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© IWA Publishing 2014 Water Science & Technology
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Determination of methane yield of cellulose using different experimental setups Bing Wang, Ivo Achu Nges, Mihaela Nistor and Jing Liu
ABSTRACT In this work, biochemical methane potential (BMP) tests with cellulose as a model substrate were performed with the aid of three manually operated or conventional experimental setups (based on manometer, water column and gas bag) and one automated apparatus specially designed for analysis of BMP. The methane yields were 340 ± 18, 354 ± 13, 345 ± 15 and 366 ± 5 ml CH4/g VS obtained from experimental setups with manometer, water column, gas bag, and automatic methane potential test system, which corresponded to a biodegradability of 82, 85, 83 and 88% respectively. The results demonstrated that the methane yields of cellulose obtained from conventional and automatic experimental setups were comparable; however, the methane yield obtained from the automated apparatus showed greater precision. Moreover, conventional setups
Bing Wang (corresponding author) Ivo Achu Nges Jing Liu Department of Biotechnology, Lund University, Getingevägen 60, SE-221 00 Lund, Sweden E-mail: [email protected] Mihaela Nistor Jing Liu Bioprocess Control Sweden AB, Scheelevägen 22, SE-223 63 Lund, Sweden
for the BMP test were more time- and labour-intensive compared with the automated apparatus. Key words
| anaerobic digestion, automatic methane potential test system, biochemical methane potential, biodegradability, standardization
INTRODUCTION Anaerobic digestion (AD) is a complex process by which microorganisms break down biodegradable materials such as the organic fraction of solid wastes, agriculture residues, wastewaters, etc. in the absence of oxygen. AD is a complex microbiological degradation process that can be described by four key steps, i.e., hydrolysis, acidogenesis, acetogenesis and methanogenesis. In the final step, the methanogens utilize and convert the intermediate products of the preceding steps into biogas (CH4 and CO2) and trace gases such as ammonia, hydrogen sulphide, etc. Anaerobic biodegradability (BD) of a material is defined as the fraction that can be converted to biogas under anaerobic conditions (Guwy ). Understanding the BD of waste materials has a significant impact on the selection of substrates to digest, the optimal process design and biogas plant operation. A conventional biochemical methane potential (BMP) test provides a preliminary indication of the BD of a substrate and of its potential to produce methane via AD. Standards for anaerobic BD tests are extensive, such as ISO standards (ISO- ) or ASTM standards (D- ; E- ). There are also many alternative methods for the BD test (Angelidaki doi: 10.2166/wst.2014.275
et al. ; Esposito et al. ). However, to date, there is still no standard protocol for performing a BMP test. In fact, different methods and instrumentations are routinely used for BMP assays in different laboratories, which has led to a large disparity in the methane yields for similar substrates reported in the literature (Kreuger et al. ; Raposo et al. ). Standardization, in terms of analytical procedure and experimental setup requirements will help to ensure that the results from different laboratories are comparable as well as to allow information sharing in the biogas sector. Various ISO or published protocols intend to solve the standardization of analytical procedure; however, standardization of the requirements for experimental setup has not been studied extensively. In this study, methane yields of cellulose were determined by three conventional manually operated experimental setups, i.e., a pressure-based gas measuring system aided by manometer, a water-column-based measuring system, a gasbag-based measuring system and one automatically operated experimental platform, i.e., the second generation automatic methane potential test system (AMPTS II) (Bioprocess Control, Sweden).
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Methane yield determination using different experimental setups
MATERIALS AND METHODS Inoculum and substrate The anaerobic inoculum used to carry out the BMP test was collected from a mesophilic biogas plant (Eslöv, Ellinge, Sweden), which receives municipal wastewater (20%) and vegetable residue from the food industry (80%). It was incubated at 37 C for 5 days to decrease the background gas production (ISO- ). Prior to the start of the experiment, the inoculum was characterized by a volatile solids (VS) content of 1.7% (w/w) and pH of 7.7. Microcrystalline cellulose (Sigma-Aldrich) was used as standard substrate for the BMP test, which had a VS content of 97.2% (w/w). W
Experimental setups Pressure-based gas measuring system aided by manometer Systems based on the measurement of pressure (Figure 1) aided by manometer are among the most conventional experimental setups for biogas determination reported in the literature (Ferrer et al. ; Hosseini Koupaie et al. ). The pressure built up by the produced biogas in the headspace of the reactor was measured by a manometer and the methane content was determined by gas chromatography (GC).
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Figure 1
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Schematic diagram of pressure-based gas measuring system.
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at the top and filled up with distilled water to a desired marked level (Figure 2). The methane produced passed as bubbles through the water layer and pushed the water level down in the column.
Gas-bag-based measuring system Gas bags are commonly used for collecting gas produced during AD processes at laboratory scale (Mshandete et al. ; Zhu et al. ). The gas bags used in this study were made of a gas-tight and impermeable material (FlextrusTransofoil® EL-OPET/PE, Sweden) (Figure 3).
Automatic methane potential test system AMPTS II is the second generation automatic methane potential test system specially designed for BMP analysis. AMPTS II was developed with extensive BMP evaluation in research groups, and it has been cited by an increasing number of publications in recent years (Badshah et al. ; Shen et al. ). The methane volume measuring device (Figure 4) of the AMPTS II embedded data acquisition system with multi-flow cells works according to the principle of liquid displacement and buoyancy.
