Environmental Engineering and Management Journal November 2013, Vol.12, No. S11 Supplement, 93-96

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

“Gheorghe Asachi” Technical University of Iasi, Romania

ANAEROBIC DIGESTION OF POTATO INDUSTRY BY-PRODUCTS ON

A PILOT-SCALE PLANT UNDER THERMOPHILIC CONDITIONS

Extended abstract

Agata Pistis1, