European Biomass Conference and Exhibition 2017 ...

25th European Biomass Conference and Exhibition, 12-15 June 2017, Stockholm, Sweden

CONVERSION OF FOOD WASTE INTO ENERGY: IMPACT OF THERMAL PRE-TREATMENT ON HYDROGEN AND METHANE PRODUCTION Agata Gallipoli1, Camilla M. Braguglia1, Marco Di Carlo1, Andrea Gianico1, Daniele Montecchio1, Pamela Pagliaccia1, Carlo Pastore2 and Fausto Gironi3 1Istituto di Ricerca sulle Acque (IRSA-CNR), Area di Ricerca RM1-Montelibretti, Via Salaria km 29.300, 00015 Monterotondo (Roma), Italy ([email protected]; [email protected]) 2Istituto di Ricerca sulle Acque (IRSA-CNR), Viale F. de Blasio, 5, 70123 Bari, Italy 3Department of Chemical Engineering, University of Rome “La Sapienza”

ABSTRACT: In the framework of the waste circular economy, anaerobic digestion (AD) is a promising treatment option, due to both renewable energy and fertilizer production. Nevertheless, in mesophilic conditions a part of the organic carbon fed is not degraded, reducing the possibility to fully exploit the waste energy potential, and opening the research to advanced processes that can increase AD efficiency. In this study, AD of food waste was investigated in thermophilic conditions. Scope of this work was to evaluate the efficiency of a mild thermal pre-treatment on the solubilisation of complex organics and the digestion enhancement potential in terms of H 2 and CH4 conversion rates. Thermal pre-treatment promoted complex organics solubilisation (soluble COD up to +40%) in particular with reference to starch and hemi-cellulose fraction. The high amount of released sugars was rapidly transformed into H 2 in the first hours of AD, with high yields (up to 2.6 mol H2/mol glucose) and significant gain with respect to untreated waste. Ultimate methane yield was not affected by the substrate pre-treatment, but the positive impact was shown by the increase in anaerobic biodegradability, and kinetics. Keywords: anaerobic digestion, organic waste, pretreatment, methane, hydrogen, modelling

1

INTRODUCTION

conditions, a part of the organic carbon introduced may result recalcitrant, reducing the possibility to fully exploit the waste energy potential, and opening the research to advanced processes that can increase AD efficiency. Complex heterogeneous material such as FW ranges from highly recalcitrant material to extremely biodegradable compounds, so the best option to improve the AD performances is a case-specific and properly designed strategy considering not only FW characteristics and composition but also the final aim of the study. It is in fact impossible to standardize the AD process: a case-by-case approach is required in order to calculate yields and composition of the biogas produced. An accurate characterization of the feedstock is hence an essential step in food waste digestion [4]. Anyway, no clear information is available, neither about an exhaustive characterization of the organic waste nor about the possible impacts on degradation kinetics and conversion rates. In order to increase the digestion performances, one strategy could be the introduction of a thermal pretreatment before digestion. Thermal pre-treatment is a process of heating waste to a certain temperature and pressure in order to maximize the degradation of the lignocellulosic matrix increasing consequently food waste biodegradability [5, 6] Scope of this work was to evaluate the efficiency of mild thermal pre-treatment on the solubilisation of complex organics and the digestion enhancement potential in terms of CH4 and/or H2 conversion rates and kinetics.

Food waste was defined by the UN Food and Agriculture Organization (FAO) and includes any healthy or edible substance that is wasted, lost, degraded at every stage of the food supply chain. The total Food Waste quantity produced each year in Europe has been estimated to be around 90 million tons Mt in 2012, of which an estimated 47 Mt is collected from household (average of 92 kg per capita). The United Kingdom generates most FW in Europe, namely 14 Mt, corresponding to almost 135 kg/capita yearly against the 62 kg per capita per year wasted in Italy. The EU Commission estimates an increase in the FW amounts to 120 Mt by 2020, which is mainly due to increased FW generation in households, that almost doubled from 2004 to 2012, while FW from food manufacturing and agriculture showed decreasing trend. The composition of the food wasted at household and food service level (restaurants, canteen, etc) varies from region to region in the world. In Europe, FW is composed by 40% of vegetables and fruit, 33% pasta and bread, 17% of dairy products and around 10% of meat and fish residues [1]. Although reduction is the most preferred option in the FW management hierarchy, sustainable approaches such as recovery in terms of waste-to-energy also require attention and technical development from the research community in order to promote a comprehensive virtuous FW management system. In fact, nowadays, most of the FW is disposed in landfills or incinerated, practices associated with different issues, such as rising costs of waste disposal, lack of space, leaching, public environmental concern and emission of toxic and greenhouse effect gases [2]. In this context, anaerobic digestion (AD) seems to be a promising option that permits an effective and environmental friendly treatment of waste and its valorization in terms of energy production (H2 and CH4), adding a surplus value to the substrate [3]. In general, food waste is characterized by a high volatile solid and moisture content, which makes it a perfect substrate for AD. Nevertheless, in mesophilic

