Appl Biochem Biotechnol DOI 10.1007/s12010-014-0724-6
Anaerobic Digestion of Yard Waste with Hydrothermal Pretreatment Wangliang Li & Guangyi Zhang & Zhikai Zhang & Guangwen Xu
Received: 5 October 2013 / Accepted: 2 January 2014 # Springer Science+Business Media New York 2014
Abstract The digestibility of lignocellulosic biomass is limited by its high content of refractory components. The objective of this study is to investigate hydrothermal pretreatment and its effects on anaerobic digestion of sorted organic waste with submerged fermentation. Hydrothermal pretreatment (HT) was performed prior to anaerobic digestion, and three agents were examined for the HT: hot compressed water, alkaline solution, and acidic solution. The concentrations of glucose and xylose were the highest in the sample pretreated in acidic solution. Compared with that of the untreated sample, the biogas yields from digesting the samples pretreated in alkaline solution, acidic solution, and hot water increased by 364, 107, and 79 %, respectively. The decrease of chemical oxygen demand (COD) in liquid phase followed the same order as for the biogas yield. The initial ammonia content of the treated samples followed the order sample treated in acidic solution > sample treated in alkaline solution > sample treated in hot water. The concentrations of volatile fatty acids (VFAs) were low, indicating that the anaerobic digestion process was running at continuously stable conditions. Keywords Anaerobic digestion . Hydrolysis . Hydrothermal . Biogas . Municipal solid wastes
Introduction Municipal solid wastes can cause serious environmental pollution, but they are also regarded as a new available energy source. Generally, biogas recovery by anaerobic digestion is seen as an ideal way to dispose biomass wastes. In comparison to all other technologies of energy production including biological and thermal-chemical processes, methane fermentation technology appears to be the most efficient way for waste handling and energy generation in terms of energy output/input ratio (28.8 MJ/MJ) [1, 2]. Anaerobic digestion process can be divided into the following steps: degradation of biomass waste into small molecules through W. Li : G. Zhang : Z. Zhang : G. Xu (*) State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China e-mail: [email protected] Z. Zhang University of Chinese Academy of Sciences, Beijing 100049, China
Appl Biochem Biotechnol
hydrolysis; fermentation of the hydrolysis product to volatile fatty acids (acidogenesis) and conversion of the acids to acetate, hydrogen, and carbon (acetogenesis); and production of methane and carbon dioxide from acetate or carbon dioxide with hydrogen by methanogenic archaea [3, 4]. However, lignocellulosic biomass has a rigid structure with tight association between lignin and carbohydrates, which makes the carbohydrates resistant to access of enzymes [5]. The hydrolysis of lignocellulosic biomass is considered as the rate-limiting step in anaerobic digestion for biogas [6, 7]. Effective pretreatment can degrade lignin fraction into a smaller molecule, making the cellulose and hemicelluloses more accessible and more readily degradable to anaerobic microorganisms [8]. Among the reported biomass pretreatment methods, hydrothermal process has been studied in pretreatment of lignocellulosic biomass for enhanced bioethanol and biogas production [2, 9]. Water under high temperature and high pressure can penetrate into biomass by hydrating cellulose and removing most of hemicellulose and part of lignin. This can improve the hydrolysis of the treated biomass and increase the reaction rate of digestion. However, phenolic compounds and furan (hydroxymethylfurfural (HMF)) can be formed under the conditions of hydrothermal pretreatment. For anaerobic digestion, these products are undesirable. HMF and phenolic compounds present a loss of fermentable sugars and are inhibitors of microorganisms [10, 11]. While literature studies on anaerobic digestion of the furans alone showed that no inhibition was observed, methane was obtained from furans (430 mL CH4 per gram of furfural and 450 mL CH4 per gram of HMF) [12]. Janzon also found that furans produced under optimal process conditions had no inhibitory effect on biogas production [13]. The novel concern of the present study was to understand the hydrolysis behavior of hydrothermal treatment in hot compressed water and alkaline and acidic solutions and to present the suitability and effectiveness of the hydrothermal pretreatment for improving biogas fermentation. The process parameters such as alkalinity, pH value, chemical oxygen demand (COD), and volatile fatty acids (VFAs) of the substrate were investigated in batch anaerobic digestion.
Materials and Methods Materials and Pretreating Methods Poplar leaf was used as yard waste, which was collected from a lawn in Beijing, China. The collected leaves were firstly screened to remove plastics, sticks, and metals, followed by natural drying before the experiment. Then, the poplar leaf was milled into 5–10 mm. The main properties of the substrate used are shown in Table 1. The yard waste was used as feedstock. Hydrothermal treatment was carried out in three agents: hot compressed water and alkaline and acidic solutions. The untreated sample (US) Table 1 Properties of the untreated poplar leaf
Total solids is based on sample weight, and the others are based on dry biomass
Parameter
Value
Total solids (%)
90.88
Volatile solids (%) Ash (%)
81.84 16.98
Total carbon (%)
66.87
Total nitrogen (%)
2.35
Carbon to nitrogen (C/N) ratio
28.45
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was the dried poplar leaf without any pretreatment, which was chosen as the basis for comparison. The poplar leaf sample pretreated in hot compressed water was marked as WS. Similarly, the samples pretreated in H2SO4 and NaOH solutions at the pH values 2.0 and 13.0 were marked as AcS and AlS, respectively. Hydrothermal treatment was carried out in a 1,000-mL stainless kettle which could be heated by a detachable electric heating jacket. The temperature could be adjusted by a temperature controller connected to a thermocouple equipped inside the kettle. About 35.0 g poplar leaf and 350.0 g water were treated for 30 min at 180 °C. The parameters for hydrothermal treatment are shown in Table 2. After the hydrothermal treatment, the kettles were taken out from the outer electric heating jacket and put into a cool water bath to quench the reaction. Finally, the samples were collected from the kettle, and NaOH or H2SO4 was added to adjust the pH value due to harmful effects of extreme pH on methanogenesis. Digestion Tests and Characterization Batch tests of anaerobic digestion of the yard wastes were carried out in a 2,500-mL biogas digester at mesophilic conditions. The temperature of the digestion was 37 °C. The anaerobic inoculum seeds were collected from the secondary treatment pool of a municipal wastewater treatment plant of Beijing. The inoculum were enriched, selected and inoculated with 100 mL seed cultures, and digested for 32 days. The ratio of inoculum to substrate was 1.0 mL/g substrate. The biogas was collected with Tedlar bag and measured every day. The solid samples were taken once a week. For all the samples, the pH value was adjusted with NaOH solution on the 8th day of digestion. For the AlS sample, the pH was adjusted again on the 16th day. The TS and volatile solid (VS) were determined at the beginning and end of digestion according to the Standard Methods for the Examination of Water and Wastewater [14]. The total carbon (TC) and total nitrogen (TN) were analyzed with an elementary analyzer (VarioEl III). After the samples were centrifuged at 10,000 r/min for 10 min, the resulting supernatant was used for further analysis. The pH was measured by a pH meter (Mettler Toledo FE20). The volume of biogas produced was measured by water displacement method, and its composition was determined by gas chromatography (Agilent Micro3000-GC) equipped with a thermal conductivity detector (TCD) using argon as carrier gas. The temperature of the injector and detector were 100 and 150 °C, respectively. Monomeric sugars were analyzed by HPLC (Agilent 1260 series) equipped with a Bio-Rad Aminex HPX-87P column and a refractive index detector (RID). The temperature of the column and the RID were maintained at 80 and 55 °C, respectively [15]. The composition and concentration of volatile fatty acids (VFAs) were analyzed by GC (Agilent 7890) using a FFAP column. Nitrogen was used as carrier gas and FID as detector. The oven temperature was from 50 to 220 °C. The total alkalinity (TA) was evaluated by titration with standardized 0.1 mol/L HCl and expressed as milligrams of CaCO3/L [16]. The indicator was methyl orange for TA measurements with color change in the Table 2 Operation parameters of hydrothermal pretreatment
US untreated sample, WS hot water-treated, AlS alkalinetreated, AcS acidic-treated
Sample
Reagent
Time (min) and temperature (°C)
US
–
–
WS
Hot compressed water
30 and 180
AlS
2.1 g NaOH
30 and 180
AcS
1.4 mL H2SO4
30 and 180
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pH region 3.1–4.4. The COD was tested by a desktop analyzer (AM-02, China) with the spectrophotometry method, a quick and easy method for determining COD based on the law between the concentration of trivalent chromium and absorbance.
