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Biomethane Production from co-fermentation of agricultural wastes
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 5 3 5 5 e5 3 6 4
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Biomethane Production from co-fermentation of agricultural wastes Saowaluck Haosagul a,b,c, Siriorn Boonyawanich a,b,**, Nipon Pisutpaisal a,b,* a
Department of Agro-Industrial, Food and Environmental Technology, Faculty of Applied Science, King Mongkut's University of Technology North Bangkok, 10800, Thailand b The Biosensor and Bioelectronics Technology Centre, King Mongkut's University of Technology North Bangkok, 10800, Thailand c The Joint Graduate School of Energy and Environment (JGSEE), King Mongkut’s University of Technology Thonburi, Bangkok, 10140, Thailand
article info
abstract
Article history:
Biomethane (CH4) was recovered from co-digestion process of waste glycerol and banana
Received 13 August 2018
wastes. The wastes used contain waste glycerol with varying concentrations from 7.5 to
Received in revised form
90 g L1 and banana peel in the range 2.5e10 %w$v1. The co-substrate mixture ratio was
25 August 2018
implemented in 0.5 L batch reactor operated at 37 C and pH 7 for 120 h. The composition of
Accepted 11 September 2018
biogas gas and liquid samples (COD, VFA, pH, alkalinity) were analyzed every 12 and 24 h,
Available online 8 November 2018
respectively. The optimum condition to produce CH4 was found at 7.5 g L1 waste glycerol
Keywords:
0.281 m3 kg1 COD and 652 mL, respectively.
mixed with 7.5% banana peel. The highest CH4 yield and CH4 production potential were Fermentation
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Waste glycerol Banana waste Biofuel Biomethane
Introduction Banana is a popular fruit grown in tropical regions of the world, with a 175% increase in yield over the past 30 years. India is the largest producer with 32 million tons, and other countries such as China, Philippines, Ecuador, Brazil, Indonesia, Tanzania, and Thailand. Banana production in Asia
has quadrupled to 62 million tons [1]. In 2013, Thailand had an area of 77% banana plantations. Report's published by the Department of Agricultural Extension from 2009 to 2013 showed that the banana production increased from 156,367 tons in 2009 to 773,732 tons in 2013, increased 36.3% per year [2]. Bananas can be consumed and processed in many ways such as flour, chips, and beverages. The Department of
* Corresponding author. Department of Agro-Industrial, Food and Environmental Technology, Faculty of Applied Science, King Mongkut's University of Technology North Bangkok, 10800, Thailand. ** Corresponding author. Department of Agro-Industrial, Food and Environmental Technology, Faculty of Applied Science, King Mongkut's University of Technology North Bangkok, 10800, Thailand. E-mail addresses: [email protected] (S. Boonyawanich), [email protected] (N. Pisutpaisal). https://doi.org/10.1016/j.ijhydene.2018.09.080 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Table 1 e Physical and chemical characteristics of pure and waste glycerol. Parameter
Pure glycerol
Waste glycerol
Unit
995 7.54 0.38 780,142 124,113
639 8.76 529 443,262 115,248
g L1 e mS cm1 mg L1 mg L1
275.3 2612.2 NA NA NA NA NA NA
1526.3 2668.5 5222 0.36 0.03 0.02 0.07 414,666
mM mM mg L1 %v/v %w/w %w/w %w/w ppm
Concentration pH Conductivity CODT CODS VFA Ethanol Lactic FOG Methanol Monoglyceride Diglyceride Triglyceride TOC NA¼Not Analyzed.
Agriculture found that during the last 10 years, the quantity of banana peel about 28.58% becomes worthless organic waste and when its degradation occurs this result in toxic gasses such as ammonia and hydrogen sulfide that causes foul odor and has a detrimental effect on the environment. Due to the increasing population and the growth of global industry in the last century, the energy consumption has increased steadily. CH4 is a renewable biofuel increasingly used to reduce the amount of imported oil and reduce greenhouse gas emissions. Biodiesel is of interest at the moment in that it burns well and completes combustion better than diesel refined from petroleum. The productivity of biodiesel worldwide increased to 37 billion gallons in 2016 (average annual growth of 42%). While Thailand had a productivity of biodiesel of about 3.26 million liters per day in 2015 and continued to rise steadily until the year 2016 [3], which means about 0.32 million gallons of
glycerol waste was produced [4,5]. Therefore, the yield of glycerol waste from biodiesel about 10% (w/w) with 50e60% purity, is required through a filtering process, chemical addition, and vacuum distillation to accomplish 95e99% purity before used as components in food, pharmaceutical, and cosmetic industries. Since the purification of waste glycerol is too costly an alternative method for its disposal is strongly required [6]. Applying waste glycerol as a carbon source for CH4 production is a promising alternative use for this highstrength waste. Banana peel and waste glycerol were immensely tested as a co-substrate for anaerobic digestion under mesophilic conditions. Previous studies [7] reported that fermented banana peel chopped and powered (10% w/v) with cattle dung digester (10% v/v) at a hydraulic residence time (HRT) for 40 days yielded CH4 219 L CH4 kg1 TSadded (chopped), while the yield was improved to 231 L CH4 kg1 TSadded after banana peel was dehydrated and powdered. This result is consistent with the research [8] which examined the peel of eight cultivars of banana, where 0.5 g of banana peel (powdered) was inoculated with sludge from an existing bench-scale digester and can produce CH4 up to 266 L CH4 kg1 TSadded. The previous research [9] reported that particle size of banana peel affects the CH4 production rate but does not affect CH4 yield. Co-digestion of glycerol has been widely studied [10e13] using glycerol waste fermented with other materials such as pig's manure, industrial wastewater, agriculture waste and food waste. They found that the addition of glycerol waste improves CH4 yield and CH4 production rates forasmuch as the glycerol waste can increase the amount of COD. Additionally, the addition of glycerol 5e15% fermented with pig manure demonstrated that it inhibits the CH4 production and increases carbon dioxide gas. This research aimed to analyse the CH4 potential from the co-digestion of agricultural wastes, glycerol waste from biodiesel production process and banana peel.
Table 2 e The physical and chemical characteristics of banana peel. Characteristics Total calories Carbohydrate (Excluding fiber) Moisture Ash Total Nitrogen Protein* Fat Crude Fiber Sugar
Test method
Value
Units per 100 g
NFI T 943 based on Laboratory Manual for Food Canners and Processors, Volume Two. (1978) 3th ed. NFI T 943 based on Laboratory Manual for Food Canners and Processors, Volume Two. (1978) 3th ed. AOAC (2005), 964.22 (NFI T 923) AOAC (2005), 964.22 (NFI T 924) In-house method based on AOAC (2005), 991.20 In-house method based on AOAC (2005), 991.20 NFI T 966 based on AOAC (2005), 922.06 AOAC (2005), 978.10 (NFI T 942) Reducing Sugar by Dinitrosalicylic Acid Method
36.45
kcal
4.52
g
88.0 1.65 0.17 1.06 1.57 3.20 0.064
g g g g g g g
Certified by: National Food Institute of Thailand (NFI), Bangkok, Thailand (www.nfi.or.th).
