Environmental Engineering and Management Journal November 2013, Vol.12, No. S11, Supplement, 105-108

http://omicron.ch.tuiasi.ro/EEMJ/

“Gheorghe Asachi” Technical University of Iasi, Romania

BIOMETHANE PRODUCTION FROM GRAPE POMACES:

A TECHNICAL FEASIBILITY STUDY

Extended abstract

Stefano Rebecchi1, Lorenzo Bertin1, Veronica Vallini2, Giacomo Bucchi3, Fabrizio Bartocci4, Fabio Fava 1

1University of Bologna, DICAM, via Terracini 28, I-40131, Bologna, Italy

2Eridania Sadam S.p.A, Via degli Agresti 4/6, I-40123, Bologna, Italy 3 Sebigas S.p.A., Via Santa Rita, 21057, Olgiate Olona (VA), Italy

4 Sadam Engineering s.r.l., Via degli Agresti 4/6, I-40123, Bologna, Italy

Background

Agricultural wastes, obtaining by processing fruits and vegetables, nowadays are a huge environmental problem. On the other hand, they can represent sources for the production of high added value products, such as biofuels and fine chemicals. Winemaking is one of the most common fruit processing industries in the world: during the 2011 campaign 293 million hL of wine were produced, 46 million hL of which in Italy, which is the second producer of wine in the world after France (FAOSTAT, 2013). Vinification is a traditional and seasonal process which differs depending on the kind of wine to be produced. However, usually, it mainly consists of the same following phases: harvest, steaming, crushing, pressing, fermentation, sedimentation decanting, stabilization, bottling. The winemaking residues can be divided in liquid and solid ones. The former are composed by wastewaters used to clean and sterilize winery equipment.

The solid wastes consist mainly of grape stalks obtained from the destemmed grapes, pomaces or marcs obtained after pressing and lees from sedimentation of the fermented grape juice. Grape pomace (GP) is the most abundant winemaking waste, and it represents about the 20% (w/w) of grapes used for the production of wine (Schieber et al., 2001). During the 2011 campaign, about 1.1 Mt of GPs were produced in Italy (FAOSTAT, 2013). GP typically consist of skins (peels), stalks and seeds. Its composition depends on a lot of parameters, among which geographic production region, clime and winemaking process.

However, it is a fibrous solid waste rich in lignin, cellulose, hemicellulose and pectine due to the major presence of the fruit skin, usually characterized by 40% of TS and a low pH (around 4). GP is not rich in sugars, while it contains a high amount of tannins and polyphenols, which are high added value compounds well known for their antioxidant characteristics (Dinuccio et al. 2010; Bustamante et al. 2008). In Italy, GPs are sent to distilleries, where they are processed for the production of distillates. Alternatively, GPs could be valorized in different ways, by taking advantage of their chemical features and the presence of a lot of high added value substances.

According to literature, the main obtainable products and processes applicable to GPs can be summarized as follows: (a) polyphenols extraction; (b) animal feeding; (c) composting; (d) fermentation for the production of fine chemicals; (e) incineration; (f) biomethane. In particular, the last one is produced during anaerobic digestion, which is a biological process usually referring to an oxygen-free fermentation of biomass which lead to the production of biogas, i.e., a mix of gases represented by up to 70% of methane. The anaerobic digestion for the production of biogas from GPs can be a highly attractive process for the winery industry, by coupling the opportunity of generating income from waste with that of abating disposal costs. Author to whom all correspondence should be addressed: e-mail: stefano.rebecchi3@unibo.it

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Objectives

The objective of this work was to evaluate the technical feasibility of producing biomethane by exploiting GPs as the substrate for anaerobic digestion processes. GPs obtained from both red and white grapes were tested. For this purpose, the experimental matrices biomethane potential (BMP), defined as the maximum amount of methane obtained from discontinued operating conditions (batch), was evaluated. Methods Experimental set up

Micro-batch anaerobic reactors (microcosms) with a total volume of approximately 110 mL were set up to determine the BMP of the experimental matrices. The microcosms consisted of a Pyrex glass bottle hermetically closed with a double silicon septum inside of a perforated cap, which was used for sampling the head space gas. All tests were conducted incubating the microcosms statically at the temperature of 55°C (termophilic conditions). Each bottle was filled under nitrogen purging with a total final mass amount of about 55 g. This was composted by substrate (GP), anaerobic inoculum and osmotic water in different percentages, as reported below. Employed Red GPs came from the production of wines “San Giovese” and “Montepulciano”, while white GPs derived from Trebbiano and Verdicchio winemaking processes. Both GP types were obtained from Cantine Moncaro, Jesi, Italy. The methanogenic inoculum was previously selected and microbiologically characterized (Bertin et al., 2012), and it had a low content of total (3.15% w/w) and volatile solids (1.34% w/w).