Results and Discussion Impact of Hydrothermal Pretreatment on Substrate The hydrolysis and dissolution of lignocellulosic fractions can be improved by hydrothermal pretreatment. Solids were partially liquefied and degraded into smaller molecules such as monosaccharides and VFAs [10, 17]. Table 3 gives the content of monosaccharides produced in the process of hydrothermal treatment. The glucose contents of the treated poplar leaf were 0.50, 0.35, and 1.25 mg/mL in WS, AlS, and AcS, respectively. The xylose contents in WS, AlS, and AcS were 1.25, 1.00, and 2.25 mg/mL, respectively. Both the concentrations of glucose and xylose in AcS were the highest among the three pretreated samples. Arabinose was found only in WS. The monosaccharides can be easily used as substrates for anaerobic digestion, which can increase the reaction rate of digestion. Table 4 shows the contents of small molecular compounds after hydrothermal treatment. The concentrations of furfural in WS, AlS, and AcS were 0.08, 0.00, and 0.87 mg/mL, and the concentrations of HMF were 0.09, 0.00, and 0.90 mg/mL, respectively. The concentrations of formic, acetic, and glycolic acids in WS were 4.00, 2.75, and 1.30 mg/mL, respectively, while in AlS they were 3.00, 2.35, and 1.30 mg/mL and in AcS 2.55, 1.80, and 1.45 mg/mL, respectively. Furfural and HMF were produced in WS and AcS, which is in accordance with the conclusion drawn by Tekin [18]. The reason is that furfurals under alkaline hydrothermal conditions are not thermally stable and they decompose to form other oxygenated hydrocarbons [19]. Furthermore, the concentrations of formic and acetic acids in WS were higher than those in AlS and AcS. The concentration of glycolic acid in AcS was the highest. Effect of Hydrothermal Pretreatment on Biogas Yield The pretreatment in hot compressed water and alkaline and acidic solutions can change the pH value of the samples. The pH value of WS, AlS, and AcS was 4.59, 7.60, and 3.44, respectively. Before the anaerobic digestion, the pH value was adjusted to ca. 7.60. Figure 1 describes the cumulative biogas yield from the poplar leaf treated with hydrothermal method. It can be seen that the biogas yields of the pretreated samples were higher than that of the untreated sample. For the pretreated samples, the biogas yield followed the order AlS > AcS > WS. Compared with that of the untreated sample, the biogas yield of the pretreated samples increased sharply within the first 5 days. The cumulative biogas yields of AlS, AcS, WS, and untreated sample were 45, 22, 18, and 0 mL/g VS, respectively. After this stage, the cumulative Table 3 Monosaccharide content in samples after hydrothermal treatment
Sample
Monosaccharide (mg/mL) Glucose
Xylose
Arabinose
WS AlS
0.50 0.35
1.25 1.00
1.00 0.00
AcS
1.25
2.25
0.00
Appl Biochem Biotechnol Table 4 Contents of small molecular compounds after hydrothermal treatment Sample
Furfural (mg/mL)
HMF (mg/mL)
Formic acid (mg/mL)
Acetic acid (mg/mL)
Glycolic acid (mg/mL)
WS
0.08
0.09
4.00
2.75
1.30
AlS
0.00
0.00
3.00
2.35
1.30
AcS
0.87
0.90
2.55
1.80
1.45
Cumulative biogas yield, mL/g VS
biogas yield increased steadily. The cumulative biogas yields of the AlS, AcS, WS, and untreated sample were 65, 29, 25, and 14 mL/g VS, respectively, after 32 days of fermentation. Compared with that of the untreated sample, the biogas yields of AlS, AcS, and WS increased by 364, 107, and 79 %, respectively. The hydrothermal pretreatment can improve the hydrolysis of poplar leaf and produce a large amount of small molecular compounds which can be easily used to produce biogas. As mentioned above, furfural and HMF were produced when the leaves were pretreated in hot water and acidic solution, which is reported to be harmful to the fermentation process. Although the contents of the monosaccharide were higher in hot compressed water and acidic solution, the cumulative biogas yields of WS and AcS were lower than that of AlS. In other words, it is only the optimal method concerning the three examined methods. Figure 2 shows the concentration of methane in the biogas. In the first 5 days of digestion, the methane concentrations were all very low. The methane concentration followed the order AlS > WS > AcS > untreated sample. The methane concentration in the biogas of AlS was basically the highest among the four samples, which increased sharply to 80 (vol/vol) %. The concentration of methane in the biogas of WS first increased then decreased after the 15th day. The methane concentration of the untreated sample was the lowest. Both the methane concentrations of the untreated sample and AcS increased steadily to 10 (vol/vol) %. The methane yield can be increased significantly by hydrothermal pretreatment with the presence of 5 % NaOH. Zhu et al. observed that the concentration of NaOH had significant effects on the alkaline pretreatment of corn stover and 5 % NaOH resulted in a 37.0 % higher biogas yield than that of the untreated corn stover [20].