2

MATERIALS AND METHODS

2.1 Food waste Origin KW was collected in a single acquisition, from the cafeteria of the CNR research area “Roma 1”. The waste consisted of mixed cooked and uncooked food such as pasta and bread (15%), cheese (15%), fruit and vegetable peelings (70%). KW, manually chopped and shredded by a food processor, was then stored at (-20) °C.

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Before the experiment, one part of the KW sample was thawed, and mixed with tap water at the weight ratio 1:5, in order to ease the grinding phase and to maximize the effectiveness of the pre-treatment. 2.2 Thermal pre-treatment After grinding phase, one portion of the waste was thermally pre-treated in a bench scale autoclave Laboklav 25b, with total capacity of 25 L and able to work at Tmax=134°C and pmax=3.2 bar. Retention time was set at 20 min on 300 mL of sample. 2.3 Analytical Methods Total and volatile solids (TS and VS) were determined according to standard methods. To analyse the soluble phase, the particulate sludge matter was removed by centrifugation (10 min at 5000 rpm) and the centrate was filtrated through 0.45 μm filters. Soluble and total COD was determined by means of COD Cell Test by Spectroquant Merck (EPA method 410.4). Total and soluble nitrogen was measured by photometric determination using the Nitrogen cell test by Spectroquant (Merck). Soluble proteins content was measured by means of the Modified Lowry Kit for Protein Determination (Sigma–Aldrich P 5656), while total proteins were estimated by multiplying organic nitrogen by 6.25. Total and soluble carbohydrates determination was based on a modified Dubois method using fructose as standard. Lipids content was calculated by difference. The biogas composition was measured using a PerkinElmer Auto System Gas Chromatograph equipped with a thermal conductivity detector (TCD). NRELTP-510-42618 method for “Determination of Structural Carbohydrates and Lignin in Biomass” was partially adapted and applied with the aim of determining free sugars, hemicellulose, starch and cellulose contents. ellulose and lignin. Dry solids (100 mg) were dissolved and kept under agitation at 303 K for 60 min in 3 mL of 72% sulphuric acid. Then, with 84 mL of distilled water, the acid solution was transferred into a 250 mL glass balloon and kept under reflux for 1 h. The suspension was cooled and filtered on a filtering crucibles previously prepared and weighted. The filtered solution was diluted 50 times (20 mL) into 1 mL of mQ water) and analysed for sugars determinations. On the other side, filtered solids were abundantly washed with water and dried for 24 h at 378 K and then weighted. Insoluble lignin was calculated by the difference between this weight and the respective ashes obtained after putting the same filtering crucible into an oven at 823 K for 3 h [7].

Figure 1: Automated Methane Potential Testing System The gas is measured through water displacement using flow cells (Figure 1, Unit C) that give a signal for approximately every 10 mL of produced gas. Temperature and pressure sensors are used to normalise the gas volume to 0°C, 1 atm and dry gas conditions at each measurement point. For the analysis of biogas composition, gas samples were collected at regular intervals with a gas-tight syringe and analysed using a Perkin Elmer Auto System Gas Chromatograph (GC) equipped with a thermal conductivity detector (TCD). The data were recorded by AMPTS II software and automatically transferred to a MS Excel™ file for subsequent analysis and visualization. Each reactor was filled with KW (either untreated or pre-treated) and inoculum, with different ratio F/I (Food/Inoculum), namely 0.5 and 1, on a VS basis according to the optimized results of our preliminary assessment. Blank tests were performed with same inoculum and water, instead of KW. The inoculum was characterized by a TS concentration of 21.5±0.5g L-1, with VS/TS of 47±1% and soluble COD of 450 mg L-1. For each KW digestion test (test #1 raw (F/I=0.5); test #2 pre-treated (F/I=0.5); test #3 raw (F/I=1); test #4 pre-treated (F/I=1)) four replicate reactors were monitored, and run for over 35 days until no significant methane production was observed. All tests were performed in duplicates. The COD mass balance for all the batches was computed considering the initial and final COD data, and the equivalent COD of methane and hydrogen (0.395 mlCH4/gCOD e 0.467 mlH2/gCOD at T=35°C and p=1 atm) which indicated a closure at 95– 98%, thus emphasizing data reliability. The theoretical methane potential is widely used to have an idea of the methane production of a specific organic substrate. In this study, the selected units used for expressing the methane potential were mainly L CH4 g−1 VSfed. On the basis of the organic fraction composition (carbohydrates, proteins and lipids), methane yield was estimated using the following general equation (1):