A control experiment for each experimental condition was prepared within microcosms containing only water and inoculum, in order to subtract the amount of any eventually observed biogas to that produced in corresponding microcosms containing the GP, thus determining the BMP only relating to the pomace. At the same time the pH was correct with a NaOH solution moving from 4 (i.e., the initial pH of the broth), to a weakly alkaline pH (between 7 and 8), to promote the onset of methanogenic conditions.

1st Experiment

The objective of this experiment was to compare the experimental GPs in terms of BMP. GPs were dried and milled before being processed, to enhance the substrate bioavailability. The used GPs had high contents of TS and VS (White GP: TS 92.63 %, VS 87.53%; Red GP: TS 93.90 %, VS 84.35%). The microcosms were set up by filling them with 10 g of GP, 50 mL of water and 5 g of inoculum. All conditions were carried out in triplicate. The experiment lasted 12 weeks.

2nd Experiment

The influence of the ratio between inoculums and substrate was evaluated by employing the matrix, which gave rise to the best results during the first experiment (namely, the red GP). In particular, by keeping constant the amount of inoculum, different quantities of substrate were tested and vice versa. All conditions were carried out without varying the total amount of anaerobic digestion broth. The reproducibility of results obtained within the 1st experiment was also verified by repeating one experimental condition, which was set up during the 1st one. Table 1 summarizes the composition of the different developed microcosms and the used acronyms. The condition, which replaces the one of previous experiment is called I5S10. All conditions were carried on in duplicate. The experiment lasted 12 weeks.

Table 1. Composition of microcosms developed within the 2nd experiment

Acronym Inoculum (g) GP (g) Water (ml) TS (g) S (g)

I5S10 5 10 40 9.5 8.5 I5S5 5 5 45 4.9 4.3 I5S1 5 1 49 1.1 0.9 I5S15 5 15 35 14.2 12.7 I2S10 2 10 43 9.5 8.5 I10S10 10 10 35 9.7 8.6 I15S10 15 10 30 9.9 8.6 I20S10 20 10 25 10.0 8.7

Monitoring Microcosms were monitored periodically, at the beginning every 2 days, then by higher growing time

intervals, till monthly. Each monitoring consisted of the evaluation of the produced biogas, in terms of volume and composition, and of the volatile fatty acids (VFA) production, indices of acid-acetogenic fermentation that typically precedes the methanogenic respiration during the anaerobic digestion process. To those aims, microcosms were taken out from the incubator and let cool till ambient temperature.

Biomethane production from grape pomaces: a technical feasibility study

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The septum of the microcosms was perforated with a needle connected to a cilinder-Mariotte system to measure the produced biogas. Assuming that the composition of the biogas in the head of the bottle was still constant and no oxygen came inside, the septum were perforated another time by a needle connected to a microGC-TCD system, by which the composition of the produced biogas was determined. Afterwards, the cap was opened under a flux of nitrogen and an aliquot of the supernatant was sampled for VFA analyses. Then, if necessary, the pH was correct to the initial value (between 7 and 8). Finally, the end the bottle is closed with the cap and incubated again.

Analytical methods

Biogas composition was measured by a MicroGC 3000 Agilent Tecnologies coupled with a TCD detector (injector temperature 90 °C; column temperature 60 °C; sampling time 20 s; injection time 50 ms; column pressure 25 psi; run time 44 s; carrier gas, N2). Volatile Fatty Acids were analyzed by an Agilent GC-FID (model 7890A) equipped with an Agilent J&W GC column, 30 m x 0,25 mm x 0,25 µm (injection volume 1 µl; injector temperature 250°C; column head pressure 5 psi; column initial temperature 40 °C; 1 min isotherm; temperature rate 25°C/min; final temperature 150°C; 6 min isotherm; temperature rate 4°C/min; final temperature 180°C; temperature rate 25°C/min; final temperature 240°C; detector temperature 280°C). The samples were centrifuged for 5 min at 14000 rpm then the supernatant diluted with an oxalic acid solution (60 mM) in the ratio 1:4 or more in order to reduce all carboxyl terminations of VFAs. This method allowed the monitoring of the following VFAs: acetic, propionic, isobutyric, butyric, isovaleric, valeric, isocaproic, caproic and eptanoic acid. Results and discussion

The 1st experimental set suggested that the matrix, which allowed a higher biomethane production, was the red drayed-milled GP. 573-mL of biogas, 62.5% of which represented by methane, were produced after 3 months of incubation; as a whole, this corresponds to 41 ml of CH4 per g of VS. No methane production was observed within control microcosms, this meaning that all the above reported production was due to biomethanization of the substrates (GPs). Anyway, results were very low, if compared to available literature, where a BMP of 116 for red/white GPs was reported (Dinuccio et al., 2010).

