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Apr 11, 2016 - soluble organic materials (soluble sugar, 10.8 g/L; and soluble protein, 1.61 g/L) were ... food waste (FW) is also a highly desirable ...
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Enhancing Food Waste Hydrolysis and the Production Rate of Volatile Fatty Acids by Prefermentation and Hydrothermal Pretreatments Xiaoqin Yu,†,‡ Jun Yin,*,†,‡ Kun Wang,†,‡ Dongsheng Shen,†,‡ Yuyang Long,†,‡ and Ting Chen†,‡ †

School of Environmental Science and Engineering, Zhejiang Gongshang University, Hangzhou 310012, People’s Republic of China Zhejiang Provincial Key Laboratory of Solid Waste Treatment and Recycling, Hangzhou 310012, People’s Republic of China

‡

ABSTRACT: The effects of prefermentation pretreatment (PF) alone and prefermentation combined with hydrothermal (HT) pretreatments (PF-HT) on the production of volatile fatty acids (VFAs) by the anaerobic fermentation of food waste (FW) were investigated. The results showed that PF could enhance the dissolution and hydrolysis of organic materials. The greater levels of soluble organic materials (soluble sugar, 10.8 g/L; and soluble protein, 1.61 g/L) were achieved after the PF-HT pretreatment. In the subsequent anaerobic fermentation tests, for the PF and PF-HT groups, the production rates of VFAs increased 1.3 times and 2.0 times, respectively, as compared to the control over the first 5 days. The maximum concentrations of VFAs for the PF and PF-HT groups on the 15th day of fermentation were 32.9 and 35.5 g COD (chemical oxygen demand)/L, respectively, with corresponding increases in the yields of VFAs of 12% and 21% over the control group. It is suggested that the high content of lipid and cellulose in the FW prevented any further improvement in the yields of VFAs. Butyric (Bu) and acetic acids (Ac) were the prevalent VFAs, regardless of the pretreatment used. However, the metabolic pathways for the VFAs production were different for the two pretreatments: with PF pretreatment, the FW was converted into VFAs via lactic acid, but with PF-HT pretreatment, it was transformed directly into VFAs. Economic analysis indicates anaerobic fermentation with appropriate PF and HT pretreatments is an earning-effective approach for FW to bioenergy such as VFAs.

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improve the biodegradability of organic materials.17−19 However, most pretreatments will consume resources and energy, leading inevitably to economic and environmental losses.20 Prefermentation (PF), with no additional chemicals or energy, has gained more attention recently. PF has been introduced as an alternative pretreatment strategy, which makes use of indigenous microorganisms in the wastes. Mahmoud et al.21 found that using PF on landfill leachate could promote the degradation of organic material, thereby increasing the content of VFAs. Geffroy et al.22 showed that PF was beneficial for wine production, raising the content of VFAs in the final product. Moldes et al.23 found that the starch in FW (during the PF phase) was apt to saccharify, therefore increasing the amount of soluble substances. In the current engineering practice, hydrothermal (HT) pretreatment with no added external chemicals has been universally used to dispose of FW.24 Many studies of its effect on the AD from sludge have shown that it can convert organic molecules to soluble substrates.25,26 The advantages of PF and the wide application of HT pretreatment lead to the need to explore their combined effect on the VFAs production from FW in an anaerobic fermentation process. Previous studies on PF pretreatment have focused only on its effect on dissolved organic materials,22,23 with its impact on acidogenic fermentation processes from FW remaining unreported. Therefore, the aim of the present study is to

