hardness (2493 mg/L), and trace metals (As, Ba, Cd, Cr, Cu, Hg, Mo, Ni, Pb, Se, and Zn). Vinasse has a great potential to serve as a soil conditioner, animal feed, combustible material, etc. (Robertiello, 1982; Espan˜a-Gamboa et al., 2011).
3
Pretreatment of SCW
3.1 Physical methods Common physical pretreatment methods used for the pretreatment of sugarcane waste are discussed in this section (Fig. 4), starting with mechanical grinding, which is used for reducing the crystallinity of cellulose. It mostly includes chipping, grinding, and/or milling techniques. Chipping can reduce the biomass size to 10–30 mm only while grinding and milling can reduce the particle size up to 0.2–0.4 mm, which is significant for hydrolysis (Chang et al., 1997). Ultrasonication is a relatively new technique used for the pretreatment of lignocellulosic biomass and it is a feasible pretreatment option at the lab scale. Ultrasound treatment leads to the formation of small cavitation bubbles, which rupture the cellulose and hemicellulose fractions and thereby increase
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Fig 4 Various pretreatment methods of sugarcane waste.
the accessibility to cellulose-degrading enzymes for effective breakdown into simpler reducing sugars (Kumar and Sharma, 2017). Additionally, microwave irradiation is also a widely used method for lignocellulosic feedstock pretreatment because of its low energy requirement, high heating capacity in a short duration of time, minimum generation of inhibitors, and the ability to degrade the structural organization of the cellulose fraction. Moreover, addition of mild-alkali reagents is preferred for more effective breakdown. A study on microwave-based alkali pretreatment of switch grass yielded nearly 70%–90% sugars (Hu and Wen, 2008). Steam explosion is also a promising method of pretreatment. It is a thermomechanical process (200–270°C) that exposes biomass to a high pressure of steam (14–16 bar). As a result, it penetrates the biomass by diffusion for short periods of time (20 s to 20 min), followed by a sudden decompression generating shear forces that hydrolyze the glycosidic and hydrogen bonds between the glucose chains (Rojas et al., 2015).
3.2 Chemical methods Acid-catalyzed pretreatment is the most widely used conventional pretreatment of lignocellulosic biomass. The corrosive and toxic nature of most acids requires a suitable material for building the reactor, which can sustain the required experimental conditions and corrosive nature of acids (Saha et al., 2005). Still it is the most widely employed pretreatment method on an industrial scale. In some cases, an enzymatic hydrolysis step could easily be avoided as acid itself hydrolyzes the biomass into fermentable sugars. However, extensive washing is necessary to remove acid before fermentation of sugars (Sassner et al., 2008). Alkali-catalyzed pre treatment methods can
Valorization of sugarcane waste: Prospects of a biorefinery
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also be used at ambient temperature and pressure. The most commonly used alkali reagents are the hydroxyl derivatives of sodium, potassium, calcium, and ammonium salts. Among these hydroxyl derivatives, sodium hydroxide was found to be the most effective (Kumar and Wyman, 2009). Alkali reagents degrade the side chains of esters and glycosides, leading to structural modification of lignin, cellulose swelling, cellulose decrystallization, and hemicellulose (Cheng et al., 2010; Ibrahim et al., 2011; McIntosh and Vancov, 2010; Sills and Gossett, 2011). Additionally, studies were reported using lime-catalyzed conditions for SCW pretreatment due to its slower reactivity, which produces a lower amount of fermentation inhibitors (Rabelo et al., 2011a,b). This method is considered as a low-cost alternative for lignin solubilization, removing approximately 33% of lignin and 100% of acetyl groups. The action of lime is of low cost and its safe handling makes it attractive. Oxidizing agents such as hydrogen peroxide, ozone, oxygen, or air were used for the treatment of the lignocellulosic biomass (Nakamura et al., 2004). A number of chemical reactions such as electrophilic substitution, side chain displacements, and oxidative cleavage of the aromatic ring ether linkages may take place during oxidative pretreatment. This process causes delignification by converting lignin to acids. A major downside of oxidative pretreatment is that it damages a significant amount of hemicellulose, making it unavailable for fermentation (Lucas et al., 2012). The most commonly employed oxidizing agent is hydrogen peroxide and it has the great advantage of not leaving residue in the biomass as it degrades into oxygen and water. It has been found that hydrolysis of hydrogen peroxide leads to the formation of hydroxyl radicals that are responsible for the degradation of lignin and the production of low molecular weight products; removal of lignin from lignocellulose exposes cellulose and hemicellulose, leading to increased enzymatic hydrolysis (Hammel et al., 2002; Rabelo et al., 2011a,b). Some studies were also reported with the use of ionic liquids for SCW pretreatment (Zavrel et al., 2009; Behera et al., 2014). The ionic liquid breaks intramolecular hydrogen bonds, whereas the cations attack the O atom of the ―OH, and anions attack the hydrogen atoms of the ―OH group. Imidazolium salts are the most commonly used ILs. ILs are assumed to compete with lignocellulosic components for hydrogen bonding, thereby disrupting its network (Moultrop et al., 2005).
3.3 Enzymatic method In contrast to conventional chemical and physical pretreatment methods, biological pretreatment is considered an efficient, environmentally friendly, and cost-effective process. Nature has abundant bacterial species that can abundantly produce the cellulolytic and hemicellulolytic enzymes. The use of enzymes specially targets the β-glycosidic bonds of cellulose or hemicellulose and results in relatively effective pretreatment than other methods (Vats et al., 2013). Biological pretreatments are carried out by microorganisms such as brown, white, and soft-rot fungi, which mainly degrade lignin and hemicellulose and small amounts of cellulose (Sa´nchez, 2009). Though the biological pretreatment is highly intriguing, the rate of hydrolysis of lignocellulosic fractions is too slow, which influences adversely during pretreatment at an industrial
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scale (Sun and Cheng, 2002). In order to make biological techniques on par with other pretreatment methods, integration with mechanical treatment or chemical pretreatments is to be performed for sustainable processes.
