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date: 21 November 2019

Food Waste and Biomass Recovery

Summary and Keywords

Agriculture waste can be a significant issue in waste management as its impact can be felt far from its place of origin. Post-harvest crop residues require clearance prior to the next planting and a common practice is burning on the field. The uncontrolled burning results in air pollution and can adversely impact the environment far from the burn site. Agriculture waste can also include animal husbandry waste such as from cattle, swine, and poultry. Animal manure not only causes odors but also pollutes water if discharged untreated. However, agricultural activities, particularly on a large scale, are typically at some distance from urban centers. The environmental impacts associated with production may not be well recognized by the consumers. As the consumption terminal of agricultural produce, urban areas in turn generate food waste, which can contribute significantly to municipal solid wastes. There is a correlation between the quantity of food waste generated and a community’s economic progress.

Managing waste carries a cost, which may illustrate cost transfer from waste generators to the public. However, waste need not be seen only as an unwanted material that requires costly treatment before disposal. The waste may instead be perceived as a raw material for resource recovery. For example, the material may have substantial quantities of organic carbon, which can be recovered for energy generation. This offers opportunity for producing and using renewable and environment-friendly fuels. The “waste” may also include quantities of recoverable nutrients such as nitrogen and phosphorus.

Keywords: agricultural waste, food waste, resource recovery, recycling

1. Introduction

Agricultural and food processing residues and discarded food are largely organic and biodegradable and may include important nutrients other than carbon. Inappropriate disposal of these have caused considerable adverse environmental impacts on air and water, and continue to do so in 21st-century Asia. While water pollution issues have often been local or national in nature, air pollution arising from the burning of post-harvest crop residues has caused trans-boundary concerns. Pollution control by means of treating the wastes for safe disposal need not, however, be the only approach, given that such an approach is seen as a cost by parties required to manage their wastes and so may not engender enthusiasm. Given the nature of the “waste” under discussion, a shift in mindset—toward these being another resource—may make its management more attractive because there is then possibility for economic gain.

Such shifts in mindset are already evident, and large-scale application efforts can be seen in Europe and North America, where the methods discussed have been practiced and there continues to be much research and development to improve methods. Asia’s interest in such applications has also grown since the last century. This is not surprising given the scale of its agriculture industry and urban centers—that is, there is much agricultural and food residues to address appropriately. Given the social (and technological) difficulties experienced in Asia until more recent times, feeding populations has taken precedence. This, however, is changing, and coupled with economic progress there is growing consciousness on the need for sustainability—not only in consumption but also production, and hence the growing interest in appropriate management of agricultural and food residues. This phenomenon is growing in East, Southeast, and South Asia.

One approach is the conversion of such wastes into added-value products through thermo-chemical (e.g., pyrolysis, gasification, and hydro-thermal) processes and resulting in products such as fuel gas, pyrolytic oil, and biochar. Biotechnology can often also be a suitable technique for such resource recovery. There is in addition the argument that conversion of waste to added-value products is possibly less harmful than is treatment to carbon dioxide and water in the longer term. Agricultural and food processing residues, given their high organic content, have been fermented to yield fuel gases such as methane (anaerobiosis and methanogenesis) and hydrogen (dark fermentation), and liquids such as butanol and ethanol. Biodiesel, produced from biomass by transesterification, is already used as a fuel to power vehicular engines. Bio-methanol is considered another biofuel with promise given its high energy content. These represent opportunities for recovering energy.

Derivable from the wastes’ organic content is a spectrum of other products, which can include useful microbes, phytohormones and antibiotics, enzymes, and polyhydroxyalkanoates (PHAs). Application of fermentation technologies on the wastes for production of added-value materials in itself results in residues. The latter with its still high organic carbon content and possibly significant quantities of phosphorus, potassium, and nitrogen can be used as a delivery platform (with some fertilizing value) for beneficial microbes for application on agricultural land damaged by excessive use of chemical fertilizers and pesticides and so rebalance the soil’s microbial ecology. The latter has become an issue requiring urgent attention as damaged farm soils result in lower crop yields. This need not only be an issue faced in rural farming or large-scale industrial farming but also in urban farming, where the bioactives biologically recovered from resources such as discarded food can find use in hydro- and aeroponics. The application of recovered resources from resides represents a tangible example of the circular economy. There are possibly two main routes toward recovering useful resources from the “waste”—biological and physico-chemical.

2. Routes to Resource Recovery

2.1 Bioprocesses

Bioprocesses involve use of microorganisms or enzymes under specific conditions to produce the desired products. The various steps in bioprocesses can include: (i) substrates preparation, (ii) inoculum introduction, (iii) the transformation of the substrate into the products, and (iv) end-products recovery. Bioprocesses for agriculture and food waste can be categorized into anaerobic and aerobic processes. In the anaerobic process, which occurs in the absence of oxygen, much of the chemical energy in the substrate would be transferred to the end products such as ethanol and methane. By contrast, the aerobic process utilizes oxygen as an electron acceptor so that the chemical energy in the substrate would be extracted by microbes for respiration and reproduction, and generates products such as carbon dioxide and water (Madigan, 2012). Anaerobic processes are often deployed to recover energy and pretreat raw material while aerobic processes are utilized to remove organic contaminants and treat effluent streams.

2.1.1 Bio-Gasification

Anaerobic digestion (AD), where digestion involves dissolved and also particulate materials, is often used for not only its lower energy requirements but also the possibility for energy recovery from biogas (Cao & Pawłowski, 2012). The AD process comprises four metabolic phases: (i) hydrolysis, wherein complex organic compounds (carbohydrates, proteins, lipids) are solubilized and converted to sugars, amino acids, and long chain fatty acids by hydrolytic bacteria, (ii) acidogenesis, where the hydrolyzed fractions are transformed into hydrogen, formate, acetate, and other volatile fatty acids (VFAs), (iii) acetogenesis, where acidogenesis byproducts are converted to acetic acid, as well as carbon dioxide and hydrogen by acetogenic bacteria, and (iv) methanogenesis, in which methanogens (acetoclastic and hydrogenotrophic) produce biogas (a mixture of methane, CH4, and carbon dioxide, CO2) from hydrogen, formate, and acetate (Figure 1). Biogas comprises 60–70% methane, 30–40% carbon dioxide, and trace amounts of hydrogen sulfide, hydrogen, nitrogen, and water vapor. In AD, about 14% of the energy available in the substrate is utilized for microbial growth (10% by fermentative bacteria, and 4% by methanogens). Most of the substrate’s energy, that is, about 86%, is therefore transferred to the end product, methane (Schink, 1997). AD process performance depends on parameters such as pH, temperature, alkalinity, residual VFAs, and ammonia. All these parameters need to be kept within their optimum ranges for efficient digestion and methane production (Appels et al., 2008).

Food Waste and Biomass RecoveryClick to view larger

Figure 1. Schematic of fate of organic matter in anaerobic digestion.

Methane

Table 1 summarizes methane yields from various agricultural and food wastes and the yield ranged from 0.12 m3-CH4/kg volatile solids (VS) to 0.68 m3-CH4/kg VS. Methane yields up to 0.68 m3-CH4/kg VS were reported for kitchen waste (KW) and the organic fraction of municipal solid waste (OFMSW) that had a high fraction of food waste. With primary and secondary wastewater sludge, 0.36–0.61 and 0.28–0.38 m3 biogas/kg VS have been reported, respectively. The various biogas yields can be linked to substrate composition in terms of the proteins, carbohydrates, and lipids present and operating conditions such reaction time, reactor type, and temperature (Kougias & Angelidaki, 2018).