INTRODUCTION Volatile fatty acids (VFAs) such as acetic (Ac), propionic (Pr), butyric (Bu), and valeric acids (Va) are typical sources of bioenergy. They are of increasing interest due to their wide range of applications, which include biological nitrogen removal and bioplastics synthesis.1,2VFAs accumulation can be achieved by organic matters degradation during anaerobic fermentation. Several studies have shown that waste-activated sludge, collected from sewage treatment plants, can be used as a potential substrate for anaerobic fermentation.3,4 At present, food waste (FW) is also a highly desirable substrate for anaerobic fermentation because of its high levels of biodegradability and nutrient contents.5,6 The main biochemical components of FW are carbohydrate (simple sugar and polysaccharides), protein, and lipids. Among these waste compositions, the degradation of polysaccharides (cellulose, hemicellulose, and lignin) was limited due to the complex structure. Therefore, to enhance the degradation efficiency of polysaccharides is of great importance. Many interesting and valuable works on degradation of cellulose for biofuels and valuable compounds have been studied, including hydrothermal conversion, pyrolysis,7−9 enzymatic degradation,10 photocatalyzed degradation,11 electrochemical degradation,12 ionic liquids degradation,13 microwave irradiation oxidation,14 etc. Anaerobic digestion (AD) proceeds in three stages: hydrolysis/acidification, acetogenesis, and methanogenesis.15 However, VFAs are only produced during the first phase. Eastman and Ferguson found that during the AD process, hydrolysis was typically a rate-limiting step.16 Several studies have shown that pretreatments using crushing, heat, Fenton oxidation, ozone, acid, alkali, and ultrasonic method can © 2016 American Chemical Society

Received: January 12, 2016 Revised: April 8, 2016 Published: April 11, 2016 4002

DOI: 10.1021/acs.energyfuels.6b00077 Energy Fuels 2016, 30, 4002−4008

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temperature of 35 °C. Detection was performed at a wavelength of 210 nm. The hydrolysis of FW and accumulation of VFAs in the present study are expressed as g COD per liter of liquid. The conversion factors are 1.5 g COD/g protein, 1.06 g COD/g carbohydrate, 1.07 g COD/g Ac, 1.51 g COD/g Pr, 1.82 g COD/g Bu, and 2.04 g COD/g Va.33

investigate the effect of PF pretreatment on the hydrolysis of FW and the VFAs production. The combination of PF and HT pretreatments will also be evaluated. This knowledge will help to improve the potential production of VFAs under optimum conditions and accelerate its industrialization.

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EXPERIMENTAL SECTION

RESULTS Effect of Pretreatments on FW. The characteristics of FW before and after pretreatments are shown in Table 2. As

Substrate and Inoculum. The FW, containing mainly rice, vegetables, and meat, was collected from a canteen of Zhejiang Gongshang University (Hangzhou, China). Before use, any bones, metals, or hard items were removed from the FW. The FW was then cut into small pieces and stored frozen at −18 °C before the fermentation test. The anaerobic granular sludge inoculum, taken from a up-flow anaerobic sludge blanket (UASB) reactor of the Xihu Brewery (Hangzhou, China), would have to be reactivated with culture medium before use. The main characteristics of the FW and the anaerobic sludge are listed in Table 1.

Table 2. Characteristics of FW before and after Pretreatments volatile solid (%) total nitrogen (%) lipid (%) cellulose (%) soluble sugar (g/L) soluble protein (mg/L)

Table 1. Characteristics of FW and Anaerobic Sludge

a

parameter

FW

anaerobic sludge

pH TS (%) VSS/TS (%) TN (%) total protein (%)

6.1 22.0 96.3a 1.9a 11.9a

6.8 4.0 79.0a − −

a

control

PF

PF-HT

96.3 1.9a 24.8a 31.5a 8.8 228.8

94.8 1.4a 20.0a 29.7a 16.3 431.2

− − 22.7a − 19.6 1839.0

As based on dry weight. “−”, undetected.