3.4 Biological processes 3.4.1 Acidogenic fermentation Hydrogen is a promising alternative to carbon-based fuels because it is clean, renewable, and has a high energy yield of 122 kJ/g (Lin and Chang, 2004). To explore the viability of biohydrogen production from sugarcane waste, for example, press mud (acidic fibrous waste with 5%–15% sugar), various studies were performed (Partha and Sivasubramanian, 2006). Press mud was codigested with sewage (1:10) in UASB reactors for hydrogen production and a maximum biohydrogen production of 7960 mL/day (187 mL g VS1) was observed (Radjaram and Saravanane, 2011a). In another study, press mud mixed with water for biohydrogen production was performed and a maximum gas yield of 2240 mL d–1 was reported on the 78th day of the UASB operation (Radjaram and Saravanane, 2011b). Pretreated press mud slurry (10% of total solids) was used as a feedstock with enriched mixed microbial consortia and 0.04 g/g; 0.27 g/g of bioethanol and VFA were respectively obtained through acidogenic anaerobic fermentation (Kuruti et al., 2015). The pretreatment of sugarcane filter cake (FC) was carried out using NaOH and an improved VFA yield was observed with the productivity of 37% (0.44 g VFA g VS1), mainly consisted of n-butyric and acetic acids ( Janke et al., 2016).
3.4.2 Biomethanization Another alternative for the use of SCW is biogas production, which can be a sustainable solution for organic matter removal from the sugar industry, for example, sugarcane waste effluent and press mud (77% volatile solids). Sugar molasses also has a methane potential (i.e., CH4 per ton of raw material) of 230 m3. The volume of biogas generated of press mud through conventional digestion is around 65 m3 while through integrated biomethanation, it is about 80 m3/t of press mud. Press mud, along with bagasse, generates biogas that contains 52% methane content. The rate of biogas production was increased from press mud by the supplementation of trace elements and by codigestion with kitchen waste (Sundaranayagi et al., 2017). Rouf et al. (2010) evaluated the biogas production using mixing ratio of 1:1 press mud; sugarcane straw and biogas production were increased by 58%, respective to the press mud in monodigestion. The anaerobic digestion from vinasse and press mud separately has been reported previously as reaching methane yields of 0.344 m3 kg1 COD removed and 0.250 m3 kg1 COD removed, respectively (Harada et al., 1996; Sa´nchez et al., 1996). The conversion efficiency of sugarcane press mud and methane yield was further increased up to 64% by the codigestion with the vinasse in stirred-tank reactors (Gonza´lez et al., 2017). To enhance the codigestion of degradation and improve the
Valorization of sugarcane waste: Prospects of a biorefinery
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biomethane production potential, sugarcane bagasse and filter mud were pretreated (using heat and alkali) and a cumulative yield of biomethane (195.8 mL g VS1) was observed (Talha et al., 2016). 0.68 m3 biogas production (CH4—67%) from press mud was reported using a continuous-process floating drum-type anaerobic digester (1 m3) (Sathish and Vivekanandan, 2015).
3.4.3 Solventogenesis Pretreatment techniques generally improved the formation of sugars, avoiding the degradation of carbohydrates and the formation of byproducts inhibitory to the subsequent hydrolysis and fermentation process of sugarcane waste (Nimbalkar et al., 2017). After detoxification/neutralization, the effective utilization of press mud for bioethanol production was successfully achieved using acid-hydrolyzed press mud, resulting in a sugar release of 21 g/L that was further converted into 11.65 g/L of ethanol using Saccharomyces cerevisiae NRRL Y-12632 (Pawar et al., 2017). Ethanol production generates great amounts of residuals, both liquid and solid. These byproducts can be used for bioresource and energy recovery. Bioethanol production and coproduction of electrical power and biogas using dry bagasse as a substrate was also investigated in several studies reported before (Rabelo et al., 2011a,b).
3.5 Fertilizer production Sugarcane waste, mainly press mud, can also be used as a soil conditioner and soil fertilizer and is very useful for agricultural crops and horticulture because of its richness in various micronutrients and nitrogen. Composting of organic wastes from the sugarcane industry is necessary to reduce the lignin and cellulose contents, thereby the nutrient availability can be improved. Processes used to improve the fertilizer value include composting, treatment with microorganisms, and mixing with distillery effluents (Nasir, 2006). Generally, vermicomposting of press mud is an efficient method of waste disposal and is used as manure in India by sugarcane farmers, enabling recycling of organic matter for solid waste treatment. Nowadays, toxic distillery effluent, spent wash having high BOD and COD content, is being utilized by spraying and mixing with press mud to enrich the press mud in its nutritional values and make one of the best organic manures. The organic manure made out of press mud maintains soil health, sustains sugarcane and sugar production, improves soils physical properties, retains soil moisture, and reduces erosion hazards (Baskar et al., 2003; Rath et al., 2010; Sangwan et al., 2010).
3.6 Animal feed In animal husbandry, cane bagasse and press mud have been used as feed ingredients, notably for ruminants, because of their sugar and mineral content. Filter press mud was used as a filler in ruminant maintenance diets at a level of 10%–30%, together with poultry manure, final molasses, ground cane, urea, and minerals (containing
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13% crude fiber, 8.8% crude protein, and 31.7% ash) (Rodriguez and Gonzalez, 1973). Sugarcane press mud was considered to be a potential low-energy feed ingredient (6.6% crude protein) in poultry diets in countries such as Sri Lanka, the Philippines, etc. (Rajaguru and Ravindran, 1985; Abrigo and Gerpacio, 1986). Press mud, which has a chemical composition similar to that of cattle dung, was used as a fish pond fertilizer in India (Keshavanath et al., 2005). In China, a carp and dace feed ingredient has been produced from press mud (Yu, 1990).
