Feasibility of using kitchen waste as future substrate for bioethanol production: A review

Renewable and Sustainable Energy Reviews 74 (2017) 671–686

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Feasibility of using kitchen waste as future substrate for bioethanol production: A review

MARK

⁎

Halimatun Saadiah Hafida, Nor’ Aini Abdul Rahmana, , Umi Kalsom Md Shaha, Azhari Samsu Baharuddinb, Arbakariya B. Ariffc a Department of Bioprocess Technology, Faculty of Biotechnology and Biomolecular Sciences, Universiti Putra Malaysia (UPM), 43400 Serdang, Selangor, Malaysia b Department of Process and Food Engineering, Faculty of Engineering, Universiti Putra Malaysia (UPM), 43400 Serdang, Selangor, Malaysia c Bioprocessing and Biomanufacturing Research Centre, Department of Bioprocess Technology, Faculty of Biotechnology and Biomolecular Sciences, Universiti Putra Malaysia (UPM), 43400 Serdang, Selangor, Malaysia

A R T I C L E I N F O

A BS T RAC T

Keywords: Kitchen waste Pretreatment Saccharification Fermentable sugar Fermentation Bioethanol

This review highlights the utilization of kitchen wastes as substrates for bioethanol production. Kitchen wastes are commonly renewable, cheap and produced in large quantities daily. Kitchen wastes also contain a significant amount of organic matters particularly carbohydrates that can be converted into fermentable sugars for subsequent use in bioethanol fermentation. However, the advantages of kitchen wastes in biofuel production are indeed an untapped resource and poorly documented due to the challenges in the handling and disposal of kitchen wastes. Hence, a proper pretreatment and hydrolysis of the kitchen wastes by physical, chemical and biological methods is explored to increase the concentration of fermentable sugar released during the hydrolysis by enzymatic saccharification, thereby, improve the efficiency of the whole process. Furthermore, the advantages and drawbacks of each technology, challenges associated with feedstock handling and storage, government policies, and applications at commercial scale are critically discussed.

1. Introduction In Malaysia, there are approximately 2500 t of municipal solid wastes (MSW) generated per day in major cities with an average per person of 1.2 kg/day [1] consisting mainly of fermentable organic materials and kitchen wastes (71.6%), plastics (13.3%) and papers (5.8%), which comprise 80% of the overall weight [2]. It has been recorded that Malaysians produced 33,000 t of solid waste daily in 2012, exceeding the government's projected waste production of 30,000 t daily by 2020 [3]. At present, landfill system is the only waste management option for MSW in Malaysia. In order to divert as much as possible waste from landfill, MSW are recycled. However, they are not fully recovered and recycled due to the limited source separation and lack of proper recycling activity [4]. Since plastics, papers, and glasses are widely used, they become the most common recyclable items. The daily generation of kitchen waste is accelerated with substantial increase in volume due to rapid urbanisation, rapid growth of population and increase in food consumption rate. These organic wastes are discharged from various sources including households, restaurants and

leftovers from food industries that consists of uneaten food as well as food preparation residues comprising of rice, meats, vegetables, fruits, bakery and dairy products [5]. The amount of kitchen waste is projected to increase due to the rapid economic expansion and population growth, especially in the Asian countries (Fig. 1) [6]. Asian economic giant, which is China has produced approximately 19,500×104 t/year of food waste. Meanwhile, other countries such as the United States, India, Japan, and Korea have also followed a similar trend, discarding between 624 – 3500×104 t/year of food waste. For instant, the developing South-eastern Asia countries including Thailand, Vietnam, and Malaysia, have generated about 440 – 712×104 t/year of food waste. Kitchen waste generation is a topic concern by most countries including Malaysia. This might be due to the unpleasant odour, vermin attraction and toxic gas emission generated by the decomposed kitchen waste containing high biodegradable organic compounds [7]. The heterogeneous composition of kitchen waste causes the specific content to be extremely unpredictable. Besides, in the landfill, kitchen waste with high percentage of moisture will generate leachate and require secondary wastewater treatment system [8].

⁎

Corresponding author. E-mail addresses: hs_hafi[email protected] (H.S. Hafid), [email protected] (N.A.A. Rahman), [email protected] (U.K.M. Shah), [email protected] (A.S. Baharuddin), [email protected] (A.B. Ariff). http://dx.doi.org/10.1016/j.rser.2017.02.071 Received 28 September 2015; Received in revised form 20 January 2017; Accepted 21 February 2017 1364-0321/ © 2017 Elsevier Ltd. All rights reserved.

672

nd nd 10.6 nd 16.1 69.3 49.9 12.5

nd not determined

nd 3.9

12.9

2.63 2.8 3.54 30.25 50 46.67 28.85 nd 23.3 ± 0.45 8.61 nd nd 14.71 nd nd 33.28 nd nd 11.5 17.8 13.2 ± 0.2 17.9 16.7 17.1 ± 0.6 77.39 82.8 nd nd 4.1 6.5 ± 0.2

22.61 17.2 18.1 ± 0.6

nd 2.75 ± 0.03 2.35 nd 51.83 ± 0.27 32.85 nd 30.27 ± 0.21 33.82 ± 5.04 nd 17.64 ± 0.52 nd nd 14.97 ± 0.35 16.88 ± 1.24 nd 46.43 ± 0.32 nd 31.18 ± 1.37 18.88 ± 0.09 13.98 nd nd 77.83 4.2 ± 0.23 nd 5.08

In recent years, research have been focusing on the production of second-generation biofuels including ethanol to reduce the world reliance and dependency on the supply of fossil fuel [21,22]. Many countries such as Brazil, the United States, Japan, China and Europe are interested in producing internal biofuels due to the large incentive given to biofuel to be use as a replacement for gasoline. Such interest is mainly due to the increase in the oil prices, recognition in depletion of

Moisture Content (% w/w)

3. Bioethanol production

pH

Table 1 Characteristics of kitchen waste.

Total Solid (% w/ w)

Total Volatile Suspended Solid (% w/ w)

Carbon / Nitrogen (C/N)

Nutritional Value

The organic fraction of kitchen waste is heterogeneous based on its composition and source. Thus, the specific content of kitchen waste can be extremely different and unpredictable in different countries [10]. Typically, the main fraction of kitchen waste consists of rice, meats, and vegetables. Kitchen waste is characterised by high organic and biodegradable materials and consists of approximately 60% carbohydrate, 20% protein and 10% lipid [11]. The composition of carbohydrate polymers (starch, cellulose and hemicellulose), proteins, lipid, fiber and other inorganic matters makes kitchen waste a promising raw material for various biotechnological processes [12]. The characteristics of kitchen waste are summarized in Table 1. The most remarkable characteristic of kitchen waste is high in moisture content and humidity with high calorific value [13]. The pH and moisture contents of kitchen waste were observed to be ranging from pH 4–6 and 70– 80% (w/w), respectively (Table 1). The average total carbohydrate and protein content in the kitchen waste are 70% (w/w) and 20% (w/w), respectively. The potential of applying kitchen waste in industry is possible through advance biotechnology engineering approaches as it could generate several added products with new value that can be recovered during downstream processes as shown in Fig. 2. Considering the complexity composition of kitchen waste, it is suggested that kitchen waste should be utilized for the production of high valuable materials such as organic acids, biodegradable plastics and enzymes. Additionally, the hydrolysis of kitchen waste might release fermentable sugars, which are amenable to fermentation by microorganisms for the production of biofuels such as ethanol, hydrogen, and methane.

95.69 ± 1.27 17.24 ± 0.08 17.87 ± 1.28

2. General characteristics of kitchen waste

Total Carbohydrate (%/TS)

Total Kjeldahl Nitrogen (%/TS)

Crude Fiber (%/TS)

Furthermore, kitchen wastes have also been employed as the animal feed and fertilizer. The major constraint in utilizing kitchen waste as animal feed and fertilizer is the high salt content from the traditional food culture [8]. The components of kitchen waste such as proteins, amino acids and organic acids can also be utilized as substrates and nutrients for fermentation and enzymatic conversion processes [9]. Therefore, it is imperative to overcome the stated problem by recycling method which further converts the organic fraction into valuable products.

23.19 ± 0.54 18.81 ± 0.12 22.17 ± 1.57

References Nitrogen (% w/ w) Total Fat (%/TS)

Carbon (% w/w)

Elemental analysis

Fig. 1. : Kitchen waste generation from several countries [6].