Water-column-based measuring system This system is based on the water displacement principle (Mallik et al. ; Okeh et al. ). Gradated transparent glass columns (3.5 cm diameter) were inserted into one tank filled with distilled water. These columns were closed
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Figure 2
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Schematic diagram of water-column-based measuring system.
Figure 3
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Schematic diagram of gas-bag-based measuring system.
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Figure 4
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Methane yield determination using different experimental setups
Schematic diagram of automatic methane potential test system II.
Performance of BMP test The experiments for the determination of the BMP of cellulose were performed in all four systems in triplicate in 500 ml standard bottles (Schott, Germany) with a total working volume of 590 ml. Inoculum and cellulose were added at a ratio of 2 based on VS. Background methane production of inoculum only was evaluated in blank bottles. For the pressure-based gas measurement system, a small amounts of inoculum and cellulose (177 g in total) were added to ensure a large headspace (i.e. 70%) to prevent high pressure built up by the biogas produced in each bottle. However, for water-column- and gas-bag-based systems and AMPTS II, the production unit was separated from the gas collection unit; therefore, 400 g of inoculum and substrate were added to each bottle since there was no need for a large headspace. Although CO2 removal is not often reported in the literature, 3 M NaOH was included in order to enable fair comparison among different experimental setups, except for the pressure-based measuring system in the current study. Each bottle was equipped with a mechanical agitator, which runs in a continuous mode to avoid mass transfer limitations. Pure nitrogen was used to flush the headspace of each bottle for one minute to create anaerobic conditions. All the bottles were incubated in the thermostatic water bath at 37 C. The AD processes were terminated after 20 days, when the gas production was lower than 2 ml/bottle per day.
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every 2 to 3 days. The pressure was measured using a manometer (AZ® Instrument Corp., RS232-82100) and the gas composition was measured by GC (Agilent 6890N, TCD) after sampling at each pressure measurement. During the AD process, gas sampling and pressure measurement were carried out 13 times, and each gas sample was analyzed for ca. 5.5 min by GC. For the water-column-based measuring system, the height difference of the water level in the glass column was measured every day for methane yield calculation. The accumulated methane volume in the gas bag was measured with a gradated 100 ml gas-tight glass syringe (Fortuna, Germany). For these three manually operated systems, room temperature and pressure were recorded manually at each gas volume measurement for gas volume normalization. For AMPTS II, the methane produced in each bottle was passed to the volume-measuring device and then stored under each flow cell; ca. 10 ml of methane can lead to the flow cell opening because of buoyancy. Real-time temperature, pressure and gas volume were recorded automatically at each flow cell opening. At the end of the process, a report was generated, presenting the normalized gas flow rate and accumulated methane volume.
DATA PRESENTATION All methane yields were expressed as ml CH4 at standard temperature and pressure (STP: 273.15 K, 101.325 kPa) conditions per gram of organic substrate added (ml CH4/g VSadded) (Hansen et al. ; Raposo et al. ). The gas production from the inoculum only was subtracted from the gas production of the test bottle prior to the determination of the methane yields. Gas volume normalization
W
The gas volume normalization was based on the ideal gas law
Analytical method PV ¼ nRT The total solids (TS) and VS were determined according to standard protocols (APHA ). The pH of the inoculum was measured using a TitraLabTM 80 titrator (Radiometer, Copenhagen, Denmark). Daily pressure measurement was necessary during the first week of digestion because of the high biogas production and consequent high pressure inside the bottle. Subsequently, it was sufficient just to measure the pressure
(1)
where P is the pressure of the gas, V is the volume of the gas, n is the number of moles of gas, T is the temperature of the gas and R [0.08206 atm L/(mol K)] is the ideal gas constant. For the experimental setups with the gas bag and water column, the actual room temperature (Tr) and atmospheric pressure (Pr) were recorded at the same time as the gas volume (V ) was measured; these values were used for gas
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volume normalization under standard temperature (T0) and pressure (P0) according to Equation (2)
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according to the following formula BDð%Þ ¼ (BMPexp =BMP0 ) 100
V T0 Pr ¼ Tr P0
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(5)
(2) Statistical analysis
For experimental setup with the pressure-measuring device, the gas was accumulated in the headspace (Vh), so that the gas temperature (T ) was the same as the incubation temperature (37 C or 310.15 K). The methane content (CH4%) of the produced gas was measured by GC. The difference (ΔP) between the atmospheric pressure and the pressure built up by the gas accumulated in the bottle was measured by manometer. According to Equations (1) and (2), VSTP was calculated as follows: W
Grubb’s test was performed to check for outliers in the BMP test. The statistical difference in methane yields was evaluated by analysis of variance (single-factor ANOVA, P 0.05) in Excel (Microsoft Excel, 2010).