80
US WS AlS AcS
70 60 50 40 30 20 10 0
0
5
10
15
20
25
Time, day Fig. 1 Cumulative biogas yield vs. volatile solid of the pretreated poplar leaf
30
35
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Methane concentration, %
90 US WS AlS AcS
80 70 60 50 40 30 20 10 0
0
5
10
15
20
25
30
35
Time, day Fig. 2 Variation of methane concentration in produced biogas
pH Value in Digestion VFAs are important intermediate products and the substrates during the process of digestion for biogas; cumulative VFAs can improve the biogas production but at the same time can also decrease the pH value of the digestion system. The accumulation of VFAs is thus even detrimental for methanogens and leads to the decrease of methane yield [21, 22]. Figure 3 presents the variation of pH during hydrolysis of the untreated sample, WS, AcS, and AlS over 35 days of retention period. For all the samples, the pH value drops to 5.5–6.5 within 7 days, which was due to the hydrolysis of biomass and accumulation of VFAs and CO2. The first drop in pH occurred within the first 24 h, which showed the hydrolysis of easily soluble cellulose, fat, and crude protein [23]. The pH continued dropping and reached the lowest value on the 6th or 7th day, which showed partial hydrolysis of mainly cellulosic components. Therefore, the pH value was adjusted on the 8th day. After adjustment with NaOH solution, 11 US WS AlS AcS
10
pH value
9 8 7 6 5
0
5
10
15
20
Time, day Fig. 3 Variation of pH value in substrate during anaerobic digestion
25
30
35
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the pH value of WS and AcS kept almost constant in the following days, while the pH value of AlS decreased sharply from the 16th to 20th days and then kept almost constant. It is evident that the hydrolysis of the untreated sample is slower than that of the pretreated samples. As reported, alkaline pretreatment could keep the pH value almost constant and lead to a high VFA concentration [24]. − Alkalinity in water results from the presence of carbonate (CO2− 3 ), bicarbonate (HCO3 ), and − hydroxides (OH ) of calcium, sodium, potassium, magnesium, and ammonia. Alkalinity resists changes in pH and may maintain a fairly stable pH even in the presence of VFAs. Total alkalinity (TA) has been considered as an insensitive parameter for indicating process instability since an increase in VFA concentration will cause a decrease in bicarbonate concentration and as a result a fairly constant TA value [25]. Thus, in order to study the effect of alkalinity on the anaerobic digestion, the changes of alkalinity and the effluent pH were monitored. Figure 4 shows the changes of alkalinity during the process of anaerobic digestion. From the figure, we can see that the alkalinity of the untreated sample was the lowest. After the 3-day digestion, the alkalinity of the untreated sample kept constant. For WS, the alkalinity increased from 12.56 to 20.12 mg CaCO3/L within 8 days of digestion and then decreased to 12.41 mg CaCO3/L after 32 days of digestion. For AlS, the alkalinity increased from 14.74 to 21.11 mg CaCO3/L within 5 days of digestion and then decreased to 14.56 mg CaCO3/L and kept constant. For AcS, the alkalinity increased steadily from 3.01 to 17.43 mg CaCO3/L during the 32 days of anaerobic digestion. Variation of COD in Digestion The effects of the hydrothermal pretreatment on COD solubilization under various conditions were compared and the results are shown in Fig. 5. The initial COD of AlS was the highest, about 36 g/L. The initial COD of WS was similar to that of AcS, and the COD of the untreated sample was the lowest. During the process of digestion, the COD of AlS increased within the first 5 days and then decreased sharply, which was attributed to the consumption of small molecular compounds. The decrease of COD of the AlS sample was the highest. The decrease of COD of AcS was the second highest. The decrease of COD of WS was the lowest among the pretreated samples. The decrease of COD followed the same order as that of the biogas yield. For the untreated sample, the COD increased in the first 12 days, which was due to the 21
Alkalinity, g CaCO3/L
18 15 12 9 US WS AlS AcS
6 3 0
0
5
10
15
20
Time, day Fig. 4 Changes of substrate alkalinity during anaerobic digestion
25
30
35
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40 US WS AlS AcS
35
COD, g/L
30 25 20 15 10 5
0
5
10
15
20
25
30
35
Time, day Fig. 5 Changes of COD in substrate during anaerobic digestion
hydrolysis of particulate organic compounds in the acidogenic stage [26]. However, in the following days of digestion, the COD of the effluents decreased, meaning that the fermentation was in the acidification and methanation stages and consumed the COD for biogas production. The COD of AlS decreased from 40.5 to 20.4 g/L within 32 days. The COD of AcS decreased from 31.7 to 18.2 g/L within 32 days. Compared with the biogas yield of 65 and 29 mL/g VS for AlS and AcS, respectively, it can be seen that the decrease of COD is much higher than the biogas yield. The main reason is that some of the total organics cannot be converted into biogas. Variation of Ammonia Nitrogen Various components such as protein, carbohydrates, lignin, and fat of organic wastes produce distinct spectra of compounds during hydrothermal treatment. Among them, protein can be converted into ammonia and cyclic organic compounds like pyridines and pyrroles [27]. Figure 6 shows the changes of ammonia nitrogen during the process of anaerobic digestion.