The composition of agricultural waste varies with crop type. Typically, grain crops waste has moderately low moisture content and a high volatile solids (VS) fraction, but the readily degradable component may not be high. In contrast, kitchen waste can be more biodegradable given its higher moisture (>80%) and VS (~95% of total solids, TS) content. However, both wastes can have low nitrogen and phosphorous content relative to their high carbon content (Surendra et al., 2014). Thus, co-digestion of agricultural and food wastes with nitrogen rich wastes such as sewage sludge and animal manure improves the nutrients balance (i.e., biochemical oxygen demand (BOD):N:P = 100:3.5:0.5) and microbial activity. There is, however, also potential for process inhibition. Inhibitors can include sodium chloride, long chain fatty acids (LCFAs), and degradation products such as ammonia and VFAs. To mitigate such effects, blending helps dilute the inhibitory compounds, and provides better buffering capacity. The latter improves process stability and biogas generation (Tyagi et al., 2018; Bhatia et al., 2018).

Table 1. Methane Yields From Agricultural and Food Wastes (Adapted From Kougias & Angelidaki, 2018)

Organic Wastes

Type

Methane Yield (m3-CH4/kg VS)

Food waste

Kitchen waste (KW)

0.54–0.68

Organic fraction of municipal solid waste (OFMSW)

0.30–0.57

Agricultural waste

Barley

0.32–0.34

Corn silage

0.27–0.30

Fruit and vegetable waste

0.15–0.34

Meadow grass

0.28–0.39

Palm oil mill effluents

0.38–0.50

Rice straw

0.27–0.28

Ryegrass

0.14–0.36

Switchgrass

0.12–0.25

Wheat

0.25–0.32

Biogas is used as a fuel and substitute for natural gas or synthetic gas. Conventional gas burners need to optimize the air-to-gas ratio to burn biogas. If upgraded to natural gas quality, biogas could drive vehicles (Appels et al., 2008; SenterNovem, 2008). On a larger scale, biogas is combusted for heat to drive gas turbines or to generate steam for steam turbines to generate electricity. Energy recovery can be improved with combined heat and power (CHP) configurations, and gas turbines deployed then can range in size from the micro (<100KW) to the large (~4000KW). The CHP Partnership had argued that if CHP was implemented at the 544 U.S. wastewater treatment facilities with influent flow rates exceeding 5 million gallons per day (mgd) and that operate anaerobic digesters, approximately 340 mega watt hour (MWh) electricity could be generated (National Association of Clean Water Agencies [NACWA], 2010). The fuel cell can lead to distributed power generation, although it is still largely under development with limited commercial application. Biogas can be the fuel for hot fuel cells (>800°C). A key requirement is that the carbon dioxide generated does not inhibit the electrochemical process but dissipates heat.

Hydrogen

Hydrogen offers the prospect of reduced use of hydrocarbons (Rifkin, 2003). It is eco-friendly, as its combustion byproduct with oxygen is water. Hydrogen’s energy yield at 122 kJ/g is 2.75 times higher than that of fossil fuels. Its production uses dark fermentation (Li et al., 2008; Tyagi et al., 2014). Dark fermentation is the conversion process of organic substrates (mainly carbohydrates) to hydrogen in the absence of light sources. In this process (acidogenesis), acidogenic bacteria converts the complex organic feedstocks to H2, CO2, alcohols, and VFAs as byproducts (Figure 2). Anaerobes from anaerobic sludge digesters or a compost heap, or a pure culture (Clostridium sp.) could be the inoculum. The growth of hydrogen consumers (i.e., the methane producers) need to be suppressed in dark fermentation (Stabnikova et al., 2010).

Food Waste and Biomass RecoveryClick to view larger

Figure 2. Organic matter transformation to hydrogen via dark fermentation mediated by facultative and obligate microbiome (adapted from Roy & Das, 2015).

Hydrogen production from carbohydrates based wastes has been reported to be 20 times higher than that from fat- and protein-based wastes (Show et al., 2012). Food waste (FW) is therefore potentially a readily available and suitable feedstock (Tyagi et al., 2014). Hydrogen yields from food waste ranged from 0.057 m3 H2/kg VS to 0.160 m3 H2/kg VS (Kiran et al., 2014; Braguglia et al., 2018). Factors such as composition of a food waste, operating conditions (process temperature, time, organic loading), and pretreatment impact on the yield and rate of hydrogen production. Temperature, which regulates the metabolic pathways, affects composition of the soluble microbial products (SMPs). For example, lactate is prevalent at 35ºC, while butyrate is found at 50ºC (Braguglia et al., 2018). Thermophilic conditions improve hydrogenase activity by Clostridia, while suppressing the activity of H2 consumers. The pH governs the metabolic pathway, hydrogenase activity, and prevailing microbial community. A pH range of 5.0–6.5 favors acetate and butyrate production. However, pH above this range helps ethanol and propionate generation (Braguglia et al., 2018). Shorter hydraulic retention times (HRT) favor the faster reproducing hydrogen producers while longer HRTs support the hydrogen consumers (Kiran et al., 2014). Thus, process optimization requires hydrogen consumers to be washed out. HRTs of 1–4 days resulted in higher H2 yield (Braguglia et al., 2018).

Reactor configuration—that is, batch, semi-continuous, continuous, and one- or multiple-stage reactors (Show et al., 2012)—has an impact on hydrogen yields because of the plug-flow and complete-mix or the extent of realizing these conditions. The up-flow anaerobic sludge blanket (UASB) and anaerobic sequencing batch reactors (anSBR) have been effective in hydrogen production due to the higher biomass concentration retained (Kim, Kim, & Shin, 2009). Gavala, Skiadas, and Ahring (2006) reported that UASB hydrogen production rate (19.05 mmole H2/h.L) was higher than that of the continuous stirred tank reactor (8.42 mmole H2/h/L) at low 2h HRT. Wu and Chang (2007) used a composite material comprising polymethyl methacrylate (PMMA), collagen, and activated carbon to entrap biomass and observed a seven-fold yield increase in continuous over batch fermentation. The C/N ratio affected hydrogen production, and Lin and Lay (2004) reported that a C/N ratio of 47 provided for optimal hydrogen production. The specific hydrogen production rate increased with temperature from 33 to 39°C, then decreased as temperature was increased to 41°C, suggesting a mesophilic microbial consortium (Mu et al., 2006; Tyagi & Lo, 2013). Co-digestion of rice straw and sewage sludge (SS) (raw and heat- treated), which enhanced the C/N ratio (at 25), was studied by Kim et al. (2012). Tyagi and Lo (2013) reported high and stable hydrogen production (with 58% hydrogen and at 0.74 mmol H2/g VS added straw). Co-digestion of OFMSW and SS had 70% improvement in hydrogen yield compared with OFMSW fermentation only (Tyagi et al., 2014).

Overall conversion of organic wastes to hydrogen is, however, low at around 33% of the chemical oxygen demand (COD) removed. This is lower than the theoretical yield of 12 mol H2/mol glucose because a large fraction of the substrate’s energy is converted to organic acids (Kim & Kim, 2013). It has therefore been suggested that H2 production be integrated with bio-methanation and ethanol production to enhance overall economic feasibility (Kiran et al., 2014).