3.7 Other applications Other industrial applications are reported for sugarcane waste and press mud in cement and paint manufacturing, as a foaming agent, in wax production, etc. (Van der Poel et al., 1998). Wax and its hydrolysis product, fatty alcohols (C25 C30), have applications as nutraceutical. Press mud is also a useful biosorbent for metal adsorption from wastewater. Filter press mud can also be used as a compacting and wetting agent in surface silos (Perez, 1990; Ansari and Gaikar, 2014). Press mud is also used for the production of citric acid by Aspergillus niger in a solid-state fermentation system that initially contained 12.5% (w/w) effective sugar and 0.1% (w/w) NH4NO3 (Show et al., 2015).
3.8 Struvite precipitation Increased consumption and no alternative sources other than the rock phosphate of mineral phosphorus make phosphorus limited (Rahman et al., 2011). Alternatively, struvite can be used as a phosphate fertilizer to provide sustainable phosphorus usage. Struvite is a crystalline substance consisting of magnesium, ammonium, and phosphorus in equal molar concentrations (MgNH4PO4) (Munir et al., 2017). Struvite forms orthorhombic (straight prisms with a rectangular base) crystals in the ratio of 1:1:1 among the magnesium, ammonium, and phosphate (Eq. 1). NH4 + + Mg2 + + HPO4 2 + 6H2 O ! NH4 MgPO4 + 6H2 O
(1)
Apart from the chemical synthesis, waste/wastewater rich in phosphorus and nitrogen could be used as a resource for struvite synthesis. Press mud/wastewater contains a good amount of nitrates and phosphates, the recovery of struvite from which offers sustainability to the current sugarcane industries. However, the recovery of struvite from press mud offers the recovery of an agriculturally important fertilizer (P) apart from waste remediation. Additionally, struvite does not burn the roots due to its slow-releasing characteristics, which is common with traditional ammonium-phosphate fertilizers (Yan et al., 2018). Conventional waste treatment technologies limit the P removal and the recovery of P in the form of struvite offers sustainability to the current wastewater treatment units and accounts for economical benefits.
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Limitations and future prospective
Increasing sugar consumption demands greater sugarcane cultivation. Simultaneously, higher sugarcane cultivation is increasing the load on the cane-processing industries, thus leading to higher waste discharge. In this direction, the waste originating from sugar industries needs to be addressed with sustainable processes to avoid further environmental pollution. The presence of fiber, wax, and proteins makes sugarcane bagasse the most recalcitrant in nature, which limits its degradability and conversion efficiency. Hence, it cannot be easily separated into readily utilizable components and the available pretreatment techniques are the most expensive and least technologically mature steps available to convert biomass to fermentable sugars. Liquid residuals from pretreatment processes are rich in pentoses, soluble and insoluble lignin, etc. This offers a great potential for improvement in efficiency and the reduction of costs through future research and development. Currently, sugarcane molasses is widely used as a resource for bioethanol production and this leaves behind other secondary materials such as pentoses, vinasse, and effluent. The best use for the pentoses in the pretreatment liquor is ethanol production. However, the current processes using existing microorganisms lead to extremely low yields as a waste. In this context, utilization of sugarcane waste originating from different processes could be considered a renewable resource for the synthesis of various products such as MCC, NCC, xylitol, ethanol, hydrogen, methane, hythane, struvite, etc. This multiproduct approach using sugarcane waste as a resource in the biorefinery framework facilitates sustainability to the existing sugarcane industries.
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Moultrop, J.S., Swatloski, R.P., Moyna, G., Rogers, R.D., 2005. High resolution 13-C NMR studies of cellulose and cellulose oligomers in ionic liquid solutions. Chem. Commun, 1557–1559. Munir, M.T., Li, B., Boiarkina, I., Baroutian, S., Yu, W., Young, B.R., 2017. Phosphate recovery from hydrothermally treated sewage sludge using struvite precipitation. Bioresour. Technol. 239, 171–179. https://doi.org/10.1016/j.biortech.2017.04.129. Nakamura, Y., Daidai, M., Kobayashi, F., 2004. Ozonolysis mechanism of lignin model compounds and microbial treatment of organic acids produced. Water Sci. Technol. 50 (3), 167–172. Nasir, N.M., 2006. Better Management Practices for Cotton and Sugarcane. WWF—Pakistan, Pakistan. Nimbalkar, P.R., Khedkar, M.A., Gaikwad, S.G., Chavan, P.V., Bankar, S.B., 2017. New insight into sugarcane industry waste utilization (press mud) for cleaner biobutanol production by using C. acetobutylicum NRRL B-527. Appl. Biochem. Biotechnol. 183 (3), 1008–1025. Partha, N., Sivasubramanian, V., 2006. Recovery of chemicals from pressmud—a sugar industry waste. Indian Chem. Eng. 48 (3), 160–163. Paturau, J.M., 1989. By-Products of Sugarcane Industry, third ed. Elsevier, Amsterdam. Pawar, S.S., Chavan, P., Bankar, S., 2017. Production of bio-ethanol from biomass (press mud). Int. J. Adv. Res. Ideas Innov. Technol. 3 (3), 1550–1555. Perez, R., 1990. Intensive livestock production systems based on local resources in Cuba. In: Speedy, A.W. (Ed.), Developing World Agriculture. Grosvenor Press International, London, pp. 230–237. Rabelo, H.T., Bezerra, L.A., Terra, D.F., Lima, R.M., Silva, M.A., Leite, T.K., de Oliveira, R.J., 2011a. Effects of 24 weeks of progressive resistance training on knee extensors peak torque and fat-free mass in older women. J. Strength Cond. Res. 25 (8), 2298–2303. Rabelo, S.C., Carrere, H., Maciel Filho, R., Costa, A.C., 2011b. Production of bioethanol, methane and heat from sugarcane bagasse in a biorefinery concept. Bioresour. Technol. 102 (17), 7887–7895. Radjaram, B., Saravanane, R., 2011a. Assessment of optimum dilution ratio for biohydrogen production by anaerobic co-digestion of press mud with sewage and water. Bioresour. Technol. 102 (3), 2773–2780. Radjaram, B., Saravanane, R., 2011b. Start up study of UASB reactor treating press mud for biohydrogen production. Biomass Bioenergy 35 (7), 2721–2728. Rahman, M.M., Liu, Y., Kwag, J., Ra, C., 2011. Recovery of struvite from animal wastewater and its nutrient leaching loss in soil. J. Hazarous Mater. 186, 2026–2030. Rais, M., Sheoran, A., 2015. Treatment of sugarcane industry effluents: science & technology issues. Int. J. Eng. Res. Appl. 5(1). Rajaguru, A.S.B., Ravindran, V., 1985. Metabolisable energy values for growing chicks of some feedstuffs from Sri Lanka. J. Sci. Food Agric. 36 (1), 1057–1064. Rath, P., Pradhan, G., Mishra, M.K., 2010. Effect of sugar factory distillery spent wash (dsw) on the growth pattern of sugarcane (Saccharum officinarum) crop. J. Phytology 2 (5), 33–39. Robertiello, A., 1982. Upgrading of agricultural and agroindustrial wastes: the treatment of distillery effluents (vinasses) in Italy. Agri. Wastes 4, 387–395. Rodriguez, V., Gonzalez, S., 1973. The use of filter cake mud in integral diets for milk production. Cuba. J. Agric. Sci. 7 (1), 29–32. Rojas, J., Bedoya, M., Ciro, Y., 2015. Current trends in the production of cellulose nanoparticles and nanocomposites for biomedical applications. https://doi.org/10.5772/61334.