US WS AlS AcS
Ammonia nitrogen, mg/L
600 500 400 300 200 100 0
0
5
10
15
20
25
Time, day Fig. 6 Variation of ammonia nitrogen in substrate during anaerobic digestion
30
35
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It can be seen that the initial ammonia content of the treated samples follows the order AcS > AlS > WS. Ammonia nitrogen is more prone to be produced in acidic solution than in alkaline solution and hot compressed water, while after the anaerobic digestion, the ammonia content of AcS decreased from 625 to 333 and to 200 mg/L after 1 and 2 days of digestion, respectively. Then, the ammonia content was lowest on the 15th day of digestion and then increased to 182 mg/L on the 26th day. The ammonia content of AlS showed the same tendency as that of AcS, which decreased from 307 to 34 mg/L and reached the lowest on the 16th day and then increased to 284 mg/L after 32 days of digestion. For WS, the ammonia content decreased from 448 to 102 mg/L within 32 days. For the untreated sample, the ammonia content increased from 173 to 374 mg/L from the 3rd day to the 26th day and then decreased to 307 mg/L on the 36th day. As mentioned above, the concentrations of glucose and xylose in AcS were higher than those in AlS, but the biogas yield of AcS was lower than that of AlS. This may be attributed to the inhibition of ammonia because ammonium ion (NH4+) and free ammonia (NH3) are the most significant inhibitors among the accumulating chemicals. Both forms can directly and indirectly cause inhibition in an anaerobic digestion system [28]. Furthermore, most evidence suggests that the acetoclastic methanogens, which degrade acetate and produce methane and carbon oxides, are more sensitive to free ammonia than hydrogenotrophic methanogens [29]. Variation of VFA Concentration The accumulation of VFAs has a significant influence on the biogas production. On one hand, VFAs are used as substrate for the methanation process, which means that a certain amount of VFAs is necessary for the biogas production. On the other hand, a high concentration of VFAs can inhibit the activity of the methanogenic bacteria. Viéitez and Ghosh [29] pointed out that anaerobic digestion would stop when the concentration of VFAs exceeds 13,000 mg/L. The variations of VFA concentration during the process of anaerobic digestion are critical for the biogas production. Figure 7 shows the variations of VFA concentration in the process of anaerobic digestion. The composition and concentration of VFAs were analyzed by GC-MS. It was found that acetic acid, propionic acid, i-butyric acid, and n-butyric acid appeared in the aqueous phase. From Fig. 7, we can see that there were no VFAs in the untreated sample at the beginning of the experiment. The concentration of acetate was higher than those of propionate, i-butyrate, and n-butyrate, which indicated that acetate was prone to be produced in the process of anaerobic digestion. The concentration of acetate first increased then decreased. After adjustment of pH value on the 16th day, the concentration of acetate increases, which verified that the neutral or weak basic atmosphere is suitable for the production of acetate, while for propionate, the concentration increased steadily. The concentration of i-butyrate and n-butyrate kept constant after 3 days of anaerobic digestion. For WS, the concentrations of VFAs were low in the first 15 days of digestion, while after the adjustment of pH value, the concentration of VFAs increased sharply. After 32 days of digestion, the concentration of acetate, propionate, i-butyrate, and n-butyrate was 0.40, 0.35, 0.20, and 0.37 g/L, respectively. For AcS, the concentration of acetate was high after hydrothermal pretreatment, which was about 0.65 g/L. After 2 days of digestion, the concentration increased to 0.97 g/L. Then, the concentration of acetate decreased sharply to 0.45 g/L after one more day of digestion and to 0.23 g/L after 12 days of digestion. This is because that the acetate was used as the main substrate for biogas production. The concentrations of other VFAs were low, which were not higher than 0.2 g/L. For AlS, the concentration of acetate increased sharply after 8 days of digestion. Within the first 10 days, the concentration of n-butyrate was the highest among the four types of VFAs. The concentration of n-butyrate and propionate increased throughout the
Appl Biochem Biotechnol 0.8
0.6
US
0.40
0.5 0.4 0.3
0.30
0.20 0.15 0.10
0.1
0.05 0.00
0
5
10
15
20
25
30
35
0
40
5
10
Time, day
AcS
0.30
0.9 Acetate Propionate i-Butyrate n-Butyrate
0.7 0.6
20
25
30
35
0.5 0.4 0.3 0.2
AlS
Acetate Propionate i-Butyrate n-Butyrate
0.25
VFA content, g/L
0.8
15
Time, day
1.0
VFA Content, g/L
WS
0.25
0.2
0.0
Acetate Propionate i-Butyrate n-Butyrate
0.35
VFA Content, g/L
VFA concent, g/L
0.45
Acetate Propionate i-Butyrate n-Butyrate
0.7
0.20 0.15 0.10 0.05
0.1 0.00
0.0 0
5
10
15
20
25
30
35
0
5
10
Time, day
15
20
25
30
35
Time, day
Fig. 7 Variations of VFAs concentration in substrate during anaerobic digestion
digestion process. It was also found that only the concentration of acetate decreased after the 20th day, which can be explained by that only acetate was consumed as substrate for biogas. The concentrations of VFAs were invariably low; the fact proved that the anaerobic digestion process was running at continuously stable conditions [30]. The concentration of VFAs in AcS was higher than those in AlS and WS, the possible reason of which was that the elevated ammonia/ammonium concentration provided a buffering effect that allowed digester operation at higher concentrations of VFAs. It is only when the buffering system is disrupted by the accumulation of VFAs that the pH drops to a point where conditions are unfavorable for methanogenesis [31]. The production of VFAs differed greatly among the various temperature and reaction conditions. Acetic acid seemed to be produced from the early experimental period onward. By contrast, butyric acid was produced later in the experiment, and propionic acid was observed at concentrations less than 1,000 mg/L throughout [32].
Conclusion The hydrothermal pretreatment of poplar leaf was carried out in three agents: hot compressed water and alkaline and acidic solutions. Monosaccharide and organic acids were produced after hydrothermal pretreatment with or without the presence of acid and alkali. The concentrations of glucose and xylose in the acid-treated sample were the highest among the three hydrothermally pretreated samples. Furfural and HMF were produced by the pretreatment in hot water and acid-pretreated samples. Compared with that of the untreated sample, the biogas yields of the samples pretreated in alkali, acid, and hot water increased by 364, 107, and 79 %, respectively. For all the samples, the decrease of COD of the samples followed the same order as for the biogas yield. The ammonia in the sample pretreated in acidic solution was the
Appl Biochem Biotechnol
highest, which tended to inhibit the anaerobic digestion process and to decrease the biogas yield. The concentration of acetate in all the samples was higher than those of propionate, i-butyrate, and n-butyrate. Their total concentrations of VFAs were low, indicating that the anaerobic digestion process ran continuously and stably. All of these results are believed to be helpful for identifying the effective hydrolysis pretreatment method and, in turn, for improving the design and operation of a high-efficiency large-scale anaerobic digestion system for yard waste. Acknowledgments The authors acknowledge the financial support from Hi-Tech Research and Development Program of China (863 Program, 2012AA021401), National Natural Science Foundation of China (21161140329), and National Key Technology R&D Program National Key Technology Support Program (2012BAC03B05).