Thus, with the proposal to enhance the biomethane production, a second set of experiment was carried out, with the aim of understanding the role of the ratio inocula/substrate. The profiles of BMPs and VFAs as a function of the time are shown in Figs. 1 and 2. BMPs are expressed as the cumulative amount of produced methane per the organic matter initially introduced in the microcosms (expressed as volatile solids).

Fig. 1. BMPs of matrices employed in the 2nd experiment

Fig. 2. Volatile Fatty Acids profile in the 2nd

experiment

After 90 days of incubation, the best result was reached by the microcosms I5S5, where 43±10 mL/gVS of CH4 and 391±57 mL of biogas were produced. Then, the I5S10 microcosms reached a production of 26±3 mL/gVS of CH4 and 571±50 mL of biogas. The obtained methane corresponded to a percentage of the 48 and 41%, respectively, of the total produced biogas. The microcosms I5S1, which had the lower quantity of TS, i.e., 1.1 g, gave rise to the lowest production of VFAs (Fig. 2), but not to the lowest BMP, which was achieved by microcosms I5S15 containing the highest amount of TS, i.e., 14.7 g. In these bottles about 32 g/L of total VFAs were achieved after 30 days of incubation, and that amount remained constant over time.

The best condition was the I5S5, whose TS content was 4.5 g. In such microcosms, the maximum concentration of produced VFAs was about 12.5 g/L.

Both values were among the lowest. The other conditions were characterized by more or less similar TS contents (around 9.5 g) and showed a trend in terms of production of acids and BMP approximately similar, and intermediate between the best and worst situations mentioned above. In general, the results were not satisfactory, being BMP values very low if compared to those cited above (Dinuccio et al., 2010) and regarding other solid vegetable matrices (Raposo et al., 2011).

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Actually, it was observed that during the phases of acid-acetogenesis, that precedes the methanogenesis, the production of acids was very high and the presence of such concentrations can not allow the achievement of suitable BMP. As for the concentration of single acids when maximum production of VFAs was reached, the mixture composition was about the same in each experiment and mainly consisted in about 60% of acetic acid, 30% of butyric acid, 5% of propionic acid and traces of the other acids.

In the I5S15 microcosms amounts of 19.6±2.8 g/L of acetic acid, 9.7±1.1 g/L butyric acid and 2.0±0.1 g/L propionic acid were observed after one month of incubation. Those data indicate that the acetogenesis was predominant with respect to the acidogenesis phase.

Concluding remarks

The anaerobic digestion for the production of methane was probably affected by the very high production of

VFAs, which was closely related to the content of TS. Thus, the performance of the anaerobic digestion strictly depended on total solids content. References Bertin L., Bettini C., Zanaroli G., Fraraccio S., Negroni A., Fava F., (2012), Acclimation of an anaerobic consortium capable of

effective biomethanization of mechanically-sorted organic fraction of municipal solid waste through a semi-continuous enrichment procedure, Journal of Chemical Technology and Biotechnology, 87, 1312-1319.

Bustamante M.A., Moral R., Paredes C., Pe´rez-Espinosa A., Moreno-Caselles J., Perez-Murcia M.D., (2008), Agrochemical characterisation of the solid by-products and residues from the winery and distillery industry, Waste Management, 28, 372–380.

Dinuccio E., Balsari P., Gioelli F., Menardo S., (2010), Evaluation of the biogas productivity potential of some Italian agro-industrial biomasses, Bioresource Technology, 101, 3780–3783.

FAOSTAT, (2013), Online la: http://faostat3.fao.org. Raposo F., De la Rubia M.A., Fernández-Cegrí V., Borja R., (2011), Anaerobic digestion of solid organic substrates in batch

mode: An overview relating to methane yields and experimental procedures, Renewable and Sustainable Energy Reviews, 16, 861–877.

Schieber A., Stintzing F.C., Carle R., (2001), By-products of plant food processing as a source of functional compounds: recent developments, Trends in Food Science & Technology, 12, 401–413.