INFLUENCE OF pH AND INOCULUM ADDITION ON
INFLUENCE OF pH AND INOCULUM ADDITION ON BIOHYDROGEN PRODUCTION FROM THE ORGANIC FRACTION OF MUNICIPAL WASTE I. PECORINI*, M. AKHLAGHI°°, F. BALDI**, E. ALBINI**, A. ROSSI°°, D. BACCHI*, A. POLETTINI°°, L. LOMBARDI°, R. POMI°° * DIEF, Department of Industrial Engineering, University of Florence, via Santa Marta 3, 50139 Florence, Italy ** PIN s.c.r.l., Servizi didattici e scientifici per l’Università di Firenze, Prato, Italy ° UNICUSANO, University Niccolò Cusano, Rome, Italy °°DICEA - Department of Civil and Environmental Engineering, University of Rome “La Sapienza”, Rome, Italy
SUMMARY: Batch fermentative assays were performed in order to evaluate hydrogen production under different pH conditions and inoculum dosages. Biochemical hydrogen potential tests were carried out with and without automatic pH control. Different substrate to inoculum ratio (or Food to Microorganism, F:M ratio, by weight) namely 1:1, 1:3 and 3:1, were tested at initial pH of 5.5 and 6.5 (no automatic pH control) and pH of 5.5, 6.5 and 7.5 (with automatic pH control). Results highlighted that the continuous pH control improved the conversion of organic waste into H2. The best process performance were attained at pH 5.5 and F:M equal to 1:1 and 1:3 (100.11 and 102.52 l H2/kgTVSOF, respectively, at standard conditions).
1. INTRODUCTION The recently renewed interest in anaerobic digestion (AD) of biodegradable residues has moved the scientific, technical and industrial communities towards further development and optimization of the process. For instance, bio-hydrogen production during the acidogenic phase of AD is nowadays regarded as a key topic by many researchers due to its potential benefits on both the energy balance and the environmental profile of the whole process. As a matter of fact, hydrogen is characterized by a high specific energy content per unit mass and no greenhouse gases emissions result from its combustion. Moreover, the combined (sequential) production of bio-H2 and bio-CH4 has been demonstrated to improve the gasification yield of organic substrates as compared to the conventional, single-stage AD process. Such potential benefits are further improved if bio-H2 is produced through the biochemical conversion of biodegradable wastes (Ghimire et al., 2015). Several substrates have been tested for hydrogen production
Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
(Ghimire et al., 2015); among them, the organic fraction of municipal waste (OFMW) appears to be a promising feedstock due to its biodegradability characteristics, as well as wide availability (De Gioannis et al., 2013, Cappai et al., 2014). The aim of the present research was to identify the optimal operating conditions in terms of pH and food to microorganisms (F:M) ratio for bio-H2 production from dark fermentation of OFMW. The bio-H2 production process was studied under mesophilic conditions (39 ± 1°C) by means of two different experimental set-up, operated under a batch hydraulic regime. The experiments allowed to assess the Biochemical Hydrogen Potential (BHP) of the investigated waste within the adopted range of operating parameters.
2. MATERIALS AND METHODS 2.1 Substrate and inoculum Source-separated OFMW from a Tuscanian city, hereinafter referred to as OF, collected by means of a door-to-door system, was manually sorted and homogenised. The Total Solids (TS) content of the homogeneous samples was then adjusted by adding tap water to a TS content of 5.1% by weight. Activated sludge (AS) collected from the aerobic unit of a municipal wastewater treatment plant was used as the inoculum. According to previous studies (Alibardi and Cossu, 2016; Cappai et al., 2014; Li and Fang, 2007), in order to harvest the hydrogen-producing bacteria and inhibit hydrogenotrophic methanogens, AS samples were heat-shocked at 105°C for 30 minutes before the start of each experiment. The characteristics of OF and AS in terms of TS, Total Volatile Solids (TVS), TOC and pH (see Table 1) were determined according to standard methods (APHA, 2006).