compared to the control, after PF pretreatment the contents of TN, lipids, and cellulose (dry weight) decreased by 0.5%, 4.7%, and 1.8%, respectively. This showed that PF promoted the hydrolysis and dissolution of part of the organic material in the solid phase. In addition, the content of VS reduced from 96.3% to 94.8% during prefermentation, which was mainly ascribed to inevitable biological consumption of organic matters. After the PF-HT pretreatment, the content of lipids was 0.7 g more than the PF group. Proteins and carbohydrates can produce biocrude (synthetic oil) by HT to enhance the percentage of lipids as described by Savage et al.34 Remarkably, the contents of soluble sugar with PF and PF-HT pretreatment were 7.5 and 10.8 g/L higher than the control, respectively. Similarly, with PF treatment, the soluble protein content at 202.4 mg/L was higher than that of the control. However, the dissolution of protein with PF-HT treatment was enhanced greatly, being 4.3 times that of the PF group. Effect of Pretreatments on Fermentation for VFAs Production. The concentrations of soluble sugar, protein, and NH4+−N during the anaerobic fermentation process are shown in Figure 1. At the beginning of fermentation (0−5 days), the soluble sugar concentrations of the three groups decreased to less than 1 g COD/L, but the uptake rate of sugar was different in each group as follows: control (1.7 g/L·d) < PF (3.9 g/L·d) < PF-HT (4.7 g/L·d). After day 5, the soluble sugar remained at a relatively low concentration for all three groups. As illustrated in Figure 1b, from day 0 to day 5, the concentration of soluble protein in the PF-HT group increased slowly at first, and then decreased rapidly at a rate of 337.5 mg/ L·d. After 5 days of fermentation, the amount of soluble protein remained at about 1400 mg COD/L. However, the soluble protein concentration in the PF group remained at about 400 mg COD/L, a value similar to the control during the whole fermentation. The greater soluble protein dissolution after pretreatments did not lead to a greater degradation of the soluble protein. The surplus soluble protein would have been expected to degrade further. The NH 4 + −N concentration increased linearly with fermentation time after 3 days (Figure 1c). As previously

As based on dry weight. “−”, undetected.

Prefermentation and Hydrothermal Pretreatments. The PF pretreatment of FW was performed at a constant temperature (about 25 °C) using the indigenous microorganisms in the FW. The pH of the FW was monitored during the PF process. When the pH had decreased to about 3.5, the PF of FW was considered complete. The HT pretreatment of FW was carried out as described by Yin et al.27 The temperature and duration of the HT were set at 160 °C and 30 min, respectively. Fermentation Experiments for VFAs Production. The pretreated FW was fermented in amber wide-mouth bottles with a working volume of 500 mL. FW with no pretreatment was used as the control. The total solids (TS) content in each reactor was adjusted to 7%. For fermentation, all of the reactors were filled with 80% pretreated FW and 20% sludge (dry weight), and then stirred mechanically at 120 rpm using a magnetic stirrer. The experimental temperature was controlled at 30 ± 2 °C and the pH at 6.0 by adding 4.5 M HCl or NaOH. Each reactor was duplicated and operated for 21 days. Analysis Methods. During fermentation, samples were collected from the reactor every 2 days. The TS, volatile solids (VS), volatile suspended solids (VSS), total nitrogen (TN), cellulose, lipids, soluble chemical oxygen (SCOD), soluble sugar, soluble protein, ammonia nitrogen (NH4+−N), VFAs, and lactate contents were all determined. The fermented broth was separated from the residue by centrifuging at 10 000 rpm for 5 min and filtered using a filter membrane (0.45 μm). TS, VS, VSS, TN, NH4+−N, and SCOD were all analyzed using standard methods.28 Cellulose was assayed using the Van Soest detergent method.29 Lipids were extracted by Soxhlet extraction.30 Soluble protein was quantified by the Lowry−Folin method using bovine serum albumin as the standard, and carbohydrate was determined using the phenol-sulfuric acid method with glucose as the standard.31,32Volatile fatty acids (VFAs, C2−C5), including Ac, Pr, n-Bu, iso-Bu, n-Va, and iso-Va, were determined using a GC7890-II gas chromatograph (Tianmei Co., Shanghai, China) equipped with a 3 m × 2 mm stainless steel packed column filled with GDX-103 as the stationary phase and a flame ionization detector. The temperatures of the column, injector, and detector were 180, 230, and 250 °C, respectively. Lactate was measured by high-performance liquid chromatography (Waters Corp., Milford, MA) using a C-18 column with 5 mM acid as a mobile phase at a flow rate of 1 mL/min and a 4003