Table 3 e Kinetics parameters of CH4 production from banana peel. Banana peel (%TS) 2.5 5.0 7.5 10
CH4 (%) 54.08 ± 58.43 ± 63.97 ± 60.02 ±
4.6 2.9 1.8 2.8
Hmax (mL) 163 170 181 158
± 46 ±2 ± 19 ± 20
Rmax (mL h1) 2.91 3.68 5.37 5.33
± 0.4 ± 0.6 ± 1.4 ± 1.8
Y (L kg1 TSadded) 170 177 188 164
± 48 ±3 ± 20 ± 21
l (h) 3.67 0.83 2.61 1.86
± 1.5 ± 0.6 ± 1.0 ± 0.6
CODT removal (%) 7 ± 0.6 24 ± 2.1 28 ± 1.9 16 ± 0.3
Alkalinity (mg CaCO3 L1) 1818 1919 2090 2160
± 43.88 ± 198.4 ± 131.6 ± 21.94
pHf 7.45 7.48 7.32 7.40
± 0.13 ± 0.10 ± 0.04 ± 0.14
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70
8.0
A
60 pH
% Methane
50 40
7.4
20
7.2
10
7.0
0 60
12
24
36
48 60 Time (hr)
72
84
96
0
108
12
Acetic 0.8 B
B VFA (mM)
50 Methane (mL)
7.6
30
0
40 30 20 10
Buty t ric
24
36
48 60 Time (hr)
Valeric
Eth t anol
72
Propionic
84
96
108
1,3 Propan a ediol
0.6 0.4 0.2 0.0
0 0
12
24
36
48 60 Time (hr)
72
84
96
2.5
108
Alkalinity (mg CaCO3 L-1)
C
200 150 100 50 0 0
12
24
36
48 60 Time (hr)
72
84
96
▪
7.5
10
7.5
10
C
2,000 1,500 1,000 500 0 2.5
108
Fig. 1 e The percentage (A) volume (B) and cumulative CH4 (C) from the fermentation of banana peel concentrations of 2.5 (B); 5.0 ( ); 7.5 (,) and 10 ( ) % w v-1. Histograms represent average value (n ¼ 2) and I-bars represent standard deviation.
5.0 BP (%TS)
2,500
250 Cumulative methane (mL)
A
7.8
5.0 BP (%TS)
Fig. 2 e pH (A) volatile fatty acids and ethanol (B) and alkalinity (C) from the fermentation of banana peel concentrations of 2.5 (B); 5.0 ( ); 7.5 (,) and 10 ( ) % w v-1 after 108 h fermentation. Histograms represent average value (n ¼ 2) and I-bars represent standard deviation.
▪
Inoculum
Materials and methods Glycerol Glycerol waste were obtained from Trang Plam Oil Co., Ltd. (Trang, Thailand) was brown color with glycerol purity of 63.9% while analytical grade pure glycerol was purchased from Qrec (New Zealand) with glycerol purity of 99.5%. The physical and chemical properties of both glycerols are summarised in Table 1.
Banana peel Fresh banana peel was collected from the deep-fried banana shop at the King Mongkut's University of Technology North Bangkok, and minced into small pieces 0.5 1 cm. Its composition was analyzed and summarised in Table 2.
The microbial sludge was collected from the up-flow anaerobic sludge blanket (UASB) process treating cassava processing wastewater (Eiamburapa Co., Ltd., Sa Kaeo province). The sludge was washed with tap water to remove dirt through a 500 mm mess sieve. The sludge was fed with cassava starch medium in a 5 L semi-batch reactor at 37 C fermentation until the sludge reached steady state with respect to CH4 content (%) and daily CH4 production (mL). The steady-state sludge was collected and wahsed in tap water prio to use.
Single- and co-substrate fermentation The experiment was set up by (1) single substrate digestion of banana peel with varying total solids (TS) of banana peel from 2.5 to 10% w v1, (2) single substrate digestion of glycerol waste with varying glycerol concentrations from 7.5 to 90 g L1; and (3) co-digestion between 7.5% banana peel and pure/waste glycerol with varying glycerol concentration from 7.5 to
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Fig. 3 e Lag phase time (A) CH4 production potential (B), rate (C) and yield (D) after108 h fermentation of banana peel. Histograms represent average value (n ¼ 2) and I-bars represent standard deviation.
90 g L1 . The experiments were conducted with and without the addition of 0.1 M NaHCO3. Banana or glycerol and microbial sludge were mixed in a glass bottle (working volume 0.5 L) at ratio 3:1, fermented at 37 C and pH 7 for 120 h. Each bottle contained 7.9 g TVS microbial sludge. The bottles were sealed with the silicone rubber stopper and flushed with nitrogen gas for 5 min to generate anaerobic condition. All bottles were placed on a magnetic stirrer plate equipped within the incubator. During the fermentation, the gas composition was collected every 12 h with a gastight syringe (Hamilton, USA) to analyze H2, CH4, and CO2 by gas chromatography. Total gas volume was measured by water displacement method. Liquid samples were analyzed every 24 h for pH, VFA, COD, glycerol, and solids (TS and TVS).
Analytical techniques Chemical oxygen demand (COD) was analyzed by closed reflux colorimetric method. Alkalinity, pH, total solids (TS), total volatile solids (TVS) were determined according to standard methods [14]. Glycerol concentration was determined according to chromatropic acid method [15]. The supernatant volatile fatty acids (VFA) was transferred into the glass vials and adding 70 ml of 17% H3PO4 (pH < 3) and analyzed by Shimazu gas chromatography (GC 2010, Japan) equipped with a flame ionization detector (FID, 230 C) and Restek Stabilwax DA capillary column (80 C). Helium gas was used as a carrier gas followed by hydrogen, air zero, and nitrogen gas, respectively. H2, CH4, and CO2 content were measured by a Shimadzu gas chromatography (GC 2014,
Japan) equipped with thermal conductivity detectors (TCD, 150 C) fitted with a column packed with Unibeads C 80/100 mesh (80 C). Helium was used as a carrier gas flow rate of 50 mL min1.
Kinetics of CH4 production Kinetics of CH4 production were calculated from the cumulative CH4 production versus time data., SigmaPlot version 11.0 was used to fit the data with modified Gompertz equation (Eq. (1)), [16]. Rmax e ðl tÞ þ 1 H ¼ Hmax exp exp Hmax
(1)
where H is the cumulative CH4 production (mL) at the time (t) of fermentation to reach a steady state (h), Hmax is the CH4 production potential (mL), Rmax is the maximum production rate (mL h1), l is a lag phase time (h) and e is a numerical constant equal to 2.71828. The CH4 yield (Y) of banana peel was calculated by dividing the CH4 production potential (Hmax) at 108 h with the amount of TSadded while the yield of co-digestion was calculated by dividing Hmax with the amount of total COD removal (CODT) at 120 h. The theoretical value of CH4 per gram of glycerol can be calculated by using the Buswell formula (Eq. (2)) and the ideal gas law per gram of glycerol is equal to 0.484 L CH4. C3 H8 O3 /1:75CH4 þ 1:25CO2 þ 0:5H2 O
(2)
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9.3 ± 0.8 11.3 ± 0.0 14.1 ± 0.5 11.4 ± 0.4 10.2 ± 0.7 1.28 0.14 0.42 0.96 0.00 4.47 ± 4.96 ± 6.80 ± 6.01 ± 2.54 ±
Net present ValueðNPVÞ ¼
Xn 1
Net Period Cash Flow time periods
ð1 þ Rate of returnÞ
Y (mL g1 CODT removal)
102 ± 2.3 45.3 ± 2.6 13.3 ± 2.5 7.9 ± 0.8 3.2 ± 2.3
Rmax (mL hr1)
10.3 ± 1.9 5.0 ± 0.7 5.5 ± 2.0 3.3 ± 0.2 0.85 ± 0 8.0 2.9 9.0 3.8 12
(3)
72.9 ± 51.7 ± 38.6 ± 27.1 ± 17.9 ± ±0 ±1 ±2 ±1 ±1 44.3 35.4 30.3 29.4 34.0 11.7 ± 0.6 22.2 ± 0.1 17.5 ± 1.4 18.0 ± 1.1 7.3 ± 0.0 332 ± 8.5 198 ± 4.2 68.7 ± 5.9 89.6 ± 2.4 163 ± 4.9 7.5 15 30 45 90
47.0 ± 51.8 ± 44.5 ± 47.8 ± 55.5 ±
4 1 0 2 0
233 377 188 381 526
± 7.7 ± 7.9 ± 0.9 ± 18 ± 16
7.9 ± 0.3 16.8 ± 0.7 7.7 ± 0.1 15.8 ± 0.1 24.5 ± 0.1
9.89 ± 0.01 10.9 ± 0.39 7.43 ± 0.09 9.70 ± 1.02 11.6 ± 0.28
CH4 (%) CODT removal (%) l (hr) Y (mL g CODT removal) Hmax (mL) CH4 (%)
Payback period ¼ yearstart payback þ
NPVat the beginning of the project NPVnet cash flow generated per year (4)
Rmax (mL hr1)
1
Pure glycerol
To calculate economic value, the analysis assumes that the production of CH4 from a banana peel, waste glycerol and the banana peel mixed with waste glycerol can be replaced LPG as an alternative fuel in households and electricity, which is calculated as follows (Eqs. (3) and (4)).