Table 1. Organic Fraction of Municipal Waste and inoculum characteristics. Concentrations are expressed as average values and related standard deviation. OF AS
TS (%) 5.1 ± 0.7 1.3 ± 0.2
TVS/TS (%) 92.0 ± 0.9 78.2 ± 0.7
pH 3.81 ± 0.01 7.08 ± 0.01
TOC (gC/l) 25.75 ± 3.3 -
2.2 Batch fermentation assays 2.2.1 Biochemical Hydrogen Potential (BHP) tests without automatic pH control The first experimental set-up consisted of 1-l (0.5-l working volume) fermentation reactors for BHP assays without automatic pH control. The fermentation tests were conducted in duplicate using stainless steel batch reactors tightly closed by a special cap provided with a ball valve to enable gas sampling (Pecorini et al., 2016). The vessels were placed on a magnetic stirrer and incubated in a water jacket under mesophilic conditions (T = 39.0 ± 1 °C) for 4 days. The reactors were run at two different OF/AS ratios (1:1 and 1:3 on a wet weight basis, corresponding to F:M ratios of 4.00 and 1.33 g VS(OF)/gVS(AS)). The reactors were initially fed
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
with the inoculum, the substrate, as well as a MES (2-N-Morpholino-EthaneSulfonic acid, Sigma-Aldrich) buffer solution and 2.5M HCl to set the initial pH at 5.5 and 6.5 (Alibardi and Cossu, 2015). After filling, the reactors were flushed with N2 for a few minutes to ensure anaerobic conditions. Gas production was estimated by measuring the pressure evolution in the headspace of each reactor and then converting it to a gas volume by means of the ideal gas law. Pressure was measured using a membrane pressure gauge (Model HD2304.0, Delta Ohm S.r.L., Italy). The measured values of pressure were converted into biogas volume as follows (Eq. (1)): (1) where: § Vbiogas: biogas volume at Standard Temperature and Pressure (STP, with T = 273.15 K and P
=105 Pa), (l); § Pmeasured: headspace pressure before gas sampling, (Pa); § Tr and Vr: temperature (K) and volume (l) of the reactor’s headspace; The BHP was determined as the cumulated hydrogen production divided by the initial TVS content of each batch. The hydrogen content of the biogas was measured by using a gas microchromatograph equipped with thermal conductivity detectors and Poraplot Q and Molsieve columns (3000 Micro GC, INFICON, Switzerland). 2.2.2 Biochemical Hydrogen Potential (BHP) tests with automatic pH control The second set of batch tests was carried out at F:M ratios of 1:3, 1:1 and 3:1, corresponding to 0.9, 2.7 and 8.1 gTVSOF/gTVSAS. Each mixture was tested in duplicate at pH values of 5.5, 6.5 and 7.5. The fermentation tests were carried out at 39 ± 1 °C in 1-l (working volume = 0.5 l) glass reactors equipped with magnetic stirring and connected to eudiometers for gas measurement on the basis of the volume displacement principle. The eudiometers were filled with a NaClsaturated solution, acidified with HCl to pH = 2 to prevent gas dissolution and connected to an electronic balance that periodically weighed the volume of solution displaced from the eudiometers. The electronic balance was interfaced with an automatic system recording the total biogas volume produced over time. The measured gas volume was corrected for ambient temperature and pressure, and converted to STP conditions. The reactors were connected to an automatic system for data acquisition and continuous pH control through NaOH addition. Similarly to the first set of tests, before the start of the experiments the reactors were flushed with N2 gas to drive off air from the reactor headspace. During the tests, gas samples were periodically collected through an air-tight syringe connected to the eudiometer sampling port and analysed for H2, N2, CO2 and CH4 content by a Varian 3600 CX gas chromatograph equipped with a thermal conductivity detector and a 2-m stainless column packed with Porapak Q (50/80 mesh) at operating temperatures of injector, oven and detector of 250, 80 and 130 °C. He was used as the carrier gas.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
2.3 Kinetic analysis The kinetics of the H2 production process was evaluated by fitting the experimental cumulative hydrogen production data with the modified Gompertz model in Eq. 2 (Van Ginkel et al., 2005; Pan et al., 2008), widely applied to describe H2 production over time. A two-stage model, derived from the Gompertz equation, was also developed to take into account the presence of a double plateau on the experimental curve of H2 production vs. time (De Gioannis et al., 2014; Akhlaghi et al., 2017): (2)
(3) where: § H(t): cumulative H2 production at time t; § Hmax: maximum H2 production; § Hmax,1 and Hmax,2: maximum H2 production of the first and second stage; § R: maximum H2 production rate; § R1, R2: maximum H2 production rate of the first and second stage; § λ: lag phase duration; § λ1 and λ2: lag phase duration of the first and second stage; § t: time.
The time needed to attain 95% of the maximum hydrogen yield (t95), was also derived to estimate the overall duration of the hydrogenogenic process. The cumulative H2 production data were fitted with Equations (2) or (3) using TableCurve 2D® v. 5.01.