Zhai et al. [14] Tian et al. [15] Wang et al. [16] Shen et al. [17] Xiao et al. [18] Zhang et al. [19] Zhang et al. [20]

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directly by microbial without requiring any complex pretreatment. The use of such materials require simple technology but this type of substrate is only available at quantity, which may increase the cost of industrial production. The second category of substrate is the one required enzymatic hydrolysis for the conversion into fermentable sugars prior to use in fermentation. The types of substrate include corn, wheat, sago starch, cassava and other grains. However, the use of this type of substrate would bring the risk to the human due to competition for food sources. The third category is the material that required pretreatments prior to acid or/and enzymatic hydrolysis into fermentable sugars, which is then use in ethanol fermentation. Lignocellulosic materials are the best example of substrate in this third category. Another alternative source of raw materials for bioethanol production can be obtained by utilizing kitchen waste. It has the advantages for being a low-cost, abundant and renewable substrate that has attracted the research interest in investigating its potential to be converted to bioethanol. Nonetheless, there is yet little information available in the literature regarding the utilization of kitchen waste for bioethanol production. Kim et al. [31] reported that kitchen waste contains carbohydrate approximately 65% of its total solid, which is rich in carbon content and therefore, can be considered as an ideal substrate for bioethanol production. Carbohydrate in the kitchen waste is usually converted into fermentable sugars, particularly glucose, through the combination of pretreatments, followed by acid hydrolysis or/and enzymatic saccharification. The fermentable sugar is subsequently fermented to bioethanol by the ethanol-fermenting microorganisms such as Saccharomyces cerevisiae. The bioethanol in fermentation broth are separated and purified, usually by distillation-rectification-dehydration process, to obtain high purity and quality bioethanol [13,23,26]. Industrially, S. cerevisiae is widely used for the conversion of kitchen waste hydrolysate to bioethanol [9]. The bacterium that is acid-tolerant-ethanol, Zymomonas mobilis that is also being used for fermentation of kitchen waste for ethanol production. Study on the feasibility of using kitchen waste as substrate for bioethanol production have been conducted by several researchers [7,8,25,32]. The bioethanol yield based on dry kitchen waste consumed of 0.23 gg−1 with a final bioethanol concentration of 29.1 g/L has been reported [7]. In another study, it was claimed that a total of 80 g/L of bioethanol was produced by utilizing 164.8 g/L of fermentable sugar, obtained from the enzymatic saccharification of kitchen waste using glucoamylase [8]. Uncu and Cekmecelioglu [25] reported that 32.2 g/L of bioethanol was produced using 64 g/L of sugars, obtained from enzymatic saccharification of kitchen waste using amylase. Yan et al. [32] also claimed that very high bioethanol production using laboratory scale (93.86 g/L) and semi-pilot scale (98.88 g/L) facilities were achieved by the utilization of sugars obtained from enzymatic saccharification of kitchen waste. The conversion of kitchen waste to bioethanol is highly dependent on concentration of fermentable sugar obtained from the enzymatic saccharification. High concentration of sugars significantly increased the overall bioethanol process productivity, minimizing the production costs due to a decrease in volumes of liquid to handle and the reduction in volume of the equipment [23]. In this sense, the challenge for bioethanol production is to enhance the high concentration of sugars especially from enzymatic saccharification of kitchen waste in large scale at a competitive level by developing more efficient technologies at pretreatment, hydrolysis and enzymatic saccharification stages. At the same time, the optimal conditions for both fermentable sugars and bioethanol production should be integrated. The economically viable technology for fermentable sugar and bioethanol production should be strongly evaluated as it can be considered as promising strategy for the future development of bioethanol to be applied worldwide.

Fig. 2. : Possibilities of products converted from kitchen waste (— intermediate products; — final products).

the global oil reserves, the requirements of Kyoto Protocol to meet the carbon emission reduction, and also because of the aim to increase interest in the economic utilization of biomass [23,24]. Production of fuel ethanol, also known as bioethanol, generally constitutes 60% of world total ethanol production via fermentation process [25]. It is a clean alternative for transportation fuel, which can reduce negative environmental impacts including the greenhouse gas and carbon dioxide (CO2) emission. Although the energy obtained from ethanol combustion is slightly lower (~68%) than that of petroleum fuel, ethanol burning can reduce carbon emission by more than 80% with lower emission of toxic substances that include sulphur dioxide [23]. The world production of ethanol recorded from 2009 to 2010 has nearly reached 100 billion litre by the consumption for fuel (68%), industrial (21%), and potable purposes (11%) [26]. The use of bioethanol for fuel has partially replaced the gasoline to produce the gasoline-ethanol mixtures, E15 (15% bioethanol and 85% gasoline) and E85 (85% bioethanol and 15% gasoline). Bioethanol is also widely used as a chemical feedstock to produce ethylene essential in the production of polyethylene plastics [13], beverages industry [25], pharmaceutical and cosmetic industries [27] and as a feedstock for producing a compound of acetaldehyde, butadiene, ethyl acetate and acetic acid. Brazil and the United States are the major bioethanol producers accounting for 62% of the world's bioethanol production. For the first generation of biofuels, industrial scale bioethanol in America and China was produced mainly from corn starch, while in Brazil it is produced from sugarcane [8,28]. However, the utilization of either sugars or starches as the food sources for biofuel production has been criticised since it has led to the dramatic increase of their prices worldwide causing problem for the countries with low income [22]. In fact, the high demand of bioethanol in the last few years has also increased the price of corn and sugarcane, thus triggered the debate on demand for food sources versus substrates for biofuels. Therefore, the availability and feasibility of corn and sugar caned for use as a feedstock became limited. In addition, it was reported that the cost of raw materials is accounted about 70% of the expenses of the product [29]. All these concerns led to the increase in research to find the lowcost, high quantity, rich in carbon content, and renewable resources of substrates for use in bioethanol production. Raw materials involved in bioethanol production can be categorised into three main types [30]. The first category is sugar such as molasses and fine sugar, where they can be converted to bioethanol 673

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Kitchen waste

Pretreatment (Physical, chemical, biological)

improve the formation of sugars by enzymatic hydrolysis to avoid the degradation or loss of carbohydrate. The availability of biomass for the enzyme attack of polysaccharide is important to increase the yield of total sugar produced. In addition, pretreatment has to take into consideration the formation of hydrolysis and fermentation inhibitory byproducts that may react against the ethanol fermenting yeast used in the subsequent fermentation process [37]. Therefore, physical, chemical, physico-chemical, and biological treatment are the four fundamental types of pretreatment technique normally employed for the production of fermentable sugar from kitchen waste [31,38].

Bioethanol

Enzymatic hydrolysis

Bioethanol fermentation

Distillation

Solid separation

Fig. 3. : General scheme for the conversion of kitchen waste to bioethanol.

4.2.1. Physical treatment Physical treatment involves the disruption of the biomass structure [39], which aimed at maximizing the accessible surface area for chemical and enzymatic reactions to increase rate of hydrolysis and/ or enzymatic saccharification. This can be achieved by reducing the particle size using chipping, grinding or milling technique [10]. Smaller particle size increases the surface area available for enzyme accessibility leading to a more rapid hydrolysis. Izumi et al. [40] investigated the effects of particle size reduction and solubilisation on biogas production of food waste. They claimed that the reduction of the mean particle size of substrate from 0.843 mm to 0.391 mm, increased the solubilisation by about 30% and thus improved the biogas yield by 28%. Biomass with fine particle size may generate clumps during the pretreatments and enzymatic hydrolysis, which lead to the negative effects of total sugars produced [24]. In addition, very fine particle size may reduce the void space between particles resulting in decreasing of microbial growth and subsequently reduces the production of valuable byproducts. Energy consumption from the physical and mechanical treatment operation is primarily based on the estimation of biomass pretreatment energy efficiency (ηpretreatment = Total sugar recovery (kg)/Total energy consumption (MJ)) [39]. Although the pretreatment period through physical method is low, the required voltage is very high especially in industrial scale and the rise in energy demand make the economic feasibility of this method to be questionable [10]. Liquid hot water, also known as hydrothermal pretreatment, is one of the pretreatment methods of solid waste that improves the digestibility of materials. The hydrothermal pretreatment is aimed at altering the structure of insoluble fraction to make it more amenable to biodegradability [24,41]. By considering the facts that hydrothermal pretreatment requires no acid or chemical and is conducted at temperatures ranging from 60 to 170 °C, it is considered as an economically interesting methods for treating biomass and it is also environmental friendly. Kuo and Cheng [42] investigated the hydrothermal pretreatment for treating kitchen waste at various temperatures (37, 50, and 60 °C) to examine the effect of hydrolysis on anaerobic digestion of kitchen waste. The results from their study indicated that the most effective temperature was observed at 60 °C, hydrolysing approximately 27.3% of solid kitchen waste with 37.7% of oil and grease removal. Organic fraction of MSW, kitchen waste, was subjected to a hydrothermal pretreatment at 160 °C from 5 to 50 min, resulting in the increase of glucose concentration from 37.5% to 43.9%, whereas the xylose content was remained at about 5% for all time tested [43]. The kitchen waste hydrolysate was used in a simultaneous saccharification and fermentation (SSF) and fed-batch SSF using cellulases and amylases producing a final bioethanol concentration of 30 g/L. This shows that hydrothermal method is a potential pretreatment in obtaining high concentration of fermentable sugars as a feedstock for bioethanol production.