DOI: 10.1021/acs.energyfuels.6b00077 Energy Fuels 2016, 30, 4002−4008

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Figure 2. (a) VFAs production and (b) lactate production during the fermentation with different treatments.

production increased at a slow rate in the PF-HT group, while in the control and PF groups, it increased more rapidly, reaching 23.8 g COD/L and 27.9 g COD/L on the ninth day, respectively. From day 10, the VFAs production in each group remained relatively stable. At the end of fermentation, the VFAs concentration declined. The highest VFAs production was 35.5 g COD/L in the PF-HT group on the 15th day, followed by 32.9 g COD/L in the PF group (15th day) and 29.4 g COD/L in the control group (13th day). VFAs Composition. The composition of VFAs at the end of fermentation is shown in Figure 3. Six types of fatty acid were detected: Ac, Pr, iso-Bu, n-Bu, iso-Va, and n-Va. For the control group, the percentage of Va was less than 5%, and the

Figure 1. (a) Soluble sugar, (b) protein, and (c) ammonia nitrogen concentrations in reactors.

reported,27 the NH4+−N release rate was calculated by a linear fitting of the NH4+−N concentration to the fermentation time. From day 3, the NH4+−N release rates were 17.2, 23.2, and 26.7 mg/L·d for the control, PF, and PF-HT groups, respectively. This pattern of results demonstrated that the degradation rates of soluble protein had increased in the PFHT and PF groups. Combining the results of soluble protein and NH4+−N, it can be seen that more soluble proteins were converted in the PF-HT group than in the other groups during fermentation. VFAs Production. As shown in Figure 2a, at the beginning of fermentation, the VFAs concentration was low for all three groups. However, 2 days later, the VFAs concentration in the PF-HT group increased rapidly to 18.8 g COD/L, while in the PF and control groups, they were still less than 5 g COD/L. The production rates of VFAs in the three groups were totally different: control (1.9 g COD/L·d) < PF (2.2 g COD/L·d) < PF-HT (9.4 g COD/L·d). From day 3 to day 9, the VFAs

Figure 3. Percentage of individual VFAs at the end of fermentation. 4004

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HT groups were 30.9%, 35.4%, and 36.7%, respectively. This result indicated that pretreatments could promote the VFAs production. However, the contents of the residual VS after anaerobic fermentation were over 50% in all three groups. The residual VS consisted of protein, lipid, sugar, sludge biomass, and cellulose. As Figure 4b displayed, the residual cellulose contents for the control, PF, and PF-HT groups were about 7.1, 6.8, and 6.6 g, respectively. As compared to the cellulose content of the original FW (8.4 g), it can be inferred the cellulose with high content in FW would inhibit its conversion to VFAs even after pretreatments.

contents of other individual VFAs were similar. However, in the PF and PF-HT groups, the Ac:Pr:Bu:Va ratios were 32:17:46:5 and 26:13:58:3, respectively. In these two groups, butyric acid was the most prevalent VFAs, followed by acetic acid and then propionic acid. The acidogenic pattern may result from the quantity of soluble sugar produced greatly after the PF and HT pretreatments. An abundance of butyric as well as acetic acids is typically characteristic of acid fermentation in a carbohydraterich substrate.35 Carbon Balance Analysis. During fermentation, the carbon in the VS was converted to the carbon in VFAs, soluble protein and sugar, lactate, alcohol, methane, and other products. The carbon balance calculation was based on the maximum VFAs production. Figure 4a shows that most of the