2.1.2 Liquid Biofuels

Liquid biofuels (e.g., biodiesel and bioethanol) are sustainable alternatives to nonrenewable petroleum fuels. Biofuels can be refined to hydrocarbon fuels and have advantages such as ease of transport, storage, and combustion, and hence greater flexibility in marketing (Domınguez et al., 2003). Furthermore, biofuels can be precursors for light aromatics such as benzene, toluene, and xylene. These have greater market value (Tian et al., 2011; Tyagi & Lo, 2013).

Bio-alcohol

Bio-alcohols (ethanol, methanol, and butanol) are fermented from sugars, starches, or cellulose derived from crops such as sugarcane and grains. These can be fuel for combustion engines (Rodionova et al., 2017).

Bioethanol—the most common alcohol used, is produced from enzymatic transformation of sugars by, typically, a yeast, Saccharomyces cerevisiae. Although starch had typically been used for industrial-scale ethanol production, corn glucose and cane sugar are used where there is abundant supply, such as in the United States and Brazil, respectively. Agriculture wastes (e.g., corn stover, rice and wheat straw, and sugarcane bagasse), food waste, and paper waste have been used, as these contain cellulose, hemicellulose, and lignin, and are low-cost substrates. Hemicellulose and cellulose account for 20–35% and 23–53% of plant matter, respectively, and form a substantial portion of the post-harvest residues. Conversion of such lignocellulosic waste into ethanol involves: (i) release of cellulose, hemicellulose, and lignin from the composite structure, (ii) sugar solution production from the released cellulose and hemicellulose, (iii) sugar fermentation for ethanol, and (iv) recovery of ethanol from the culture medium (Stabnikova et al., 2010). Ethanol production ranged from 44 to 144 g/L and 2.9 to 22 g/L for food and fruit wastes (potato and orange peel, banana stem), respectively (Bhatia et al., 2018). Given the variability in food wastes, various sources (household, cafeteria, diner) have resulted in ethanol concentrations from 8 to 88 g/L and ethanol yield up to 0.44 g/g, reducing sugars with a volumetric productivity of 0.61 to 2.85 g/L.h (Matsakas et al., 2017).

There are still few industrial-scale installations, although one to convert the organic fraction of municipal solid waste into ethanol with the OxyNol process, an acid hydrolysis process, was planned by Masada OxyNolTM. T BC International Corporation has constructed a plant in Louisiana that generated 30 million gallons of ethanol per year with sugar from sugarcane bagasse (Stabnikova et al., 2010).

Bio-butanol can serve as a fuel constituent for gasoline engines without any amendment. Compared to bioethanol, bio-butanol is less corrosive and hydrophilic, has higher calorific value, and is miscible with gasoline at higher levels up to 85%. Agricultural wastes (Cassava bagasse, rice straw, sugarcane bagasse) have been used, resulting in butanol concentrations, productivity, and yield of 2.29 to 22 g/L, 1 g/L.d to 7.2 g/L.d, and 0.11 to 0.42 g/g reducing sugars, respectively (Bhatia et al., 2018).

Bio-methanol is expected to have a significant role as a fuel. It has higher energy content per unit volume compared to compressed natural gas or liquefied petroleum gas, and minimal amendments would be required in the existing fuel distribution network. Methanol does not require anti-knock additives because of its high octane number and can reduce automotive emissions (Tyagi & Lo, 2013). Bio-methanol can be produced from starch and sugars, which are transformed into simple sugars and then to ethanol and methanol. Agricultural waste biomass and food waste can provide the starch and sugars (Rodionova et al., 2017).

Biodiesel

Biodiesel use has reduced emissions of carbon dioxide, sulfur oxides, and un-burned hydrocarbons; has easy biodegradability, less toxicity, and is safer for storage and handling (Gogate & Kabadi, 2009). Biodiesel comprises the esters of simple alkyl fatty acids that can be produced from lipids by trans-esterification with alcohol in the presence of a base, acid, enzyme, or solid catalyst (Tyagi & Lo, 2013). Biodiesel can be mixed with petroleum diesel or in pure form.

Some microbes can accumulate triacylglycerol (TAG), which is then enzymatically, chemically, or by mechanical means extracted. The recovered oil is transformed into biodiesel through transesterification. Biodiesel production from agricultural wastes such as corn stover, hardwood pulp, sugarcane bagasse, wheat straw, and oil palm biomass ranged from 2.7 (0.14 g/g glucose consumed) to 32 g/L (0.29 g/g carbon source consumed) and for food waste, fruit juice processing pulp, and restaurant oil waste from 1.3 to 7.3 g/L (Bhatia et al., 2018). Biodiesel can be used as is (e.g., B100) or blended with fossil-diesel in various proportions—for example, B2 (2% biodiesel: 98% fossil-diesel), B5 (5% biodiesel), and B20 (20% biodiesel) (Jain et al., 2011).

2.1.3 Chemical Production

Bioplastics

Polyhydroxyalkanoates (PHA) are naturally occurring and biodegradable polyesters of hydroxyalkanoic acids with mechanical strength comparable to petroleum based plastics (Lee & Yu, 1997; Akaraonye et al., 2010; Thomson et al., 2010). Poly-β‎-hydroxybutyric acid (PHB) and its copolymer poly (3-hydroxybutyrate-co-hydroxyvalerate [P(3HB-co-HV)]) are the most widely used (Chua et al., 1999) while PHA with longer chain carbon backbone (C6 and above) has been reported (Sun et al., 2007). Condensation of acetyl-CoA will synthesize short chain PHA with PHB as monomer while β‎-oxidation of fatty acids and de novo fatty acid synthesis would result in PHA with medium to long chain monomers (Tan et al., 2014). Substrates such as sugar, amino acids, short chain fatty acids, and anaerobic digestion effluent can be substrates for PHA production (Anderson et al., 1990; Yun et al., 2013; Jia et al., 2014; Sakamoto et al., 2014). PHA in microorganisms, particularly in bacteria, serves as a carbon and energy reserve and/or as sink for redundant reducing power or electrons when microbes are in stressed states (Anderson & Dawes, 1990; Lee, 1996). PHA-producing microbes gain competitive advantage when confronted with substrate scarcity. Stress conditions such as nitrogen scarcity can trigger PHA synthesis, and a few species such as Bacillus megaterium are capable of synthesizing PHA along with cell growth in nutrient rich mediums (Chen et al., 1991). Pseudomonas putida and Cupriavidus necator (formerly Ralstonia eutropha) are the most studied due to their high PHA content and diverse PHA produced. New PHA-producing bacterial strains are being discovered (López et al., 2012). PHA also play the role of a reducing power storage medium in poly-phosphate accumulating organisms under alternating aerobic and anaerobic conditions (Liu et al., 2001).