RESULTS AND DISCUSSION Methane production
VSTP
ΔP Vh T0 CH4 % ¼ P0 T
(3)
Theoretical methane potential (BMP0) Theoretical methane potential is used to predict and act as the reference for the methane production of a specific substrate with defined chemical composition. It is frequently expressed as ml CH4 at STP conditions per amount of organic material added (based on VS or chemical oxygen demand (COD)), although it can also be expressed per organic material removed. In this study, the selected units used for expressing the methane potential were ml CH4/g VSadded. For compounds containing carbon, hydrogen and oxygen (CnHaOb), the general reaction has been shown to follow Buswell’s equation (Symons & Buswell ): a b H2 O Cn Ha Ob þ n 4 2 n a b n a b ! þ CO2 þ þ CH4 2 8 4 2 8 4
Table 1 presents the methane yields and BD of cellulose obtained from different experimental setups. The methane yields of cellulose achieved in the present study from all the experimental setups were in the same range as that (350 ± 29 ml CH4/g VS) reported by (Kreuger et al. ). The BD of cellulose obtained in this work are well in agreement with theoretical data, considering that during AD about 10% of the substrate is used for both biomass growth (ca. 5%) and transformed into heat (ca. 5%) (Gerardi ). The methane yield achieved in the AMPTS II process showed a low standard deviation that demonstrates higher precision and lower random error for the automated apparatus. It is worth mentioning that AMPTS II performs realtime temperature and pressure compensation for gas flow and volume normalization in order to minimize any impacts from geographical and climatic differences, as well as changes of weather conditions, on gas flow and volume measurement. This allows the gas flow and volume to be converted at the standard condition, i.e. 0 C, 1 atm, and to free moisture that allows data comparison across time and regions. W
(4)
Assuming the total stoichiometric conversion of the organic matter to methane and carbon dioxide using Buswell’s equation, the BMP0 of cellulose (C6H10O5)n is 415 ml CH4/g VS at STP. Biodegradability The extent of anaerobic BD was calculated by experimental methane yield (BMPexp) in comparison with the BMP0
Table 1
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Methane production and BD of cellulose determined by different experimental setups
Experimental setups
Methane yield (ml CH4/g VS)
BD (%)
Manometer
340 ± 18
82
Water column
354 ± 13
85
Gas bag
345 ± 15
83
AMPTS II
366 ± 5
88
Methane yield from triplicate experiments presented by mean methane yield ± standard deviation (SD).
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Table 2
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Methane yield determination using different experimental setups
Workload of BMP test by different experimental setups
Experimental setups
Workload 1 (min)
Manometer
30–35
Workload 2 (min)
Workload 3 (min)
Total workload (min/sample)
35 × 13
80
540
Water column
(6–8) × 20
60
220
Gas bag
(6–8) × 20
60
220
AMPTS II
0
5
40
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incubation period, results are processed and a report is generated into a standard sheet format. This in turn significantly reduces the time and labour demand for carrying out the analysis and data calculations compared with the manual experimental setups for BD tests. Potential errors
The methane yield obtained from the pressure-based gas measuring system was lower compared with the methane yields obtained from other setups, but without significant difference. This is probably due to the high pressure built up by the biogas produced, which leads to an increase of gas components in the liquid phase. Furthermore, low-level gas emission and diffusion through tubes can occur under high pressure, which could also lead to the observed low methane yield for the pressure method (Hansen et al. ).
Determination of methane or biogas production using conventional setups has a high risk of inducing systematic errors, since these setups are manually operated and human errors are inevitable. For instance, by using the pressure-measuring system, potential errors might be induced when taking samples for gas component measurement, since the injected gas volume would affect the gas composition. Apart from low measurement resolution, errors may also be introduced when determining the height difference of water levels in columns or measuring the methane production in gas bags using a syringe. Human errors can be largely minimized using automated apparatus, which has been clearly demonstrated in the current study.
Properties of the different experimental setups
Advantages and drawbacks
Workload
The advantages and drawbacks of each experimental setup used in this study are briefly outlined in Table 3.
Workload 1: time for inoculum and substrate addition; workload 2: time for experimental follow-up including gas volume and composition analysis during the whole incubation; workload 3: data management and interpretation. Total workload: workload 1þ workload 2þ workload 3.
Table 2 presents the time requirement for performing the BMP test using the different experimental setups. It serves as a reference indication only for evaluating workload. The workload in the present study was evaluated in terms of time spent in performing a particular operation. It entails inoculum and substrate addition, gas volume measurement, gas content determination, data management and interpretation. With the AMPTS II, both the gas volume measurements and data logging are fully automatic during the long Table 3
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CONCLUSIONS Methane yields of cellulose determined by all experimental setups are comparable in this study. The methane yields did not differ significantly; however, the automated apparatus demonstrates higher analytical precision as evidenced by low standard deviation. Manually operated or conventional
Advantages and drawbacks of different experimental setups
Experimental setups
Advantagesþ
Drawbacks
Conventional systems: manometer, water column, gas bag
þ Simple þ Inexpensive
– Manual operation and error induction – Requirement for good air-tightness and expensive equipment, i.e. GC (manometer) – Time-consuming and labour-intensive
Automatic system: AMPTS II
þ Time- and labour-saving þ Automatic data acquisition þ Generates report with high quality data þ Real-time temperature and pressure compensation
– Limitation for measuring biogas volume
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experimental setups were time- and labour-intensive compared with the automated apparatus. In sum, the standardization of experimental setup for the BMP test is essential to ensure the results from different laboratories are comparable. It is inevitable that this will lead to an automated system that can minimize human error and workload demand, improve analytical precision and accuracy, as well as standardize data sampling, calculation and presentation, etc.