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Rouf, M.A., Bajpai, P.K., Jotshi, C.K., 2010. Optimization of biogas generation from press mud in batch reactor. Bangladesh J. Scient. Ind. Res. 45 (4), 371–376. Saha, B.C., Iten, L.B., Cotta, M.A., Wu, Y.V., 2005. Dilute acid pretreatment, enzymatic saccharification and fermentation of wheat straw to ethanol. Process Biochem. 40 (12), 3693–3700. Saleh-e-In, M.M., Yeasmin, S., Paul, B.K., Ahsan, M., Rahman, M.Z., Roy, S.K., 2012. Chemical studies on press mud: a sugar industries waste in Bangladesh. Sugar Tech. 14 (2), 109–118. Sa´nchez, C., 2009. Lignocellulosic residues: biodegradation and bioconversion by fungi. Biotechnol. Adv. 27, 185–194. Sa´nchez, E., Borja, R., Lopez, M., 1996. Determination of the kinetic constants of anaerobic digestion of sugar mill-mud-waste (SMMW). Bioresour. Technol. 56, 245–249. Sangwan, P., Kaushik, C.P., Garg, V.K., 2010. Vermicomposting of sugar industry waste (press mud) mixed with cow dung employing an epigeic earthworm Eisenia fetida. Waste Manage. Res. 28 (1), 71–75. Sassner, P., Galbe, M., Zacchi, G., 2008. Techno-economic evaluation of bioethanol production from three different lignocellulosic materials. Biomass Bioenergy 32 (5), 422–430. Sathish, S., Vivekanandan, S., 2015. Experimental investigation on biogas production using industrial waste (press mud) to generate renewable energy. Int. J. Innovat. Res. Sci. Eng. 4 (2), 388–392. Show, P.L., Oladele, K.O., Siew, Q.Y., Aziz Zakry, F.A., Lan, J.C.W., Ling, T.C., 2015. Overview of citric acid production from Aspergillus niger. Front Life Sci 8 (3), 271–283. Sills, D.L., Gossett, J.M., 2011. Assessment of commercial hemicellulases for saccharification of alkaline pretreated perennial biomass. Bioresour. Technol. 102, 1389–1398. Sun, Y., Cheng, J., 2002. Hydrolysis of lignocellulosic materials for ethanol production: a review. Bioresour. Technol. 83, 1–11. Sundaranayagi, S., Sirajunnisa, A., Ramasamy, V., 2017. Production of biogas by anaerobic digestion of press mud using iron, cobalt and nickel. Indian J. Scientific Res. 14 (1), 374–377. Talha, Z., Ding, W., Mehryar, E., Hassan, M., Bi, J., 2016. Alkaline pretreatment of sugarcane bagasse and filter mud codigested to improve biomethane production. Biomed. Res. Int. Van der Poel, P.W., Schiweck, H., Schwartz, T., 1998. Sugar Technology. Beet and Cane Sugar Manufacture. Verlag Dr. Albert Martens KG, Berlin, p. 1005. Vats, S., Maurya, D.P., Shaimoon, M., Negi, S., 2013. Development of a microbial consortium for the production of blend enzymes for the hydrolysis of agricultural waste into sugars. Indian J. Scientific Res. 72, 585–590. Whitmarsh, J., Govindjee, 1995. Singhal, G.S., Renger, G., Sopory, S.K., Irrgang, K.-D., Govindjee, (Eds.), Concepts in Photobiology: Photosynthesis and Photomorphogenesis. Narosa Publishers/Kluwer Academic/Dordrecht, New Delhi, pp. 11–51. Yan, T., Ye, Y., Ma, H., Zhang, Y., Guo, W., Du, B., Wei, Q., Wei, D., Ngo, H.H., 2018. A critical review on membrane hybrid system for nutrient recovery from wastewater. Chem. Eng. J. 348, 143–156. https://doi.org/10.1016/j.cej.2018.04.166. Yu, B.G., 1990. Protein feed granules from bagacillo for fish farming. Sugar y Azucar 85 (11), 30–40. Zavrel, M., Bross, D., Funke, M., Buchs, J., Spiess, A.C., 2009. High-throughput screening for ionic liquids dissolving (ligno-)cellulose. Bioresour. Technol. 100, 2580–2587.