There are, however, challenges for PHA production and application, particularly from agriculture and food waste. The low PHA yield increases the cost of PHA. Although PHA content in dry biomass is high (~80% by weight) (Yang et al., 2010), the overall yield of PHA from consumed substrate is only around 10% (w/w). Simultaneous synthesis and degradation (Uchino et al., 2007) resulted in the lower observed accumulated PHA. The wide distribution of agriculture and food waste could nonetheless provide abundant and cheap carbon sources for PHA production. Aerobic bioprocess for PHA production with high cell density could, however, be hindered by the oxygen mass transfer rate (Tian et al., 2009). Consequently, substrates have been fed at 10 g/L or lower. The recovery of PHA is another challenge. As PHA is intracellularly produced, its recovery requires cell membrane rupture, and this would incur energy consumption (Tan et al., 2014). Due to its cost, application of PHA is still restricted to high-end medical cardiovascular products and drug carrier materials (Keshavarz et al., 2010) although PHA for packaging materials has been reported (Chen, 2010). To further expand application, post-modification of PHA and recombinant microbial strains are potential options (Wang et al., 2014).

2.2 Thermo-Chemical Processes

Thermo-chemical processes may provide an efficient route to disposal of agricultural and food wastes and to recover energy (Panwar et al., 2012). Compared with the bioprocesses, thermo-chemical conversion of agricultural and food wastes into fuels is potentially faster and more efficient in terms of resources recovery. The processes considered include direct combustion, gasification, pyrolysis, and hydrothermal treatment (Kumar et al., 2018). These produce combinations of heat, gaseous fuel, syngas, biochar, pyrolytic oil, and simpler organic compounds, respectively (Tradler et al., 2018).

2.2.1 Incineration (Direct Combustion)

Incineration involves combustion at 800 to 1000oC. The heat generated can be used to generate electricity (Kumar et al., 2018). The relatively simple and lower cost mass-burn process is the dominant technology used (Psomopoulos et al., 2009). The process triggers pyrolysis and so partially volatilizes organic compounds to combustible gases (Panwar et al., 2012). The resulting hot flue gas flowing pass the boiler produces superheated steam, which can drive a turbine to generate electricity (Kirubakaran et al., 2009). However, there are concerns with the bottom and fly ash, and flue gas generated as these may contain pollutants such as particulates, dioxins, furans, and volatile organic carbons (Sipra, Gao, & Sarwar, 2018). Table 2 shows the bottom and fly ash characteristics. Where there is low risk of environmental contamination, the ash may be suitable for use as absorbents for environmental protection. The bottom ash may also contain quantities of plant nutrients (i.e., P, K) suitable for agriculture (Wang et al., 2017).

Table 2. Typical Characteristics of Fly and Bottom Ash

Density (kg/m3)

Size

Specific Gravity

Chemical Composition

Morphology

References

Fly ash

2300–2600

1–200 μ‎m

1.90–2.96

Network formers (SiO2, Al2O3, Fe2O3, TiO2, and P2O5); Network modifiers (CaO, MgO, Na2O, and K2O)

Irregular-shaped minerals and unburned carbon

Xu & Shi. (2018)

Bottom ash

510–2000

>100 mm after the removal of oversized fraction (20–50 mm)

2.15–2.65

SiO2 (37.4%), CaO (22.2%), Al2O3 (10.2%), Fe2O3 (8.3%), Na2O (2.8%), SO3 (2.7%), P2O5 (2.3%), MgO (2.0%), K2O (1.4%)

Irregular, angularly shaped, rough-textured particles

Dhir et al. (2018)

2.2.2 Gasification

Agricultural and food wastes can be endothermically converted into combustible gases through partial oxidation at high temperatures (700–1400oC), and in the presence of gasifying agents such as water vapor, oxygen, air, or various combinations of these (Agrela, Cabrera, Morales, Zamorano, & Alshaaer, 2019). Gasification uses less oxygen than incineration and hence has lower air movement requirements. The oxygen deficient environment reduces the generation of dioxins (Pham et al., 2015). The relatively large amounts of carbohydrates (e.g., rice, noodles, and vegetables) present in Asian food wastes can be readily gasified into syngas (Ahmed & Gupta, 2010). Early process products, such as char and tar, react with the gasifying agent (CO2 and H2O) to produce syngas, which is primarily carbon monoxide, hydrogen, and methane (McKendry, 2002). Syngas can be used as a fuel and as a feedstock for production of higher value-added chemical products (Kirubakaran et al., 2009). Nutrients in the agricultural and food wastes can be captured in the biochar and hence recovered (Silber et al., 2010). The biochar’s characteristics, for example, pH, surface functional groups, cation exchange capability, porosity, reactivity, hydrophobicity, surface charge, and structure, would be influenced by the gasification temperature. The latter is therefore adjusted to achieve the desired product quality (Kavitha et al., 2018).

2.2.3 Pyrolysis

Pyrolysis converts organic materials, in the absence of oxygen and at temperatures of 250–800oC, into char, oil/wax, tar, and syngas (Chen et al., 2014). The pyrolysis process comprises—(1) initiation (cleavage of C-C bonds in the polymer chains to form radicals), (2) propagation (hydrogen abstraction, and β‎-decomposition reactions), and (3) termination (second-order termination reactions) (Sipra et al., 2018). Pyrolysis can be classified as slow (heating rates at < 10oC/min, and residence time in minutes or hours) or fast (heating rate at ~1000oC min–1, and residence time in seconds or a few minutes) (Pham et al., 2015). Char (35%), tar (30%), and syngas (35%) are the products in slow pyrolysis, whereas bio-oil is the predominant product in fast pyrolysis (around 70% of raw biomass) (Opatokun et al., 2015). Char is the devolatilized organic solid, while tar refers to the condensable volatiles (Demirbaş, 2002). The char can be used for soil amendment and as adsorbents for removal of heavy metals in acidic wastewaters (Dai et al., 2018), and phosphorus (Yao et al., 2011), furan (Monlau et al., 2015), and cadmium in aqueous solution (Liu et al., 2011). The tar can be used as feedstock for specialty chemical production or combusted directly for heat (Li & Suzuki, 2010). The syngas comprises hydrogen (H2) and carbon monoxide (CO), with minor proportions of methane (CH4), carbon dioxide (CO2), and water, as well as trace amounts of volatile organic compounds (Dai et al., 2018). It is used as a fuel, and as building blocks for producing chemicals and fuels (e.g., methanol) (Pham et al., 2015). For example, syngas has been used as feed for fermentative microorganisms generating bioethanol, butanol, and hydrogen (Monlau et al., 2015). Almost 50% of raw food wastes can be converted into bio-oils (Opatokun et al., 2015). The composition of bio-oil includes carbamic acid, phenyl ester, silane, benzene, hexanoic acid, trimethylsilyl ester, propanoic acid, ethenone, undecane, and benzofuran (Opatokun et al., 2015). The pyrolytic products vary in terms of yields and compositions due to operating parameters such as temperature, heating rate, residence time in the reaction zone, material size, contact time, pressure, and the catalyst used (Williams & Slaney, 2007). Typical characteristics of the pyrolytic products are outlined in Table 3.

Table 3. Characteristics of Bio-oil, Char, and Syngas Pyrolyzed From Various Agricultural and Food Wastes (Adapted From Kan, Strezov, & Evans, 2016)

Pyrolysis Species

Pyrolysis Conditions

Product Yields (wt%)

Oil Composition

Bio-oil

Char

Syngas

Single-pass corn stover (ensiled)

500 °C in a free-fall pyrolyzer

55

25.5

16.2

46.7% C,

14.7% H,

0.5% N,

38.1% O

Corn stover

400°C for 20 min in a batch pressure reactor

31

37

15

78% C,

9% H,

1.9% N,

10.6% O

Corn stover

Fast pyrolysis at 500 °C in a fluidized bed

61.6

17

21.9

54% C,

6.9% H,

1.2% N,

37.9% O

Wheat straw

400 °C in a circulating fluidized bed reactor

46

47

7

34.5% C,

8.2% H,

0.8% N,

56.5% O

Acid-treated wheat straw

400 °C and τ‎=1 s in a circulating fluidized bed reactor

57

38

5

41.3% C,

7.5% H,

0.9% N,

50.3% O

Soybean cake

5°C/min to 550 °C in sweeping N2

30 (oil) + 20 (water)

25

25

62.2% C,

8.3% H,

7.5% N,

22.0% O

Soybean (Glycinemax L.)