3. RESULTS 3.1 Biochemical Hydrogen Potential (BHP) tests without automatic pH control Figure 1 and Table 2 present the cumulative hydrogen production over time and the kinetic parameters calculated using the modified Gompertz equation (Eq. 2), respectively. The different kinetics of the two mixtures were mirrored by the t95 values, which were approximately equal to 48 h for F:M 1:1 at both pHs and equal to 15.1 and 32.3 h for F:M = 1:3 at pH 5.5 and 6.5, respectively. The inoculum heat pre-treatment prior to the DF process was effective since no methane was detected in the gas during the test. Hydrogen production was found to be in the range 32.3-77.3 l H2/kgTVSOF. Previous studies on batch fermentation of similar substrates without automatic pH control showed similar results. Alibardi and Cossu (2015) determined H2 production yields in the range 25-85 l H2/kgVS while Pecorini et al. (2017) reported a hydrogen production of 55.0 Nl H2/kgTVSOF. The highest hydrogen generation was observed for the mixtures with F:M ratios equal to 1:1 and 1: 3 at pH =
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
5.5. At pH = 6.5, the test carried out at the highest substrate content (F:M = 1:1) showed a better response compared to that at F:M = 1:3. Concerning the process kinetics, the Gompertz model fitted well the experimental data with high correlation coefficients (Table 2). The lag phase lasted few hours (1.4-2.6 h) while the maximum hydrogen production rate was found in the range 2.0-3.8 l H2/kgTVSOF h. These results are coherent with previous researches. De Gioannis et al. (2017) reported a lag phase of 4.15 h with an R value of 3.84 l H2/kgTVSOF h. Similarly, Pan et al. (2008) observed lag phase durations in the range 0.05-4.89 h performing batch tests with OF as the substrate.
Figure 1. Hydrogen production over time for batch fermentative assays without automatic pH control. Solid lines indicate Gompertz model curves. Error bars represent standard deviation.
Table 2. Model results for batch fermentation assays without automatic pH control.
F:M 1:1 – pH 5.5 F:M 1:3 – pH 5.5 F:M 1:1 – pH 6.5 F:M 1:3 – pH 6.5
Hmax (l H2/kgTVSOF) 77.3 70.4 65.6 32.3
R (l H2/kgTVSOF h) 2.4 3.3 2.0 3.8
λ (h) 2.1 1.4 1.7 2.6
t95 (h) 48.3 32.3 48.5 15.1
R2 0.996 0.995 0.993 0.995
3.2 Biochemical Hydrogen Potential (BHP) tests with automatic pH control In Figure 2, the cumulative specific H2 production is reported for the investigated F:M ratios and pHs. If compared to the results shown in Figure 1, it is evident that the continuous control of the pH conditions exerted a beneficial effect on both process yields and kinetics under the whole set of experimental conditions tested. As expected from previous research on various substrates (Cappai et al., 2014), the operating pH and the F:M ratio were found to affect both the H2 yield and the kinetics.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 2. Hydrogen production over time for batch fermentative test with automatic pH control. Solid lines indicate the Gompertz model curves. The highest specific H2 production yield, equal to approximately 100 l H2/kgTVSOF, was attained at pH 5.5 and F:M ratios of 1:3 and 1:1. Under the same pH conditions, a decrease in the amount of inoculum added to the mixture (F:M = 3:1) resulted into a halved H2 yield (equal to 50.6 l H2/kgTVSOF), probably due to the onset of an unfavourable environment for microbial activity. This may be ascribed to an excess of substrate and to the known adverse effect (Baronosky et al., 1984) of undissociated acetic acid accumulation at low pHs into the microorganism cell and the consequent intracellular pH drop. Such an effect was not observed at the highest pH values. In particular, at pH = 6.5, the H2 yield appeared to be unaffected by the increase of added inoculum, with an H2 production of approximately 80 l H2/kgTVSOF at all the F:M values tested. This result further supported the hypothesis that the negative effect on the microbial activity observed at pH=5.5 was due to presence of undissociated acetic acid in the system; such a detrimental effect was reduced as the pH values increased to 6.5 and 7.5. The increase in the F:M ratio was beneficial at pH = 7.5, at which a specific yield of ∼60 l H2/kgTVSOF was attained, corresponding to a threefold increase compared to pH = 5.5. By comparing the results in Tables 2 and 3, it is evident that continuous control of the pH conditions also produced a significant effect on the kinetics of bio-H2 production process. Under the whole set of the experimental conditions tested, the t95 values for the experiments with automatic pH control were lower than those of the corresponding test with no automatic control. In particular, a t95 of 17.9 and 25.6 hours was found at pH = 5.5 and F:M equal to 1:1 and 1:3, when automatic pH control was adopted. Under the same pH conditions but without automatic pH control, at F:M ratios of 1:1 and 1:3 t95 values of 48.3 and 32.3 hours were calculated, thus suggesting the pivotal role of accurate pH control in promoting microbial activity and substrate availability to microorganisms. Similar results were observed at pH = 6.5 and F:M = 1:1, when a t95 rose fourfold when no automatic pH control was applied (t95 = 11.4 and 48.5 hours with and without automatic pH control). In spite of the generally lower H2 yield observed at all the investigated F:M ratios, at pH = 7.5 the process kinetics appeared to be faster, with t95 values always below 17 hours. Such a result is in agreement with previous microbiological studies on metabolic activities of Clostridium (Baronofsky et al., 1984).