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Appl Biochem Biotechnol 20. Zhu, J., Wan, C., & Li, Y. (2010). Enhanced solid-state anaerobic digestion of corn stover by alkaline pretreatment. Journal of Bioresource Technology, 101, 7523–7528. 21. Buyukkamaci, N., & Filibeli, A. (2004). Volatile fatty acid formation in an anaerobic hybrid reactor. Process Biochemistry, 39, 1491–1494. 22. Nguyen, P. H. L., Kuruparan, P., & Visvanathan, C. (2007). Anaerobic digestion of municipal solid waste as a treatment prior to landfill. Bioresource Technology, 98, 380–387. 23. Chen, Y., Cheng, J. J., & Creamer, K. S. (2008). Inhibition of anaerobic digestion process: a review. Bioresource Technology, 99(10), 4044–4064. 24. Hashimoto, A. G. (2004). Pretreatment of wheat straw for fermentation to methane. Biotechnology and Bioengineering, 28, 1857–1866. 25. Björnsson, L., Murto, M., Jantsch, T. G., & Mattiasson, B. (2001). Evaluation of new methods for the monitoring of alkalinity, dissolved hydrogen and the microbial community in anaerobic digestion. Water Research, 35(12), 2833–2840. 26. Fezzani, B., & Cheikh, R. B. (2010). Two-phase anaerobic co-digestion of olive mill wastes in semicontinuous digesters at mesophilic temperature. Bioresource Technology, 101(6), 1628–1634. 27. Toor, S. S., & Rudolf, L. (2011). Hydrothermal liquefaction of biomass: a review of subcritical water technologies. Energy, 36, 2328–2342. 28. Yenigün, O., & Demirel, B. (2013). Ammonia inhibition in anaerobic digestion: a review. Process Biochemistry, 48, 901–911. 29. Viéitez, E. R., & Ghosh, S. (1999). Biogasification of solid wastes by two-phase anaerobic fermentation. Biomass of Bioenergy, 16(5), 299–309. 30. Athanasoulia, E., Melidis, P., & Aivasidis, A. (2012). Optimization of biogas production from waste activated sludge through serial digestion. Renewable Energy, 47, 147–151. 31. Walker, M., Iyer, K., Heaven, S., & Banks, C. J. (2011). Ammonia removal in anaerobic digestion by biogas stripping: an evaluation of process alternatives using a first order rate model based on experimental findings. Chemical Engineering Journal, 178, 138–145. 32. Komemoto, K., Lim, Y. G., Nagao, N., Onoue, Y., Niwa, C., & Toda, T. (2009). Effect of temperature on VFA’s and biogas production in anaerobic solubilization of food waste. Waste Management, 29(12), 2950–2955.
Anaerobic Digestion of Yard Waste with Hydrothermal Pretreatment Wangliang Li & Guangyi Zhang & Zhikai Zhang & Guangwen Xu
Received: 5 October 2013 / Accepted: 2 January 2014 # Springer Science+Business Media New York 2014
Abstract The digestibility of lignocellulosic biomass is limited by its high content of refractory components. The objective of this study is to investigate hydrothermal pretreatment and its effects on anaerobic digestion of sorted organic waste with submerged fermentation. Hydrothermal pretreatment (HT) was performed prior to anaerobic digestion, and three agents were examined for the HT: hot compressed water, alkaline solution, and acidic solution. The concentrations of glucose and xylose were the highest in the sample pretreated in acidic solution. Compared with that of the untreated sample, the biogas yields from digesting the samples pretreated in alkaline solution, acidic solution, and hot water increased by 364, 107, and 79 %, respectively. The decrease of chemical oxygen demand (COD) in liquid phase followed the same order as for the biogas yield. The initial ammonia content of the treated samples followed the order sample treated in acidic solution > sample treated in alkaline solution > sample treated in hot water. The concentrations of volatile fatty acids (VFAs) were low, indicating that the anaerobic digestion process was running at continuously stable conditions. Keywords Anaerobic digestion . Hydrolysis . Hydrothermal . Biogas . Municipal solid wastes
Introduction Municipal solid wastes can cause serious environmental pollution, but they are also regarded as a new available energy source. Generally, biogas recovery by anaerobic digestion is seen as an ideal way to dispose biomass wastes. In comparison to all other technologies of energy production including biological and thermal-chemical processes, methane fermentation technology appears to be the most efficient way for waste handling and energy generation in terms of energy output/input ratio (28.8 MJ/MJ) [1, 2]. Anaerobic digestion process can be divided into the following steps: degradation of biomass waste into small molecules through W. Li : G. Zhang : Z. Zhang : G. Xu (*) State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China e-mail: [email protected] Z. Zhang University of Chinese Academy of Sciences, Beijing 100049, China
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hydrolysis; fermentation of the hydrolysis product to volatile fatty acids (acidogenesis) and conversion of the acids to acetate, hydrogen, and carbon (acetogenesis); and production of methane and carbon dioxide from acetate or carbon dioxide with hydrogen by methanogenic archaea [3, 4]. However, lignocellulosic biomass has a rigid structure with tight association between lignin and carbohydrates, which makes the carbohydrates resistant to access of enzymes [5]. The hydrolysis of lignocellulosic biomass is considered as the rate-limiting step in anaerobic digestion for biogas [6, 7]. Effective pretreatment can degrade lignin fraction into a smaller molecule, making the cellulose and hemicelluloses more accessible and more readily degradable to anaerobic microorganisms [8]. Among the reported biomass pretreatment methods, hydrothermal process has been studied in pretreatment of lignocellulosic biomass for enhanced bioethanol and biogas production [2, 9]. Water under high temperature and high pressure can penetrate into biomass by hydrating cellulose and removing most of hemicellulose and part of lignin. This can improve the hydrolysis of the treated biomass and increase the reaction rate of digestion. However, phenolic compounds and furan (hydroxymethylfurfural (HMF)) can be formed under the conditions of hydrothermal pretreatment. For anaerobic digestion, these products are undesirable. HMF and phenolic compounds present a loss of fermentable sugars and are inhibitors of microorganisms [10, 11]. While literature studies on anaerobic digestion of the furans alone showed that no inhibition was observed, methane was obtained from furans (430 mL CH4 per gram of furfural and 450 mL CH4 per gram of HMF) [12]. Janzon also found that furans produced under optimal process conditions had no inhibitory effect on biogas production [13]. The novel concern of the present study was to understand the hydrolysis behavior of hydrothermal treatment in hot compressed water and alkaline and acidic solutions and to present the suitability and effectiveness of the hydrothermal pretreatment for improving biogas fermentation. The process parameters such as alkalinity, pH value, chemical oxygen demand (COD), and volatile fatty acids (VFAs) of the substrate were investigated in batch anaerobic digestion.