400 °C with a heating rate of 50 °C/min

25.81

23.56

Others

67.9% C,

7.8% H,

10.8% N,

13.5% O

Soybean cake

300 °C/min to 550 °C in sweeping N2

39 (oil) + 22 (water)

21

18

67.2% C,

9.0% H,

10.8% N,

13.0% O

Switchgrass (Panicum virgatum)

6°C/min to 600 °C and then 20 min at 600 °C

37

25

26 (syn-gas)

50% C,

9.3% H,

1.5% N,

0.6% S,

37% O

Switchgrass (Cave-in-Rock variety)

480 °C in fluidized bed

60.7

12.9

11.3 (non- condensable)

/

2.2.4 Hydrothermal Treatment

The hydrothermal process degrades organic materials into simpler compounds in the presence or absence of oxidants at elevated temperatures (120–550 °C) and pressures (20–150 bar) and solubilizes these in water (Baroutian et al., 2013). Direct processing of waste in the forms of liquid and slurry without prior treatment is the advantage of the hydrothermal process (Baroutian et al., 2016). There are four hydrothermal processes: thermal hydrolysis, wet oxidation, hydrothermal carbonization, and hydrothermal liquefaction (Munir et al., 2018). The operating parameters for these are listed in Table 4.

Table 4. Operating Parameters for Four Types of Hydrothermal Processes (Adapted From Munir et al., 2018)

Temperature (oC)

Pressure (Bar)

Reaction Type

Final Products

Thermal hydrolysis

120–200

20–150

Non-oxidative

Valuable chemicals (acetic, formic-butyric acids, methanol) and natural fertilizers (nitrogen, phosphorus)

Wet oxidation

150–300

20–150

Oxidative

Valuable chemicals (e.g., acetic formic, maleic acid, alcohols, acetone, volatile fatty acids), nitrogen, phosphorus

Hydrothermal carbonization

150–350

13–70

Non-oxidative

Biochar, sugars, nano-structured carbon, and organic acids

Hydrothermal liquefaction

250–550

50–250

Non-oxidative

Crude oil, sugar, amino acids, biofuels, bio-methane, bioethanol, and fatty acids

Hydrothermal processes function through oxidation, hydrolysis, thermal decomposition, and dehydration (Baroutian et al., 2013). Performance is determined by reaction temperature and time, feedstock type, and pressure (Munir et al., 2018). However, understanding of the reaction and mass transfer kinetics is not clear yet. Hydrothermal treatment is therefore still largely at the laboratory or demonstration scales, with few full-scale commercial systems reported (Akhtar et al., 2012). Transformation of batch hydrothermal processing into a continuous self-regulated process would be desirable. Such a configuration would, however, incur costs associated with continuous feeding at high pressure and high temperatures (Elliott et al., 2015). To reduce the impact of capital costs, process integration and process intensification through combining with other processes such as AD, alkaline treatment, and struvite precipitation may be necessary (Sun et al., 2015; Yao et al., 2016).

3. Full-Scale Experiences in Asia

Bioenergy, a major renewable energy source, accounted for only 10% of global energy supply (573 exajoule, EJ). The biomass required for bioenergy generation is provided by forestry (87%), agriculture (10%), and municipal solid waste including landfill gas (3%) (International Energy Agency [IEA], 2014; Kummamuru, 2017). Asia leads (26 EJ) among the continents in deriving primary energy supply from biomass, followed by Africa (15.4 EJ), the Americas (10.7 EJ), Europe (6.71 EJ), and Oceania (0.26 EJ). Much of the municipal solid waste–derived bioenergy is in Europe (0.82 EJ), followed by Asia (0.40 EJ). The Americas are the world leaders in producing bioenergy from liquid biofuels (2.3 EJ), followed by Europe (0.65 EJ). In terms of biogas, Europe leads (0.63 EJ). Of the total biomass-based energy supply (59 EJ), the highest comes from solid biomass (52.6 EJ) followed by liquid biofuels (3.2 EJ), municipal waste (1.32 EJ), biogas (1.27), and industrial waste (0.80) (IEA, 2014; Kummamuru, 2017).

3.1 East Asia (China)

Agricultural wastes such as post-harvest crop residues, livestock manure, harvest processing residues, and general rural wastes are sources of organic and other useful components (Dai et al., 2018). Post-harvest residues and manure are the major components of agricultural wastes, and the provinces of Henan, Shandong, and Heilongjiang are the largest waste straw producers (Jiang et al., 2012). The annual crop residues production in China is 600 million–800 million tons (Jiang et al., 2012). This is projected to increase to 850 million tons by 2020 (Jia et al., 2018). China’s Ministry of Agriculture reported that 2.14% of crop residues was used for mushroom production, 2.37% for industrial material production, 14.78% application as fertilizer, 18.72% bioenergy, and 30.69% animal feed. A balance of 31.30% of crop residues was left on the fields (Jia et al., 2018), and this contributed to on-field burning. Composting has been recommended as an economical way to convert manure and crop residues to value-added products (Bernal et al., 2009). Large-scale composting plants have been established to reduce the volume, odor, moisture, and pathogen content in agricultural waste (Onwosi et al., 2017). It has been reported that 24.9 million tons of commercial organic fertilizer was produced from 3,021 factories in 2008 (Jia et al., 2018).

The presence of lignocellulose compounds and the tight structures in cellulose and hemicellulose make crop residues difficult for AD and result in low methane yield. By the end of 2010, only 273 straw biogas plants (47 medium- and large-scale) had been built (Jia et al., 2018). Hence, the pretreatment of crop residues is a prerequisite before AD. Alkaline hydrolysis, steam explosion, and microbial/enzymatic pretreatment are the methods used.