ACKNOWLEDGEMENTS The China Scholarship Council (CSC, File No. 2011704033), Swedish International Development Agency (SIDA) and Swedish Energy Agency (STEM) are gratefully acknowledged for their financial support. The authors are also grateful to guest researcher, Dr Shaochuan Shen, at the Department of Biotechnology in Lund University for his assistance.
REFERENCES Angelidaki, I., Alves, M. M., Bolzonella, D., Borzacconi, L., Campos, J. L., Guwy, A. J., Kalyuzhnyi, S., Jenicek, P. & Lier, J. B. v. Defining the biomethane potential (BMP) of solid organic wastes and energy crops: a proposed protocol for batch assays. Water Science and Technology 59 (5), 927– 934. APHA Standard Methods for the Examination of Water and Wastewater. 19th edn, American Public Health Association/ American Water Works Association/Environment Federation, Washington, DC, USA. Badshah, M., Lam, D. M., Liu, J. & Mattiasson, B. Use of an Automatic Methane Potential Test System for evaluating the biomethane potential of sugarcane bagasse after different treatments. Bioresource Technology 114, 262–269. D5210-92 Standard Test Method for Determining the Anaerobic Biodegradation of Plastic Materials in the Presence of Municipal Sewage Sludge. ASTM, West Conshohocken, Pennsylvania, USA. E2710-01 Standard Test Method for Determining Anaerobic Biodegradation Potential of Organic Chemicals Under Methanogenic Conditions. ASTM, West Conshohocken, Pennsylvania, USA. Esposito, G., Frunzo, L., Liotta, F., Panico, A. & Pirozzi, F. Bio-methane potential tests to measure the biogas production from the digestion and co-digestion of complex organic substrates. The Open Environmental Engineering Journal 5, 1–8.
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Ferrer, I., Ponsá, S., Vázquez, F. & Font, X. Increasing biogas production by thermal (70 C) sludge pre-treatment prior to thermophilic anaerobic digestion. Biochemical Engineering Journal 42 (2), 186–192. Gerardi, M. H. The Microbiology of Anaerobic Digesters. Wiley.com. Guwy, A. J. Equipment used for testing anaerobic biodegradability and activity. Re/Views in Environmental Science & Bio/Technology 3 (2), 131–139. Hansen, T. L., Schmidt, J. E., Angelidaki, I., Marca, E., Jansen, J. I. C., Mosbæk, H. & Christensen, T. H. Method for determination of methane potentials of solid organic waste. Waste Management 24 (4), 393–400. Hosseini Koupaie, E., Barrantes Leiva, M., Eskicioglu, C. & Dutil, C. Mesophilic batch anaerobic co-digestion of fruit-juice industrial waste and municipal waste sludge: process and cost-benefit analysis. Bioresource Technology 152, 66–73. ISO-11734 Water Quality–Evaluation of the ‘Ultimate’ Anaerobic Biodegradability of Organic Compounds in Digested Sludge–Method by Measurement of the Biogas Production. In: 1st edn, International Organization for Standardization, Geneva, Switzerland. Kreuger, E., Sipos, B., Zacchi, G., Svensson, S.-E. & Björnsson, L. Bioconversion of industrial hemp to ethanol and methane: The benefits of steam pretreatment and coproduction. Bioresource Technology 102 (3), 3457–3465. Mallik, M. K., Singh, U. K. & Ahmad, N. Batch digester studies on biogas production from Cannabis sativa, water hyacinth and crop wastes mixed with dung and poultry litter. Biological Wastes 31 (4), 315–319. Mshandete, A., Björnsson, L., Kivaisi, A. K., Rubindamayugi, M. S. T. & Mattiasson, B. Effect of particle size on biogas yield from sisal fibre waste. Renewable Energy 31 (14), 2385–2392. Okeh, O. C., Onwosi, C. O. & Odibo, F. J. C. Biogas production from rice husks generated from various rice mills in Ebonyi State, Nigeria. Renewable Energy 62, 204–208. Raposo, F., Fernández-Cegrí, V., De la Rubia, M. A., Borja, R., Béline, F., Cavinato, C., Demirer, G., Fernández, B., Fernández-Polanco, M., Frigon, J. C., Ganesh, R., Kaparaju, P., Koubova, J., Méndez, R., Menin, G., Peene, A., Scherer, P., Torrijos, M., Uellendahl, H., Wierinck, I. & de Wilde, V. Biochemical methane potential (BMP) of solid organic substrates: evaluation of anaerobic biodegradability using data from an international interlaboratory study. Journal of Chemical Technology & Biotechnology 86 (8), 1088–1098. Shen, S., Nges, I. A., Yun, J. & Liu, J. Pre-treatments for enhanced biochemical methane potential of bamboo waste. Chemical Engineering Journal 240, 253–259. Symons, G. E. & Buswell, A. M. The methane fermentation of carbohydrates 1,2. Journal of the American Chemical Society 55 (5), 2028–2036. Zhu, J., Zheng, Y., Xu, F. & Li, Y. Solid-state anaerobic codigestion of hay and soybean processing waste for biogas production. Bioresource Technology 154, 240–247. W
First received 9 April 2014; accepted in revised form 29 May 2014. Available online 11 June 2014
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© IWA Publishing 2014 Water Science & Technology
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Determination of methane yield of cellulose using different experimental setups Bing Wang, Ivo Achu Nges, Mihaela Nistor and Jing Liu
ABSTRACT In this work, biochemical methane potential (BMP) tests with cellulose as a model substrate were performed with the aid of three manually operated or conventional experimental setups (based on manometer, water column and gas bag) and one automated apparatus specially designed for analysis of BMP. The methane yields were 340 ± 18, 354 ± 13, 345 ± 15 and 366 ± 5 ml CH4/g VS obtained from experimental setups with manometer, water column, gas bag, and automatic methane potential test system, which corresponded to a biodegradability of 82, 85, 83 and 88% respectively. The results demonstrated that the methane yields of cellulose obtained from conventional and automatic experimental setups were comparable; however, the methane yield obtained from the automated apparatus showed greater precision. Moreover, conventional setups