Methane production from banana peel
[Glycerol] (g L1)
Table 4 e Kinetics parameters of CH4 production from pure and waste glycerol.
Economic value analysis
Results and discussion
Hmax (mL)
Waste glycerol
l (hr)
CODT removal (%)
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CH4 content was obtained from banana peel fermentation in the range of 40e64% (Table 3). The highest CH4 content in biogas was 64% at 7.5% banana peel (Fig. 1A). No hydrogen gas was found in the reactor headspace. The maximum CH4 production was approximately 28e49 mL during a 24 h fermentation (Fig. 1B). 7.5% banana peel generated the maximum cumulative methane (Fig. 1C). Fermentation pH was maintained in the range of 7.2e7.7 in all cases (Fig. 2A), which are suitable for the CH4 fermentation under mesophilic condition [17]. VFAs and ethanol were slightly accumulated during the fermentation (Fig. 2B). Alkalinity was found in the range 1800 to 2100 mg CaCO3 L1 (Fig. 2C), which are optimum for the CH4 production [18]. The modified Gompertz equation used to analyze the kinetic data from CH4 fermentation. The S curve of cumulative methane versus time fits in with the equation, results showed R2 values more than 0.97. Lag phase time of banana peel, TS 2e5 to 7.5% was approximately 0.5e3.5 h (Fig. 3A). The trend of Hmax, Rmax, and Y was increased as a function of the peel in the range of 2.5e7.5% TS (Fig. 3B, 3C, 3D). Hmax and Rmax were up to 181 mL and 5.37 mL h1, respectively for banana peel 7.5% (Fig. 3B, 3C). The highest CH4 yield was 188 L kg1 TSadded obtained from a banana peel, TS 7.5% (Fig. 3D). The data of Hmax, Rmax, l, and yield indicated that banana peel had a high potential for produce CH4.
Methane production from pure and waste glycerol CH4 content produced from pure and waste glycerol was 45e55% and 30e44% (Table 4). The highest CH4 production potential (Hmax) and rate (Rmax) apparently increases as increased concentrations of pure glycerol in contrast to waste glycerol (Fig. 4A and 4B). CH4 yield (Fig. 4C) dropped significantly when the glycerol concentration exceeded 7.5 g L1. At the end of the fermentation process, pH and alkalinity were decreased to less than 4.5 and 1500 mg CaCO3 L1, respectively. The accumulation of VFAs and ethanol, increased markedly in the waste glycerol (Fig. 5), caused the inhibition of methanogenic activity, and consequently dropped the CH4
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▪
Fig. 4 e CH4 production potential (A), rate (B) and yield (C) from the fermentation pure (,) and waste glycerol ( ) after 48 h fermentation. Histograms represent average value (n ¼ 2) and I-bars represent standard deviation.
production corresponding to the previous study [19,20]. At 7.5 g L1 glycerol, pure and waste glycerol gave CH4 yield 83.39% and 25.53% of the theoretical yield, respectively. These results suggest that the initial concentration of glycerol and glycerol purity incluenced the potential of bacteria to convert glycerol to CH4.
Methane production from co-digestion of banana peel and pure/waste glycerol Table 5 summarised the kinetics parameters of the co-disgestion of banana peel and pure/waste glycerol. The addition
of biocarbonate appeared to improve CH4 content, CH4 production potential (Hmax), CH4 yield (Y) and lag phase time. The trend of CH4 production potential and rate versus initial concentration of pure and waste glycerol were similar to that of the CH4 yield (Fig. 6B, 6C, 6D). The highest CH4 production potential and rate obviously decreased when concentrations of the glycerol pure and glycerol waste were increased (Fig. 6B, 6C): 7.5 g L1 Pure glycerol (with biocarbonate addition) showed Hmax 467 mL, and Rmax 8.04 mL h1, respectively, while those from waste glycerol were 652 mL and 17.47 mL h1, respectively. pH and alkalinity dropped significantly due to the glycerol concentration exceeding 7.5 g L1
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Fig. 5 e Concentrations of VFAs and ethanol in the fermentation of waste glycerol after 48 h fermentation. Histograms represent average value (n ¼ 2) and I-bars represent standard deviation.
Table 5 e Kinetics parameters of CH4 production from co-digestion of banana peel and pure glycerol (PG) /waste glycerol (WG). Type of Glycerol PG PG PG PG WG WG WG WG
[Glycerol] (g L1)
[BP] (%TS)
[NaHCO3]
CH4 (%)
Hmax (mL)
Rmax (mL h1)
Y (mL g1 CODT removal)
l (h)
CODT removal (%)
Alkalinity (mg CaCO3 L1)
pHf
7.5 7.5 90 90 7.5 7.5 90 90
7.5 7.5 7.5 7.5 7.5 7.5 7.5 7.5
e 0.1 M e 0.1 M e 0.1 M e 0.1 M
59 62 30 38 49 65 7 5
242 467 290 325 519 652 27.1 28.3
167 8.0 16 5.7 28 17.5 20.2 20.9
117 151 19 31 198 281 16 2
22.0 2.37 11.3 6.81 13.2 1.57 26.4 7.24
19.4 30.8 37.6 28.1 20.0 20.2 3.92 35.3
276 1691 181 1112 732 1853 675 722
5.02 7.27 4.66 5.23 5.55 7.62 5.99 6.19
Fig. 6 e Lag phase time (A) CH4 production potential (B), rate (C) and yield (D) in the fermentation of 7.5% banana peel mixed with varying concentrations of pure (,) and waste glycerol ( ) under conditions with buffer (B) and without biocarbonate addition (NB) after 120 h fermentation. Histograms represent average value (n ¼ 2) and I-bars represent standard deviation.