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Ranaprathap Katakojwala*,†, A. Naresh Kumar*,†, Debkumar Chakraborty*, S. Venkata Mohan* *Bioengineering and Environmental Sciences Lab, EEFF Department, CSIR-Indian Institute of Chemical Technology (CSIR-IICT), Hyderabad, India, †Academy of Scientific and Innovative Research (AcSIR), Hyderabad, India
1
Introduction
Sugarcane is one of the most important agricultural crops and the day-to-day population increment, demands greater sugar consumption, therefore increasing the sugarcane cultivation area (Rabelo et al., 2011a,b) (Fig. 1). Sugarcane cultivation requires a tropical or temperate climate, with a minimum of 60 cm (24 in.) of annual moisture. It is one of the most efficient photosynthesizers in the plant kingdom. It is a C4 plant, able to convert up to 1% of incident solar energy into biomass (Whitmarsh and Govindjee, 1995). Sugarcane has also been shown to utilize a wider range of wavelengths of solar radiation within the visible spectrum than most other plants. Physiological adaptations also include sucrose as a principal photosynthetic storage product which facilitates easy translocation of carbohydrates for growth of the plant. As a consequence of such adaptations, sugarcane fixes around four times as much solar energy as most temperate crops, and can consequently yield around 50 t dry matter/ha/year (Paturau, 1989). Globally, Brazil stands in first place followed by India with a production rate of 170.7 and 167 metric tons, respectively (Fig. 2). Sugarcane fields produce mainly two types of biomass: cane trash and bagasse. Cane trash is the field residue remaining after harvesting the cane stalk and bagasse is the milling byproduct that remains after extracting the sugar from the stalk (Rabelo et al., 2011a,b). The potential energy value of these residues has traditionally been ignored by various industries. However, unavoidable rises in fossil fuel prices and depletion of resources have led to viewing the waste as a valuable renewable energy resource (Margeot et al., 2009). In this context, sugarcane molasses is being used as a resource for bioethanol production (Luo et al., 2011). Additionally, during the course of sugarcane processing, various kinds of other wastes are also being produced, such as bagasse, press mud, washwater, and vinasse. Utilization of this waste as a resource facilitates additional benefits to sugarcane industries. In this context, the present chapter discusses the composition of various sugarcane industrial wastes and their significant potential toward various bio-based product synthesis in the biorefinery framework.
Industrial and Municipal Sludge. https://doi.org/10.1016/B978-0-12-815907-1.00003-9 © 2019 Elsevier Inc. All rights reserved.
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Production
Consumption
200
200
195
195
190
190
185
185
180
180
175
175
170
170
165
2016
2018
2020
2022
2024
2026
Consumption (Mt tn)
Production (Mt tn)
205
2028
Year
Fig 1 Sugar production and consumption profiles. Source: OECD-FAO Agricultural Outlook 2018–2027.
2
Characterization of sugarcane waste (SCW)
The sugarcane industry produces various waste materials at the different processing stages of sugar production (Fig. 3). To utilize this waste as a resource, prior characterization needs to be understood. Therefore, in this section the detailed characterization of sugarcane industrial waste (SCW) is discussed (Table 1). Sugarcane bagasse (SCB), which is an abundant waste fibrous pulp material, results after the juice extraction from sugarcane stalks. The typical chemical composition of SCB is cellulose 40%–45%, hemicellulose 27%–29%, lignin 19%–21%, extractives 3%–4%, and other ashes 2%–4% (Karp et al., 2013). The cellulose, hemicellulose, and lignin fractions of SCB are considered natural polymers. Polysaccharides such as cellulose is composed of branched glucose (C6) units and hemicellulose is composed of hexoses (C6) and pentoses (C5) units, respectively. The most significant character of SCB is its complete biodegradable and compostable nature. Additionally, the sugar content of the SCB (cellulose/hemicellulose) could be extracted and used as a pillar material for various biological and chemical processes. Lignin is a constituent of the cell walls of almost all dry land plant cell walls, the second-most abundant natural polymer in the world, which is a complex polymer of aromatic alcohols. The high organic content of wastewater from the sugarcane industry makes it a good organic resource for biological valorization. In general in the sugarcane industry, wastewater contains (1–4.3 g/L) of COD, but the maximum COD standard for discharge of effluents is 150–250 mg/L (Hampannavar and Shivayogimath, 2010; Rais and Sheoran, 2015). Sugarcane press mud is the residue resulting after the filtration of sugarcane juice. It is soft, spongy, amorphous in nature, and dark brown in color. Press mud is characterized (on dry weight basis) by 9%–14% of wax, 10%–18% protein, 11%–17% cellulose, 15%–27% hemicelluloses, 9%–14% lignin, oil, resin, etc.
Valorization of sugarcane waste: Prospects of a biorefinery
49
Fig 2 Global contribution in sugarcane production with respect to country. Source: FAO report.
Sugarcane
Washing
Milling
Waste water
Bagasse
Filtration
Sugar
Press mud Molasses
Alcohol
Distillation
Fermentation
Vinasse
Fig 3 Various processing methods for the sugarcane industry and the corresponding waste generation.
50
Table 1 Physical properties and chemical composition of sugarcane bagasse, press mud, molasses, and vinasse S. no.
Bagasse
Press mud
Molasses
Vinasse
References
1 2 3 4
pH Total solid (%) Total VS (%) C.O.D.