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DISCUSSION Performance of FW Fermentation by Pretreatments. From these results, it is feasible to produce a higher level of soluble substrates and accelerate the VFAs production using these two pretreatments. PF can promote the hydrolysis of carbohydrates, which contributes to the saccharification of polysaccharides (such as starch) by the indigenous microorganisms in FW. Moldes et al. also found that during the prefermentation phase, starch in FW was apt to saccharify, therefore increasing the amount of soluble sugar.23 At high temperatures between 160 and 200 °C, complex substrates (i.e., starch, protein, lipids, and some cellulose) can be hydrolyzed into small molecules. Therefore, FW produced more soluble organic materials after HT treatment.36,37 During fermentation, soluble sugar was converted rapidly and finally remained at a low concentration (Figure 1a). However, because protein is not susceptible to protease cleavage in its native folded conformation,38 it was still found to accumulate in FW even after fermentation by pretreatments. Chen et al. also reported that some soluble proteins remained in the system when the VFAs production had reached a maximum during the AD of residual activity sludge.3 Similarly, Liu et al.19 found that only 40−50% of protein was converted into VFAs after a thermalalkaline pretreatment. The accumulation of VFAs was related to the bioconversion of soluble organic substrates.39 During the first 5 days, the production rates of VFAs in the control, PF, and PF-HT groups were 2.8, 3.5, and 5.4 g COD/L·d, respectively. After 5 days, the production rates of VFAs decreased when most of the soluble organic substrates had been consumed. The dissolution and hydrolysis of organic materials were improved by the pretreatments, thus increasing the production rate of VFAs. It was difficult for microorganisms to make use of particulate matters. So to enhance the dissolution of solid materials is of great importance in anaerobic fermentation. Influence of FW Composition on the VFAs Production. As Table 3 shows, several pretreatments have been used to enhance the VFAs yields from FW, such as ultrasonic, acid, enzymatic, and heat treatment.40−42 As compared to some of

Figure 4. (a) Carbon balance of each reactor at the optimum VFAs production; and (b) the residual contents of FW constituents with different treatments.

reduced VS was converted to VFAs. Also, the carbon conversion effciencies for VFAs in the control, PF, and PF-

Table 3. Comparison of VFAs Production from Different Types of Fermentation fermentation substrate food food food food food a

waste waste waste waste waste

seeding

optimal condition

VFAs production

refs

mesophilic anaerobic sludge anaerobic sludge food waste anaerobic sludge anaerobic sludge

ultrasonic 480 W/L, 15 min; TS = 100 g/L 0.1% enzyme dosage ultrasonic (79 kJ/g TS) and acid HT (160 °C, 30 min) PF-HT (160 °C, 30 min)

103.1 g COD/L 5.7 g COD/L 16.9 g COD/L 0.91 g/g VSSremoval (51.3 g COD/L) 0.74 g/g VSSremoval (35.5 g COD/L)

Jiang et al.a Kim et al.b Elbeshbishy et al.c Yin et al.d this study

As a reference in the text.40 bAs a reference in the text.41 cAs a reference in the text.42 dAs a reference in the text.27 4005

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activities of other microorganisms will recover. Meanwhile, a certain concentration of lactate will suppress the activity of Lactobacillus. What is more, Guido et al. found that the organic materials in FW can first be degraded to lactic acid,55 and then converted into acetic, propionic, and butyric acids. A possible metabolic pathway for VFAs production from organic materials has been reported.3 Fermentation substrates, such as polysaccharides and proteins, are first hydrolyzed into monosaccharides and amino acids, respectively, and then into pyruvates. Pyruvates can be transformed directly into VFAs or lactate, and then lactate can also be converted to VFAs (Table 4). Consequently, the method of producing VFAs from FW, during the initial period of fermentation using PF as the pretreatment, was substrate− lactate−VFAs.