▪
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Fig. 7 e pH (A) and alkalinity (B) in the fermentation of 7.5% banana peel mixed with varying concentration of pure (,) and waste glycerol ( ) under conditions with buffer (B) and without buffer (NB) after 120 h fermentation. Histograms represent average value (n ¼ 2) and I-bars represent standard deviation.
Fig. 8 e VFAs concentrations in the fermentation of 7.5% banana peel mixed with varying concentration of pure (,) and waste glycerol ( ) under conditions with buffer (B) and without buffer (NB) after 120 h fermentation. Histograms represent average value (n ¼ 2) and I-bars represent standard deviation.
▪
▪
(Fig. 7A and B) which was unsuitable for the CH4 production [18]. The accumulation of VFAs and ethanol in the fermentation of pure and waste glycerol slightly increased when concentration of glycerol exceeded 7.5 g L1 (Fig. 8A, 8B). This results are in accordnce with the previous studies [19, 20] reported ethanol increased significantly when pH were in the range of 4.5e6.5. At this pH range, the dominant metabolic product was 1,3 propanediol, which suppressed the activity of methanogenic bacteria and the CH4 production.
The CH4 yield dropped significantly after the glycerol concentration exceeded 7.5 g L1. At 7.5 g L1 pure/waste glycerol mixed with banana peel (with bicarbonate addition), the highest CH4 yield was 150 mL g1 CODT removal and 280 mL g1 CODT removal (Fig. 4D), which are 1.3 and 1.4 times higher than those without the biocarbonate addition. Previous studies [11e13, 21], reported that when the glycerol concentration exceeded 5%, the CH4 production was likely to decline significantly. The results suggested that the efficiency of CH4
Table 6 e Comparative CH4 production from co-digestion of banana peel and waste glycerol compared to previous studies. Co-Substrate waste water derived from biodiesel manufacturing cattle manure hog manure hog manure hog manure banana peel banana peel
[Waste Glycerol] [Pure Glycerol] % CH4 Hmax (mL) Rmax (mL d1)
Yield Reference (m3 kg1 CODremoval)
1.00% (w/v)
e
e
332
e
0.310
[22]
4.00% (v/v) e 1.00% (w/v) e 0.75% (w/v) e
e 1.00% (w/v) e 2.00% (w/v) e 0.75% (w/v)
58 63 67 66 65 62
e e e e 652 467
e e e e 192 420
0.365 0.370 0.360 0.390 0.281 0.151
[23] [24] [24] [24] This study This study
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Table 7 e Economic value analysis. CH4 (Substrate)
Cost (Baht L1)
NPV (10 years)
PBP (year)
LPG (kg yr1)
Electricity (kWh yr1)
0 408 0 7 408 0 7
229.94 0.03 173.83 172.09 225.47 480.43 478.70
<1 >10 <1 1.01 1.16 <1 1.00
31.57 55.67 17.06 17.06 25.32 47.15 47.15
84.6 149.28 45.72 45.72 67.92 126.36 126.36
BP PG WG WG BP þ PG BP þ WG BP þ WG
production can be better improved when waste glycerol was co-digested with banana peel than using single substrate. The comparative kinetics parameters from the co-digestion of glycerol waste with various materais were summarised in Table 6.
Mongkut's University of Technology North Bangkok (KMUTNB-61-KNOW-042), Research and Researchers for Industries Program (PHD60I0028), and Joint Graduate School of Energy and Environment, King Mongkut’s University of Technology Thonburi (JGSEE739) for the financial support.
Economic values Estimating economic value for this study (Table 7) shows that CH4 production from banana peels, waste glycerol, and banana peel mixed with waste or pure glycerol as substrates are worth investment (NPV> 0) due to the payback period (PBP) which is estimated at less than 1.2 years. Conversely, the economic value of producing CH4 from pure glycerol found that the project is unlikely to attract investment because less CH4 can be produced when compared to the cost of high purity glycerol that shows a payback period of over 10 years. The codigestion of waste glycerol and banana peel to produce CH4 is the best condition to carry an investment due to the high yields and short payback time when compared to other substrates.
Conclusions This study successfully demonstrated the CH4 production from co-digestion of pure glycerol/waste glycerol and the banana peel. The highest CH4 yield was obtained from banana peel 7.5% TS mixed with waste glycerol 7.5 g L1 (in the presence of buffer), which yielded 281 mL g1 CODT removal slightly higher than for the banana peel 7.5% mixed with pure glycerol 7.5 g L1 (in the presence of buffer; 151 mL g1 CODT removal) and only banana peel (188 mL g1 CODT removal). Unlike pure glycerol, waste glycerol favors for the is higher than when the pure glycerol was used due to it being rich in vitamins and other nutrients such as methanol, soluble COD, which easily biodegradable used by methanogenic bacteria, resulting in higher metabolic activity. Our results suggest that glycerol waste is a by-product of biodiesel production could be used as a co-substrate in a fermentation without removing impurities before being used. In particular, the concentration of glycerol exceeded 0.75% (w/v) was important impacts of methanogenic bacteria on the utilization of the substrate to produce gas.
Acknowledgements The authors would like to express gratitude to Thailand Research Fund (DBG6280001), The Royal Society, King
references
[1] Consultative Group on International Agricultural Research (CGIAR) [Internet]. Thailand: Banana and plantain, [updated 2011 October 17; cited 2011 November 10]. Available from: http://www.cgiar.org/our-research/crop-factsheets/bananas/. [2] Department of Agriculture. Strategy for banana research, 2016 e 2020. 2016 [Internet] [updated 2016 May 16; cited 2017 December 10]. Available from: http://www.doa.go.th. [3] Department of Energy. Situation and trend in 2015. [Internet] [updated 2015 January 20; cited 2015 December 13]. Available from: http://www.energy.go.th/sites/all/files/situation58_ trend 59.pdf. [4] Wang L, Du W, Liu DH, Li LL, Dai NM. Lipase-catalyzed biodiesel production from soybean oil deodorizer distillate with absorbent present in test-butanol system. J Mol Catal B Enzym 2006;43:29e32. [5] Fan X, Burton R, Zhou Y. Glycerol (byproduct of biodiesel production) as a source for fuels and chemicals e mini review. Open Fuel Energy Sci J 2010;3:17e22. [6] Pachauri N, He B. Value-added utilization of crude glycerol from biodiesel production. A survey of current research activities. Am Soc Agric and Biol Eng 2006;066223:1e16. [7] Bardiya N, Somayaji D, Khanna S. Biomethanation of banana peel and pineapple waste. Bioresour Technol 1996;58:73e6. [8] Gunaseelan N. Biochemical methane potential of fruits and vegetable solid waste feedstocks. Biomass Bioenergy 2004;26:389e99. [9] Tumutegyereize P, Muranga FI, Kawongolo J, Nabugoomu F. Optimization of biogas production from banana peels. Effect of particle size on methane yield. Afr J Biotechnol 2011;10:18243e51. [10] Fountoulakis MS, Manios T. Co-digestion of sewage sludge with glycerol to boost biogas production. Waste Manag 2010;30:1849e53. [11] Amon TH, Amon B, Kryvoruchko V, Bodiroza V, Potsch E, Zollitsch W. Optimising methane yield from anaerobic digestion of manure: effects of dairy systems and of glycerine supplementation. Int Congr 2006;1293:217e20. [12] Limsuk S, Peantum P, Petiraksakul A. Biogas production from food waste added with crude glycerin produced from biodiesel production. KKU Eng J 2011;38:101e10. [13] Powalowska DS, Kubiak P, Lewicki A. The methane fermentation medium as an attractive source of bacteria from genus Clostridium capable of converting glycerol into 1,3-propylene glycol. J Bio Comput Bio Bionanotechnol 2012;93:68e77.