Bing Wang (corresponding author) Ivo Achu Nges Jing Liu Department of Biotechnology, Lund University, Getingevägen 60, SE-221 00 Lund, Sweden E-mail: [email protected] Mihaela Nistor Jing Liu Bioprocess Control Sweden AB, Scheelevägen 22, SE-223 63 Lund, Sweden
for the BMP test were more time- and labour-intensive compared with the automated apparatus. Key words
| anaerobic digestion, automatic methane potential test system, biochemical methane potential, biodegradability, standardization
INTRODUCTION Anaerobic digestion (AD) is a complex process by which microorganisms break down biodegradable materials such as the organic fraction of solid wastes, agriculture residues, wastewaters, etc. in the absence of oxygen. AD is a complex microbiological degradation process that can be described by four key steps, i.e., hydrolysis, acidogenesis, acetogenesis and methanogenesis. In the final step, the methanogens utilize and convert the intermediate products of the preceding steps into biogas (CH4 and CO2) and trace gases such as ammonia, hydrogen sulphide, etc. Anaerobic biodegradability (BD) of a material is defined as the fraction that can be converted to biogas under anaerobic conditions (Guwy ). Understanding the BD of waste materials has a significant impact on the selection of substrates to digest, the optimal process design and biogas plant operation. A conventional biochemical methane potential (BMP) test provides a preliminary indication of the BD of a substrate and of its potential to produce methane via AD. Standards for anaerobic BD tests are extensive, such as ISO standards (ISO- ) or ASTM standards (D- ; E- ). There are also many alternative methods for the BD test (Angelidaki doi: 10.2166/wst.2014.275
et al. ; Esposito et al. ). However, to date, there is still no standard protocol for performing a BMP test. In fact, different methods and instrumentations are routinely used for BMP assays in different laboratories, which has led to a large disparity in the methane yields for similar substrates reported in the literature (Kreuger et al. ; Raposo et al. ). Standardization, in terms of analytical procedure and experimental setup requirements will help to ensure that the results from different laboratories are comparable as well as to allow information sharing in the biogas sector. Various ISO or published protocols intend to solve the standardization of analytical procedure; however, standardization of the requirements for experimental setup has not been studied extensively. In this study, methane yields of cellulose were determined by three conventional manually operated experimental setups, i.e., a pressure-based gas measuring system aided by manometer, a water-column-based measuring system, a gasbag-based measuring system and one automatically operated experimental platform, i.e., the second generation automatic methane potential test system (AMPTS II) (Bioprocess Control, Sweden).
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Methane yield determination using different experimental setups
MATERIALS AND METHODS Inoculum and substrate The anaerobic inoculum used to carry out the BMP test was collected from a mesophilic biogas plant (Eslöv, Ellinge, Sweden), which receives municipal wastewater (20%) and vegetable residue from the food industry (80%). It was incubated at 37 C for 5 days to decrease the background gas production (ISO- ). Prior to the start of the experiment, the inoculum was characterized by a volatile solids (VS) content of 1.7% (w/w) and pH of 7.7. Microcrystalline cellulose (Sigma-Aldrich) was used as standard substrate for the BMP test, which had a VS content of 97.2% (w/w). W
Experimental setups Pressure-based gas measuring system aided by manometer Systems based on the measurement of pressure (Figure 1) aided by manometer are among the most conventional experimental setups for biogas determination reported in the literature (Ferrer et al. ; Hosseini Koupaie et al. ). The pressure built up by the produced biogas in the headspace of the reactor was measured by a manometer and the methane content was determined by gas chromatography (GC).
Water Science & Technology
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Figure 1
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Schematic diagram of pressure-based gas measuring system.
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2014
at the top and filled up with distilled water to a desired marked level (Figure 2). The methane produced passed as bubbles through the water layer and pushed the water level down in the column.
Gas-bag-based measuring system Gas bags are commonly used for collecting gas produced during AD processes at laboratory scale (Mshandete et al. ; Zhu et al. ). The gas bags used in this study were made of a gas-tight and impermeable material (FlextrusTransofoil® EL-OPET/PE, Sweden) (Figure 3).
Automatic methane potential test system AMPTS II is the second generation automatic methane potential test system specially designed for BMP analysis. AMPTS II was developed with extensive BMP evaluation in research groups, and it has been cited by an increasing number of publications in recent years (Badshah et al. ; Shen et al. ). The methane volume measuring device (Figure 4) of the AMPTS II embedded data acquisition system with multi-flow cells works according to the principle of liquid displacement and buoyancy.