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Table 3. Kinetic parameters of H2 production according to Eq. 3, ST1: first stage and ST2: second stage; STtot: whole process. λ t95 Hmax R F:M R2 (h) (h) (l H2/kgTVSOF) (l H2/kgTVSOF h) F:M 1:1 – pH 5.5 ST1 73.10 18.81 3.55 0.999 ST2 27.01 5.75 13.39 STttot 100.11 17.86 F:M 1:3 – pH 5.5 ST1 46.36 23.89 3.53 0.999 ST2 56.20 2.76 0.53 STtot 102.56 25.59 F:M 3:1 – pH 5.5 ST1 46.62 3.56 31.47 0.999 ST2 2.34 0.07 78.53 STtot 48.96 65.39 F:M 1:1 – pH 6.5 ST1 63.39 30.67 3.26 0.999 ST2 16.33 3.28 7.23 STtot 79.72 11.41 F:M 1:3 – pH 6.5 ST1 37.15 2.22 0.67 0.999 ST2 45.06 38.00 3.18 STtot 82.21 20.06 F:M 3:1 – pH 6.5 ST1 36.38 20.29 4.55 0.999 ST2 38.15 2.68 2.88 STtot 74.53 20.00 F:M 1:1 – pH 7.5 ST1 48.87 19.40 2.63 0.999 ST2 0.69 23.23 -90.88 STtot 49.56 6.30 F:M 1:3 – pH 7.5 25.81 2.35 ST1 20.73 0.999 0.73 10.35 ST2 0.53 STtot 26.54 4.53 F:M 3:1 – pH 7.5 20.06 4.26 ST1 52.60 0.999 1.58 11.67 ST2 10.00 STtot 62.60 16.29
4. CONCLUSIONS Two different experimental campaigns of batch fermentative assays were carried out using sorted organic fraction from municipal waste as the substrate. Hydrogen production was evaluated by means of Biochemical Hydrogen Potential tests with and without automatic pH control. The experiments were performed by varying the pH conditions (5.5, 6.5 and 7.5) and the food to microorganisms ratio (1:1, 1:3 and 3:1). The tests where pH was controlled using a buffer solution yielded the highest hydrogen production (77.3 l H2/kgTVSOF) under acidic conditions (pH = 5.5) and a food to microorganisms
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
ratio of 1:1. In general, the tests at pH = 5.5 attained better results than those at pH = 6.5. Similarly, better performances were achieved at higher substrate additions (F:M = 1:1). The adoption of an automatic pH control system improved the metabolic pathways in terms of both H2 yield and kinetics. Bio-H2 yields were observed to increase by up to three times when acidic conditions (pH = 5.5) were adopted, while the process kinetics was faster by up to four times compared to the test without automatic pH control.
AKNOWLEDGEMENTS The research has been funded with support from the MIUR-MISE-Regione Toscana DGRT 758/2013 PAR FAS 2007-2013.
REFERENCES Akhlaghi, M., Boni, M.R., De Gioannis, G., Muntoni, A., Polettini, A., Pomi, R., Rossi, A. Spiga, D. (2017). A parametric response surface study of fermentative hydrogen production from cheese whey. Bioresour. Technol., in press. Alibardi, L. and Cossu, R. (2015). Composition variability of the organic fraction of municipal solid waste and effects on hydrogen and methane production potentials. Waste Manage 36, 147-155. Alibardi, L. and Cossu, R. (2016). Effects of Carbohydrate, protein and lipid content of organic waste on hydrogen production and fermentation products. Waste Manage. 47, 69-77. APHA (2006). Standard Methods for the Examination of Water and Wastewater, Eighteenth ed. American Public Health Association, 2006, Washington, DC. Baronofsky, J.J., Schreurs, W.J.A. and Kashket, E.R. (1984). Uncoupling by Acetic Acid Limits Growth of and Acetogenesis by Clostridium thermoaceticum. Appl Environ Microbiol., 1134– 1139. Cappai, G., De Gioannis, G., Friargiu, M., Massi, E., Muntoni, A., Polettini, A., Pomi, R. and Spiga, D. (2014). An experimental study on fermentative H2 production from food waste as affected by pH. Waste Manage 34, 1510-1519. De Gioannis, G., Muntoni, A., Polettini, A. and Pomi, R. (2013). A review of dark fermentative hydrogen production from biodegradable municipal waste fractions. Waste Manage 33, 13451361. De Gioannis, G., Friargiu, M., Massi, E., Muntoni, A., Polettini, A., Pomi, R., Spiga, D. (2014). Biohydrogen production from dark fermentation of cheese whey: influence of pH, Int. J. Hydrogen Energ., 39(36), 20930-20941. De Gioannis, G., Muntoni, A., Polettini, A., Pomi, R. and Spiga, D. (2017). Energy recovery from one- and two-stage anaerobic digestion of food waste. Waste Manage., in press. Ghimire, A., Frunzo, L., Pirozzi, F., Trably, E., Escudie, R., Lens, P. N. L. and Esposito, G. (2015). A review on dark fermentative biohydrogen production from organic biomass: process parameters and use of by-products. Appl Energ 144, 73-95. Lenth, R.V. (2009). Response-surface methods in R, using rsm. J. Stat. Softw. 32, 1–17. Li, C. and Fang, H.H.P. (2007). Fermentative hydrogen production from wastewater and solid