2.4 Anaerobic Digestion tests The Biomethane Potential (BMP) tests for determining the anaerobic biodegradability in terms of methane (and hydrogen, if feasible) yield of KW were carried out by using the Automatic Methane Potential Test System (AMPTS) by Bioprocess (Sweden). The apparatus consists of 15 parallel batch reactors of 500 mL capacity in an incubator unit with controlled thermophilic (55°C) temperature (Figure 1, Unit A). Each reactor was mechanically stirred. The CO2-fixing unit vials were filled with NaOH 3N (Figure 1, Unit B).

BMPth=0.415 L/g carb+0.496 L/g prot+ 1.014 L/g lip (1) The biodegradable fraction was estimated by the ratio between experimental and theoretical BMP value.

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3

RESULTS AND DISCUSSION

and total organics composition, indicating that no mineralization or evaporation occurred. Because of the comparable composition in terms of organic constituents (Tables 1 and 2), the theoretical biomethane potential for pre-treated KW was 0.483±0.010 Nm3/kg VSfed, not statistically different for the one obtained for the untreated KW.

3.1 Characterization of KW and effect of thermal pretreatment Total solids in raw KW was around 19% with high content of water and organic substance (VS/TS= 95 %), highlighting the high transformation potential of this substrate in AD process. The COD soluble fraction (around 20%) was mainly due to shredding treatment of the KW, carried out to homogenize the sample before chemical characterization, and was mainly composed of soluble carbohydrates (Table 1). The KW particulate organic fraction was principally composed by complex sugars (by about 50%), and by a significant portion of lignin (around 15%), typically recalcitrant in anaerobic conditions. For this reason in the calculation of the theoretical biomethane potential (by means of equation (1)), lignin was not accounted for, and the BMPth was equal to 0.479±0.010 Nm3/kg VSfed, according with the data reported in literature [8, 9] for this type of waste.

3.2 Experimental Hydrogen and Methane Yields form untreated and pre-treated KW Biogas production varies with many factors such as inoculum, volatile solids, ammonia, VFA, pH and temperature. The pH is one of the most important parameters influencing AD, because all the involved microorganisms are very sensitive to pH variations, and each process step shows a different pH sensitivity. For fermentative bacteria, a comprehensive pH range from 4 to 8.5 is suitable, while most methanogens work optimally in a pH range from 6.5 to 7.2, with a methanogenesis step failure below pH of 5.5. A method to source and determine the feasibility of a material to serve as a substrate in anaerobic digestion is the Biochemical Methane Potential (BMP) test. The BMP test was used as a tool for evaluating the energy potential and KW biodegradability, and the related impact of the thermal pre-treatment, in particular as regards the soluble compounds trend. The applied F/I ratio, relevant parameter affecting generally the performances of the BMP test of food waste as regards the acidification phase, ranged between 0.5 and 1 [9]. It is important to note that the initial pH for all the tests was around 7-7.5 and during the first 48 hours a pH drop, down to 6.3-6.7, was observed. The initial soluble COD (principally due to released proteins and carbohydrates) in the pre-treated mixture was considerably higher than that of the untreated one demonstrating the effectiveness of the thermal pretreatment (Figure 2 and Figure 3). For the untreated FW however, soluble COD slightly increased in the first 48 hours of the process and was then removed in the following digestion days (Figure 2). On the contrary, as regards the pre-treated KW, the trend of soluble COD was completely different, because most part of the high initial COD was rapidly removed from the beginning of the digestion process (Figure 3).