4.5–5.5 59 97.6 NA
4.95 47.87 34.8 284.5(g/kg)
4.8 5.3 78.6 62.3 (g/L)
5
B.O.D. (5 d, 27°C) Total nitrogen (%) Cellulose (%) Hemi cellulose (%) Lignin (%) Protein (%) Sugars (g/kg) Fiber (%) N NH4 + (g kg1) Phosphorous (%) Potassium (%) Crude wax (%) Ash
NA
222.0 (g/kg)
5–5.5 46 48.1 70–100 (g/ L) 40–40 (g/L)
Espan˜a-Gamboa et al. (2011), Gupta et al. (2011), Radjaram and Saravanane (2011a,b), Saleh-e-In et al. (2012), Janke et al. (2015), Janke et al. (2016), Talha et al. (2016), Sundaranayagi et al. (2017); Gonza´lez et al. (2017), Ansari and Gaikar (2014), and Kumar and Thankamani (2016)
1.86
1.68
<0.2
0.6
40–45 27–29
11.4 27.1
– –
– –
19–21 1.4 NA 35 –
9.3 3.08 22.5 15–30 –
– 1.25 49.9% – 200–215
– 0.4 31.6 – 5.6 103
0.345 2.1 <1 2%–4%
2.5–3.0 0.30–1.80 5–14 19.2%
3 2.89 – 10.25%
0.1 2.06 – 21.4%
6 7 8 9 10 11 12 13 14 15 16 17
5.05 mg/L
Industrial and Municipal Sludge
Parameters
Valorization of sugarcane waste: Prospects of a biorefinery
51
(Gonza´lez et al., 2017). Press mud also contains a sizable quantity of macro- and micronutrients, besides 20%–25% of organic carbon. The X-ray fluorescent spectrometric analysis of the elemental composition carried out by Saleh-e-In et al. (2012) showed the presence of calcium (Ca), magnesium (Mg), silicon (Si), phosphorus (P), sulphur (S), aluminium (Al), and potassium (K) in press mud. The chemical composition may vary based on the cane variety, soil condition, nutrients applied in the field, the process of clarification adopted, and other environmental factors. The disposal of press mud is one of the major issues in the sugarcane industries because higher amounts of press mud are left in piles and their drainage causes the pollution of nearby water bodies. However, due to its high organic content, press mud can be used as a viable resource for secondary product synthesis using chemical and biological processes. Molasses is the syrup remaining after the crystallization of sugar from cane or beet juice. It is separated from the sugar crystals by the repeated centrifugation process. There are several grades of molasses and the final extraction yield is known as black strap molasses, which is heavy, viscous, and dark colored. The chemical composition of black strap molasses is primarily composed of total soluble solids (83.2%), followed by total sugars (49.9%), ash (10.25%), calcium (54%), sodium (54%), and potassium (2.89%) (pH 5.6–6.0) (Abubaker et al., 2012). Molasses is widely used as animal feed and for the synthesis of various bio-based product synthesis such as ethanol, vinegar, glycerin, inositol, lactic acid, succinic acid, glutamic acid, aconitic acid, kojic acid, and single cell proteins (SCP). Additionally, vinasse is a liquid fraction discharged during the refinement and distillation of ethanol, which is 20-fold higher than the ethanol produced. Vinasse is sulfur-rich in nature with a low pH (3.8–4.0), dark in color and an odorous effluent. The typical chemical composition of vinasse is like BOD (5046 mg/L), potassium (2056 mg/L), sodium (50.2 mg/L), sulfate (710 mg/L), calcium (719 mg/L), magnesium (237 mg/L), phosphorus (190 mg/L), hardness (2493 mg/L), and trace metals (As, Ba, Cd, Cr, Cu, Hg, Mo, Ni, Pb, Se, and Zn). Vinasse has a great potential to serve as a soil conditioner, animal feed, combustible material, etc. (Robertiello, 1982; Espan˜a-Gamboa et al., 2011).
3
Pretreatment of SCW
3.1 Physical methods Common physical pretreatment methods used for the pretreatment of sugarcane waste are discussed in this section (Fig. 4), starting with mechanical grinding, which is used for reducing the crystallinity of cellulose. It mostly includes chipping, grinding, and/or milling techniques. Chipping can reduce the biomass size to 10–30 mm only while grinding and milling can reduce the particle size up to 0.2–0.4 mm, which is significant for hydrolysis (Chang et al., 1997). Ultrasonication is a relatively new technique used for the pretreatment of lignocellulosic biomass and it is a feasible pretreatment option at the lab scale. Ultrasound treatment leads to the formation of small cavitation bubbles, which rupture the cellulose and hemicellulose fractions and thereby increase
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Fig 4 Various pretreatment methods of sugarcane waste.
the accessibility to cellulose-degrading enzymes for effective breakdown into simpler reducing sugars (Kumar and Sharma, 2017). Additionally, microwave irradiation is also a widely used method for lignocellulosic feedstock pretreatment because of its low energy requirement, high heating capacity in a short duration of time, minimum generation of inhibitors, and the ability to degrade the structural organization of the cellulose fraction. Moreover, addition of mild-alkali reagents is preferred for more effective breakdown. A study on microwave-based alkali pretreatment of switch grass yielded nearly 70%–90% sugars (Hu and Wen, 2008). Steam explosion is also a promising method of pretreatment. It is a thermomechanical process (200–270°C) that exposes biomass to a high pressure of steam (14–16 bar). As a result, it penetrates the biomass by diffusion for short periods of time (20 s to 20 min), followed by a sudden decompression generating shear forces that hydrolyze the glycosidic and hydrogen bonds between the glucose chains (Rojas et al., 2015).