4. Conversion of kitchen waste to bioethanol 4.1. Effective pretreatment overview The basic biochemical conversion process steps in producing bioethanol from biomass or kitchen waste include pretreatment, hydrolysis or saccharification and fermentation, followed byproduct separation/distillation (Fig. 3). However, the hydrolysis and/or enzymatic saccharification of solid fraction in kitchen waste is still recognised as a rate-limiting step during the whole conversion process to value-added products. Therefore, pretreatment is necessary to enhance the digestibility of solid content for improving hydrolysis. Many factors including particle size, structure, and composition of substrate may influence the rate of hydrolysis especially during the biodegradation of high solid content substrate [33]. Harsh pretreatment may not necessary during the conversion of kitchen waste to fermentable sugar [13]. However, the appropriate choice of pretreatment has a large impact on subsequence steps in the overall conversion scheme particularly in organic matter solubilisation, digestibility, generation of toxic compound, energy demand in the downstream process and wastewater treatment [34]. The pretreatment of kitchen waste can be classified into physical (milling and grinding), chemical (alkali, dilute acid and organic solvent), and biological (fungal pretreatment and enzymatic process) [35]. The pretreatment of kitchen waste is aimed at altering and disrupting some structural characteristics of kitchen waste to increase the accessibility of enzyme to be hydrolysed into sugars monomers. Effective pre-treatment of kitchen waste is essential to produce up to 100% fermentable sugars at low cost. The concentration of sugars from the combination of pretreatment and enzymatic hydrolysis should be above 10% to ensure the recovery of sugars and other downstream process cost to be manageable [34]. It is worth noted that the harsh condition of pretreatment lead to partial degradation of sugars and generation of inhibitory compound derived from sugar decompositions that might latter affect the subsequent fermentation steps. Degraded products such as weak acids (lactic acid, acetic acid and formic acid), phenolic compounds (alcohols, aldehydes and ketones), and furan derivatives (furfural and 5-hydroxymethylfurfural) are usually detected in the kitchen waste hydrolysate [9,34]. The product of enzymatic hydrolysis or saccharification of pretreated kitchen waste is known as kitchen waste hydolysate. Another important factor is the compatibility of fermentable sugars presence in the kitchen waste (glucose, pentoses and hexoses) to the ethanol-fermenting microorganism for an efficient bioconversion to bioethanol [36]. Different strains have different abilities in the utilization of different fermentable sugar for the conversion into bioethanol during the fermentation, 4.2. Pretreatment of kitchen waste

4.2.2. Physico-chemical pretreatment Chemical pretreatment is one of the most frequently used methods to enhance the biodegradability of organic substrate especially lignocellulosic materials by increasing the susceptibility to enzymatic hydrolysis or saccharification. The effect of chemical pretreatment is

Pretreatment methods refer to the solubilisation and separation of one or more components of biomass that makes the remaining solid biomass more accessible for further chemical or biological treatment [24]. The effective pretreatment strategies of kitchen waste should 674

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Hydrolysis of kitchen waste by acid pretreatment is to break the glycosidic bond of polysaccharides emerging oligosaccharides and maltodextrins to produce monomeric sugars, which are amenable to be fermented into valuable products [47]. Cekmecelioglu and Uncu [54] reported that the acid hydrolysis of kitchen waste particularly using HCl and H2SO4 can produced higher amount of ethanol from kitchen waste hydrolysate. The effect of pretreatment method (hot water, dilute H2SO4, and a control) on enzymatic hydrolysis of kitchen waste was evaluated using kinetic model approach. The kinetic model results showed that the maximum glucose was accumulated after each pretreatment method and that the enzymatic saccharification was higher for dilute H2SO4 (77.52 g/L glucose), followed by control (40.54 g/L) and hot water pretreatment (38.90 g/L). The dilute H2SO4 pretreatment has significantly improved further performance of enzymatic saccharification of kitchen waste into fermentable sugars. Khraisheh and Li [55] reported that kitchen waste soaked with 1% H2SO4 in an autoclave at 121 °C for 60 min followed by enzymatic saccharification by β-glucosidase produced glucose with a yield of 85%. A glucose yield of 72.8% was obtained when the kitchen waste is subjected with 4% H2SO4 followed by steam treatment at 121 °C for 15 min and then enzymatic hydrolysis using cellulose from Trichoderma viridae [48]. Concurrent pretreatment of dilute H2SO4 and steam can effectively improve enzymatic hydrolysis, decrease the production of inhibitory compounds and lead to a more complete hydrolysis of hemicellulose. Besides dilute H2SO4, high hydrolysis yield could also be obtained by the pretreatment of kitchen waste with dilute HCl. Acid pretreatment of kitchen waste using 1% HCl for 94 min at 100 °C has increased the soluble sugars concentration by 120% as compared to that of untreated kitchen waste [12]. The thermo-chemical pretreatment does not only results in a high total soluble sugars yield but also facilitates subsequence biochemical conversion by enzymatic hydrolysis, thus, enhancing the enzymatic digestibility of kitchen waste to fermentable sugars at optimum combined thermo-chemical-enzymatic conditions. Direct conversion of kitchen waste to bioethanol in a thermo-acidic pretreatment using 1 M HCl with pH adjusted up to pH 1 has yielded about 0.36 g ethanol/g starch, which is corresponds to 64% of the theoretical value [56]. These promising result showed that dilute acid pretreatment has a high potential to be used to convert kitchen waste to fermentable sugars, for subsequence use in bioethanol fermentation.

depended on the type of methods used and the characteristics of substrates. Chemical pretreatments is widely applied on waste sludge and lignocellulosic substrates to remove lignin and/or hemicelluloses, which in turn, decrease the cellulose crystalline structure, reducing the degree of polymerisation (DP), increasing the pore size and surface area; primarily to make the cellulosic biomass amenable to the action of cellulolytic enzymes for its degradation to monomers sugar [35,44]. However, there are only limited studies that have been conducted on organic fraction of MSW. It has been reported by Ariunbaatar et al. [45] that chemical pretreatment is not suitable for easily biodegradable substrates containing high amount of carbohydrate. In the anaerobic digestion process, chemical pretreatment can initiate the rapid degradation and accelerate the accumulation of volatile fatty acid which leads to the failure of methanogenesis steps. Thus, the most prevalence chemical treatment, acid hydrolysis is explored in this section. Acid hydrolysis is considered as an important chemical pretreatment method due to its ability to adapt with wide a variety of feedstocks [44]. The concentration of released sugars during acid hydrolysis pretreatment is highly depending on the type of biomass, compositions of substrates, temperature, time, acid concentration, solid to liquid ratio and the reactor employed in the process [46]. The exploitation of carbohydrate rich organic waste and kitchen waste, which are relatively easy to be converted into valuable products such as ethanol, lactic acid, and hydrogen through various biological processes is the focussed of many studies [47]. However, the utilization of kitchen waste is limited by the hydrolysis of solids in kitchen waste that is yet recognised as a rate-determining steps of its application. Therefore, chemical hydrolysis of acid pretreatment is aimed at the modification of the structures of solid materials to increase the accessibility of cellulose to enzymatic hydrolysis for the conversion of kitchen waste to fermentable sugars [35,48]. Currently, bioethanol production involving acid hydrolysis and enzymatic saccharification by cellulase enzyme has been reported [49,50]. Acid pretreatment does not only breakdown the lignin component, but also provides a favourable condition for the growth of microorganisms in the biomass hydrolysate [45]. The reaction occurred helps in hydrolysing hemicellulose fraction of biomass into monosaccharides; xylose, mannose, glucose and galactose [34,35,45]. High yield of sugars can be obtained through acid hydrolysis using inorganic hydrochloric acid (HCl) and sulphuric acid (H2SO4) at either dilute or concentrated acid [51]. Dilute H2SO4 acid pretreatment has been extensively studied for its applicability in treating biomass due to its high catabolic activity and low cost chemical. Acid hydrolysis of biomass can be occurred at high temperatures (~180 °C) during a short period of time or at slightly lower temperature (~120 °C) for a longer period (~30−90 min) [34]. Dilute acid is more economic as the process is performed at relatively mild temperatures and moderate pressures with longer retention time, which providing the advantage of high efficiency of sugar recovery [51]. Industrially, dilute acid pretreatment is more attractive due to its inexpensive operational and process maintenance, with lower amount of fermentation inhibitors generated as well as the ability to recover hemicellulose sugars up to 80% of the initial amount [44,51,52]. Acid hydrolysis using concentrated acid is less attractive especially for bioethanol production due to the formation of large quantities of degraded by-products and undesirable inhibitory compounds such as hydroxymethyl furfural (HMF), furfural, and aromatic lignin degradation compound that might affect the metabolism of the ethanolfermenting microorganism in the fermentation steps [34,44]. Other disadvantages include the loss of fermentable sugars due to increase in sugar degradation at high temperatures and additional cost for neutralizing the acidic conditions prior to bioethanol fermentation [45,53]. Furthermore, corrosiveness of equipment and acid recovery that increase the operation and maintenance costs are also considered as the major drawbacks preventing the application of pretreatment using concentrated acid [52].

4.2.3. Biological pretreatment The conversion of kitchen waste to bioethanol depends on the hydrolysis of the carbohydrate as the ethanol-fermenting microorganisms normally are not able to directly ferment starch or cellulose to bioethanol [13]. The hydrolysis involves the breakdown of polysaccharides into simple sugars such as glucose, which is usually catalysed by enzymatic reaction [26]. Particle size, moisture content, pretreatment time and temperature could affect the biomass degradation and enzymatic hydrolysis yield [52]. Other factors such as substrate concentration, enzymes loading, pH and mixing rates also affect the rate of enzymatic hydrolysis [24]. For carbohydrate-rich-organic waste such as kitchen waste containing high amount of starch, a mixture of αamylase can specifically catalyse the hydrolysis of α−1,6-glucosidic linkages of branched polysaccharides producing a linear oligosaccharides. Fermentable sugars such as glucose, sucrose, fructose and maltose can be produced in the saccharification process using glucoamylase [57]. In the hydrolysis process of the biomass containing high composition of cellulose, at least three groups of cellulolytic enzymes are involved; endoglucanase which attack low crystallinity region of cellulose fiber resulting in free chain-ends, exoglucanase that degrades and removes the cellobiose units free chain-ends and β-glucosidase which hydrolyse cellobiose to produce glucose [26]. Enzymatic hydrolysis offers more advantages over physical and/or chemical pretreatment by giving higher sugar yield than acid catalysed hydrolysis. In addition, the enzymes are highly specific to the target 675