Materials and Methods Materials and Pretreating Methods Poplar leaf was used as yard waste, which was collected from a lawn in Beijing, China. The collected leaves were firstly screened to remove plastics, sticks, and metals, followed by natural drying before the experiment. Then, the poplar leaf was milled into 5–10 mm. The main properties of the substrate used are shown in Table 1. The yard waste was used as feedstock. Hydrothermal treatment was carried out in three agents: hot compressed water and alkaline and acidic solutions. The untreated sample (US) Table 1 Properties of the untreated poplar leaf
Total solids is based on sample weight, and the others are based on dry biomass
Parameter
Value
Total solids (%)
90.88
Volatile solids (%) Ash (%)
81.84 16.98
Total carbon (%)
66.87
Total nitrogen (%)
2.35
Carbon to nitrogen (C/N) ratio
28.45
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was the dried poplar leaf without any pretreatment, which was chosen as the basis for comparison. The poplar leaf sample pretreated in hot compressed water was marked as WS. Similarly, the samples pretreated in H2SO4 and NaOH solutions at the pH values 2.0 and 13.0 were marked as AcS and AlS, respectively. Hydrothermal treatment was carried out in a 1,000-mL stainless kettle which could be heated by a detachable electric heating jacket. The temperature could be adjusted by a temperature controller connected to a thermocouple equipped inside the kettle. About 35.0 g poplar leaf and 350.0 g water were treated for 30 min at 180 °C. The parameters for hydrothermal treatment are shown in Table 2. After the hydrothermal treatment, the kettles were taken out from the outer electric heating jacket and put into a cool water bath to quench the reaction. Finally, the samples were collected from the kettle, and NaOH or H2SO4 was added to adjust the pH value due to harmful effects of extreme pH on methanogenesis. Digestion Tests and Characterization Batch tests of anaerobic digestion of the yard wastes were carried out in a 2,500-mL biogas digester at mesophilic conditions. The temperature of the digestion was 37 °C. The anaerobic inoculum seeds were collected from the secondary treatment pool of a municipal wastewater treatment plant of Beijing. The inoculum were enriched, selected and inoculated with 100 mL seed cultures, and digested for 32 days. The ratio of inoculum to substrate was 1.0 mL/g substrate. The biogas was collected with Tedlar bag and measured every day. The solid samples were taken once a week. For all the samples, the pH value was adjusted with NaOH solution on the 8th day of digestion. For the AlS sample, the pH was adjusted again on the 16th day. The TS and volatile solid (VS) were determined at the beginning and end of digestion according to the Standard Methods for the Examination of Water and Wastewater [14]. The total carbon (TC) and total nitrogen (TN) were analyzed with an elementary analyzer (VarioEl III). After the samples were centrifuged at 10,000 r/min for 10 min, the resulting supernatant was used for further analysis. The pH was measured by a pH meter (Mettler Toledo FE20). The volume of biogas produced was measured by water displacement method, and its composition was determined by gas chromatography (Agilent Micro3000-GC) equipped with a thermal conductivity detector (TCD) using argon as carrier gas. The temperature of the injector and detector were 100 and 150 °C, respectively. Monomeric sugars were analyzed by HPLC (Agilent 1260 series) equipped with a Bio-Rad Aminex HPX-87P column and a refractive index detector (RID). The temperature of the column and the RID were maintained at 80 and 55 °C, respectively [15]. The composition and concentration of volatile fatty acids (VFAs) were analyzed by GC (Agilent 7890) using a FFAP column. Nitrogen was used as carrier gas and FID as detector. The oven temperature was from 50 to 220 °C. The total alkalinity (TA) was evaluated by titration with standardized 0.1 mol/L HCl and expressed as milligrams of CaCO3/L [16]. The indicator was methyl orange for TA measurements with color change in the Table 2 Operation parameters of hydrothermal pretreatment
US untreated sample, WS hot water-treated, AlS alkalinetreated, AcS acidic-treated
Sample
Reagent
Time (min) and temperature (°C)
US
–
–
WS
Hot compressed water
30 and 180
AlS
2.1 g NaOH
30 and 180
AcS
1.4 mL H2SO4
30 and 180
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pH region 3.1–4.4. The COD was tested by a desktop analyzer (AM-02, China) with the spectrophotometry method, a quick and easy method for determining COD based on the law between the concentration of trivalent chromium and absorbance.
Results and Discussion Impact of Hydrothermal Pretreatment on Substrate The hydrolysis and dissolution of lignocellulosic fractions can be improved by hydrothermal pretreatment. Solids were partially liquefied and degraded into smaller molecules such as monosaccharides and VFAs [10, 17]. Table 3 gives the content of monosaccharides produced in the process of hydrothermal treatment. The glucose contents of the treated poplar leaf were 0.50, 0.35, and 1.25 mg/mL in WS, AlS, and AcS, respectively. The xylose contents in WS, AlS, and AcS were 1.25, 1.00, and 2.25 mg/mL, respectively. Both the concentrations of glucose and xylose in AcS were the highest among the three pretreated samples. Arabinose was found only in WS. The monosaccharides can be easily used as substrates for anaerobic digestion, which can increase the reaction rate of digestion. Table 4 shows the contents of small molecular compounds after hydrothermal treatment. The concentrations of furfural in WS, AlS, and AcS were 0.08, 0.00, and 0.87 mg/mL, and the concentrations of HMF were 0.09, 0.00, and 0.90 mg/mL, respectively. The concentrations of formic, acetic, and glycolic acids in WS were 4.00, 2.75, and 1.30 mg/mL, respectively, while in AlS they were 3.00, 2.35, and 1.30 mg/mL and in AcS 2.55, 1.80, and 1.45 mg/mL, respectively. Furfural and HMF were produced in WS and AcS, which is in accordance with the conclusion drawn by Tekin [18]. The reason is that furfurals under alkaline hydrothermal conditions are not thermally stable and they decompose to form other oxygenated hydrocarbons [19]. Furthermore, the concentrations of formic and acetic acids in WS were higher than those in AlS and AcS. The concentration of glycolic acid in AcS was the highest. Effect of Hydrothermal Pretreatment on Biogas Yield The pretreatment in hot compressed water and alkaline and acidic solutions can change the pH value of the samples. The pH value of WS, AlS, and AcS was 4.59, 7.60, and 3.44, respectively. Before the anaerobic digestion, the pH value was adjusted to ca. 7.60. Figure 1 describes the cumulative biogas yield from the poplar leaf treated with hydrothermal method. It can be seen that the biogas yields of the pretreated samples were higher than that of the untreated sample. For the pretreated samples, the biogas yield followed the order AlS > AcS > WS. Compared with that of the untreated sample, the biogas yield of the pretreated samples increased sharply within the first 5 days. The cumulative biogas yields of AlS, AcS, WS, and untreated sample were 45, 22, 18, and 0 mL/g VS, respectively. After this stage, the cumulative Table 3 Monosaccharide content in samples after hydrothermal treatment
Sample
Monosaccharide (mg/mL) Glucose
Xylose
Arabinose
WS AlS
0.50 0.35
1.25 1.00
1.00 0.00
AcS
1.25
2.25
0.00
Appl Biochem Biotechnol Table 4 Contents of small molecular compounds after hydrothermal treatment Sample
Furfural (mg/mL)
HMF (mg/mL)
Formic acid (mg/mL)
Acetic acid (mg/mL)
Glycolic acid (mg/mL)
WS
0.08
0.09
4.00
2.75
1.30
AlS
0.00
0.00
3.00
2.35
1.30
AcS
0.87
0.90
2.55
1.80
1.45
Cumulative biogas yield, mL/g VS
biogas yield increased steadily. The cumulative biogas yields of the AlS, AcS, WS, and untreated sample were 65, 29, 25, and 14 mL/g VS, respectively, after 32 days of fermentation. Compared with that of the untreated sample, the biogas yields of AlS, AcS, and WS increased by 364, 107, and 79 %, respectively. The hydrothermal pretreatment can improve the hydrolysis of poplar leaf and produce a large amount of small molecular compounds which can be easily used to produce biogas. As mentioned above, furfural and HMF were produced when the leaves were pretreated in hot water and acidic solution, which is reported to be harmful to the fermentation process. Although the contents of the monosaccharide were higher in hot compressed water and acidic solution, the cumulative biogas yields of WS and AcS were lower than that of AlS. In other words, it is only the optimal method concerning the three examined methods. Figure 2 shows the concentration of methane in the biogas. In the first 5 days of digestion, the methane concentrations were all very low. The methane concentration followed the order AlS > WS > AcS > untreated sample. The methane concentration in the biogas of AlS was basically the highest among the four samples, which increased sharply to 80 (vol/vol) %. The concentration of methane in the biogas of WS first increased then decreased after the 15th day. The methane concentration of the untreated sample was the lowest. Both the methane concentrations of the untreated sample and AcS increased steadily to 10 (vol/vol) %. The methane yield can be increased significantly by hydrothermal pretreatment with the presence of 5 % NaOH. Zhu et al. observed that the concentration of NaOH had significant effects on the alkaline pretreatment of corn stover and 5 % NaOH resulted in a 37.0 % higher biogas yield than that of the untreated corn stover [20].