the present research (Table 3), PF-HT pretreatment may be a good strategy for acidogenic fermentation because of less chemical and energy costs and higher VFAs accumulation. Although the PF and HT pretreatments significantly improved the dissolution of FW and the rates of hydrolysis and acid production, the maximum VFAs production from the PF-HT group (0.742 g/g VSSremoval) was less than that in our previous study (0.908 g/g VSSremoval) using HT treatment.27 Comparing the two experimental conditions, there existed a difference in the composition of the FW. The soluble sugar and soluble protein contents were similar, but the contents of lipid and cellulose in the present study were 14.8% and 7.3%, respectively, more than those previously.27 Some polysaccharides such as cellulose and hemicellulose are known to be hard to hydrolyze. Also, the inhibitory effect of lipids on anaerobic biogenesis is universal.43 Thus, the influence of lipids and cellulose on the VFAs production should be taken into account. Table 2 shows the level of lipids conversion after PF and PFHT pretreatments. It can be seen that these pretreatments play small roles in the dissolution and hydrolysis of lipids. Longchain fatty acids (LCFA) are one type of hydrolysis product from grease during anaerobic fermentation, which can be related to lipid suppression.44 LCFA not only inhibits the organic mass transfer process through adsorption onto microbial cell membranes,45−47 but can even weaken the activity of some bacteria.48,49 Jiang et al.40 found that grease did not prevent the organic materials from hydrolyzing to soluble molecules. However, due to the toxicity of LCFA, small molecules are possible to be prevented from being converted to VFAs. The FW in the present and previous study27 had almost the same C/N ratio, but the amounts of cellulose were different. Although Yen et al.50 explored the effect of the initial C/N ratio on the production of methane, providing a high initial C/N ratio could effectively increase the yield of VFAs. Liu et al. confirmed that the components of FW could play important roles in the yield of VFAs even if the C/N ratios were similar.51 The composition of the residual materials (Figure 4b) also suggested that cellulose could hardly be converted during fermentation even after PF and HT pretreatment. This may stop more substrates from being converted to VFAs. As Ravindran and Jaiswal have shown, chemicals can be released from cellulose, hemicellulose, and lignin during pretreatments, which can inhibit enzyme activity, microbial growth, and metabolism.52 Siegert and Banks found that VFAs inhibited cellulase activity and cellulose hydrolysis at its concentrations above 2 g/L.50 The results on the accumulation of VFAs show that it was possible that VFAs had restrained the hydrolysis of cellulose to limit the yield of VFAs. Therefore, once the biodegradable proportion of the FW decreased, the yield of VFAs also decreased, even after PF and HT pretreatments. Acidogenic Pathway Analysis. Figure 2b shows that the initial lactic acid content was 7.4 g/L after PF. For the PF group, the maximum amount of lactic acid reached 13.96 g/L on the second day, but after day 3 it was almost undetectable. However, the yield of VFAs increased with fermentation time. This phenomenon indicated there existed the change of microbial community between Lactobacillus and Clostridium.53The possible explanation is the inhibitory reaction of lactate, which existed based on the adjustment of pH and the concentration of lactate in the system. A high level of lactate will be produced when the pH control was in the range of 5− 7.54That is to say, when the pH is above or below the range, the

Table 4. Comparison of FW Composition and VFAs Production VS (%) TN (%) lipid (%) cellulose (%) soluble carbohydrate (g/L) soluble protein (g/L) pretreatment optimal condition fermentation time (d) VFA production (g/ gVSS removal) a

previous studya

this study

93.3b 1.8b 11.0b 24.3b 19.8 1.1 HT (160 °C, 0.5 h) pH 6.0, 30 °C 15 0.908

96.3b 1.9b 24.8b 31.5b 19.6 1.8 PF-HT (170 °C, 0.5 h) pH 6.0, 30 °C 15 0.742

As a reference in the text.27 bAs based on dry weight.