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[14] Standard APHA. Methods for the examination of water and wastewater. 21st ed. Washington, DC, USA: American Public Health Association, American Water Works Association, Water Environment Federation; 2005. [15] Handel VE, Zilversmit DB. Micromethod for the direct determination of serum triglycerides. J Lab Clin Med 1957;50:152e7. [16] Housagul S, Sirisukpoka U, Pisutpaisal N. Enhancement of biohydrogen yield by Co-digestion of waste glycerol and glucose. Energy Procedia 2014;61:2249e53. ve J, Dewil R. Principles and [17] Appels L, Baeyens J, Degre potential of anaerobic digestion of waste-activated sludge. Prog Energy Combust Sci 2008;34:755e81. [18] Osman NA, Delia TS. Effect of alkalinity on the performance of a simulated landfill bioreactor digesting organic solid wastes. Chemosphere 2005;59:871e9. [19] Vlassis T, Antonopoulou G, Stamatelatou K, Lyberatos G. Anaerobic treatment of glycerol for methane and hydrogen production. Global NEST J 2012;14:149e56.
[20] Wang Y, Zhang Y, Wang J, Meng L. Effects of volatile fatty acid concentrations on methane yield and methanogenic bacteria. Biomass Bioenergy 2009;33:848e53. ~ o RC, Santaella ST. [21] Viana MB, Freitas AV, Leita Biodegradability and methane production potential of glycerol generated by biodiesel industry. Water Sci Technol 2012;66:2217e22. [22] Siles JA, Martı´n MA, Chica AF, Martı´n A. Anaerobic codigestion of glycerol and wastewater derived from biodiesel manufacturing. Bioresour Technol 2010;101:6315e521. n L, Ferna ndez-Nava Y, Ormaechea P, Maran ~o n E. [23] Castrillo Optimization of biogas production from cattle manure by pre-treatment with ultrasound and co-digestion with crude glycerin. Bioresour Technol 2011;102:7845e9. [24] Wohlgemut O, Cicek N, Oleszkiewicz J, Sparling R. Codigestion of hog manure with glycerol to boost biogas and methane production. Trans. ASAE 2011;54:723e7.
Available online at www.sciencedirect.com
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Biomethane Production from co-fermentation of agricultural wastes Saowaluck Haosagul a,b,c, Siriorn Boonyawanich a,b,**, Nipon Pisutpaisal a,b,* a
Department of Agro-Industrial, Food and Environmental Technology, Faculty of Applied Science, King Mongkut's University of Technology North Bangkok, 10800, Thailand b The Biosensor and Bioelectronics Technology Centre, King Mongkut's University of Technology North Bangkok, 10800, Thailand c The Joint Graduate School of Energy and Environment (JGSEE), King Mongkut’s University of Technology Thonburi, Bangkok, 10140, Thailand
article info
abstract
Article history:
Biomethane (CH4) was recovered from co-digestion process of waste glycerol and banana
Received 13 August 2018
wastes. The wastes used contain waste glycerol with varying concentrations from 7.5 to
Received in revised form
90 g L1 and banana peel in the range 2.5e10 %w$v1. The co-substrate mixture ratio was
25 August 2018
implemented in 0.5 L batch reactor operated at 37 C and pH 7 for 120 h. The composition of
Accepted 11 September 2018
biogas gas and liquid samples (COD, VFA, pH, alkalinity) were analyzed every 12 and 24 h,
Available online 8 November 2018
respectively. The optimum condition to produce CH4 was found at 7.5 g L1 waste glycerol
Keywords:
0.281 m3 kg1 COD and 652 mL, respectively.
mixed with 7.5% banana peel. The highest CH4 yield and CH4 production potential were Fermentation
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Waste glycerol Banana waste Biofuel Biomethane
Introduction Banana is a popular fruit grown in tropical regions of the world, with a 175% increase in yield over the past 30 years. India is the largest producer with 32 million tons, and other countries such as China, Philippines, Ecuador, Brazil, Indonesia, Tanzania, and Thailand. Banana production in Asia
has quadrupled to 62 million tons [1]. In 2013, Thailand had an area of 77% banana plantations. Report's published by the Department of Agricultural Extension from 2009 to 2013 showed that the banana production increased from 156,367 tons in 2009 to 773,732 tons in 2013, increased 36.3% per year [2]. Bananas can be consumed and processed in many ways such as flour, chips, and beverages. The Department of
* Corresponding author. Department of Agro-Industrial, Food and Environmental Technology, Faculty of Applied Science, King Mongkut's University of Technology North Bangkok, 10800, Thailand. ** Corresponding author. Department of Agro-Industrial, Food and Environmental Technology, Faculty of Applied Science, King Mongkut's University of Technology North Bangkok, 10800, Thailand. E-mail addresses: [email protected] (S. Boonyawanich), [email protected] (N. Pisutpaisal). https://doi.org/10.1016/j.ijhydene.2018.09.080 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Table 1 e Physical and chemical characteristics of pure and waste glycerol. Parameter
Pure glycerol
Waste glycerol
Unit
995 7.54 0.38 780,142 124,113
639 8.76 529 443,262 115,248
g L1 e mS cm1 mg L1 mg L1
275.3 2612.2 NA NA NA NA NA NA
1526.3 2668.5 5222 0.36 0.03 0.02 0.07 414,666
mM mM mg L1 %v/v %w/w %w/w %w/w ppm
Concentration pH Conductivity CODT CODS VFA Ethanol Lactic FOG Methanol Monoglyceride Diglyceride Triglyceride TOC NA¼Not Analyzed.
Agriculture found that during the last 10 years, the quantity of banana peel about 28.58% becomes worthless organic waste and when its degradation occurs this result in toxic gasses such as ammonia and hydrogen sulfide that causes foul odor and has a detrimental effect on the environment. Due to the increasing population and the growth of global industry in the last century, the energy consumption has increased steadily. CH4 is a renewable biofuel increasingly used to reduce the amount of imported oil and reduce greenhouse gas emissions. Biodiesel is of interest at the moment in that it burns well and completes combustion better than diesel refined from petroleum. The productivity of biodiesel worldwide increased to 37 billion gallons in 2016 (average annual growth of 42%). While Thailand had a productivity of biodiesel of about 3.26 million liters per day in 2015 and continued to rise steadily until the year 2016 [3], which means about 0.32 million gallons of
glycerol waste was produced [4,5]. Therefore, the yield of glycerol waste from biodiesel about 10% (w/w) with 50e60% purity, is required through a filtering process, chemical addition, and vacuum distillation to accomplish 95e99% purity before used as components in food, pharmaceutical, and cosmetic industries. Since the purification of waste glycerol is too costly an alternative method for its disposal is strongly required [6]. Applying waste glycerol as a carbon source for CH4 production is a promising alternative use for this highstrength waste. Banana peel and waste glycerol were immensely tested as a co-substrate for anaerobic digestion under mesophilic conditions. Previous studies [7] reported that fermented banana peel chopped and powered (10% w/v) with cattle dung digester (10% v/v) at a hydraulic residence time (HRT) for 40 days yielded CH4 219 L CH4 kg1 TSadded (chopped), while the yield was improved to 231 L CH4 kg1 TSadded after banana peel was dehydrated and powdered. This result is consistent with the research [8] which examined the peel of eight cultivars of banana, where 0.5 g of banana peel (powdered) was inoculated with sludge from an existing bench-scale digester and can produce CH4 up to 266 L CH4 kg1 TSadded. The previous research [9] reported that particle size of banana peel affects the CH4 production rate but does not affect CH4 yield. Co-digestion of glycerol has been widely studied [10e13] using glycerol waste fermented with other materials such as pig's manure, industrial wastewater, agriculture waste and food waste. They found that the addition of glycerol waste improves CH4 yield and CH4 production rates forasmuch as the glycerol waste can increase the amount of COD. Additionally, the addition of glycerol 5e15% fermented with pig manure demonstrated that it inhibits the CH4 production and increases carbon dioxide gas. This research aimed to analyse the CH4 potential from the co-digestion of agricultural wastes, glycerol waste from biodiesel production process and banana peel.