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wastes by mixed cultures. Crit. Rev. Environ. Sci. Technol. 37, 1-39. Pan, J., Zhang, R., El-Mashad, R.H., Sun, H. and Ying, Y. (2008). Effect of food to microorganism ratio on biohydrogen production from food waste via anaerobic fermentation. Int J Hydrogen Energy 33, 6968-6975. Pecorini, I., Baldi, F., Carnevale, E.A. and Corti, A. (2016). Biochemical methane potential tests of different autoclaved and microwaved lignocellulosic organic fractions of municipal solid waste. Waste Manage 56, 143-150. Pecorini, I., Bacchi, D., Albini, E., Baldi, F., Galoppi, G., Rossi, P., Paoli, P., Ferrari, L., Carnevale, E.A., Peruzzini, M., Lombardi, L., Ferrara, G. (2017). The Bio2Energy project: bioenergy, biofuels and bioproducts from municipal solid waste and sludge. EUBCE 2017 Proceedings, 25th Edition European Biomass Conference & Exhibition, 12-15 June 2017 Stockholm (S). R Development Core Team (2009). R: A language and environment for statistical computing. Vienna, Austria: R Foundation for Statistical Computing. Van Ginkel, S.W., Oh, S.-E. and Logan, B.E. (2005). Biohydrogen gas production from food waste processing and domestic wastewaters. Int J Hydrogen Energy 30, 1535-1542.
SUMMARY: Batch fermentative assays were performed in order to evaluate hydrogen production under different pH conditions and inoculum dosages. Biochemical hydrogen potential tests were carried out with and without automatic pH control. Different substrate to inoculum ratio (or Food to Microorganism, F:M ratio, by weight) namely 1:1, 1:3 and 3:1, were tested at initial pH of 5.5 and 6.5 (no automatic pH control) and pH of 5.5, 6.5 and 7.5 (with automatic pH control). Results highlighted that the continuous pH control improved the conversion of organic waste into H2. The best process performance were attained at pH 5.5 and F:M equal to 1:1 and 1:3 (100.11 and 102.52 l H2/kgTVSOF, respectively, at standard conditions).
1. INTRODUCTION The recently renewed interest in anaerobic digestion (AD) of biodegradable residues has moved the scientific, technical and industrial communities towards further development and optimization of the process. For instance, bio-hydrogen production during the acidogenic phase of AD is nowadays regarded as a key topic by many researchers due to its potential benefits on both the energy balance and the environmental profile of the whole process. As a matter of fact, hydrogen is characterized by a high specific energy content per unit mass and no greenhouse gases emissions result from its combustion. Moreover, the combined (sequential) production of bio-H2 and bio-CH4 has been demonstrated to improve the gasification yield of organic substrates as compared to the conventional, single-stage AD process. Such potential benefits are further improved if bio-H2 is produced through the biochemical conversion of biodegradable wastes (Ghimire et al., 2015). Several substrates have been tested for hydrogen production
Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
(Ghimire et al., 2015); among them, the organic fraction of municipal waste (OFMW) appears to be a promising feedstock due to its biodegradability characteristics, as well as wide availability (De Gioannis et al., 2013, Cappai et al., 2014). The aim of the present research was to identify the optimal operating conditions in terms of pH and food to microorganisms (F:M) ratio for bio-H2 production from dark fermentation of OFMW. The bio-H2 production process was studied under mesophilic conditions (39 ± 1°C) by means of two different experimental set-up, operated under a batch hydraulic regime. The experiments allowed to assess the Biochemical Hydrogen Potential (BHP) of the investigated waste within the adopted range of operating parameters.