Organic fraction MSW lignocellulosic MSW

Kitchen waste

Kitchen waste

Kitchen waste

Kitchen waste

1

3

4

5

6

676

Kitchen waste

Kitchen waste

Kitchen waste

Kitchen waste

Potato peel waste (PPW) Potato peel waste (PPW) Banana waste

Kitchen waste

8

9

10

11

12

15

14

13

Kitchen waste

7

2

Feedstock

No

dried, ground, sieved at particle size 500–1000 mm soak with 1% H2SO4 at 90 °C for 90 min Mechanical pretreatment (cuts materials into 2– 3 in. using stainless steel knife) soak with 5% H2SO4 and digested in microwave at 180 °C, 700 W, 25 min Mechanical pretreatment (milled to 0.2–1.2 mm size) soak with 1% H2SO4 autoclave 121 °C, 60 min

Mechanical pretreatment (crush into small sizes) thermo-drying at 105 °C Mechanical pretreatment (blend and chopped into small pieces) Mechanical pretreatment (grinded and screen to 0.2 mm) 1% H2SO4, 60 °C, 180 min mix with 120 mL HCl, autoclave at 121 °C, 15 min

Mechanical pretreatment (crush with compact chopper), spray with lactic acid bacteria Mechanical pretreatment spray with lactic acid bacteria Mechanical pretreatment

Mechanical pretreatment (blend with water and ground in a chopper into small pieces)

Mechanical pretreatment (blend with water and ground in a chopper)

Mechanical pretreatment (sieve 2×2 mm)

Thermal pretreatment (autoclave 165.5 °C, 45 min)

Thermal pretreatment (autoclave 160 °C, 30 min)

Pretreatment

50 °C, pH 4.8, 48 h

50 °C, pH 3.7–5, 24 h

cellulase β-glucosidase

α-amylase cellulase

50 °C, pH 4.8, 96 h

cellulose β-glucosidase

ternamyl (85 °C, pH 6, 1 h) viscozyme (44 °C, pH 4.6, 2.5 h) celluclast (50 °C, pH 5, 2 h) 37 °C, 14 h, 200 rpm

55 °C, pH 4.5, 150 rpm, 2 h

α-amylase glucoamylase

ternamyl (amylase) viscozyme celluclast

45–50 °C, pH 4.5–6, shaking waterbath

Carbohydrase glucoamylase protease cellulase α-amylase amyloglucosidase protease

Ternamyl glucoamylase cellulase

glucoamylase

ternamyl at 85 °C, 30 min glucoamylase 60 °C, 150 rpm, 2 h cellulase at 50 °C, 150 rpm, 2 h 30 °C, pH 5, 30 min

liquefaction by α-amylase at 95 °C, pH 5.5, 1 h saccharification by amyloglucosidase, cellulase, β-glucosidase at 55 °C liquefaction by α-amylase at 50–60 °C, pH 4– 6.5, saccharification with glucoamylase at 55– 60 °C, pH 4–4.5 60 °C, 150 rpm, 2 h

α-amylase amyloglucosidase cellulase β-glucosidase α-amylase glucoamylase

50 °C, pH 4.5, 3 h

Amyloglucosidase carbohydrase (arabinose, cellulase, β-glucanase, hemicellulase, xylanase)

40 °C, pH 4.8, 12 h

50 °C, 72 h

cellulose β-glucosidase Trichoderma reesei cellulose

Hydrolysis conditions

Enzymes

Table 2 Combined physical-chemical pretreatment and enzymatic hydrolysis of different food-based feedstock for fermentable sugar production.

85% glucose yield

36.84% reducing sugar yield

69g/L reducing sugar

18g/L reducing sugar

72.8% glucose yield

200g/L reducing sugar

135g/L glucose with ~0.83g glucose/g total solid 600g glucose/kg kitchen waste

74.1g/L glucose with 97.5% glucose recovery 86% glucose recovery

164.8g/L reducing sugar

64.7g/L glucose in 6 h

265 mg sugar/g substrate with 53% conversion of cellulose and hemicellulose 44g/L of glucose, 63g/L of reducing sugar

36g/L of glucose

Outcome

Khraisheh and Li [55]

Gabhane et al. [64]

Khawla et al. [63]

Arapoglou et al. [62]

Li et al. [48]

Yan et al. [61]

Hong and Yoon [60]

Kim et al. [31]

Koike et al. [28]

Tang et al. [59]

Yan et al. [8]

Uncu and Cekmecelioglu [25]

Moon et al. [7]

Li et al. [58]

Ballesteros et al. [43]

Ref

H.S. Hafid et al.

Renewable and Sustainable Energy Reviews 74 (2017) 671–686

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The proper planning of integrated pretreatment methods resulted to the most technical and economical feasible approach at enhancing fermentable sugar production. 4.3. Effect of hydrolysis on kitchen waste composition An efficient bioconversion process of kitchen waste to bioethanol depends on the extent of carbohydrate saccharification [65]. It is known that, on a dry basis, kitchen waste contains ~64% carbohydrate, 22% protein, 13% fat and the remaining of 1% of fiber that can be hydrolysed into monomeric sugars such as glucose and xylose [8,66]. However, the behaviour of the kitchen waste that is naturally very complex and heterogeneous in their composition causes the low efficiency of overall hydrolysis process. In addition, the effects of each components presence in kitchen was on the rate and degree of hydrolysis are not yet clearly understood by biochemical means [67]. Carbohydrate, protein, fat and fiber have different biodegradability owing to the physico-chemical and structural characteristics factors [68].

Fig. 4. : Partial chemical structures of carbohydrate [71].

substrate and can be carried out in a mild condition with temperatures ranging from 40 °C to 50 °C. Therefore, low energy is required for hydrolysis to take place with less generation of toxic compounds in the hydrolysate [35]. However, the enzymatic hydrolysis faces the technoeconomic challenges and less attractive at commercial scale due to the slow reaction rates and requires careful control of reaction conditions [26,35]. Furthermore, excess cost of enzymes that may affect the overall process limits the application of enzymes as a sole pretreatment and not economically viable [12]. As mentioned earlier, hydrolytic enzymes suffers from slow reaction rate due to the natures and recalcitrant of biomass that makes the penetration of enzymes to the active sites become difficult [36]. This requires a pretreatments to break the structure of recalcitrant biomass to expose the starch and cellulose to enzymatic action. Therefore, the combination of pretreatment has been considered as a promising approach for sugars production with high efficiency, decreasing the formation of inhibitory and a short processing time [52]. The various pretreatments prior to enzymatic hydrolysis process are summarized in Table 2. These reported work indicated the interest of many researchers in utilizing kitchen waste as a substrate through combined physical, chemical and biological pretreatment prior to enzymatic hydrolysis. Hong and Yong [60] employed α-amylase and glucoamylase for further liquefaction and saccharification steps after mechanical grinding. The results showed that the glucose concentration released at 600 g glucose/kg kitchen waste. Another contribution of biological pretreatment was conducted by Tang et al. [59] and Koike et al. [28] showed that the use of lactic acid bacteria as a microorganism pretreatment agent on ground kitchen waste resulted in an increase in the release of 97.5% and 86% of glucose, respectively, after additional of combine cellulase and glucoamylase. In this case, microbial pretreatment was effectively increased accessibility of kitchen waste to enzymatic hydrolysis. The ethanol productivity produced is remarkably increased to 7 g/L/h and 24 g/L/h as reported by Tang et al. [59] and Koike et al. [28], respectively. The employed microorganisms specifically act as a mediator to preserve the freshness of the kitchen waste from being degraded to organic acid. The use of biological agent is regards as an attractive pretreatment due to the less harmful effect on environment, mild condition and low energy needed during operation. Aiming at increasing the efficiency of enzymatic hydrolysis, the research include not only physical and biological agent pretreatment methods, but also the chemical pretreatment. The synergistic effect of 1% of H2SO4 treated on kitchen waste with cellulase and glucoamylase enzyme was studied by Khraisheh and Li [55]. Compared to control, the yield of glucose was increased to 85% indicated that the polysaccharides chain of kitchen waste was partially degraded by H2SO4 with the cleavage of glycosidic linkages, increasing accessibility of feedstock for enzyme attack. Unlike microorganism pretreatment, the use of chemical pretreatment is approachable due to cheaper price and durable. However, the use of chemical reactant may randomly degrade the polysaccharide to unnecessary monomers that latter will reduce the concentration of desired and actual sugars. The combination method is necessary to overcome the economic challenges of using only enzyme pretreatment due to the enzyme cost.