Table 2 e The physical and chemical characteristics of banana peel. Characteristics Total calories Carbohydrate (Excluding fiber) Moisture Ash Total Nitrogen Protein* Fat Crude Fiber Sugar
Test method
Value
Units per 100 g
NFI T 943 based on Laboratory Manual for Food Canners and Processors, Volume Two. (1978) 3th ed. NFI T 943 based on Laboratory Manual for Food Canners and Processors, Volume Two. (1978) 3th ed. AOAC (2005), 964.22 (NFI T 923) AOAC (2005), 964.22 (NFI T 924) In-house method based on AOAC (2005), 991.20 In-house method based on AOAC (2005), 991.20 NFI T 966 based on AOAC (2005), 922.06 AOAC (2005), 978.10 (NFI T 942) Reducing Sugar by Dinitrosalicylic Acid Method
36.45
kcal
4.52
g
88.0 1.65 0.17 1.06 1.57 3.20 0.064
g g g g g g g
Certified by: National Food Institute of Thailand (NFI), Bangkok, Thailand (www.nfi.or.th).
Table 3 e Kinetics parameters of CH4 production from banana peel. Banana peel (%TS) 2.5 5.0 7.5 10
CH4 (%) 54.08 ± 58.43 ± 63.97 ± 60.02 ±
4.6 2.9 1.8 2.8
Hmax (mL) 163 170 181 158
± 46 ±2 ± 19 ± 20
Rmax (mL h1) 2.91 3.68 5.37 5.33
± 0.4 ± 0.6 ± 1.4 ± 1.8
Y (L kg1 TSadded) 170 177 188 164
± 48 ±3 ± 20 ± 21
l (h) 3.67 0.83 2.61 1.86
± 1.5 ± 0.6 ± 1.0 ± 0.6
CODT removal (%) 7 ± 0.6 24 ± 2.1 28 ± 1.9 16 ± 0.3
Alkalinity (mg CaCO3 L1) 1818 1919 2090 2160
± 43.88 ± 198.4 ± 131.6 ± 21.94
pHf 7.45 7.48 7.32 7.40
± 0.13 ± 0.10 ± 0.04 ± 0.14
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70
8.0
A
60 pH
% Methane
50 40
7.4
20
7.2
10
7.0
0 60
12
24
36
48 60 Time (hr)
72
84
96
0
108
12
Acetic 0.8 B
B VFA (mM)
50 Methane (mL)
7.6
30
0
40 30 20 10
Buty t ric
24
36
48 60 Time (hr)
Valeric
Eth t anol
72
Propionic
84
96
108
1,3 Propan a ediol
0.6 0.4 0.2 0.0
0 0
12
24
36
48 60 Time (hr)
72
84
96
2.5
108
Alkalinity (mg CaCO3 L-1)
C
200 150 100 50 0 0
12
24
36
48 60 Time (hr)
72
84
96
▪
7.5
10
7.5
10
C
2,000 1,500 1,000 500 0 2.5
108
Fig. 1 e The percentage (A) volume (B) and cumulative CH4 (C) from the fermentation of banana peel concentrations of 2.5 (B); 5.0 ( ); 7.5 (,) and 10 ( ) % w v-1. Histograms represent average value (n ¼ 2) and I-bars represent standard deviation.
5.0 BP (%TS)
2,500
250 Cumulative methane (mL)
A
7.8
5.0 BP (%TS)
Fig. 2 e pH (A) volatile fatty acids and ethanol (B) and alkalinity (C) from the fermentation of banana peel concentrations of 2.5 (B); 5.0 ( ); 7.5 (,) and 10 ( ) % w v-1 after 108 h fermentation. Histograms represent average value (n ¼ 2) and I-bars represent standard deviation.
▪
Inoculum
Materials and methods Glycerol Glycerol waste were obtained from Trang Plam Oil Co., Ltd. (Trang, Thailand) was brown color with glycerol purity of 63.9% while analytical grade pure glycerol was purchased from Qrec (New Zealand) with glycerol purity of 99.5%. The physical and chemical properties of both glycerols are summarised in Table 1.
Banana peel Fresh banana peel was collected from the deep-fried banana shop at the King Mongkut's University of Technology North Bangkok, and minced into small pieces 0.5 1 cm. Its composition was analyzed and summarised in Table 2.
The microbial sludge was collected from the up-flow anaerobic sludge blanket (UASB) process treating cassava processing wastewater (Eiamburapa Co., Ltd., Sa Kaeo province). The sludge was washed with tap water to remove dirt through a 500 mm mess sieve. The sludge was fed with cassava starch medium in a 5 L semi-batch reactor at 37 C fermentation until the sludge reached steady state with respect to CH4 content (%) and daily CH4 production (mL). The steady-state sludge was collected and wahsed in tap water prio to use.
Single- and co-substrate fermentation The experiment was set up by (1) single substrate digestion of banana peel with varying total solids (TS) of banana peel from 2.5 to 10% w v1, (2) single substrate digestion of glycerol waste with varying glycerol concentrations from 7.5 to 90 g L1; and (3) co-digestion between 7.5% banana peel and pure/waste glycerol with varying glycerol concentration from 7.5 to
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Fig. 3 e Lag phase time (A) CH4 production potential (B), rate (C) and yield (D) after108 h fermentation of banana peel. Histograms represent average value (n ¼ 2) and I-bars represent standard deviation.
90 g L1 . The experiments were conducted with and without the addition of 0.1 M NaHCO3. Banana or glycerol and microbial sludge were mixed in a glass bottle (working volume 0.5 L) at ratio 3:1, fermented at 37 C and pH 7 for 120 h. Each bottle contained 7.9 g TVS microbial sludge. The bottles were sealed with the silicone rubber stopper and flushed with nitrogen gas for 5 min to generate anaerobic condition. All bottles were placed on a magnetic stirrer plate equipped within the incubator. During the fermentation, the gas composition was collected every 12 h with a gastight syringe (Hamilton, USA) to analyze H2, CH4, and CO2 by gas chromatography. Total gas volume was measured by water displacement method. Liquid samples were analyzed every 24 h for pH, VFA, COD, glycerol, and solids (TS and TVS).