Water-column-based measuring system This system is based on the water displacement principle (Mallik et al. ; Okeh et al. ). Gradated transparent glass columns (3.5 cm diameter) were inserted into one tank filled with distilled water. These columns were closed
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Figure 2
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Schematic diagram of water-column-based measuring system.
Figure 3
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Schematic diagram of gas-bag-based measuring system.
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Figure 4
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Methane yield determination using different experimental setups
Schematic diagram of automatic methane potential test system II.
Performance of BMP test The experiments for the determination of the BMP of cellulose were performed in all four systems in triplicate in 500 ml standard bottles (Schott, Germany) with a total working volume of 590 ml. Inoculum and cellulose were added at a ratio of 2 based on VS. Background methane production of inoculum only was evaluated in blank bottles. For the pressure-based gas measurement system, a small amounts of inoculum and cellulose (177 g in total) were added to ensure a large headspace (i.e. 70%) to prevent high pressure built up by the biogas produced in each bottle. However, for water-column- and gas-bag-based systems and AMPTS II, the production unit was separated from the gas collection unit; therefore, 400 g of inoculum and substrate were added to each bottle since there was no need for a large headspace. Although CO2 removal is not often reported in the literature, 3 M NaOH was included in order to enable fair comparison among different experimental setups, except for the pressure-based measuring system in the current study. Each bottle was equipped with a mechanical agitator, which runs in a continuous mode to avoid mass transfer limitations. Pure nitrogen was used to flush the headspace of each bottle for one minute to create anaerobic conditions. All the bottles were incubated in the thermostatic water bath at 37 C. The AD processes were terminated after 20 days, when the gas production was lower than 2 ml/bottle per day.
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every 2 to 3 days. The pressure was measured using a manometer (AZ® Instrument Corp., RS232-82100) and the gas composition was measured by GC (Agilent 6890N, TCD) after sampling at each pressure measurement. During the AD process, gas sampling and pressure measurement were carried out 13 times, and each gas sample was analyzed for ca. 5.5 min by GC. For the water-column-based measuring system, the height difference of the water level in the glass column was measured every day for methane yield calculation. The accumulated methane volume in the gas bag was measured with a gradated 100 ml gas-tight glass syringe (Fortuna, Germany). For these three manually operated systems, room temperature and pressure were recorded manually at each gas volume measurement for gas volume normalization. For AMPTS II, the methane produced in each bottle was passed to the volume-measuring device and then stored under each flow cell; ca. 10 ml of methane can lead to the flow cell opening because of buoyancy. Real-time temperature, pressure and gas volume were recorded automatically at each flow cell opening. At the end of the process, a report was generated, presenting the normalized gas flow rate and accumulated methane volume.
DATA PRESENTATION All methane yields were expressed as ml CH4 at standard temperature and pressure (STP: 273.15 K, 101.325 kPa) conditions per gram of organic substrate added (ml CH4/g VSadded) (Hansen et al. ; Raposo et al. ). The gas production from the inoculum only was subtracted from the gas production of the test bottle prior to the determination of the methane yields. Gas volume normalization
W
The gas volume normalization was based on the ideal gas law
Analytical method PV ¼ nRT The total solids (TS) and VS were determined according to standard protocols (APHA ). The pH of the inoculum was measured using a TitraLabTM 80 titrator (Radiometer, Copenhagen, Denmark). Daily pressure measurement was necessary during the first week of digestion because of the high biogas production and consequent high pressure inside the bottle. Subsequently, it was sufficient just to measure the pressure
(1)
where P is the pressure of the gas, V is the volume of the gas, n is the number of moles of gas, T is the temperature of the gas and R [0.08206 atm L/(mol K)] is the ideal gas constant. For the experimental setups with the gas bag and water column, the actual room temperature (Tr) and atmospheric pressure (Pr) were recorded at the same time as the gas volume (V ) was measured; these values were used for gas
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Methane yield determination using different experimental setups
volume normalization under standard temperature (T0) and pressure (P0) according to Equation (2)
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according to the following formula BDð%Þ ¼ (BMPexp =BMP0 ) 100
V T0 Pr ¼ Tr P0
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(5)
(2) Statistical analysis
For experimental setup with the pressure-measuring device, the gas was accumulated in the headspace (Vh), so that the gas temperature (T ) was the same as the incubation temperature (37 C or 310.15 K). The methane content (CH4%) of the produced gas was measured by GC. The difference (ΔP) between the atmospheric pressure and the pressure built up by the gas accumulated in the bottle was measured by manometer. According to Equations (1) and (2), VSTP was calculated as follows: W
Grubb’s test was performed to check for outliers in the BMP test. The statistical difference in methane yields was evaluated by analysis of variance (single-factor ANOVA, P 0.05) in Excel (Microsoft Excel, 2010).