4.3.1. Carbohydrate Carbohydrate in the kitchen waste is in the form of starchy and cellulosic materials that are not readily fermentable for bioethanol production by microorganisms [69]. Therefore, prior to bioethanol production, carbohydrate polymers of kitchen waste have to be hydrolysed into fermentable sugar. Being a major component in the kitchen waste, carbohydrate hydrolysis is regarded as limiting factors for an efficient conversion of kitchen waste to fermentable sugars [70]. The complex carbohydrate polymers covalently linked together by glycosidic linkages as shown in Fig. 4 to form several structural types: linear (e.g. amylose, cellulose), branched (e.g. amylopectin, glycogen), interrupted (e.g. pectin), block (e.g. alginate) and alternate repeat (e.g. agar, carrageenan) [71]. Pretreatment by physical, chemical and enzymatic reactions are usually proposed to depolymerise and open the chain structure of the carbohydrate during the hydrolysis stage to form monosaccharides before being converted into bioethanol by the ethanol-fermenting microorganisms [72]. Under acidic conditions, the hydrolysis of carbohydrate produces hetero-composition of monosaccharides (glucose and fructose) and disaccharides sugars (sucrose and maltose) [67]. The carbonyl group of sugars reacts with the hydroxyl group of acid and water forming carbon-oxygen bonds called O-glycosidic bonds. The partial hydrolysis of carbohydrate produces disaccharides, which consist of two monosaccharides joined by O-glycosidic bonds are produced. Melikoglu [73] reported that the hydrolytic enzymes plays an important roles of breaking down the high molecular weight of disaccharides into simple sugars such as glucose. Therefore, study on the saccharification of carbohydrate in kitchen waste is currently focuses on utilizing of amylase enzyme that comprises of two major classes; α-amylase (EC 3.2.2.1) and glucoamylase (EC 3.2.1.3). Both classes are directly involved in the hydrolysis of kitchen waste carbohydrate. α-amylase hydrolyses carbohydrate into maltose, glucose and maltotriose by cleaving the α−1,4-glucosidic linkages which is the linear molecule of carbohydrate, while glucoamylase act to cleave α−1,6-glucosidic linkages at the branching point of amylopectin as well as α−1,4-glucosidic linkages, yielding glucose as the end product [74]. 4.3.2. Protein Proteins are the polymers of amino acids that covalently linked by peptide bonds. As the kitchen waste contains meats, fishes, eggs, and chickens, hence, substantial amount of protein can be detected. The rate of protein degradation is found to be slowest compared to carbohydrate and fat [75]. Hydrolysis of protein into amino acids and small peptides can be accomplished by chemical under acidic and alkaline conditions or enzymatic means [76]. The amino acids is 677

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and become more enzyme-access friendly after undergoing physical and chemical pretreatments for further conversion to fermentable sugars [58]. Approximately 40–60% of glucose could be recovered after pretreatment of kitchen waste with acid [86].

further catabolised to produce volatile fatty acids (lactic acids, acetic acids, propionic acids and butyric acids), branched-chain fatty acids (isobutyrate and isovaleric acid) together with a small amount of methane, carbon dioxide, hydrogen, ammonia and reduced sulphur [67,77]. The introduction of rich proteinaceous biomass alone for bioethanol production is not recommended due to the high risk of inhibition to the ethanol-fermenting microorganisms by ammonia that might reduce the overall productivity. The ammonia released enters the microbial cells through the cell's lipid membranes in the unprotonated form and become toxic to the microorganism. The accumulation of ammonia inhibits the activity of the cytosol enzymes thus affecting the metabolism of the microorganisms [78].

4.3.5. Salts and metal ions Kitchen waste contained substantial amount of metal ions in the form of salt particularly sodium chloride (NaCl), which is normally used as an additive in Asian countries to enhance food flavour. Moon et al. [7] reported that the presence of salt hindered the potential use of the treated kitchen waste as animal feed due to the uncertainty with regard to the safety. In addition, its demand as a fertilizer decreases, which make the recycling activities of kitchen waste become challenging. Salt and metal ions such as Fe3+ and Al3+ greatly affect bioethanol production by Saccharomyces cerevisae, in which the excessive concentrations of metal ions inhibit yeasts growth due to the increase in osmotic pressure in the culture [87]. The inhibition effect of salt and Na+ on methane production during anaerobic digestion process of kitchen waste has been reported by Kuo and Cheng [42]. The presence of Na+ at a concentration of 2817 mg/L, greatly reduced the gas production by about 14% lower than that obtained in control fermentation (without Na+).

4.3.3. Fat Crude fat account for 12–15% (w/w) of the total composition of kitchen waste [8,59]. The hydrolysis of fats, also known as lipolysis, is widely studied in the anaerobic digestion process for the production of methane and hydrogen. It is an endothermic reaction in which hydrolysis rate increase with increasing temperatures. It should be noted that free fatty acids and glycerol are the main products from fats hydrolysis [79]. In the β-oxidation pathways, fatty acids are more susceptible to oxidise resulting in more volatile free fatty acids after being released from the glycerol backbones [80]. Fats and oil will undergo undesired thermal decomposition more quickly at high temperatures and in the presence of water that leads to an oxidation of fats and oil to a reduced yield of free fatty acids with low molecular weight of acidic products such as acetic acid [81,82]. The degradation of crude fat is quite low as compared to carbohydrate, therefore, fats is generally the rate-limiting-steps in the hydrolysis of kitchen waste particularly to methane and hydrogen production [83]. The presence of fats in kitchen waste generates problems of flotation, clogging and mass transfer in the reactor, which hindering the efficiency of the hydrolysis process [67].

4.4. Effect of pre-treatment on formation of byproducts and inhibitors Hydrolysis of carbohydrate-rich-biomass releases the major type of hexoses and pentoses sugars such as glucose, fructose, maltose, sucrose and xylose [88]. For easily biodegradable waste such as kitchen waste, hydrolysis is not necessarily the rate limiting step, thus the increased hydrolysis due to extreme pretreatment conditions may also generates various degradation characteristics such as the formation of 5-hydroxymethylfurfural (HMF) and furfural as well as the accumulation of VFA that lead to a decrease in pH, which subsequently inhibit the hydrolysis process [45,89].

4.3.4. Fibers Apart from the readily available carbohydrate, kitchen waste also contains insoluble carbohydrates such as cellulose and hemicellulose fibers, which are mainly come from the vegetables and fruits peels residues. Cellulose fibers are made of parallel unbranched D-glucopyranose units linked by β−1,4-glycosidic bonds that form crystalline and highly organised microfibrils through inter and intramolecular hydrogen bonds and Van der Waals forces. The structure of cellulosic fibers is the repeating unit of consecutive glucose chains [84] whereas the hemicellulose is the polysaccharides with pentose as the backbone [85]. Theoretically, cellulose and hemicellulose fibers can be hydrolysed into monomeric sugars, as the feedstock for bioethanol production [60]. However, the cellulosic fibers are not readily fermentable for microorganisms’ uptake and metabolisms. Furthermore, cellulose-fibers are hardly degraded due to highly stable crystalline and rigid structure [67]. Hence, the inclusion of various pretreatments (physical, chemical and enzymatic reactions) have been developed to enhance the hydrolysis of oligosaccharides and the liberation of more fermentable sugars particularly glucose [69,86]. Cellulosic fibers is partially hydrolysed

Carbohydrate

4.4.1. By-products and inhibitors At high temperatures, carbohydrate compound starts to reacts with protein compound specifically amino acid in the Maillard reaction, which further leads to the formation of melanoidins or Amadori compounds, that can be easily detected by the brown-colouredproducts of the hydrolysates. The formation of Amadori compounds is initiated by the condensation of reducing sugars and amino acids in the kitchen waste followed by the reaction of carbonyl group of the sugars with the nucleophilic amino group of the amino acid, where this phenomena are occurs at high temperatures ( > 100 °C) [71]. The Maillard reaction is expected as the main cause of the sugar loss during the hydrolysis. The derivatives from sugars decomposition cause by Maillard reaction are VFAs and furans such as HMF, which is the main products of hexoses degradation (C6 sugars) and the conversion of HMF in the presence of acid catalysts gives levulinic acid and formic acid at 1:1 mol ratio [90]. The pentoses (C5 sugars) which are mainly xylose and arabinose are degraded to furfural forming formic acid and

C6 sugars

HMF

Levulinic Formic acid acid Decomposition products

C5 sugars

Furfural

Formic acid

Fig. 5. : Reaction pathways of hydrolysis of carbohydrate in kitchen waste [91].

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tion of toxic compounds and inhibitors is lessen, become non-toxic and compatible to the selected microorganism responsible of converting sugars to bioethanol in further fermentation steps [34]. In contrast, lignocellulosic materials commonly used harsh conditions in which operated at high temperature ranging from 150 to 250 °C with concentrated acid reactant during its pretreatment to facilitate deformation of recalcitrant components; lignin, hemicellulose and cellulose, thereby entailing high capital investment of pretreatment process to make cellulose more accessible to enzyme. The harshness of the pretreatment lead to a partial degradation of hemicellulose and reduce the degree of crystallinity. Additionally, the severity pretreatment resulting in the formation of toxic compounds such as 5hydroxymethylfurfural (HMF) and furfural from hexose and pentose decomposition, respectively. The amount and type of degradation product could greatly affect the proceeding enzymatic hydrolysis and further bioethanol production. While the physical pretreatment by milling and grinding to reduce the particle size are reporting to be energy-intensive and costly technologies, however, the combination with the chemical pretreatment would soften the rigid components of lignocellulosic, catalyse the lignin removal and openly exposed the fibrils of cellulose for higher concentration of fermentable sugar produced at the end of enzymatic hydrolysis. Nevertheless, the lignocellulosic materials from oil palm biomass is still regards as a promising resources in Malaysia that has been spotlight as a valuable feedstock for bioethanol production. Due to high yield of crops with million palm tree planted annually ensure the sustainable supply of biomass. The in-situ operation of oil palm mill with excess steam generated during processing could reduce the energy consumption during pretreatment of biomass prior to enzymatic hydrolysis. The understanding of material's nature is undeniable helps in the selection of an appropriate pretreatment that well-suited with overall biorefinery concept.