Analytical techniques Chemical oxygen demand (COD) was analyzed by closed reflux colorimetric method. Alkalinity, pH, total solids (TS), total volatile solids (TVS) were determined according to standard methods [14]. Glycerol concentration was determined according to chromatropic acid method [15]. The supernatant volatile fatty acids (VFA) was transferred into the glass vials and adding 70 ml of 17% H3PO4 (pH < 3) and analyzed by Shimazu gas chromatography (GC 2010, Japan) equipped with a flame ionization detector (FID, 230 C) and Restek Stabilwax DA capillary column (80 C). Helium gas was used as a carrier gas followed by hydrogen, air zero, and nitrogen gas, respectively. H2, CH4, and CO2 content were measured by a Shimadzu gas chromatography (GC 2014,
Japan) equipped with thermal conductivity detectors (TCD, 150 C) fitted with a column packed with Unibeads C 80/100 mesh (80 C). Helium was used as a carrier gas flow rate of 50 mL min1.
Kinetics of CH4 production Kinetics of CH4 production were calculated from the cumulative CH4 production versus time data., SigmaPlot version 11.0 was used to fit the data with modified Gompertz equation (Eq. (1)), [16]. Rmax e ðl tÞ þ 1 H ¼ Hmax exp exp Hmax
(1)
where H is the cumulative CH4 production (mL) at the time (t) of fermentation to reach a steady state (h), Hmax is the CH4 production potential (mL), Rmax is the maximum production rate (mL h1), l is a lag phase time (h) and e is a numerical constant equal to 2.71828. The CH4 yield (Y) of banana peel was calculated by dividing the CH4 production potential (Hmax) at 108 h with the amount of TSadded while the yield of co-digestion was calculated by dividing Hmax with the amount of total COD removal (CODT) at 120 h. The theoretical value of CH4 per gram of glycerol can be calculated by using the Buswell formula (Eq. (2)) and the ideal gas law per gram of glycerol is equal to 0.484 L CH4. C3 H8 O3 /1:75CH4 þ 1:25CO2 þ 0:5H2 O
(2)
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9.3 ± 0.8 11.3 ± 0.0 14.1 ± 0.5 11.4 ± 0.4 10.2 ± 0.7 1.28 0.14 0.42 0.96 0.00 4.47 ± 4.96 ± 6.80 ± 6.01 ± 2.54 ±
Net present ValueðNPVÞ ¼
Xn 1
Net Period Cash Flow time periods
ð1 þ Rate of returnÞ
Y (mL g1 CODT removal)
102 ± 2.3 45.3 ± 2.6 13.3 ± 2.5 7.9 ± 0.8 3.2 ± 2.3
Rmax (mL hr1)
10.3 ± 1.9 5.0 ± 0.7 5.5 ± 2.0 3.3 ± 0.2 0.85 ± 0 8.0 2.9 9.0 3.8 12
(3)
72.9 ± 51.7 ± 38.6 ± 27.1 ± 17.9 ± ±0 ±1 ±2 ±1 ±1 44.3 35.4 30.3 29.4 34.0 11.7 ± 0.6 22.2 ± 0.1 17.5 ± 1.4 18.0 ± 1.1 7.3 ± 0.0 332 ± 8.5 198 ± 4.2 68.7 ± 5.9 89.6 ± 2.4 163 ± 4.9 7.5 15 30 45 90
47.0 ± 51.8 ± 44.5 ± 47.8 ± 55.5 ±
4 1 0 2 0
233 377 188 381 526
± 7.7 ± 7.9 ± 0.9 ± 18 ± 16
7.9 ± 0.3 16.8 ± 0.7 7.7 ± 0.1 15.8 ± 0.1 24.5 ± 0.1
9.89 ± 0.01 10.9 ± 0.39 7.43 ± 0.09 9.70 ± 1.02 11.6 ± 0.28
CH4 (%) CODT removal (%) l (hr) Y (mL g CODT removal) Hmax (mL) CH4 (%)
Payback period ¼ yearstart payback þ
NPVat the beginning of the project NPVnet cash flow generated per year (4)
Rmax (mL hr1)
1
Pure glycerol
To calculate economic value, the analysis assumes that the production of CH4 from a banana peel, waste glycerol and the banana peel mixed with waste glycerol can be replaced LPG as an alternative fuel in households and electricity, which is calculated as follows (Eqs. (3) and (4)).
Methane production from banana peel
[Glycerol] (g L1)
Table 4 e Kinetics parameters of CH4 production from pure and waste glycerol.
Economic value analysis
Results and discussion
Hmax (mL)
Waste glycerol
l (hr)
CODT removal (%)
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 5 3 5 5 e5 3 6 4
CH4 content was obtained from banana peel fermentation in the range of 40e64% (Table 3). The highest CH4 content in biogas was 64% at 7.5% banana peel (Fig. 1A). No hydrogen gas was found in the reactor headspace. The maximum CH4 production was approximately 28e49 mL during a 24 h fermentation (Fig. 1B). 7.5% banana peel generated the maximum cumulative methane (Fig. 1C). Fermentation pH was maintained in the range of 7.2e7.7 in all cases (Fig. 2A), which are suitable for the CH4 fermentation under mesophilic condition [17]. VFAs and ethanol were slightly accumulated during the fermentation (Fig. 2B). Alkalinity was found in the range 1800 to 2100 mg CaCO3 L1 (Fig. 2C), which are optimum for the CH4 production [18]. The modified Gompertz equation used to analyze the kinetic data from CH4 fermentation. The S curve of cumulative methane versus time fits in with the equation, results showed R2 values more than 0.97. Lag phase time of banana peel, TS 2e5 to 7.5% was approximately 0.5e3.5 h (Fig. 3A). The trend of Hmax, Rmax, and Y was increased as a function of the peel in the range of 2.5e7.5% TS (Fig. 3B, 3C, 3D). Hmax and Rmax were up to 181 mL and 5.37 mL h1, respectively for banana peel 7.5% (Fig. 3B, 3C). The highest CH4 yield was 188 L kg1 TSadded obtained from a banana peel, TS 7.5% (Fig. 3D). The data of Hmax, Rmax, l, and yield indicated that banana peel had a high potential for produce CH4.
Methane production from pure and waste glycerol CH4 content produced from pure and waste glycerol was 45e55% and 30e44% (Table 4). The highest CH4 production potential (Hmax) and rate (Rmax) apparently increases as increased concentrations of pure glycerol in contrast to waste glycerol (Fig. 4A and 4B). CH4 yield (Fig. 4C) dropped significantly when the glycerol concentration exceeded 7.5 g L1. At the end of the fermentation process, pH and alkalinity were decreased to less than 4.5 and 1500 mg CaCO3 L1, respectively. The accumulation of VFAs and ethanol, increased markedly in the waste glycerol (Fig. 5), caused the inhibition of methanogenic activity, and consequently dropped the CH4
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▪
Fig. 4 e CH4 production potential (A), rate (B) and yield (C) from the fermentation pure (,) and waste glycerol ( ) after 48 h fermentation. Histograms represent average value (n ¼ 2) and I-bars represent standard deviation.
production corresponding to the previous study [19,20]. At 7.5 g L1 glycerol, pure and waste glycerol gave CH4 yield 83.39% and 25.53% of the theoretical yield, respectively. These results suggest that the initial concentration of glycerol and glycerol purity incluenced the potential of bacteria to convert glycerol to CH4.