Table I: Characterization of untreated Kitchen Waste Parameter pH TS (g/kgww) VS/TS (%) Soluble COD (g/kgww) Total COD (g/ kgww) COD/VS Total Nitrogen (% TS) C/N Total Carbohydrates Total Proteins (% COD) Lignin (%COD) Lipids (% COD)

Untreated KW 4.2±0.2 189±3 95±0.5 54±3 265±20 1.45±0.03 3.7±0.2 13±0.2 49±2 14±1 14±1 18±3

Table II: Characterization of pre-treated Kitchen Waste Parameter pH TS (g/kgww) VS/TS (%) Soluble COD (g/kgww) Total COD (g/ kgww) COD/VS Total Nitrogen (% TS) C/N Total Carbohydrates Total Proteins (% COD) Lignin (%COD) Lipids (% COD)

Pre-treated KW 4.1±0.2 186±3 95±0.5 76±4 270±22 1.45±0.03 3.6±0.2 12.9±0.2 50±2 15±1 13±1 18±3

Thermal pre-treatment was effective in the solubilisation of organic matter, affecting mostly the carbohydrate fraction (Table 2). Lignocellulosic and starch fraction were always found to be lower in the pretreated substrate, according to the corresponding increase in soluble COD and carbohydrates. Starch content is by far the easiest to process [12] and was found to be the one with the higher grade of transformation. As expected, lignin fraction underwent less conversion because of the reduced temperature hydrolysis of the pre-treatment. Thermal pre-treatment did not affect total solids content

Figure 2: Soluble compounds trend during digestion of untreated KW at F/I=0.5 In particular, the high amount of soluble carbohydrates released during the thermal pre-treatment (see Figure 3) was rapidly transformed into hydrogen

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during the first 48 hours, with very high conversion yields, up to 0.089 Nm3/kgVSfed (Figure 4).

recovered during the digestion period, without chemicals addition, assuring successive methane production (Table III). The alkalinity of the medium was therefore sufficient to avoid a dramatic pH drop when VFA started to accumulate at the beginning of the process. Nevertheless, methane yields for raw and pre-treated feed resulted affected by the lack of the soluble part of the carbohydrate fraction, “subtracted” for hydrogen conversion, and no longer available for methanogenesis. Overall, no process inhibition due to thermal pretreatment was observed. The methane content of the biogas produced from pre-treated and untreated substrates ranged from 44% to 59%, the rest being mostly carbon dioxide. This is a similar finding to that determined by Liu et al. [13] which measured an average methane percentage of 56.7 for mesophilic food waste digestion, while varied from 55 to 63% for autoclaved and untreated food waste semi-continuous digestion [14].

Figure 3: Soluble compounds trend during digestion of pre-treated KW at F/I=0.5

Table III: Conversion yields of KW at F/I=0.5

The applied organic load affected the hydrogen yield of the untreated KW, that increased by increasing F/I, while for the pre-treated substrate the yield remained the same (Tables III and IV). Other studies proved that F/I ratio influenced the efficiency of the process, and that efficient H2 production is strictly related to high F/I ratios [10, 11]. Under thermophilic conditions, a maximum H2 yield of 57 ml/g VS was obtained at F/I of 7, while under mesophilic conditions the highest yield of 39 ml-H2/g VS was attained at F/I of 6 [11].

Yield (L/kg VSfed)

Raw

H2 CH4

53±3 300±25

Pre-treated 89±8 304±10

Table IV: Conversion yields of KW at F/I=1 0.10

PRETREATED KW

Hydrogen yield (Nm3/kg VSfed)

0.09 0.08

Yield (L/kg VSfed)

Raw

Pre-treated

H2 CH4

67±6 313±30

88±8 328±20

+70% gain

0.07 0.06

UNTREATED KW

0.05

0.04 0.03 0.02

Methane yields of the anaerobic digestion tests and the corresponding data fitting results are reported and discussed in the following paragraph.

0.01 0.00 0

10

20

30

40

50

60

70

digestion time (h)

3.3 Data fitting In order to evaluate the effect of the pre-treatment on digestion of food waste, process kinetics was investigated, too. Kinetic parameters estimation can be performed by assuming that the overall process is represented by a first order hydrolysis model (Eq. (2)):

Figure 4: H2 yields from untreated and pre-treated KW Digestion carried out at F/I=0.5

0.10

PRETREATED KW

Hydrogen yield (Nm3/kg VSfed)

0.09 0.08

dS/dt=-KhS

+30% gain

(2)

0.07 0.06

where S is the substrate concentration and Kh is the first order hydrolysis constant. Once Eq. 2 is integrated, the relationship between substrate concentration and time can be represented by the Gompertz equation (Eq. 3):

UNTREATED KW

0.05

0.04 0.03 0.02 0.01

S = So (1-exp (Kh t))