decomposition products [88]. At molecular level, the hydrolysis of kitchen waste in a complex reaction pathways are shown in Fig. 5 [91]. The concentration of reaction products are highly affected by the hydrolysis parameters such as temperature, hydrolysis time, type of biomass and solid concentration. The formation of HMF during hydrolysis using H2SO4 (1−5% v/v) at 120 min for different types of food processing waste have been studied by El-Tayeb et al. [92]. They reported that the concentration of HMF increases with acids concentration. The HMF concentrations, initially detected are 0.021%, 0.022% and 0.03% (w/v) in corn stalks, sugar beet waste, and sugarcane bagasse, respectively. As the concentration of H2SO4 increases to 5% v/ v, the concentration of HMF in corn stalks, sugar beet waste and sugarcane bagasse hydrolysate was proportionally increased to 0.033%, 0.03%, and 0.042%, respectively. The furfural and HMF are the most important inhibitors that give negative effects on the yield and productivity of bioethanol. In the presence of furfural < 0.5 g/L, glucose was completely consumed by the yeast but the ethanol yield and productivity were decreased from 0.44 g/g to 0.41 g/g and 0.48 g/L.h to 0.44 g/L.h, respectively [93]. When the concentration of furfural and HMF was increased of above 0.5 g/L has however significant decreased in glucose consumption by the yeasts and simultaneously reduction of both ethanol yield (18– 26%) and productivity (71–73%) was observed. The inhibitory effect caused by furfural during respiratory has a great impact on yeast propagation and cellular metabolism during the fermentative growth by directly inhibiting the alcohol dehydrogenase enzymes (ADH) that contributes to acetaldehyde excretion. The accumulation of intracellular acetaldehyde was postulated responsible to the lag phase in the growth leads to a low specific growth rate of the yeast and consequently reduced the specific ethanol productivity in ethanol fermentation [93,94]. Modig et al. [95] also claimed that the addition of 4 g/L of furfural to an exponential growth phase of batch culture of S. cerevisiae, reduced the specific growth rate and the specific ethanol production rate by 93% and 68%, respectively.

4.6. Bioprocessing options for bioethanol production

4.5. Comparison of pretreatment approach between kitchen waste and other wastes

4.6.1. Fermentation strategy Improvement of ethanol fermentation performance of the hydrolysate generated from the enzymatic hydrolysis of kitchen waste is also an important strategy to ensure that the bioconversion technology is economically feasible. As mentioned earlier, the complex structure of biomass in kitchen waste makes it difficult to be depolymerised into simple sugars. Pretreatment plays a significant role in breaking the structure to increase the conversion by enzymatic saccharification to simple sugars, which will subsequently use for bioethanol production using the fermentative microorganisms [96]. Depending on the biomass source, the kitchen waste hydrolysate typically consists of glucose, fructose, sucrose and maltose whereas other source such as lignocellulosic hydrolysate is characterised by glucose, xylose, arabinose, galactose, mannose, fructose, and rhamnose [97,98]. The sugars can be used as substrates in bioethanol production by yeasts or bacteria [66], based on the molar mass as shown by Eq. (1).

As reported earlier, Malaysia generates excessive kitchen waste production which are unprocessed and directly disposed in the landfill. Another waste generated in a large quantities is recorded from oil palm waste that characterised by lignocellulosic materials. Based on the abundant of waste generated, Malaysia could possibly sustain the nation production and supply of bioethanol via biorefinery concept that being supported by Malaysia government through legislative mandates. Utilizing kitchen waste as a substrate for bioethanol is rare reported in the literature as compared to lignocellulosic materials. Therefore, the specific focus on the status of kitchen waste biorefinery is very limited. A key challenge associated with kitchen waste is the ability to obtain a standardize feedstock, given the constituents of a daily waste production is varies comparing with lignocellulosic materials, hence, make the appropriate choice of pretreatment become difficult. Notwithstanding, in regards to a typical starchy material of kitchen waste make a simple physical-chemical pretreatment is sufficient enough to distinctively provide easier access to α-amylase and glucoamylase for enzymatic hydrolysis. In addition, kitchen waste is highly digestibility pretreated solid which is soluble in water. With the present of water, heat and chemical reactant, loosen and dissolved the hydrogen bond of the long chain polysaccharides. At this stage, α-amylase randomly cut down the α−1,4 glycosidic bonds releases few glucose and other monomers. The synergistic effect of α-amylase and glucoamylase enzyme degrade linear α−1,4 linkages and branching α−1,6 linkages give better performance on polysaccharide hydrolysis yielding high concentration of sugars. It is worthy to note that pretreatment of kitchen waste normally is conducted under mild temperature ( < 100 °C), therefore, the genera-

C6H12O6 → 2C2H5OH + 2CO2 Sugar (glucose) → Ethanol + Carbon dioxide gas

(1)

Theoretically, the fermentative microorganisms are able to produce 1 kg sugars particularly glucose into 0.51 kg of ethanol and 0.49 kg of carbon dioxide [99]. 4.6.2. Bioethanol producing microorganisms An ideal microorganism for biomass-bioethanol production can be best described in terms of their ability to produce a high yield of ethanol, broad substrate utilization range, resistant to inhibitory compounds generated during the hydrolysis steps, the ability to tolerate with high sugar and ethanol concentrations, resistance to inhibitors, ability to withstand high temperatures, ability to grow at low 679

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The integrated conversion technology, SSF and CBP, are aimed at performing the hydrolysis and fermentation in a single reactor. Obviously, the key of the SSF process is its ability to rapidly convert the sugars to bioethanol, lessening their accumulation in the medium. This can be achieve by combining the enzymatic hydrolysis and fermentation into a single step in the same reactor [26,36]. In the lignocellulosic-bioethanol production, SSF is usually preferred over SHF due to the ability of the system to reduce the inhibition of cellulose and concurrently increase the rapid conversion of glucose to bioethanol by yeast, hence, resulting in a faster production rate and lower capital costs [101]. The combination of enzymatic hydrolysis and bioethanol fermentation in one reactor has kept the concentration of glucose at low level. The accumulation of ethanol in the reactor does not inhibit the cellulases as much as the high concentration of glucose is maintained, therefore, SSF provides a better option than SHF in the ability to increase the rate of overall bioethanol production [36]. Among these three strategies, CBP is still in the early stages of establishment. Recent research shows a trend towards consolidated over time that CBP might be feasible in the future. In CBP configuration, this integration allows the combination of hydrolytic enzyme production, enzymatic hydrolysis, and bioethanol fermentation in a single reactor by the community of microorganisms that makes it different than other strategies [26]. This difference has an important advantage as only a single reactor is required and also helps in reducing the cost associated with the enzyme production [102]. A comparative study on advantages and disadvantages of integrated conversion technology is presented in Table 3.

pH and robust condition with minimum medium growth requirement, and minimal by-product formations [26]. Microorganisms suitable for industrial production of bioethanol are the strains that can produce bioethanol yield of more than 90% theoretical yield, bioethanol tolerance of more than 40 g/L and bioethanol productivity of more than 1 g/L.h [97]. Some native or wild type microorganisms such as Saccharomyces cerevisiae, Escherichia coli, Zymomonas mobilis, Candida shehatae, Kluyveromyces marxianus are normally used in the bioethanol fermentation [24]. Traditionally, the preferred microorganism for industrial bioethanol fermentation process is S. cerevisiae. It is characterised by robust grown yeasts that well suited for the fermentation of various types of biomass hydrolysates containing hexoses sugars. It is capable of fermenting hexose sugars more effectively compared to pentose sugars including xylose due to lacks of enzymes that can convert xylose to xylulose [97]. Substantial amount of bioethanol (36 g/L) was produced from the 48 h fermentation of 100 g/L of kitchen waste residue in S. cerevisiae [60]. They also reported that the bioethanol production was reduced to 25 g/L in fermentation using the same kitchen waste residue without nitrogen supplements, except yeast extract. Higher bioethanol concentration (90.72 g/L) was achieved in fed-batch fermentation by S. cerevisiae using 194.43 g/L of kitchen waste hydrolysate [65]. Besides S. cerevisiae, other bacterium namely Z. mobilis, has been rising attention as an ethanologens due to its ability to produce bioethanol at high rates from hexoses sugars. In addition, Z. mobilis showed a more rapid fermentation compared to yeasts [97]. Z. mobilis and C. shehatae are able to produce about 54.2 g/L and 48 g/L of bioethanol through fermentation using kitchen waste hydrolysate, respectively [66]. Both microorganisms have potential to replace S. cerevisiae for bioethanol production based on its ability to efficiently ferment sugars to bioethanol. Apart from that, the use of thermophilic microorganisms has also been extensively studied for their potential in bioethanol production. Thermophiles, due to its ability to withstand extreme temperature have a distinct advantages as its can be applied in a variety of inexpensive biomass feedstock, increase the solubility of the substrates prior to hydrolysis, improved mass transfer and diffusion rates, high bioconversion rates and has a low risk of contamination. The thermotolerant ethanol producing yeast strain, K. marxianus IMB3, produces 20 g ethanol in fermentation at 45 °C using 100 g of NaOH-pretreated barley straw, followed by enzymatic hydrolysis using 2% cellulases [36].

4.6.4. Product recovery and distillation The recovery of bioethanol from the fermentation broth began with solid-liquid separation as a predominant primary unit operation in the downstream processing to remove the solid fraction from the hydrolysate containing residual sugars and bioethanol [103]. Indeed, the best option for solid-liquid separation is through centrifugation and filtration. A supernatant is pumped into the rotary evaporator to reduce the portion of water in the hydrolysate. The series of evaporation steps produce fairly clean condensate with the concentrated syrup containing 15–20% by weight of total solid. After the first step, the concentrated syrup containing bioethanol will be passed through the distillation unit that exploits the difference in the boiling points of bioethanol (~78 °C) and water (100 °C) in the liquid mixture. The dilute bioethanol: water mixture can be repeatedly distilled to obtain a more concentrated ( > 95%) bioethanol solution. The recovery of bioethanol through distillation unit is fixed with 99.6% efficiency to reduce the losses of bioethanol evaporated to the environment [97].