The annual production of animal manure is about 1,400 million tons, with 45.95% from fattening pigs, 25.5% from beef cattle, 15.4% from dairy cows, 9.05% from poultry layers, and 4.1% from broilers (Li, Liu, & Sun, 2016). Livestock manure has largely been channeled into biogas generation and use as fertilizer since the 1950s. Two models have been promoted: the “Pig-Biogas-Fruit” model and the “Four in One” model. The former was used in South China for ecological and sustainable agriculture. Pig manure was anaerobically digested and the biogas generated used for lighting up communities. The anaerobic digestate was used as fertilizer for fruit trees and harvested fruits fed to the pigs, forming a circular mass flow in the agricultural system. The “Four in One” model was designed for application in the colder climate of northern China. A pigpen, a toilet, and a biogas digester were built in a greenhouse wherein temperature suitable for the AD of pig manure, vegetable residues, and other organic wastes could be maintained. The biogas was burned to supply heat, carbon dioxide, and lighting for the greenhouse growing vegetables (Li et al., 2016). The Chinese government has invested 36.4 billion Yuan to support the biogas industry from 2008 to 2018. By 2010, more than 40 million biogas plants had been built, among which 27,410 medium- and large-scale plants dealt with manure (Chen et al., 2012). Almost 65% of these plants were continuous stirred tank reactors (CSTRs) and upflow solids reactors (USRs) (Chen et al., 2012). Biogas utilization in China typically involved piping the gas to households for cooking, which helped solve the rural energy shortage problem. Biogas for power generation accounted for only 3% of the total biogas produced—supplying 20 to 600 kW power with conversion ratios of 0.6–0.8 kW•h/m3 (Li et al., 2016). Representative biogas power plants include those from Minghe Animal Husbandry Company Limited in Shandong Province (500 t/d chicken manure, 60,000 kW•h/d), Mengniu Model Dairy Farm in Inner Mongolia (10,000 t/d cattle manure, 18,000 kW•h/d), and Beijing Deqingyuan Agricultural Technology Company Limited (2,600,000 t/d layers’ manure, 38,000 kW•h/d). Biogas for vehicles was initiated in 2009 by the Anyang Zhongdan Biomass Energy Company in Henan, which planned to treat 185,000 tons of organic waste and produce 4 million m3 biogas (Yin & Weng, 2011). In practice, numerous difficulties (e.g., poor maintenance and lack of proper technical support) have been associated with biogas plants in China, and only 60% of built plants had been operated as per design in 2007. As the AD of single substrates such as manure showed compromised performance due to inhibition induced by ammonia and nutrient imbalance, co-digestion with other wastes has been adopted (Chen et al., 2012). A large-scale plant has been operated in Tongzhou District, Beijing, on 30 tons of dairy manure and 30 tons of dairy wastewater at 38°C, and this generated biogas that produced 160 kW electricity with a CHP system (Yin & Weng, 2011).

Food waste production was 83.95 million tons in 2014, 91.10 million tons in 2015, and was projected to increase to 126.85 million tons by 2020 (Li, Li, & Jin, 2016). The largest portion of food waste (43.18%) is generated in eastern China, followed by western and central China (Yang, Bao, & Xie, 2019). The quantity of food waste generated could be correlated with the level of urbanization. The annual quantity of food waste has been estimated to have an equivalent food worth of more than 200 billion Yuan (Li, Li, & Jin, 2016). Chinese dietary habits result in food waste with high moisture, salinity, organic, and oil contents (Gao et al., 2017). Given that rice and noodles are main staples, the carbohydrate content varies from 30.3 to 75% of carbon content while nitrogen fluctuates from 0.8% to 30.3% (Dai et al., 2016). If not appropriately managed, food waste can release odors and greenhouse gases (Cerda et al., 2018).

Reduction, reuse, and recycling of food waste are targets the Chinese government set for a circular economy (Ghisellini et al., 2016). For example, 100 pilot projects completed in 2011–2015 considered various technologies to enhance food waste resource recovery; the treatment capacities ranged from 20 to 3,990 tons per day. Among these, 76.1% adopted AD, 3.3% combined composting with AD, 5.2% used aerobic composting, 8.9% used fast aerobic composting, and 6.4% generated fertilizers.

In China, food waste had historically been collected for use as animal feed, but increasingly hygiene and food safety awareness has reinforced the need for preparation processes such as boiling, dehydration, drying, and sterilization (Pham et al., 2015). There are, nonetheless, still potential hazards (e.g., pathogens, heavy metals, and organic pollutants). Outbreaks of the African swine fever has, at least in part, been linked to this usage. Such outbreaks resulted in banning the use of food waste as animal feed. The high water content in Chinese food wastes (about 80% w/w) adversely impacts the incineration option. Food waste composting has been a favored option due to its relative simplicity of operation, positive economic benefits, and fertilizer product (Cerda et al., 2018). Small composting machines of 1–5 tons per day capacity have been widely used in situ at restaurants or canteens. However, this technique has also met with challenges: (1) the long duration of composting for a stable product, (2) issues with acidity and odors, and (3) possible heavy metals contamination (Jin, Li, & Li, 2016). In recognition of heavy metal contamination possibility, the Ministry of Agriculture of China has limited application of compost from food wastes on garden and horticulture soils. All these reasons collectively constrain the wider use of fertilizer from food waste.

AD is the most common method used for food waste treatment in China, accounting for 76.1% of all treatment capacities. About 54.1% of the AD facilities have treatment capacities larger than 200 ton/day, while only 1.6% of the facilities have treatment capacities below 100 ton/d (Li et al., 2016). Table 5 lists examples of full-scale AD of agricultural and food waste in China. For example, the Tai-an Food Waste Treatment Plant operated by Beijing Zhongke Jieneng Environmental Engineering Technology Co., Ltd., treated 200 ton/d food waste. The biogas produced was used for electricity generation, while the liquid effluent was sent to a landfill plant for further treatment and the digestate incinerated for heat generation. AD is usually perceived as a “slow” process impacted by dynamic feed properties and inappropriate management procedures (Chen et al., 2008, Xu et al., 2018). In the case of China, preliminary separation is still not widespread, and food waste is typically mixed with municipal solid waste—a consequence of inadequate public awareness, education, legislation, and enforcement (Zhang et al., 2010). The separation of non-biodegradable impurities (such as bones, plastics, and metals) can improve overall performance in terms of process and mechanical equipment reliability (Li et al., 2016). Three-phase separation encompassing a liquid phase for AD, a solid phase for fertilizer production, and an oil phase for biodiesel production has been proposed as an efficient process for facilitating the use of food waste (Jin et al., 2016). To address issues associated with the high protein content and the consequent inhibitory effects of free ammonia and long-chain fatty acids on the anaerobic process, co-digestion of food waste with other wastes with low nitrogen and lipid content (e.g., sewage sludge and municipal solid waste) has been helpful (Pham et al., 2015). Concomitant benefits such as increased biogas yields and better buffer capacity can be achieved (Zhang et al., 2010). Thermal pretreatment is another method that can achieve sterilization, homogenization, and solubilization (Jin et al., 2016). Thermal pretreatment below 110oC has been reported as the most sustainable method for enhancing the efficiency of food waste AD (Li, Li, & Jin, 2016).

Table 5. Examples of Full-Scale AD of Agricultural and Food Waste in China (Adapted From Huanbao)

Feed Substrate

Processing Capacity (ton/d)

Final Products

Province

Year for Operation

Food waste and cooking oil

146–240

Biogas, biodiesel

Jiangsu

2015

Food waste

600

Biogas, biodiesel

Jiangsu

2012

Food waste

200

Biogas, lipid

Shanghai

2016

Food waste and other organic waste

150 ton/d food waste and 150 ton/d other organic waste

Biogas, lipid

Zhejiang

2013

Food waste and sludge

100 ton/d vegetable waste, 100 ton/d food waste, and 300 ton/d sewage sludge

Fertilizer, biogas

Shenzhen

2015

Food waste, peanut shells, corn husks, wheat bran

400

Bacterial manure

Beijing

2010

Food waste

200

Food waste

Shandong

2016

Waste oil and food waste

200–500

Biogas, biodiesel

Yunnan

2011

Food waste

167–480

Biogas, lipid

Chongqing

2014

3.2 Southeast Asia

3.2.1 Food and Horticultural Waste Management in Singapore

Singapore is a highly urbanized tropical island city-state with a small agricultural industry that contributes to less than 10% of food consumed (Agri-Food & Veterinary Authority, 2018). Agricultural residues from production common in other Asian countries are not therefore a significant issue. Food waste from consumers does, however, make up a large portion of the organic waste generated. The other significant component is the green waste generated from landscaping, categorized as horticultural waste. In 2017, 809,800 and 328,300 metric tons of food and horticultural waste, respectively, were generated in Singapore (National Environment Agency, 2018b). The recycling rates for food and horticultural waste were 16% and 67%, respectively. Non-recycled organic waste was disposed through incineration for energy recovery (National Environment Agency, 2018a) with the consequent incinerator bottom ash transported to the offshore Pulau Semakau landfill.