Methane production from co-digestion of banana peel and pure/waste glycerol Table 5 summarised the kinetics parameters of the co-disgestion of banana peel and pure/waste glycerol. The addition
of biocarbonate appeared to improve CH4 content, CH4 production potential (Hmax), CH4 yield (Y) and lag phase time. The trend of CH4 production potential and rate versus initial concentration of pure and waste glycerol were similar to that of the CH4 yield (Fig. 6B, 6C, 6D). The highest CH4 production potential and rate obviously decreased when concentrations of the glycerol pure and glycerol waste were increased (Fig. 6B, 6C): 7.5 g L1 Pure glycerol (with biocarbonate addition) showed Hmax 467 mL, and Rmax 8.04 mL h1, respectively, while those from waste glycerol were 652 mL and 17.47 mL h1, respectively. pH and alkalinity dropped significantly due to the glycerol concentration exceeding 7.5 g L1
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Fig. 5 e Concentrations of VFAs and ethanol in the fermentation of waste glycerol after 48 h fermentation. Histograms represent average value (n ¼ 2) and I-bars represent standard deviation.
Table 5 e Kinetics parameters of CH4 production from co-digestion of banana peel and pure glycerol (PG) /waste glycerol (WG). Type of Glycerol PG PG PG PG WG WG WG WG
[Glycerol] (g L1)
[BP] (%TS)
[NaHCO3]
CH4 (%)
Hmax (mL)
Rmax (mL h1)
Y (mL g1 CODT removal)
l (h)
CODT removal (%)
Alkalinity (mg CaCO3 L1)
pHf
7.5 7.5 90 90 7.5 7.5 90 90
7.5 7.5 7.5 7.5 7.5 7.5 7.5 7.5
e 0.1 M e 0.1 M e 0.1 M e 0.1 M
59 62 30 38 49 65 7 5
242 467 290 325 519 652 27.1 28.3
167 8.0 16 5.7 28 17.5 20.2 20.9
117 151 19 31 198 281 16 2
22.0 2.37 11.3 6.81 13.2 1.57 26.4 7.24
19.4 30.8 37.6 28.1 20.0 20.2 3.92 35.3
276 1691 181 1112 732 1853 675 722
5.02 7.27 4.66 5.23 5.55 7.62 5.99 6.19
Fig. 6 e Lag phase time (A) CH4 production potential (B), rate (C) and yield (D) in the fermentation of 7.5% banana peel mixed with varying concentrations of pure (,) and waste glycerol ( ) under conditions with buffer (B) and without biocarbonate addition (NB) after 120 h fermentation. Histograms represent average value (n ¼ 2) and I-bars represent standard deviation.
▪
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Fig. 7 e pH (A) and alkalinity (B) in the fermentation of 7.5% banana peel mixed with varying concentration of pure (,) and waste glycerol ( ) under conditions with buffer (B) and without buffer (NB) after 120 h fermentation. Histograms represent average value (n ¼ 2) and I-bars represent standard deviation.
Fig. 8 e VFAs concentrations in the fermentation of 7.5% banana peel mixed with varying concentration of pure (,) and waste glycerol ( ) under conditions with buffer (B) and without buffer (NB) after 120 h fermentation. Histograms represent average value (n ¼ 2) and I-bars represent standard deviation.
▪
▪
(Fig. 7A and B) which was unsuitable for the CH4 production [18]. The accumulation of VFAs and ethanol in the fermentation of pure and waste glycerol slightly increased when concentration of glycerol exceeded 7.5 g L1 (Fig. 8A, 8B). This results are in accordnce with the previous studies [19, 20] reported ethanol increased significantly when pH were in the range of 4.5e6.5. At this pH range, the dominant metabolic product was 1,3 propanediol, which suppressed the activity of methanogenic bacteria and the CH4 production.
The CH4 yield dropped significantly after the glycerol concentration exceeded 7.5 g L1. At 7.5 g L1 pure/waste glycerol mixed with banana peel (with bicarbonate addition), the highest CH4 yield was 150 mL g1 CODT removal and 280 mL g1 CODT removal (Fig. 4D), which are 1.3 and 1.4 times higher than those without the biocarbonate addition. Previous studies [11e13, 21], reported that when the glycerol concentration exceeded 5%, the CH4 production was likely to decline significantly. The results suggested that the efficiency of CH4
Table 6 e Comparative CH4 production from co-digestion of banana peel and waste glycerol compared to previous studies. Co-Substrate waste water derived from biodiesel manufacturing cattle manure hog manure hog manure hog manure banana peel banana peel
[Waste Glycerol] [Pure Glycerol] % CH4 Hmax (mL) Rmax (mL d1)
Yield Reference (m3 kg1 CODremoval)
1.00% (w/v)
e
e
332
e
0.310
[22]
4.00% (v/v) e 1.00% (w/v) e 0.75% (w/v) e
e 1.00% (w/v) e 2.00% (w/v) e 0.75% (w/v)
58 63 67 66 65 62
e e e e 652 467
e e e e 192 420
0.365 0.370 0.360 0.390 0.281 0.151
[23] [24] [24] [24] This study This study
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Table 7 e Economic value analysis. CH4 (Substrate)
Cost (Baht L1)
NPV (10 years)
PBP (year)
LPG (kg yr1)
Electricity (kWh yr1)
0 408 0 7 408 0 7
229.94 0.03 173.83 172.09 225.47 480.43 478.70
<1 >10 <1 1.01 1.16 <1 1.00
31.57 55.67 17.06 17.06 25.32 47.15 47.15
84.6 149.28 45.72 45.72 67.92 126.36 126.36
BP PG WG WG BP þ PG BP þ WG BP þ WG
production can be better improved when waste glycerol was co-digested with banana peel than using single substrate. The comparative kinetics parameters from the co-digestion of glycerol waste with various materais were summarised in Table 6.
Mongkut's University of Technology North Bangkok (KMUTNB-61-KNOW-042), Research and Researchers for Industries Program (PHD60I0028), and Joint Graduate School of Energy and Environment, King Mongkut’s University of Technology Thonburi (JGSEE739) for the financial support.
Economic values Estimating economic value for this study (Table 7) shows that CH4 production from banana peels, waste glycerol, and banana peel mixed with waste or pure glycerol as substrates are worth investment (NPV> 0) due to the payback period (PBP) which is estimated at less than 1.2 years. Conversely, the economic value of producing CH4 from pure glycerol found that the project is unlikely to attract investment because less CH4 can be produced when compared to the cost of high purity glycerol that shows a payback period of over 10 years. The codigestion of waste glycerol and banana peel to produce CH4 is the best condition to carry an investment due to the high yields and short payback time when compared to other substrates.
Conclusions This study successfully demonstrated the CH4 production from co-digestion of pure glycerol/waste glycerol and the banana peel. The highest CH4 yield was obtained from banana peel 7.5% TS mixed with waste glycerol 7.5 g L1 (in the presence of buffer), which yielded 281 mL g1 CODT removal slightly higher than for the banana peel 7.5% mixed with pure glycerol 7.5 g L1 (in the presence of buffer; 151 mL g1 CODT removal) and only banana peel (188 mL g1 CODT removal). Unlike pure glycerol, waste glycerol favors for the is higher than when the pure glycerol was used due to it being rich in vitamins and other nutrients such as methanol, soluble COD, which easily biodegradable used by methanogenic bacteria, resulting in higher metabolic activity. Our results suggest that glycerol waste is a by-product of biodiesel production could be used as a co-substrate in a fermentation without removing impurities before being used. In particular, the concentration of glycerol exceeded 0.75% (w/v) was important impacts of methanogenic bacteria on the utilization of the substrate to produce gas.
Acknowledgements The authors would like to express gratitude to Thailand Research Fund (DBG6280001), The Royal Society, King
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