2. MATERIALS AND METHODS 2.1 Substrate and inoculum Source-separated OFMW from a Tuscanian city, hereinafter referred to as OF, collected by means of a door-to-door system, was manually sorted and homogenised. The Total Solids (TS) content of the homogeneous samples was then adjusted by adding tap water to a TS content of 5.1% by weight. Activated sludge (AS) collected from the aerobic unit of a municipal wastewater treatment plant was used as the inoculum. According to previous studies (Alibardi and Cossu, 2016; Cappai et al., 2014; Li and Fang, 2007), in order to harvest the hydrogen-producing bacteria and inhibit hydrogenotrophic methanogens, AS samples were heat-shocked at 105°C for 30 minutes before the start of each experiment. The characteristics of OF and AS in terms of TS, Total Volatile Solids (TVS), TOC and pH (see Table 1) were determined according to standard methods (APHA, 2006).
Table 1. Organic Fraction of Municipal Waste and inoculum characteristics. Concentrations are expressed as average values and related standard deviation. OF AS
TS (%) 5.1 ± 0.7 1.3 ± 0.2
TVS/TS (%) 92.0 ± 0.9 78.2 ± 0.7
pH 3.81 ± 0.01 7.08 ± 0.01
TOC (gC/l) 25.75 ± 3.3 -
2.2 Batch fermentation assays 2.2.1 Biochemical Hydrogen Potential (BHP) tests without automatic pH control The first experimental set-up consisted of 1-l (0.5-l working volume) fermentation reactors for BHP assays without automatic pH control. The fermentation tests were conducted in duplicate using stainless steel batch reactors tightly closed by a special cap provided with a ball valve to enable gas sampling (Pecorini et al., 2016). The vessels were placed on a magnetic stirrer and incubated in a water jacket under mesophilic conditions (T = 39.0 ± 1 °C) for 4 days. The reactors were run at two different OF/AS ratios (1:1 and 1:3 on a wet weight basis, corresponding to F:M ratios of 4.00 and 1.33 g VS(OF)/gVS(AS)). The reactors were initially fed
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
with the inoculum, the substrate, as well as a MES (2-N-Morpholino-EthaneSulfonic acid, Sigma-Aldrich) buffer solution and 2.5M HCl to set the initial pH at 5.5 and 6.5 (Alibardi and Cossu, 2015). After filling, the reactors were flushed with N2 for a few minutes to ensure anaerobic conditions. Gas production was estimated by measuring the pressure evolution in the headspace of each reactor and then converting it to a gas volume by means of the ideal gas law. Pressure was measured using a membrane pressure gauge (Model HD2304.0, Delta Ohm S.r.L., Italy). The measured values of pressure were converted into biogas volume as follows (Eq. (1)): (1) where: § Vbiogas: biogas volume at Standard Temperature and Pressure (STP, with T = 273.15 K and P
=105 Pa), (l); § Pmeasured: headspace pressure before gas sampling, (Pa); § Tr and Vr: temperature (K) and volume (l) of the reactor’s headspace; The BHP was determined as the cumulated hydrogen production divided by the initial TVS content of each batch. The hydrogen content of the biogas was measured by using a gas microchromatograph equipped with thermal conductivity detectors and Poraplot Q and Molsieve columns (3000 Micro GC, INFICON, Switzerland). 2.2.2 Biochemical Hydrogen Potential (BHP) tests with automatic pH control The second set of batch tests was carried out at F:M ratios of 1:3, 1:1 and 3:1, corresponding to 0.9, 2.7 and 8.1 gTVSOF/gTVSAS. Each mixture was tested in duplicate at pH values of 5.5, 6.5 and 7.5. The fermentation tests were carried out at 39 ± 1 °C in 1-l (working volume = 0.5 l) glass reactors equipped with magnetic stirring and connected to eudiometers for gas measurement on the basis of the volume displacement principle. The eudiometers were filled with a NaClsaturated solution, acidified with HCl to pH = 2 to prevent gas dissolution and connected to an electronic balance that periodically weighed the volume of solution displaced from the eudiometers. The electronic balance was interfaced with an automatic system recording the total biogas volume produced over time. The measured gas volume was corrected for ambient temperature and pressure, and converted to STP conditions. The reactors were connected to an automatic system for data acquisition and continuous pH control through NaOH addition. Similarly to the first set of tests, before the start of the experiments the reactors were flushed with N2 gas to drive off air from the reactor headspace. During the tests, gas samples were periodically collected through an air-tight syringe connected to the eudiometer sampling port and analysed for H2, N2, CO2 and CH4 content by a Varian 3600 CX gas chromatograph equipped with a thermal conductivity detector and a 2-m stainless column packed with Porapak Q (50/80 mesh) at operating temperatures of injector, oven and detector of 250, 80 and 130 °C. He was used as the carrier gas.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
2.3 Kinetic analysis The kinetics of the H2 production process was evaluated by fitting the experimental cumulative hydrogen production data with the modified Gompertz model in Eq. 2 (Van Ginkel et al., 2005; Pan et al., 2008), widely applied to describe H2 production over time. A two-stage model, derived from the Gompertz equation, was also developed to take into account the presence of a double plateau on the experimental curve of H2 production vs. time (De Gioannis et al., 2014; Akhlaghi et al., 2017): (2)
(3) where: § H(t): cumulative H2 production at time t; § Hmax: maximum H2 production; § Hmax,1 and Hmax,2: maximum H2 production of the first and second stage; § R: maximum H2 production rate; § R1, R2: maximum H2 production rate of the first and second stage; § λ: lag phase duration; § λ1 and λ2: lag phase duration of the first and second stage; § t: time.