Horticultural waste can be used at biomass power plants (National Environment Agency, 2018b). The two biomass power plants are operated by ecoWise Holdings Limited. Heat from these plants is utilized for industrial drying and electricity generation with steam turbines (ecoWise Holdings Limited, 2018). Composting is the other method adopted. Two composting plants are operated by Kiat Lee Landscape & Building Private Limited. In composting, a thermophilic phase is included to deactivate pathogens and pests and hence pasteurize the product. The product compost is applied as soil conditioner at nurseries and public green spaces (Kiat Lee Landscape & Building, 2018).

Food waste generation increased at about 4.1% per annum between 2008 and 2017. This increase is expected to continue as the population and economic activities continue to expand. The Singapore National Environment Agency (NEA) manages food waste through a four-step program: (1) prevent and reduce food waste at the source, (2) redistribute excess food, (3) recycle/treat food waste, and (4) recover energy (National Environment Agency, 2018a). This program forms part of the Reduce, Reuse, Recycle (3Rs) solid waste management system. Prevention and reduction is the preferred approach. In 2014, the NEA undertook studies to understand consumer behavior on food procurement, usage, and disposal. Results indicated the top reason for food wastage in households was food approaching the “use by date.” NEA has launched outreach programs to educate consumers on ways to reduce food wastage, such as redistribution of excess food to avoid excessive storage. Businesses are encouraged to donate excess food to food distribution organizations for dispersal to the needy. Nonetheless some food waste will inevitably be generated and AD, animal feed conversion, and composting are used for resource recovery. The first full-scale anaerobic digester for food waste was operated by IuT Global Private Limited (IuTG) (Wee et al., 2015). Food waste collected from food factories, shopping malls, and public and commercial dining locations was subjected to the thermophilic (55°C) AD process to produce biogas. Phase I of the bio-methanation plant started operations in 2007 with a treatment capacity of 300 metric tons of food waste per day. Biogas produced was expected to drive gas engines to supply 3.5 MW of electricity. A second phase was planned to increase the treatment capacity to 500 metric tons of food waste per day and electricity production to 6 MW (Tong et al., 2018). Unfortunately, IuTG encountered difficulties in food waste collection and inadequate amounts were collected to meet the design capacity. For example, in March 2011 only 110 metric tons of food waste per day could be collected. The food waste was also contaminated with non-biodegradable material (e.g., metals, plastics), which needed costly manual separation. Food waste generators were then resistant to separating inorganic material at the source. These difficulties resulted in the plant’s closure in 2011 (Wee et al., 2015). In 2015, a demonstration plant was started to investigate the feasibility of AcoD of food waste with sewage sludge. This was found to produce more biogas compared to sewage sludge digestion (Dai et al., 2013) because of the food waste’s higher calorific value and complimentary nutrient content of sewage sludge. The demonstration plant treated 40 metric tons of food waste–sewage sludge mixture per day. It is anticipated that a full-scale anaerobic co-digestion plant will be built at an integrated solid waste treatment facility and water reclamation plant in the western part of Singapore (National Environment Agency, 2018a). Food waste can also be converted to animal feed, but this would require a homogenous waste stream from sources such as food factories. The material is then dried and pelletized. In a biomass power plant operated by ecoWise Holdings Limited, the heat generated is utilized to produce animal feed (ecoWise Holdings Limited, 2018). Food wastes such as spent grains and soybean residues are suitable for this application. Composting is also practiced by businesses that generate smaller amounts of heterogeneous food waste. Hotels, shopping malls, and hawker center operators operate composting facilities onsite (National Environment Agency, 2018a). At the bottom of the food waste treatment hierarchy is energy recovery by incineration. Non-recycled food waste is sent to the waste-to-energy (WTE) plants for energy recovery.

3.2.2 Palm Oil Biomass Waste in Indonesia and Malaysia

In Southeast Asia (other than Singapore), agricultural biomass waste is estimated to be able to generate more energy than forestry and livestock biomass waste. The palm oil industry is the third largest biomass source, following rice and sugarcane farming (Stich et al., 2017). Indonesia and Malaysia are the two largest palm oil producers in the world (Umar et al., 2018). In 2017, Indonesia and Malaysia produced 36 and 21 million metric tons of palm oil, respectively, which accounted for 86% of the world’s palm oil production that year (Iskandar et al., 2018). They also produced the most palm oil biomass waste, 52.4% and 41.8%, respectively. The palm oil industry has significant economic standing in the region because of its scale and status as a cash crop.

The biomass waste can be from the plantations and the mills. Direct usage as fuel for biomass power plants to generate heat, steam, and electricity is a widely applied method for palm oil biomass waste management from plantations and the mills (Shuit et al., 2009; Umar et al., 2014; Loh, 2017; Kamahara et al., 2018; Rahayu et al., 2018). Biomass power plants quickly dispose of biomass waste and meet the energy requirements of plantations and palm oil processing plants that have constrained access to the main power grid (Nasution et al., 2014).

Other than on-site usage, palm oil biomass waste is converted into fuel products that can be easily transported and traded. Fuel products in solid, liquid, and gaseous forms have been produced through various physio-chemical or biochemical processes. Solid fuel products include fuel pellets (Brunerová et al., 2018) and briquettes (Awalludin et al., 2015). Liquid fuel products include fuel oil from pyrolysis (Sulaiman et al., 2011; Awalludin et al., 2015; Yanti et al., 2018) and alcohols from fermentation (Shuit et al., 2009; Sari et al., 2018). Gaseous fuel in the form of biogas, hydrogen, and syngas has gained interest (Diah et al., 2018; Sulaiman et al., 2011; Hamidi et al., 2018; Sari et al., 2018).

Besides energy generation, palm oil biomass residues have also been made into soil conditioners and animal feed (Kamahara et al., 2018). The high nutrient and low metal content of palm oil biomass is beneficial for plant and animal growth (Loh, 2017). Application of such soil conditioners on plantations ensures that nutrients are returned to the soil and makes plantation operations more sustainable.