3.2 Chemical methods Acid-catalyzed pretreatment is the most widely used conventional pretreatment of lignocellulosic biomass. The corrosive and toxic nature of most acids requires a suitable material for building the reactor, which can sustain the required experimental conditions and corrosive nature of acids (Saha et al., 2005). Still it is the most widely employed pretreatment method on an industrial scale. In some cases, an enzymatic hydrolysis step could easily be avoided as acid itself hydrolyzes the biomass into fermentable sugars. However, extensive washing is necessary to remove acid before fermentation of sugars (Sassner et al., 2008). Alkali-catalyzed pre treatment methods can
Valorization of sugarcane waste: Prospects of a biorefinery
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also be used at ambient temperature and pressure. The most commonly used alkali reagents are the hydroxyl derivatives of sodium, potassium, calcium, and ammonium salts. Among these hydroxyl derivatives, sodium hydroxide was found to be the most effective (Kumar and Wyman, 2009). Alkali reagents degrade the side chains of esters and glycosides, leading to structural modification of lignin, cellulose swelling, cellulose decrystallization, and hemicellulose (Cheng et al., 2010; Ibrahim et al., 2011; McIntosh and Vancov, 2010; Sills and Gossett, 2011). Additionally, studies were reported using lime-catalyzed conditions for SCW pretreatment due to its slower reactivity, which produces a lower amount of fermentation inhibitors (Rabelo et al., 2011a,b). This method is considered as a low-cost alternative for lignin solubilization, removing approximately 33% of lignin and 100% of acetyl groups. The action of lime is of low cost and its safe handling makes it attractive. Oxidizing agents such as hydrogen peroxide, ozone, oxygen, or air were used for the treatment of the lignocellulosic biomass (Nakamura et al., 2004). A number of chemical reactions such as electrophilic substitution, side chain displacements, and oxidative cleavage of the aromatic ring ether linkages may take place during oxidative pretreatment. This process causes delignification by converting lignin to acids. A major downside of oxidative pretreatment is that it damages a significant amount of hemicellulose, making it unavailable for fermentation (Lucas et al., 2012). The most commonly employed oxidizing agent is hydrogen peroxide and it has the great advantage of not leaving residue in the biomass as it degrades into oxygen and water. It has been found that hydrolysis of hydrogen peroxide leads to the formation of hydroxyl radicals that are responsible for the degradation of lignin and the production of low molecular weight products; removal of lignin from lignocellulose exposes cellulose and hemicellulose, leading to increased enzymatic hydrolysis (Hammel et al., 2002; Rabelo et al., 2011a,b). Some studies were also reported with the use of ionic liquids for SCW pretreatment (Zavrel et al., 2009; Behera et al., 2014). The ionic liquid breaks intramolecular hydrogen bonds, whereas the cations attack the O atom of the ―OH, and anions attack the hydrogen atoms of the ―OH group. Imidazolium salts are the most commonly used ILs. ILs are assumed to compete with lignocellulosic components for hydrogen bonding, thereby disrupting its network (Moultrop et al., 2005).
3.3 Enzymatic method In contrast to conventional chemical and physical pretreatment methods, biological pretreatment is considered an efficient, environmentally friendly, and cost-effective process. Nature has abundant bacterial species that can abundantly produce the cellulolytic and hemicellulolytic enzymes. The use of enzymes specially targets the β-glycosidic bonds of cellulose or hemicellulose and results in relatively effective pretreatment than other methods (Vats et al., 2013). Biological pretreatments are carried out by microorganisms such as brown, white, and soft-rot fungi, which mainly degrade lignin and hemicellulose and small amounts of cellulose (Sa´nchez, 2009). Though the biological pretreatment is highly intriguing, the rate of hydrolysis of lignocellulosic fractions is too slow, which influences adversely during pretreatment at an industrial
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scale (Sun and Cheng, 2002). In order to make biological techniques on par with other pretreatment methods, integration with mechanical treatment or chemical pretreatments is to be performed for sustainable processes.
3.4 Biological processes 3.4.1 Acidogenic fermentation Hydrogen is a promising alternative to carbon-based fuels because it is clean, renewable, and has a high energy yield of 122 kJ/g (Lin and Chang, 2004). To explore the viability of biohydrogen production from sugarcane waste, for example, press mud (acidic fibrous waste with 5%–15% sugar), various studies were performed (Partha and Sivasubramanian, 2006). Press mud was codigested with sewage (1:10) in UASB reactors for hydrogen production and a maximum biohydrogen production of 7960 mL/day (187 mL g VS1) was observed (Radjaram and Saravanane, 2011a). In another study, press mud mixed with water for biohydrogen production was performed and a maximum gas yield of 2240 mL d–1 was reported on the 78th day of the UASB operation (Radjaram and Saravanane, 2011b). Pretreated press mud slurry (10% of total solids) was used as a feedstock with enriched mixed microbial consortia and 0.04 g/g; 0.27 g/g of bioethanol and VFA were respectively obtained through acidogenic anaerobic fermentation (Kuruti et al., 2015). The pretreatment of sugarcane filter cake (FC) was carried out using NaOH and an improved VFA yield was observed with the productivity of 37% (0.44 g VFA g VS1), mainly consisted of n-butyric and acetic acids ( Janke et al., 2016).
3.4.2 Biomethanization Another alternative for the use of SCW is biogas production, which can be a sustainable solution for organic matter removal from the sugar industry, for example, sugarcane waste effluent and press mud (77% volatile solids). Sugar molasses also has a methane potential (i.e., CH4 per ton of raw material) of 230 m3. The volume of biogas generated of press mud through conventional digestion is around 65 m3 while through integrated biomethanation, it is about 80 m3/t of press mud. Press mud, along with bagasse, generates biogas that contains 52% methane content. The rate of biogas production was increased from press mud by the supplementation of trace elements and by codigestion with kitchen waste (Sundaranayagi et al., 2017). Rouf et al. (2010) evaluated the biogas production using mixing ratio of 1:1 press mud; sugarcane straw and biogas production were increased by 58%, respective to the press mud in monodigestion. The anaerobic digestion from vinasse and press mud separately has been reported previously as reaching methane yields of 0.344 m3 kg1 COD removed and 0.250 m3 kg1 COD removed, respectively (Harada et al., 1996; Sa´nchez et al., 1996). The conversion efficiency of sugarcane press mud and methane yield was further increased up to 64% by the codigestion with the vinasse in stirred-tank reactors (Gonza´lez et al., 2017). To enhance the codigestion of degradation and improve the
Valorization of sugarcane waste: Prospects of a biorefinery
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biomethane production potential, sugarcane bagasse and filter mud were pretreated (using heat and alkali) and a cumulative yield of biomethane (195.8 mL g VS1) was observed (Talha et al., 2016). 0.68 m3 biogas production (CH4—67%) from press mud was reported using a continuous-process floating drum-type anaerobic digester (1 m3) (Sathish and Vivekanandan, 2015).