The time needed to attain 95% of the maximum hydrogen yield (t95), was also derived to estimate the overall duration of the hydrogenogenic process. The cumulative H2 production data were fitted with Equations (2) or (3) using TableCurve 2D® v. 5.01.
3. RESULTS 3.1 Biochemical Hydrogen Potential (BHP) tests without automatic pH control Figure 1 and Table 2 present the cumulative hydrogen production over time and the kinetic parameters calculated using the modified Gompertz equation (Eq. 2), respectively. The different kinetics of the two mixtures were mirrored by the t95 values, which were approximately equal to 48 h for F:M 1:1 at both pHs and equal to 15.1 and 32.3 h for F:M = 1:3 at pH 5.5 and 6.5, respectively. The inoculum heat pre-treatment prior to the DF process was effective since no methane was detected in the gas during the test. Hydrogen production was found to be in the range 32.3-77.3 l H2/kgTVSOF. Previous studies on batch fermentation of similar substrates without automatic pH control showed similar results. Alibardi and Cossu (2015) determined H2 production yields in the range 25-85 l H2/kgVS while Pecorini et al. (2017) reported a hydrogen production of 55.0 Nl H2/kgTVSOF. The highest hydrogen generation was observed for the mixtures with F:M ratios equal to 1:1 and 1: 3 at pH =
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
5.5. At pH = 6.5, the test carried out at the highest substrate content (F:M = 1:1) showed a better response compared to that at F:M = 1:3. Concerning the process kinetics, the Gompertz model fitted well the experimental data with high correlation coefficients (Table 2). The lag phase lasted few hours (1.4-2.6 h) while the maximum hydrogen production rate was found in the range 2.0-3.8 l H2/kgTVSOF h. These results are coherent with previous researches. De Gioannis et al. (2017) reported a lag phase of 4.15 h with an R value of 3.84 l H2/kgTVSOF h. Similarly, Pan et al. (2008) observed lag phase durations in the range 0.05-4.89 h performing batch tests with OF as the substrate.
Figure 1. Hydrogen production over time for batch fermentative assays without automatic pH control. Solid lines indicate Gompertz model curves. Error bars represent standard deviation.
Table 2. Model results for batch fermentation assays without automatic pH control.
F:M 1:1 – pH 5.5 F:M 1:3 – pH 5.5 F:M 1:1 – pH 6.5 F:M 1:3 – pH 6.5
Hmax (l H2/kgTVSOF) 77.3 70.4 65.6 32.3
R (l H2/kgTVSOF h) 2.4 3.3 2.0 3.8
λ (h) 2.1 1.4 1.7 2.6
t95 (h) 48.3 32.3 48.5 15.1
R2 0.996 0.995 0.993 0.995
3.2 Biochemical Hydrogen Potential (BHP) tests with automatic pH control In Figure 2, the cumulative specific H2 production is reported for the investigated F:M ratios and pHs. If compared to the results shown in Figure 1, it is evident that the continuous control of the pH conditions exerted a beneficial effect on both process yields and kinetics under the whole set of experimental conditions tested. As expected from previous research on various substrates (Cappai et al., 2014), the operating pH and the F:M ratio were found to affect both the H2 yield and the kinetics.