80
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70 60 50 40 30 20 10 0
0
5
10
15
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25
Time, day Fig. 1 Cumulative biogas yield vs. volatile solid of the pretreated poplar leaf
30
35
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Methane concentration, %
90 US WS AlS AcS
80 70 60 50 40 30 20 10 0
0
5
10
15
20
25
30
35
Time, day Fig. 2 Variation of methane concentration in produced biogas
pH Value in Digestion VFAs are important intermediate products and the substrates during the process of digestion for biogas; cumulative VFAs can improve the biogas production but at the same time can also decrease the pH value of the digestion system. The accumulation of VFAs is thus even detrimental for methanogens and leads to the decrease of methane yield [21, 22]. Figure 3 presents the variation of pH during hydrolysis of the untreated sample, WS, AcS, and AlS over 35 days of retention period. For all the samples, the pH value drops to 5.5–6.5 within 7 days, which was due to the hydrolysis of biomass and accumulation of VFAs and CO2. The first drop in pH occurred within the first 24 h, which showed the hydrolysis of easily soluble cellulose, fat, and crude protein [23]. The pH continued dropping and reached the lowest value on the 6th or 7th day, which showed partial hydrolysis of mainly cellulosic components. Therefore, the pH value was adjusted on the 8th day. After adjustment with NaOH solution, 11 US WS AlS AcS
10
pH value
9 8 7 6 5
0
5
10
15
20
Time, day Fig. 3 Variation of pH value in substrate during anaerobic digestion
25
30
35
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the pH value of WS and AcS kept almost constant in the following days, while the pH value of AlS decreased sharply from the 16th to 20th days and then kept almost constant. It is evident that the hydrolysis of the untreated sample is slower than that of the pretreated samples. As reported, alkaline pretreatment could keep the pH value almost constant and lead to a high VFA concentration [24]. − Alkalinity in water results from the presence of carbonate (CO2− 3 ), bicarbonate (HCO3 ), and − hydroxides (OH ) of calcium, sodium, potassium, magnesium, and ammonia. Alkalinity resists changes in pH and may maintain a fairly stable pH even in the presence of VFAs. Total alkalinity (TA) has been considered as an insensitive parameter for indicating process instability since an increase in VFA concentration will cause a decrease in bicarbonate concentration and as a result a fairly constant TA value [25]. Thus, in order to study the effect of alkalinity on the anaerobic digestion, the changes of alkalinity and the effluent pH were monitored. Figure 4 shows the changes of alkalinity during the process of anaerobic digestion. From the figure, we can see that the alkalinity of the untreated sample was the lowest. After the 3-day digestion, the alkalinity of the untreated sample kept constant. For WS, the alkalinity increased from 12.56 to 20.12 mg CaCO3/L within 8 days of digestion and then decreased to 12.41 mg CaCO3/L after 32 days of digestion. For AlS, the alkalinity increased from 14.74 to 21.11 mg CaCO3/L within 5 days of digestion and then decreased to 14.56 mg CaCO3/L and kept constant. For AcS, the alkalinity increased steadily from 3.01 to 17.43 mg CaCO3/L during the 32 days of anaerobic digestion. Variation of COD in Digestion The effects of the hydrothermal pretreatment on COD solubilization under various conditions were compared and the results are shown in Fig. 5. The initial COD of AlS was the highest, about 36 g/L. The initial COD of WS was similar to that of AcS, and the COD of the untreated sample was the lowest. During the process of digestion, the COD of AlS increased within the first 5 days and then decreased sharply, which was attributed to the consumption of small molecular compounds. The decrease of COD of the AlS sample was the highest. The decrease of COD of AcS was the second highest. The decrease of COD of WS was the lowest among the pretreated samples. The decrease of COD followed the same order as that of the biogas yield. For the untreated sample, the COD increased in the first 12 days, which was due to the 21
Alkalinity, g CaCO3/L
18 15 12 9 US WS AlS AcS
6 3 0
0
5
10
15
20
Time, day Fig. 4 Changes of substrate alkalinity during anaerobic digestion
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40 US WS AlS AcS
35
COD, g/L
30 25 20 15 10 5
0
5
10
15
20
25
30
35
Time, day Fig. 5 Changes of COD in substrate during anaerobic digestion
hydrolysis of particulate organic compounds in the acidogenic stage [26]. However, in the following days of digestion, the COD of the effluents decreased, meaning that the fermentation was in the acidification and methanation stages and consumed the COD for biogas production. The COD of AlS decreased from 40.5 to 20.4 g/L within 32 days. The COD of AcS decreased from 31.7 to 18.2 g/L within 32 days. Compared with the biogas yield of 65 and 29 mL/g VS for AlS and AcS, respectively, it can be seen that the decrease of COD is much higher than the biogas yield. The main reason is that some of the total organics cannot be converted into biogas. Variation of Ammonia Nitrogen Various components such as protein, carbohydrates, lignin, and fat of organic wastes produce distinct spectra of compounds during hydrothermal treatment. Among them, protein can be converted into ammonia and cyclic organic compounds like pyridines and pyrroles [27]. Figure 6 shows the changes of ammonia nitrogen during the process of anaerobic digestion.