RESULTS AND DISCUSSION Methane production
VSTP
ΔP Vh T0 CH4 % ¼ P0 T
(3)
Theoretical methane potential (BMP0) Theoretical methane potential is used to predict and act as the reference for the methane production of a specific substrate with defined chemical composition. It is frequently expressed as ml CH4 at STP conditions per amount of organic material added (based on VS or chemical oxygen demand (COD)), although it can also be expressed per organic material removed. In this study, the selected units used for expressing the methane potential were ml CH4/g VSadded. For compounds containing carbon, hydrogen and oxygen (CnHaOb), the general reaction has been shown to follow Buswell’s equation (Symons & Buswell ): a b H2 O Cn Ha Ob þ n 4 2 n a b n a b ! þ CO2 þ þ CH4 2 8 4 2 8 4
Table 1 presents the methane yields and BD of cellulose obtained from different experimental setups. The methane yields of cellulose achieved in the present study from all the experimental setups were in the same range as that (350 ± 29 ml CH4/g VS) reported by (Kreuger et al. ). The BD of cellulose obtained in this work are well in agreement with theoretical data, considering that during AD about 10% of the substrate is used for both biomass growth (ca. 5%) and transformed into heat (ca. 5%) (Gerardi ). The methane yield achieved in the AMPTS II process showed a low standard deviation that demonstrates higher precision and lower random error for the automated apparatus. It is worth mentioning that AMPTS II performs realtime temperature and pressure compensation for gas flow and volume normalization in order to minimize any impacts from geographical and climatic differences, as well as changes of weather conditions, on gas flow and volume measurement. This allows the gas flow and volume to be converted at the standard condition, i.e. 0 C, 1 atm, and to free moisture that allows data comparison across time and regions. W
(4)
Assuming the total stoichiometric conversion of the organic matter to methane and carbon dioxide using Buswell’s equation, the BMP0 of cellulose (C6H10O5)n is 415 ml CH4/g VS at STP. Biodegradability The extent of anaerobic BD was calculated by experimental methane yield (BMPexp) in comparison with the BMP0
Table 1
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Methane production and BD of cellulose determined by different experimental setups
Experimental setups
Methane yield (ml CH4/g VS)
BD (%)
Manometer
340 ± 18
82
Water column
354 ± 13
85
Gas bag
345 ± 15
83
AMPTS II
366 ± 5
88
Methane yield from triplicate experiments presented by mean methane yield ± standard deviation (SD).
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Table 2
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Methane yield determination using different experimental setups
Workload of BMP test by different experimental setups
Experimental setups
Workload 1 (min)
Manometer
30–35
Workload 2 (min)
Workload 3 (min)
Total workload (min/sample)
35 × 13
80
540
Water column
(6–8) × 20
60
220
Gas bag
(6–8) × 20
60
220
AMPTS II
0
5
40
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incubation period, results are processed and a report is generated into a standard sheet format. This in turn significantly reduces the time and labour demand for carrying out the analysis and data calculations compared with the manual experimental setups for BD tests. Potential errors
The methane yield obtained from the pressure-based gas measuring system was lower compared with the methane yields obtained from other setups, but without significant difference. This is probably due to the high pressure built up by the biogas produced, which leads to an increase of gas components in the liquid phase. Furthermore, low-level gas emission and diffusion through tubes can occur under high pressure, which could also lead to the observed low methane yield for the pressure method (Hansen et al. ).
Determination of methane or biogas production using conventional setups has a high risk of inducing systematic errors, since these setups are manually operated and human errors are inevitable. For instance, by using the pressure-measuring system, potential errors might be induced when taking samples for gas component measurement, since the injected gas volume would affect the gas composition. Apart from low measurement resolution, errors may also be introduced when determining the height difference of water levels in columns or measuring the methane production in gas bags using a syringe. Human errors can be largely minimized using automated apparatus, which has been clearly demonstrated in the current study.
Properties of the different experimental setups
Advantages and drawbacks
Workload
The advantages and drawbacks of each experimental setup used in this study are briefly outlined in Table 3.
Workload 1: time for inoculum and substrate addition; workload 2: time for experimental follow-up including gas volume and composition analysis during the whole incubation; workload 3: data management and interpretation. Total workload: workload 1þ workload 2þ workload 3.
Table 2 presents the time requirement for performing the BMP test using the different experimental setups. It serves as a reference indication only for evaluating workload. The workload in the present study was evaluated in terms of time spent in performing a particular operation. It entails inoculum and substrate addition, gas volume measurement, gas content determination, data management and interpretation. With the AMPTS II, both the gas volume measurements and data logging are fully automatic during the long Table 3
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CONCLUSIONS Methane yields of cellulose determined by all experimental setups are comparable in this study. The methane yields did not differ significantly; however, the automated apparatus demonstrates higher analytical precision as evidenced by low standard deviation. Manually operated or conventional
Advantages and drawbacks of different experimental setups
Experimental setups
Advantagesþ
Drawbacks
Conventional systems: manometer, water column, gas bag
þ Simple þ Inexpensive
– Manual operation and error induction – Requirement for good air-tightness and expensive equipment, i.e. GC (manometer) – Time-consuming and labour-intensive
Automatic system: AMPTS II
þ Time- and labour-saving þ Automatic data acquisition þ Generates report with high quality data þ Real-time temperature and pressure compensation
– Limitation for measuring biogas volume
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Methane yield determination using different experimental setups
experimental setups were time- and labour-intensive compared with the automated apparatus. In sum, the standardization of experimental setup for the BMP test is essential to ensure the results from different laboratories are comparable. It is inevitable that this will lead to an automated system that can minimize human error and workload demand, improve analytical precision and accuracy, as well as standardize data sampling, calculation and presentation, etc.