3.4.3 Solventogenesis Pretreatment techniques generally improved the formation of sugars, avoiding the degradation of carbohydrates and the formation of byproducts inhibitory to the subsequent hydrolysis and fermentation process of sugarcane waste (Nimbalkar et al., 2017). After detoxification/neutralization, the effective utilization of press mud for bioethanol production was successfully achieved using acid-hydrolyzed press mud, resulting in a sugar release of 21 g/L that was further converted into 11.65 g/L of ethanol using Saccharomyces cerevisiae NRRL Y-12632 (Pawar et al., 2017). Ethanol production generates great amounts of residuals, both liquid and solid. These byproducts can be used for bioresource and energy recovery. Bioethanol production and coproduction of electrical power and biogas using dry bagasse as a substrate was also investigated in several studies reported before (Rabelo et al., 2011a,b).
3.5 Fertilizer production Sugarcane waste, mainly press mud, can also be used as a soil conditioner and soil fertilizer and is very useful for agricultural crops and horticulture because of its richness in various micronutrients and nitrogen. Composting of organic wastes from the sugarcane industry is necessary to reduce the lignin and cellulose contents, thereby the nutrient availability can be improved. Processes used to improve the fertilizer value include composting, treatment with microorganisms, and mixing with distillery effluents (Nasir, 2006). Generally, vermicomposting of press mud is an efficient method of waste disposal and is used as manure in India by sugarcane farmers, enabling recycling of organic matter for solid waste treatment. Nowadays, toxic distillery effluent, spent wash having high BOD and COD content, is being utilized by spraying and mixing with press mud to enrich the press mud in its nutritional values and make one of the best organic manures. The organic manure made out of press mud maintains soil health, sustains sugarcane and sugar production, improves soils physical properties, retains soil moisture, and reduces erosion hazards (Baskar et al., 2003; Rath et al., 2010; Sangwan et al., 2010).
3.6 Animal feed In animal husbandry, cane bagasse and press mud have been used as feed ingredients, notably for ruminants, because of their sugar and mineral content. Filter press mud was used as a filler in ruminant maintenance diets at a level of 10%–30%, together with poultry manure, final molasses, ground cane, urea, and minerals (containing
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13% crude fiber, 8.8% crude protein, and 31.7% ash) (Rodriguez and Gonzalez, 1973). Sugarcane press mud was considered to be a potential low-energy feed ingredient (6.6% crude protein) in poultry diets in countries such as Sri Lanka, the Philippines, etc. (Rajaguru and Ravindran, 1985; Abrigo and Gerpacio, 1986). Press mud, which has a chemical composition similar to that of cattle dung, was used as a fish pond fertilizer in India (Keshavanath et al., 2005). In China, a carp and dace feed ingredient has been produced from press mud (Yu, 1990).
3.7 Other applications Other industrial applications are reported for sugarcane waste and press mud in cement and paint manufacturing, as a foaming agent, in wax production, etc. (Van der Poel et al., 1998). Wax and its hydrolysis product, fatty alcohols (C25 C30), have applications as nutraceutical. Press mud is also a useful biosorbent for metal adsorption from wastewater. Filter press mud can also be used as a compacting and wetting agent in surface silos (Perez, 1990; Ansari and Gaikar, 2014). Press mud is also used for the production of citric acid by Aspergillus niger in a solid-state fermentation system that initially contained 12.5% (w/w) effective sugar and 0.1% (w/w) NH4NO3 (Show et al., 2015).
3.8 Struvite precipitation Increased consumption and no alternative sources other than the rock phosphate of mineral phosphorus make phosphorus limited (Rahman et al., 2011). Alternatively, struvite can be used as a phosphate fertilizer to provide sustainable phosphorus usage. Struvite is a crystalline substance consisting of magnesium, ammonium, and phosphorus in equal molar concentrations (MgNH4PO4) (Munir et al., 2017). Struvite forms orthorhombic (straight prisms with a rectangular base) crystals in the ratio of 1:1:1 among the magnesium, ammonium, and phosphate (Eq. 1). NH4 + + Mg2 + + HPO4 2 + 6H2 O ! NH4 MgPO4 + 6H2 O
(1)
Apart from the chemical synthesis, waste/wastewater rich in phosphorus and nitrogen could be used as a resource for struvite synthesis. Press mud/wastewater contains a good amount of nitrates and phosphates, the recovery of struvite from which offers sustainability to the current sugarcane industries. However, the recovery of struvite from press mud offers the recovery of an agriculturally important fertilizer (P) apart from waste remediation. Additionally, struvite does not burn the roots due to its slow-releasing characteristics, which is common with traditional ammonium-phosphate fertilizers (Yan et al., 2018). Conventional waste treatment technologies limit the P removal and the recovery of P in the form of struvite offers sustainability to the current wastewater treatment units and accounts for economical benefits.
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Limitations and future prospective
Increasing sugar consumption demands greater sugarcane cultivation. Simultaneously, higher sugarcane cultivation is increasing the load on the cane-processing industries, thus leading to higher waste discharge. In this direction, the waste originating from sugar industries needs to be addressed with sustainable processes to avoid further environmental pollution. The presence of fiber, wax, and proteins makes sugarcane bagasse the most recalcitrant in nature, which limits its degradability and conversion efficiency. Hence, it cannot be easily separated into readily utilizable components and the available pretreatment techniques are the most expensive and least technologically mature steps available to convert biomass to fermentable sugars. Liquid residuals from pretreatment processes are rich in pentoses, soluble and insoluble lignin, etc. This offers a great potential for improvement in efficiency and the reduction of costs through future research and development. Currently, sugarcane molasses is widely used as a resource for bioethanol production and this leaves behind other secondary materials such as pentoses, vinasse, and effluent. The best use for the pentoses in the pretreatment liquor is ethanol production. However, the current processes using existing microorganisms lead to extremely low yields as a waste. In this context, utilization of sugarcane waste originating from different processes could be considered a renewable resource for the synthesis of various products such as MCC, NCC, xylitol, ethanol, hydrogen, methane, hythane, struvite, etc. This multiproduct approach using sugarcane waste as a resource in the biorefinery framework facilitates sustainability to the existing sugarcane industries.
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