US WS AlS AcS
Ammonia nitrogen, mg/L
600 500 400 300 200 100 0
0
5
10
15
20
25
Time, day Fig. 6 Variation of ammonia nitrogen in substrate during anaerobic digestion
30
35
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It can be seen that the initial ammonia content of the treated samples follows the order AcS > AlS > WS. Ammonia nitrogen is more prone to be produced in acidic solution than in alkaline solution and hot compressed water, while after the anaerobic digestion, the ammonia content of AcS decreased from 625 to 333 and to 200 mg/L after 1 and 2 days of digestion, respectively. Then, the ammonia content was lowest on the 15th day of digestion and then increased to 182 mg/L on the 26th day. The ammonia content of AlS showed the same tendency as that of AcS, which decreased from 307 to 34 mg/L and reached the lowest on the 16th day and then increased to 284 mg/L after 32 days of digestion. For WS, the ammonia content decreased from 448 to 102 mg/L within 32 days. For the untreated sample, the ammonia content increased from 173 to 374 mg/L from the 3rd day to the 26th day and then decreased to 307 mg/L on the 36th day. As mentioned above, the concentrations of glucose and xylose in AcS were higher than those in AlS, but the biogas yield of AcS was lower than that of AlS. This may be attributed to the inhibition of ammonia because ammonium ion (NH4+) and free ammonia (NH3) are the most significant inhibitors among the accumulating chemicals. Both forms can directly and indirectly cause inhibition in an anaerobic digestion system [28]. Furthermore, most evidence suggests that the acetoclastic methanogens, which degrade acetate and produce methane and carbon oxides, are more sensitive to free ammonia than hydrogenotrophic methanogens [29]. Variation of VFA Concentration The accumulation of VFAs has a significant influence on the biogas production. On one hand, VFAs are used as substrate for the methanation process, which means that a certain amount of VFAs is necessary for the biogas production. On the other hand, a high concentration of VFAs can inhibit the activity of the methanogenic bacteria. Viéitez and Ghosh [29] pointed out that anaerobic digestion would stop when the concentration of VFAs exceeds 13,000 mg/L. The variations of VFA concentration during the process of anaerobic digestion are critical for the biogas production. Figure 7 shows the variations of VFA concentration in the process of anaerobic digestion. The composition and concentration of VFAs were analyzed by GC-MS. It was found that acetic acid, propionic acid, i-butyric acid, and n-butyric acid appeared in the aqueous phase. From Fig. 7, we can see that there were no VFAs in the untreated sample at the beginning of the experiment. The concentration of acetate was higher than those of propionate, i-butyrate, and n-butyrate, which indicated that acetate was prone to be produced in the process of anaerobic digestion. The concentration of acetate first increased then decreased. After adjustment of pH value on the 16th day, the concentration of acetate increases, which verified that the neutral or weak basic atmosphere is suitable for the production of acetate, while for propionate, the concentration increased steadily. The concentration of i-butyrate and n-butyrate kept constant after 3 days of anaerobic digestion. For WS, the concentrations of VFAs were low in the first 15 days of digestion, while after the adjustment of pH value, the concentration of VFAs increased sharply. After 32 days of digestion, the concentration of acetate, propionate, i-butyrate, and n-butyrate was 0.40, 0.35, 0.20, and 0.37 g/L, respectively. For AcS, the concentration of acetate was high after hydrothermal pretreatment, which was about 0.65 g/L. After 2 days of digestion, the concentration increased to 0.97 g/L. Then, the concentration of acetate decreased sharply to 0.45 g/L after one more day of digestion and to 0.23 g/L after 12 days of digestion. This is because that the acetate was used as the main substrate for biogas production. The concentrations of other VFAs were low, which were not higher than 0.2 g/L. For AlS, the concentration of acetate increased sharply after 8 days of digestion. Within the first 10 days, the concentration of n-butyrate was the highest among the four types of VFAs. The concentration of n-butyrate and propionate increased throughout the
Appl Biochem Biotechnol 0.8
0.6
US
0.40
0.5 0.4 0.3
0.30
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0.1
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0.9 Acetate Propionate i-Butyrate n-Butyrate
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Time, day
Fig. 7 Variations of VFAs concentration in substrate during anaerobic digestion
digestion process. It was also found that only the concentration of acetate decreased after the 20th day, which can be explained by that only acetate was consumed as substrate for biogas. The concentrations of VFAs were invariably low; the fact proved that the anaerobic digestion process was running at continuously stable conditions [30]. The concentration of VFAs in AcS was higher than those in AlS and WS, the possible reason of which was that the elevated ammonia/ammonium concentration provided a buffering effect that allowed digester operation at higher concentrations of VFAs. It is only when the buffering system is disrupted by the accumulation of VFAs that the pH drops to a point where conditions are unfavorable for methanogenesis [31]. The production of VFAs differed greatly among the various temperature and reaction conditions. Acetic acid seemed to be produced from the early experimental period onward. By contrast, butyric acid was produced later in the experiment, and propionic acid was observed at concentrations less than 1,000 mg/L throughout [32].
Conclusion The hydrothermal pretreatment of poplar leaf was carried out in three agents: hot compressed water and alkaline and acidic solutions. Monosaccharide and organic acids were produced after hydrothermal pretreatment with or without the presence of acid and alkali. The concentrations of glucose and xylose in the acid-treated sample were the highest among the three hydrothermally pretreated samples. Furfural and HMF were produced by the pretreatment in hot water and acid-pretreated samples. Compared with that of the untreated sample, the biogas yields of the samples pretreated in alkali, acid, and hot water increased by 364, 107, and 79 %, respectively. For all the samples, the decrease of COD of the samples followed the same order as for the biogas yield. The ammonia in the sample pretreated in acidic solution was the
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highest, which tended to inhibit the anaerobic digestion process and to decrease the biogas yield. The concentration of acetate in all the samples was higher than those of propionate, i-butyrate, and n-butyrate. Their total concentrations of VFAs were low, indicating that the anaerobic digestion process ran continuously and stably. All of these results are believed to be helpful for identifying the effective hydrolysis pretreatment method and, in turn, for improving the design and operation of a high-efficiency large-scale anaerobic digestion system for yard waste. Acknowledgments The authors acknowledge the financial support from Hi-Tech Research and Development Program of China (863 Program, 2012AA021401), National Natural Science Foundation of China (21161140329), and National Key Technology R&D Program National Key Technology Support Program (2012BAC03B05).
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