In contrast, for the PF-HT group, lactate was only detected on the first day at a level of 2.38 g/L. The Lactobacillus in FW was killed by the HT treatment.56 In other words, the soluble organic materials were not converted to lactic acid but directly to VFAs. Also, the initial lactate accumulated by PF was converted to VFAs. Therefore, for the PF-HT group, the possible acid production pathway was substrate−VFAs. Economic Assessment. In Table 5, the economic potential of PF and PF-HT pretreatment for VFAs production from FW was analyzed on the basis of the experimental results and literature data obtained.57−59 The FWPF and FWPF‑HT are fermented to VFAs using PF and PF-HT treatment, respectively. The residual FW are then disposed of at a landfill, and the produced VFAs are assumed to be used in the market for revenue estimation. Besides, the FWcontrol is converted to VFAs without using any pretreatments, and the other processes were the same with the FWPF and FWPF‑HT. Table 5 displays that the benefits of PF and PF-HT pretreatments are due to the increased revenue from VFAs-based energy production and the lower chemical and energy consumption costs. In general, PF and PF-HT pretreatments would generate about $2005.8 and $2075.1 million earnings more than the FWcontrol per annum. Consequently, PF-HT treatment could be an excellent strategy for FW to VFAs. Nevertheless, the cost of HT treatment should not be ignored, which may diminish the beneficial role of HT in practical application. In addition, the preliminary economic assessment was conducted according to laboratory-scale data in the present research, and some unfitness and uncertainties might exist in the practical engineering. Therefore, the economic feasibility of PF and PF-HT pretreatments for 4006

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Table 5. Preliminary Economic Analysis of the Different Pretreatments for VFAs Production from FW general parameters

values

annual FW production (million ton wet solids/y) FW concentration (g TS/g wet solid) ratio of TCOD to TS initial FW temperature (°C) specific heat value of FW (kJ/kg·°C) price of VFAs ($/ton) yield coefficient of VFAs from FWcontrol (g COD/g COD) yield coefficient of VFAs from FWPF (g COD/g COD) yield coefficient of VFAs from FWPF‑HT (g COD/g COD) cost of transportation and landfill ($/ton dry solids) heat price ($/Mcal) FWcontrol reduction (%) FWPF reduction (%) FWPF‑HT reduction (%) annualized FWcontrol disposal cost (million$/y) annualized FWPF disposal cost (million$/y) annualized FWPF‑HT disposal cost (million$/y) annualized cost of HT treatment (million$/y) annualized cost of fermentation (million$/y) revenue of FWcontrol converted VFAs (million$/y) revenue of FWPF converted VFAs (million$/y) revenue of FWPF‑HT converted VFAs (million$/y) annualized net revenue (FWcontrol) (million$/y) annualized net revenue (FWPF) (million$/y) annualized net revenue (FWPF‑HT) (million$/y)

a

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

85 0.2b 1.14 25b 3.1b 997.6c 0.46d 0.52d 0.56d 250e 0.07f 27.0g 56.0g 59.5g 3102.5 1870.0 1721.3 594.9 21.1h 5928.70 6702.0 7217.5 2805.1 4810.9 4880.2

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate the Foundation of Zhejiang Educational Committee (no. Y201534397) and the Innovative Research Team in Higher Educational Institutions of Zhejiang Province (no. T200912) for providing the funding support for this project.

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a

Estimated data for China (2015). bBased on our previous study survey. cData from www.alibaba.com. dThe maximum value shown in this study. eLiterature values based on sludge disposition (Dhar et al., 2012).57 fHistorical prices (Cho et al., 2014).58 gBased on our “this study” result. hEstimated from the energy costs to operate the fermentation systems (Metcalf and Eddy, 2003)59 and the heat capture from the fermented residual (Dhar et al., 2012).57