0.00 0

20

40

60

80

100

120

(3)

140

where So is the ultimate methane potential. Nevertheless, in case of digestion tests with a significant lag phase, the “classical” Gompertz proved to be unfit to properly predict the methane production. In the literature, several simplified models have been applied for estimating performance parameters. The modified Gompertz equation is considered much more appropriate to model both hydrogen and methane

digestion time (h)

Figure 5: H2 yields from untreated and pre-treated KW digestion carried out at F/I=1 However, the acidification phase in these operative conditions was always reversible, and the declining pH

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production, because it accounts for the lag phase which may take place at the beginning of the process (Eq. 4):

S = So exp (- exp (

(λ – t) + 1))

methane yields reported in Table III with the ultimate methane potential (So) evidenced an overall agreement (R2  0.98), confirming the adequacy of the data fitting. Only for the pre-treated KW digestion carried out at F/I 1, the coefficient of determination was 0.80. Moreover, both the estimated ultimate hydrogen and methane potential for the pre-treated KW were higher than those estimated for the untreated KW, except in the case of F/I=1, where the methane was overestimated As observed experimentally, carbohydrates were consumed at higher rates for the pre-treated sample, and their uptake was accomplished by 48 hours of digestion. The presence of this easy degradable fraction allowed a very short lag phase (almost negligible) before fermentation started. The cumulative CH4 yield data were fitted by the modified Gompertz equation (Figure 6), and the function described very well the observed lag phase due to hydrogen production, for both samples; methane production increased during the first 12-15 days of digestion remaining successively almost constant until the end.

(4)

where λ is the lag phase extent and Rm represents the biogas production rate. The unknown parameters of the above mentioned model have been individuated through a standard procedure which requires the minimization of a cost function [3]. Tables V and VI presents the parameters obtained in the optimization process of the modified Gompertz model, at F/I 0.5 and 1, respectively. Table V: Parameters estimation of the Modified Gompertz model for untreated and pre-treated KW F/I 0.5

U

P

Gain (%)

Hydrogen 49.5

78.3

+58

400

Rm (L H2/kgVS/day)

4.3

11.8

+174

350

λ (h)

0.1

0.6

R2

0.99

0.98

Methane Yield (L CH4 kg-1VS)

So (L H2/kgVS)

Methane So (L CH4/kgVS)

275

304

+11

Rm (L CH4/kgVS/day)

2.6

3.4

+31

λ (h)

121

75

R2

0.98

0.99

Untreated KW

300 250 200 150 100

50 0 0

200

400

P

Methane Yield (L CH4 kg-1VS)

Table VI: Parameters estimation of the Modified Gompertz model for untreated and pre-treated KW U

Gain (%)

Hydrogen So (L H2/kgVS)

52.6

85.7

+63

Rm (L H2/kgVS/day)

5.0

10.0

+100

λ (h)

0.1

0.2

R2

0.99

0.80

1000

Pretreated KW

350

300 250 200 150 100 50 0 0

200

400

600

800

1000

digestion hours

Figure 6. Models fit (continuous line) with methane specific production experimental data (by points), for test #3 and #4 carried out at F/I 1.

Methane So (L CH4/kgVS)

359

346

-4

Rm (L CH4/kgVS/day)

1.7

2.7

+59

λ (h)

87

82

0.99

0.99

R2

800

digestion hours

400

F/I 1

600

Finally, the outcomes of this study showed a noticeable raise of the kinetics parameters thanks to thermal pre-treatment integration, in particular as regards the first fermentation phase producing hydrogen from released sugars. In literature, by thermal pre-treating synthetic organic fraction of municipal solid waste, Cano et al. [15] reported a considerable improvement of kinetics (Rm from 11.9 to 31.5 L CH4/kgVS/day) and a lag phase reduction, but no hydrogen production.

The Modified Gompertz function, taking into account the lag phase due to hydrogen production, described the experimental trend with a good agreement for both substrates. The comparison of the observed hydrogen and

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Substrate properties are very dependent on the waste composition and on its origin. Hence, a complete characterization of the waste is very important in order to select the most appropriate pre-treatment and specific tests for each one have to be performed.

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REFERENCES

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ACKNOWLEDGEMENTS

This work was supported by a PRIN 2012 project titled “Advanced Processes to convert organic wastes in innovative,sustainable and useful products” co-financed by the Italian Minister of University and Scientific Research (MIUR). Authors wish to thank all the CNR Canteen Staff for the precious support in sampling and storing the food waste investigated in this study.

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