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Figure 2. Hydrogen production over time for batch fermentative test with automatic pH control. Solid lines indicate the Gompertz model curves. The highest specific H2 production yield, equal to approximately 100 l H2/kgTVSOF, was attained at pH 5.5 and F:M ratios of 1:3 and 1:1. Under the same pH conditions, a decrease in the amount of inoculum added to the mixture (F:M = 3:1) resulted into a halved H2 yield (equal to 50.6 l H2/kgTVSOF), probably due to the onset of an unfavourable environment for microbial activity. This may be ascribed to an excess of substrate and to the known adverse effect (Baronosky et al., 1984) of undissociated acetic acid accumulation at low pHs into the microorganism cell and the consequent intracellular pH drop. Such an effect was not observed at the highest pH values. In particular, at pH = 6.5, the H2 yield appeared to be unaffected by the increase of added inoculum, with an H2 production of approximately 80 l H2/kgTVSOF at all the F:M values tested. This result further supported the hypothesis that the negative effect on the microbial activity observed at pH=5.5 was due to presence of undissociated acetic acid in the system; such a detrimental effect was reduced as the pH values increased to 6.5 and 7.5. The increase in the F:M ratio was beneficial at pH = 7.5, at which a specific yield of ∼60 l H2/kgTVSOF was attained, corresponding to a threefold increase compared to pH = 5.5. By comparing the results in Tables 2 and 3, it is evident that continuous control of the pH conditions also produced a significant effect on the kinetics of bio-H2 production process. Under the whole set of the experimental conditions tested, the t95 values for the experiments with automatic pH control were lower than those of the corresponding test with no automatic control. In particular, a t95 of 17.9 and 25.6 hours was found at pH = 5.5 and F:M equal to 1:1 and 1:3, when automatic pH control was adopted. Under the same pH conditions but without automatic pH control, at F:M ratios of 1:1 and 1:3 t95 values of 48.3 and 32.3 hours were calculated, thus suggesting the pivotal role of accurate pH control in promoting microbial activity and substrate availability to microorganisms. Similar results were observed at pH = 6.5 and F:M = 1:1, when a t95 rose fourfold when no automatic pH control was applied (t95 = 11.4 and 48.5 hours with and without automatic pH control). In spite of the generally lower H2 yield observed at all the investigated F:M ratios, at pH = 7.5 the process kinetics appeared to be faster, with t95 values always below 17 hours. Such a result is in agreement with previous microbiological studies on metabolic activities of Clostridium (Baronofsky et al., 1984).
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Table 3. Kinetic parameters of H2 production according to Eq. 3, ST1: first stage and ST2: second stage; STtot: whole process. λ t95 Hmax R F:M R2 (h) (h) (l H2/kgTVSOF) (l H2/kgTVSOF h) F:M 1:1 – pH 5.5 ST1 73.10 18.81 3.55 0.999 ST2 27.01 5.75 13.39 STttot 100.11 17.86 F:M 1:3 – pH 5.5 ST1 46.36 23.89 3.53 0.999 ST2 56.20 2.76 0.53 STtot 102.56 25.59 F:M 3:1 – pH 5.5 ST1 46.62 3.56 31.47 0.999 ST2 2.34 0.07 78.53 STtot 48.96 65.39 F:M 1:1 – pH 6.5 ST1 63.39 30.67 3.26 0.999 ST2 16.33 3.28 7.23 STtot 79.72 11.41 F:M 1:3 – pH 6.5 ST1 37.15 2.22 0.67 0.999 ST2 45.06 38.00 3.18 STtot 82.21 20.06 F:M 3:1 – pH 6.5 ST1 36.38 20.29 4.55 0.999 ST2 38.15 2.68 2.88 STtot 74.53 20.00 F:M 1:1 – pH 7.5 ST1 48.87 19.40 2.63 0.999 ST2 0.69 23.23 -90.88 STtot 49.56 6.30 F:M 1:3 – pH 7.5 25.81 2.35 ST1 20.73 0.999 0.73 10.35 ST2 0.53 STtot 26.54 4.53 F:M 3:1 – pH 7.5 20.06 4.26 ST1 52.60 0.999 1.58 11.67 ST2 10.00 STtot 62.60 16.29
4. CONCLUSIONS Two different experimental campaigns of batch fermentative assays were carried out using sorted organic fraction from municipal waste as the substrate. Hydrogen production was evaluated by means of Biochemical Hydrogen Potential tests with and without automatic pH control. The experiments were performed by varying the pH conditions (5.5, 6.5 and 7.5) and the food to microorganisms ratio (1:1, 1:3 and 3:1). The tests where pH was controlled using a buffer solution yielded the highest hydrogen production (77.3 l H2/kgTVSOF) under acidic conditions (pH = 5.5) and a food to microorganisms
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ratio of 1:1. In general, the tests at pH = 5.5 attained better results than those at pH = 6.5. Similarly, better performances were achieved at higher substrate additions (F:M = 1:1). The adoption of an automatic pH control system improved the metabolic pathways in terms of both H2 yield and kinetics. Bio-H2 yields were observed to increase by up to three times when acidic conditions (pH = 5.5) were adopted, while the process kinetics was faster by up to four times compared to the test without automatic pH control.
AKNOWLEDGEMENTS The research has been funded with support from the MIUR-MISE-Regione Toscana DGRT 758/2013 PAR FAS 2007-2013.
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