3.3 South Asia (India)

In India, the direction for agricultural wastes is conversion into sustainable, non-polluting. and economic sources of energy through the lighting up communities program. Production of renewable energy from urban, industrial, and agricultural waste residue is high on the societal improvement and political agenda of the Indian government. A gasifier system combusting forest wastes, agro-wastes, and sawmill dust at the rate of 50–150 kg/h has been installed at Nohar, Rajasthan. Approximately 25% of total fuel gas produced was consumed to operate the process, with the balance used for electricity production (Ahsan, 1999). Gasification did not, however, appear to have become widespread subsequently. The Planning Commission reported (2014) that there were 279 installations for composting, 138 for vermicomposting, 172 for biogas generation, 29 for pelletization (refuse derived fuel, RDF), and 8 WTE plants. India’s agriculture sector holds considerable promise for biomass power generation. The sector generates 686 MT agricultural waste annually, of which 234 MT (34%) has been projected as available for bioenergy production. Uttar Pradesh is the largest producer of crop residues among the 28 states and sugar cane contributes the largest component of residues, followed by rice paddies. The projected annual bioenergy potential from this agricultural biomass is 4.15 EJ, which is equivalent to 17% of India’s overall primary energy utilisation (Hiloidhari et al., 2014). The Ministry of New and Renewable Energy (MNRE) has reported 288 biomass power and co-generation plants with a combined capacity of 2665 MW. The fuel used at these facilities included sugar cane, rice husks, bagasse, straw, groundnut shells, coconut shells, cotton stalks, soya husks, jute waste, sawdust, and coffee waste.

In urban settings, the Ministry of Urban Development (MoUD) has reported seven operational WTE plants of 92.4 MW capacity total, four non-operational plants of 40.6 MW capacity total (due to technical reasons), 31 plants under construction with a capacity of 241.8 MW, and 21 plants at the tendering phase of 163.5 MW capacity. The total power generation capacity of all the WTE plants treating municipal solid waste (MSW) was anticipated to be 538.3 MW (MNRE, 2016). One such plant, the Okhla landfill WTE plant in New Delhi, combusted 1,350 t/day of solid waste and produced 16 MW of electricity (Kalyani & Pandey, 2014). The Ministry of New and Renewable Energy has estimated there is the potential for deriving some 1,700 MW of energy from urban organic waste (1,500 MW from MSW and 225 MW from sewage) along with 1,300 MW of energy from industrial waste (Mazumdar, 2013; Dhar et al., 2017).

The MNRE is looking toward AD as a means of producing renewable energy from municipal, industrial, and agricultural wastes. The first full-scale AD system treating 300 t/day of MSW to produce 5 MW electricity at Lucknow has been commissioned at a cost of 760 million rupees. The treatment stages include screening, manual sorting, magnetic separation, size reduction, ballistic separation, pulping, and grit removal prior to feeding to the digesters. Another bio-methanation system treating 30 tons of vegetable waste to produce power (230 kW/day) and compost (10 t/day) was installed at Koyambedu Wholesale Market Complex in Chennai at a cost of 50 million rupees (Joseph, 2014). Under the biogas development program, about five million domestic biogas plants (40%) have been set up, with the overall potential of 12 million domestic biogas plants (Central Statistics Office (CSO), 2014). In addition, 400 biogas off-grid power plants have been installed with electricity production capacity of 5.5 MW (MNRE, 2016). There has, however, been a report that there are, for various unstated reasons, only 56 functioning biogas-based power plants in India, and the majority of these are in the states of Maharashtra, Kerala, and Karnataka (Central Pollution Control Board (CPCB), 2013).

The total biogas generated in India is 2.07 billion m3/year, but this is low compared to the potential capacity, which is in the range of 29 billion–48 billion m3/year (Hiloidhari et al., 2014; Mittal et al., 2018). In Panjab, a biogas bottling project with 600 m3 biogas/day capacity generated from a mix of kitchen waste, cattle dung, and poultry waste, was implemented in 2011. The process is based on multistage UASB technology and upgraded biogas with 98% CH4 and has gas filled into cylinders under pressure (150 bar). Another plant of 2 metric ton/day capacity, treating kitchen waste from hotels, such as vegetables and other green wastes, was installed in Mangalore City. This produced 100–160 m3 methane/day. A bio-methanation plant of 2 metric ton/day capacity operates at Palayam, Thiruvananthapuram, generating 30 kW of electricity per day. The digester feed includes residues from the fish and vegetable market and wastes from about 1,500 households (Dhar et al., 2017). In Thiruvananthapuram, 20,000 domestic anaerobic digester units have been installed by the firm Biotech. This has diverted 2.5% of organic waste from landfills and saved 225 million Indian Rupees (INR) yearly in solid waste transportation to landfills. It has also helped to reduce the release of the equivalent of 7,000 tons of carbon dioxide into the atmosphere every year (Annepu, 2012). The scale of plants has grown over the years, presumably as confidence in the technology grows. A commercial bio-methanation plant producing 25,000 m3 biogas/day from 600 m3 of sugar cane waste has been installed in Satara, Maharashtra, while a 5,000 m3 biogas/day biogas facility treating agriculture waste such as banana stems, potato peelings, animal dung, and sugar cane waste was installed in 2015 at Sundarpur of Gujarat. The biogas generated has been supplied to neighboring industrial users.

In India, mechanical composting and vermicomposting are more popular than WTE. Composting/vermicomposting-based waste processing plants have been implemented in the Indian states of Andhra Pradesh (32), Chatishgarh (15), Delhi (3), Goa (5), Haryana (2), Gujarat (86), Himachal Pradesh (13), Karnataka (5), Kerala (29), Madhya Pradesh (4), Maharashtra (125), Meghalaya (2), Orissa (3), Punjab (2), Rajasthan (2), Tamil Nadu (3), Tripura (13), Uttarakhand (3) and West Bengal (9) (CPCB, 2013). In Vijayawada, Andhra Pradesh, a 150 t/day composting plant (ultimate design capacity: 300 t/d) based on the “build-own-operate” model, was installed in 1997. The plant initially produced 40 MT compost per day. The operating costs of the facility include: rental at INR 1/m2/year for 3.36 hectares leased for 30 years, Rs. 0.225 million per year as depreciation cost for existing infrastructure (buildings, machinery, equipment, etc.), and 2.5% royalty on net sales or Rs. 35 per metric ton of manure produced (Joseph, 2014).

Abbreviations

  • AcoD

    anaerobic co-digestion

  • AAD

    acidogenic anaerobic digestion

  • AD

    anaerobic digestion

  • AnSBR

    anaerobic sequencing batch

  • BOD

    Biochemical oxygen demand

  • CHP

    combined heat and power

  • COD

    chemical oxygen demand

  • CSTR

    continuous stirred tank reactor

  • FW

    food waste

  • HRT

    Hydraulic retention time

  • KW

    kitchen waste

  • LCFAs

    long chain fatty acids

  • MNRE

    Ministry of New and Renewable Energy

  • MoUD

    Ministry of Urban Development

  • MSW

    municipal solid waste

  • NEA

    National Environment Agency

  • OFMSW

    organic fraction of municipal solid waste

  • OLR

    organic loading rate

  • PAO

    polyphosphate accumulating organisms

  • PHA

    Polyhydroxyalkanoates

  • PHB

    Poly-β-hydroxybutyric acid

  • PMMA

    polymethyl methacrylate

  • SMPs

    soluble microbial products

  • SS

    sewage sludge

  • TAG

    triacylglycerol

  • TS

    total solids

  • UASB

    up-flow anaerobic sludge blanket

  • VFAs

    volatile fatty acids

  • VS

    volatile solids

  • ULBs

    urban local bodies

  • USR

    upflow solids reactor

  • WTE

    waste-to-energy

Suggested Reading

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Khanal, S. K. (2008). Anaerobic biotechnology for bioenergy production: Principles and application. Ames, IA: Wiley-Blackwell.Find this resource:

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