ACKNOWLEDGEMENTS The China Scholarship Council (CSC, File No. 2011704033), Swedish International Development Agency (SIDA) and Swedish Energy Agency (STEM) are gratefully acknowledged for their financial support. The authors are also grateful to guest researcher, Dr Shaochuan Shen, at the Department of Biotechnology in Lund University for his assistance.
REFERENCES Angelidaki, I., Alves, M. M., Bolzonella, D., Borzacconi, L., Campos, J. L., Guwy, A. J., Kalyuzhnyi, S., Jenicek, P. & Lier, J. B. v. Defining the biomethane potential (BMP) of solid organic wastes and energy crops: a proposed protocol for batch assays. Water Science and Technology 59 (5), 927– 934. APHA Standard Methods for the Examination of Water and Wastewater. 19th edn, American Public Health Association/ American Water Works Association/Environment Federation, Washington, DC, USA. Badshah, M., Lam, D. M., Liu, J. & Mattiasson, B. Use of an Automatic Methane Potential Test System for evaluating the biomethane potential of sugarcane bagasse after different treatments. Bioresource Technology 114, 262–269. D5210-92 Standard Test Method for Determining the Anaerobic Biodegradation of Plastic Materials in the Presence of Municipal Sewage Sludge. ASTM, West Conshohocken, Pennsylvania, USA. E2710-01 Standard Test Method for Determining Anaerobic Biodegradation Potential of Organic Chemicals Under Methanogenic Conditions. ASTM, West Conshohocken, Pennsylvania, USA. Esposito, G., Frunzo, L., Liotta, F., Panico, A. & Pirozzi, F. Bio-methane potential tests to measure the biogas production from the digestion and co-digestion of complex organic substrates. The Open Environmental Engineering Journal 5, 1–8.
Water Science & Technology
|
70.4
|
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Ferrer, I., Ponsá, S., Vázquez, F. & Font, X. Increasing biogas production by thermal (70 C) sludge pre-treatment prior to thermophilic anaerobic digestion. Biochemical Engineering Journal 42 (2), 186–192. Gerardi, M. H. The Microbiology of Anaerobic Digesters. Wiley.com. Guwy, A. J. Equipment used for testing anaerobic biodegradability and activity. Re/Views in Environmental Science & Bio/Technology 3 (2), 131–139. Hansen, T. L., Schmidt, J. E., Angelidaki, I., Marca, E., Jansen, J. I. C., Mosbæk, H. & Christensen, T. H. Method for determination of methane potentials of solid organic waste. Waste Management 24 (4), 393–400. Hosseini Koupaie, E., Barrantes Leiva, M., Eskicioglu, C. & Dutil, C. Mesophilic batch anaerobic co-digestion of fruit-juice industrial waste and municipal waste sludge: process and cost-benefit analysis. Bioresource Technology 152, 66–73. ISO-11734 Water Quality–Evaluation of the ‘Ultimate’ Anaerobic Biodegradability of Organic Compounds in Digested Sludge–Method by Measurement of the Biogas Production. In: 1st edn, International Organization for Standardization, Geneva, Switzerland. Kreuger, E., Sipos, B., Zacchi, G., Svensson, S.-E. & Björnsson, L. Bioconversion of industrial hemp to ethanol and methane: The benefits of steam pretreatment and coproduction. Bioresource Technology 102 (3), 3457–3465. Mallik, M. K., Singh, U. K. & Ahmad, N. Batch digester studies on biogas production from Cannabis sativa, water hyacinth and crop wastes mixed with dung and poultry litter. Biological Wastes 31 (4), 315–319. Mshandete, A., Björnsson, L., Kivaisi, A. K., Rubindamayugi, M. S. T. & Mattiasson, B. Effect of particle size on biogas yield from sisal fibre waste. Renewable Energy 31 (14), 2385–2392. Okeh, O. C., Onwosi, C. O. & Odibo, F. J. C. Biogas production from rice husks generated from various rice mills in Ebonyi State, Nigeria. Renewable Energy 62, 204–208. Raposo, F., Fernández-Cegrí, V., De la Rubia, M. A., Borja, R., Béline, F., Cavinato, C., Demirer, G., Fernández, B., Fernández-Polanco, M., Frigon, J. C., Ganesh, R., Kaparaju, P., Koubova, J., Méndez, R., Menin, G., Peene, A., Scherer, P., Torrijos, M., Uellendahl, H., Wierinck, I. & de Wilde, V. Biochemical methane potential (BMP) of solid organic substrates: evaluation of anaerobic biodegradability using data from an international interlaboratory study. Journal of Chemical Technology & Biotechnology 86 (8), 1088–1098. Shen, S., Nges, I. A., Yun, J. & Liu, J. Pre-treatments for enhanced biochemical methane potential of bamboo waste. Chemical Engineering Journal 240, 253–259. Symons, G. E. & Buswell, A. M. The methane fermentation of carbohydrates 1,2. Journal of the American Chemical Society 55 (5), 2028–2036. Zhu, J., Zheng, Y., Xu, F. & Li, Y. Solid-state anaerobic codigestion of hay and soybean processing waste for biogas production. Bioresource Technology 154, 240–247. W
First received 9 April 2014; accepted in revised form 29 May 2014. Available online 11 June 2014
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