4.6.3. Hydrolysis and fermentation strategies: An integrated technology The pretreatment methods (physico-chemical and enzymatic hydrolysis) and fermentation processes are the key determinants of the sustainability of bioethanol production from kitchen waste. The process development history of the technology used for the bioconversion of biomass to bioethanol is generally initiated by the conversion process that require fermentable sugars from the saccharification steps to be fermented by either ethanologenic yeast or bacteria [100]. Three types of process configuration for efficient bioethanol production from biomass has been identified: (i) separate hydrolysis and fermentation (SHF), (ii) simultaneous saccharification and fermentation (SSF), and (iii) consolidated bioprocessing (CBP) (Fig. 6). Traditionally, separate hydrolysis and fermentation (SHF) sequential steps are normally used in bioethanol production. The hydrolysate from the hydrolysis reactor enters the sugar fermentation reactor and converted to bioethanol by ethanologenic yeasts. The mixture then distillate to separate bioethanol produced, leaving the remaining fermentation broth and hydrolysate [26]. However, these approach is very costly, therefore, different techniques including SSF and CBP have been adopted to combine the enzymatic hydrolysis and fermentation in one reactor, thus reducing the overall production time, operating costs, inhibitors, which in turn, increasing the hydrolysis rate [39].

4.7. Summary of techno-economic conversion of kitchen waste to bioethanol It is known that the cost of feedstock, which accounts about 80– 90% of the overall production cost is significantly affect the bioethanol price. Kitchen waste is readily recognised as no cost resource since it is directly discarded without further utilized and its availability is undeniable. However, the main costs of kitchen waste utilization are sorting, transportation and pretreatments prior to enzymatic saccharification to produce fermentable sugars that are amenable for bioethanol production through fermentation. Therefore, techno-economic evaluation process needs to be conducted to analyse the viability of the process at commercial scale. At present, there are no comprehensive reports on the techno-economic analysis of kitchen waste to bioethanol either in small or medium scale of pilot plants. In general, techno-economic reports will offer an in-depth information on i) design and cost estimation of bioethanol plant, ii) process flow, iii) real market data, iv) financial analysis of production facility, and v) price of bioethanol [104]. 680

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Fig. 6. : Bioprocessing option for the conversion of biomass to bioethanol (SHF: separate hydrolysis and fermentation; SSF: simultaneous saccharification and fermentation; CBP: consolidated bioprocessing) [26,36].

followed by fermentation and bioethanol recovery process. The proposed process consists of i) sugar production process, ii) bioethanol plant, iii) combined heat and transfer system, and iv) wastewater treatment plant. In sugar production process, kitchen waste is converted to fermentable sugar by a series of pretreatment and hydrolysis using enzymatic reaction before being converted into bioethanol in bioethanol fermentation plant. The resulting wastewater will be treated in the wastewater treatment plant and can be used to generate biogas before being further treated to meet the final discharge standard. The generated biogas will be converted to electricity in heat and power section. The generated electricity is then supplied back to sugar production and bioethanol plant for self-sustenance. In addition, any excess electricity generated from the process can be sold to the grid,

The installation of infrastructure near to kitchen waste resource point to sort, collect and process of kitchen waste is needed for cost reduction of kitchen waste processing. Fig. 7 shows conceptual block diagram of an integrated bioethanol production from kitchen waste. A conversion technology of proposed kitchen waste-based bioethanol is adopted from Wan et al. [105] based on the case study on the conversion of sago biomass into bioethanol. Integrated process may reduce the additional cost of labour and may lower the greenhouse gas emission. Academic study using economic tools shows that the viability of bioethanol from non-food resources is highly dependent on the process flow for better economic profit. Therefore, the proposed process of kitchen waste conversion to bioethanol is involved in the pretreatment and enzymatic saccharification for sugar production

Table 3 A comparative study on advantages and disadvantages of integrated conversion technology. Fermentation configuration Separate hydrolysis and fermentation (SHF)

Advantages

Limitation

steps can be carried out under optimal • High cost • Each conditions (eg: enzymatic hydrolysis at 45due to long period process • Contamination 60°C; bioethanol fermentation at 30°C) End product inhibition minimize yield • bioethanol steps minimize interaction between • Separate steps be applied to wide range of • Can microorganisms as the process carried out

References Sarkar et al. [24] Vohra et al. [26] of

separately depending on the optimal conditions Simultaneous saccharification and fermentation (SSF)

effective optimum temperature for hydrolysis and • Cost • Different fermentation (eg: enzymatic hydrolysis at 45-60°C; number of reactor used and lower • Reduce bioethanol fermentation at 30°C) requirement of sterile condition, thus, easier operation Only applicable to thermotolerance yeasts strains • bioethanol yield due to immediate • Higher conversion of sugars to bioethanol that might

Sarkar et al. [24] Vohra et al. [26] Balat [97]

inhibit the hydrolytic enzyme activity

Consolidated bioprocessing (CBP)

product inhibition • Reduce • Shorter processing time effective as no capital and operational • Co-fermenting microorganisms need to be • Cost expenditure required for enzyme production compatible in terms of operating pH and temperature part of substrates is not deviated for • All production of enzyme Poor bioethanol yield • fermentation period (3-12 days) • Reduce product inhibition • Long range of microorganisms as only • Extensive applicable to thermophilic cellulolytic anaerobic bacteria for higher hydrolysis rate and bioethanol yield

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Biomass / kitchen waste Water Acid

Pretreatment

Steam Electricity Heat and Power System Treated water

Pretreatment slurry Saccharification tank

Electricity

Treated water

Wastewater

Mechanical vapor compressor evaporator

Wastewater Pressure filter

Electricity Electricity

Wastewater

Treated water Yeasts

Wastewater Treated water

Wastewater treatment plant

Storage tank Bioethanol production plant

Fig. 7. : Proposed integrated conversion of kitchen waste to bioethanol.

resource due to its complex composition and diverse in nature compared to other standard feedstocks such as agricultural crops and lignocellulosic materials. In addition, this complexity composition is due to different place of its generation and food habits of people in an area, therefore, proper characterisation method need to be commence for a better selection of the pretreatment process. Separation of individual components of kitchen waste such as carbon and nitrogen for value added products might end up increasing the cost of processing that is not economically feasible. Alternatively, the utilization of whole kitchen waste compound is encouraged in which simultaneous process of bioethanol production from kitchen waste that covered from pretreatment, enzymatic saccharification, bioethanol production can be developed in a single reactor, which will make the overall conversion process to become simpler and cost effective.

hence, may increase the economic performance of the integrated process. Meanwhile, the treated wastewater can be also recycled to reduce the freshwater consumption. In general, bioethanol production from kitchen waste will make the countries energy independent particularly for transportation fuel. Furthermore, it has a positive impact on economy since there will be less dependency on the political instability of fossil fuel. 5. Challenges in the utilization of kitchen waste for bioethanol production The concept of turning waste materials into valuable resources of financial, environmental and social returns has gained interest by means of protecting environment and concomitantly offers a sensible solutions in both economically and ecologically manners [106]. Currently, bioethanol is produced from agricultural feedstock by cultivation in significant large areas that has a great effect on the rural development particularly in the creation of new jobs and auxiliary incomes. It is well-known that most of ethanol production is via fermentation process using hexose sugars presence in the agricultural feedstocks such as sugarcane syrup, corn, soybeans, and palm, which tend to increase the price of the crops worldwide [7]. The use of food biofuel crops as a fuel raw material has been criticised as it leads to a dramatic shortage of global food as these crops should be designated to grow to meet the food supplication instead of being used for biofuel production [96]. In addition, the cultivation of crops in large areas leads to several environmental problems including soil erosion, loss of biodiversity and the emission of volatile compound. For all the above reasons, the researchers have decided to continuously explore an alternative non-food feedstocks, technologies and process for biofuel production. The utilization of kitchen waste as a feedstock most likely minimize the potential conflict between food and fuel. The key advantages in the utilization of kitchen waste for bioethanol production is that the unlimited natural bioresources that are geographically more evenly distributed than fossil fuel [96]. However, there are several issues for sustainable utilization of kitchen waste for bioethanol production which are summarized as below;.