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NOMENCLATURE PF = prefermentation pretreatment HT = hydrothermal pretreatment VFAs = volatile fatty acids FW = food waste Ac = acetic acid Pr = propionic acid Bu = butyric acid Va = valeric acid AD = anaerobic digestion UASB = upflow anaerobic sludge blanket TS = total solids VS = volatile solids VSS = volatile suspended solids TN = total nitrogen SCOD = soluble chemical oxygen NH4+−N = ammonia nitrogen VSSremoval = volatile suspended solids removal LCFA = long-chain fatty acids C/N ratio = carbon/nitrogen ratio REFERENCES

(1) Li, X.; Chen, H.; Hu, L. F.; Yu, L.; Chen, Y. G.; Gu, G. W. Environ. Sci. Technol. 2011, 45, 1834−1839. (2) Chen, H.; Meng, H. J.; Nie, Z. C.; Zhang, M. M. Bioresour. Technol. 2013, 128, 533−538. (3) Zhang, P.; Chen, Y. G.; Zhou, Q. Water Res. 2009, 43, 3735− 3742. (4) Jia, S. T.; Dai, X. H.; Zhang, D.; Dai, L. L.; Wang, R. C.; Zhao, J. F. Water Res. 2013, 47, 4576−4584. (5) Jiang, J. G.; Zhang, Y. J.; Li, K. M. Bioresour. Technol. 2013, 143, 525−530. (6) Xu, S. Y.; Karthikeyan, O. P.; Selvam, A.; Wong, J. W. Bioresour. Technol. 2012, 126, 425−430. (7) Jiang, G.; Nowakowski, D. J.; Bridwater, A. V. Thermochim. Acta 2010, 20, 61−6. (8) Wang, S.; Wang, K.; Liu, Q.; Gu, Y.; Luo, Z. Biotechnol. Adv. 2009, 27, 562−7. (9) Jiang, G.; Nowakowski, D. J.; Bridwater, A. V. Energy Fuels 2010, 24, 4470−5. (10) Xia, Z. Y.; Yoshida, T.; Funaoka, M. Eur. Polym. J. 2003, 39, 909−14. (11) Kansal, S. K.; Singh, M.; Sud, D. J. Hazard. Mater. 2008, 153, 412−7. (12) Tian, M.; Wen, J.; MacDonald, D.; Asmussen, R. M.; Chen, A. Electrochem. Commun. 2010, 12, 527−30. (13) Binder, J. B.; Gray, M. J.; White, J. F.; Zhang, Z. C.; Holladay, J. E. Biomass Bioenergy 2009, 33, 1122−30. (14) Ouyang, X.; Lin, Z.; Deng, Y.; Yang, D.; Qiu, X. Chin. J. Chem. Eng. 2010, 18, 695−702. (15) De La Rubia, M. A.; Raposo, F.; Rinson, B.; Borja, R. Bioresour. Technol. 2009, 100 (18), 4133−4138.

VFAs production from FW should be evaluated with a larger scale.

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CONCLUSIONS PF and PF-HT pretreatments can significantly promote the dissolution and hydrolysis of organic materials. PF mainly promotes the hydrolysis of carbohydrates, which contributes to the saccharification of polysaccharides (i.e., starch) in an acidic environment caused by Lactobacillus in FW. However, to some complex substrates (i.e., cellulose and protein), PF-HT pretreatment seems to be a better strategy for their solubilization. The production rate of VFAs increased noticeably at the beginning of anaerobic fermentation using pretreated FW. However, because of the high contents of cellulose and lipids in FW, the yield of VFAs would be limited, even after PF and HT pretreatments. The acidogenic pathway of fermentation system varied after different pretreatments. After PF pretreatment, the substrates were first converted to lactate, then produced VFAs, while VFAs were produced directly from the substrates after the PFHT pretreatment. Economic analysis indicates anaerobic fermentation with appropriate PF and HT preteatment is an earning-effective approach for FW to bioenergy such as VFAs. 4007

DOI: 10.1021/acs.energyfuels.6b00077 Energy Fuels 2016, 30, 4002−4008

Article

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DOI: 10.1021/acs.energyfuels.6b00077 Energy Fuels 2016, 30, 4002−4008