5.2. Kitchen waste separation and collection Being a part of MSW, kitchen waste already has a proper management system of collection chain that is well organised by the local authority. Hence, the utilization of kitchen waste for bioethanol production offers an alternative solution for the management of municipal waste. For the development of waste-to-wealth strategy, the organic waste from domestic households should be initially separated properly at source from other type of recyclable materials such as plastics, paper and bottles and the sorted kitchen waste can be sent to various recycling facilities to be processed as a feedstock for the biofuel production. Currently, in many countries, most residents failed to sort kitchen waste at source. Behaviour of the public make the collection and separation facilities particularly for kitchen waste and other organic waste is practically needed upon the existing waste management system [107]. A series of social campaign by society groups, government and non-government organization, are critically needed particularly in television shows and social media platform to educate and bring awareness to the public upon the value of kitchen waste and the importance of kitchen waste separation at source against the general perception of kitchen waste that it should be thrown away. Municipalities and local authority in housing department should play a significant role to execute proper plant and smooth collection system. In addition, specific budget needs to be allocated to build and manage kitchen waste recycling facilities. An official policy is necessary to persuade the people and private sector participating in the segregation of waste materials in proper ways since the separation of kitchen waste

5.1. Kitchen waste composition It is well-known that kitchen waste is recognised as non-standard 682

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and wastewater treatment [23]. Although different technologies for bioethanol production have been adopted, their application at industrial levels are however limited [12]. Despite achieving high sugar yield, the equipment configuration of handling high ratio of water to solid during the hydrolysis and the ratio of bioethanol to fermentation medium during the fermentation that require significant energy for further pretreatment and product recovery. The current effort is to develop an effective process integration during the hydrolysis and fermentation in the SSF and CBP instead of the current application using SHF strategy. The developed method offers an advantages over SHF in reducing the inhibition of enzymes at high concentration of sugars and increasing the overall bioethanol yield and productivity. However, such microorganisms are not currently available to produce all the required enzymes and ferment all sugars to bioethanol in a single reactor as the applicability of these concepts is still under study [110]. Nevertheless, the bioethanol process shows that reliable cost estimations require laboratory results in pilot plants, where all steps are integrated into a continuous process. There is also the possibility to explore the process integration configuration to reduce the number of process steps, energy consumption, and to recirculate the process streams to eliminate the use of fresh water and the amount of waste streams. The economically viable of bioethanol production is important as it would allow the sustainability and strengthen the market of international bioethanol.

is critically important for further utilized as a bioethanol feedstock. In some developing countries such as Malaysia and Thailand, in order to promote better recovery of organic materials of MSW, the government has legislated the separation of household waste using separated bins for organic and recyclable wastes [108]. 5.3. Policies encouraging of subsidy on bioethanol plant The subsidy for establishing bioethanol plants will accelerate the production of biofuels as tax credits given by the government will create the market for biofuel. The National Renewable Energy Laboratory (NREL) of United States Department of Energy targeted the ethanol yields of 400 L/day by 2030 using the developed technology [96]. In pursuit of such a goal, financial rewards including the tax breaks are awarded to catalyse and support the establishment of bioenergy research centres at a commercial scale, which indirectly enhance the development of biofuel industry. In 2013, Malaysian government had introduced a National Biomass Strategy 2020, mainly focusing on high value industries such as biomass pellet, bioethanol and biobased chemical that is expected to reach commercial scale in year 2015–2020. Many incentives being announced aimed at creating the viability of investment in this industry and the appropriate technology developed to increase the opportunity of Malaysian biomass owners to participate in the downstream processing for value creation products [109]. However, it should be noted that the demand coming from the biobased specifically bioethanol industries highly depends on the regulation and policies ruled by the government such as taxationbased policies, agricultural based policies, subsidies and fuel mandates [99].

6. Bioethanol industry: The impact Rapid urbanisation and industrial development in Malaysia had increase in gradual rise of populations that leds to a significant challenge of managing MSW especially the organic fraction of kitchen waste. With a tremendous amount of waste generated, it is inevitable that the amount of land available for disposal landfills become scarce. Therefore, a potential methods of managing MSW is reducing the dependency of landfilling method by minimizing the amount of organic kitchen waste as found placed in the landfill. The landfill method is the least preferred method of kitchen waste disposal due to its high moisture content which pose serious environmental and social threats, hence, kitchen waste should be subjected to physical, chemical and biological pretreatment for a higher value product such as bioethanol. The use of kitchen waste for bioethanol production is attracting an interest due to its potential on sustainable basis. A long term demand for bioethanol from biomass will create new opportunities for biofuel industry and give a great impact on food security, increase in the economic value and overcome the deteriorating problem of environmental quality. The first generation of bioethanol are produced from agricultural crops such as corn, soy and rapeseed has arisen many concerns and impact on food security in which an increasing application of agricultural commodities for bioethanol production leads to crops shortage and increase in food commodities price. The ethical concern have encourage research effort on potential inedible feedstock alternatives. The introduction of non-food feedstock such as kitchen waste from MSW and lignocellulosic materials that do not compete with the food provision and animal feed eliminate the bad image of bioethanol. Additionally, kitchen waste represent the alternative inexpensive resources and can be supplied on a large-scale basis, which important to produce bioethanol at a reasonable costs. The use of kitchen waste seems to be a promising approach to produce bioethanol, reduce the reliance on fossil fuel and simultaneously decrease the oil price [22]. The fluctuation in agricultural commodities that commonly closely linked to the oil price in return may be diminished. The growing bioethanol industry has a positive impact on the growth of social and economic area by creation of direct and indirect employment opportunities. The sophisticated technological quest for higher performance of process plant requires a skill manpower and offers opportunities of higher qualified workers. The establishment of

5.4. Economic consideration of integrated plant performance While the bioethanol industry deliver an efficient technology and process, economically feasible, environmentally safe and sociable accepted, the government is the primary key players that plays a significant roles to make the wealth creation from biomass a reality for the nation. A competitive bioethanol production from biomass requires research and development of an appropriate technologies and downstream processes for converting biomass feedstock to bioethanol. Therefore, a strategic collaboration among experts from multidisciplinary areas such as biotechnologist, downstream processing, chemical and biological catalysts, environmental engineering are required to ensure a sustainable integrated technology for bioethanol production. In kitchen waste-bioethanol production, while considering the fact that waste has been properly separated, it is yet important to initially establish a suitable and economically viable pretreatment and hydrolysis methods that requires the use of hydrolytic enzymes for bioconversion of biomass to sugar. Enzymes usually present an elevated costs and lower sugar yields with long reaction times which are not competitive at the commercial scale [96]. The pretreatment methods that requires high energy consumption particularly at high temperatures remains a challenge and not economically feasible [45]. Even so, the utilization of carbohydrate-rich-kitchen waste as a substrate may reduce the cost of energy consumption as it is usually conducted at mild conditions due to the structure of kitchen waste that can be easily depolymerised to monomer sugars compared to other agricultural and woody biomasses [31]. In large scale of biomass conversion to sugar, there is a need to improve the conversion rate and the yield of fermentable sugar produced, therefore, the process of scale-up at commercial scale is important to increase further bioethanol production. The final production cost of bioethanol has been estimated at US$ 0.48/L, with enzymatic and fermentation stages being the most expensive steps (31%), followed by the hydrolysate detoxification (22%), pretreatment of the raw material (12%) and the remaining of 35% involving the costs of distillation, labour, cell mass production (inoculum preparation), 683

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the new bioethanol plant increase the prospect of rural development and increased wealth creation in those region, hence, bring the society to better economic state [111]. To fully capitalise on the kitchen waste utilization, the efficiency of the plant constructed for higher value downstream activities will create a joint venture (JV) cluster to help aggregation of the biomass, alleviate the risk of downstream process and market failure and create a domestic and international investment. The increasing demand of fossil hydrocarbon resulted in the diminishing of world fuels reservoir and cause the price to increase. Pull up the present scenario of global warming due to the release of carbon dioxide (CO2) from transportation and industries, no doubt bioethanol plays an important role in reducing greenhouse gas emission through a complete combustion of fuel and reduce particulate emission that pose health threat to living materials. A reduction of approximately 50% of the annual CO2 emission by 2050 is targeted based on the bioethanol development from biomass. Kitchen waste itself is a sustainable and eco-friendly resources to overcome the problem of ecological impacts and pollution of fresh water due to fossil fuel digging activities [112]. To produce bioethanol from kitchen waste is well-thought-out as a novel concept that can assessed in relation to sustainability and ecological preservation. In conclusion, kitchen waste is an attractive low-cost feedstock that contribute to the competiveness of the bioethanol industry providing positive socioeconomy impact on the nation.

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7. Concluding remarks and future direction The consideration of kitchen waste as a substrate for bioethanol production and the current research challenge to be faced in the downstream processing demands a critical evaluations. Despite of so many advantageous discussed earlier, there are still no large-scale production facilities of bioethanol using kitchen waste as substrates. In a large scale and competitive level, major research efforts on utilizing kitchen waste as substrate for bioethanol production need to be carefully assessed in comparison with the well-established bioethanol production using other biomasses. It is also crucially important to provide an alternative solution in the separation and sorting of organic kitchen waste from the heterogeneous compound of MSW, in maximizing higher recovery of organic waste from the waste streams. A development of cost effective strategies and efficient technologies during pretreatment and hydrolysis of carbohydrate-rich-kitchen waste to fermentable sugar is necessary to obtain high rate and yield of biomass soluble sugars. Simultaneously, the hydrolysate containing sugars and inhibitory compound generated during pretreatment need to be completely separated to reduce any inhibitions to the fermentative microorganisms during the fermentation stages. A-state-of-the-art integrated conversion process need to be further explored and optimized as it might reduce and minimize the demand of overall process energy. A successful integrated process requires superior microorganisms able to metabolize all types of pentoses and hexoses sugars and simultaneously withstand the stress imposed by the potential inhibitors, extreme pH conditions and must also have an ability to survive high temperatures, giving a considerably high productivity and yield of bioethanol in larger scale. Well-thought integrated strategies are hoped to reduce the number of steps involved in the overall process of bioethanol production in an economically efficient manner. Bioethanol production requires more advanced technological developments to enhance the high net energy and reduce the production cost. A complete biofuel future scheme should be able to provide ecologically save, economically competitive and able to conduct in large scale production without affecting the provision of food. Acknowledgement The authors are thankful to the Ministry of Higher Education Malaysia for supporting of this work under research Grant no. 02–03-11-1008FR. 684

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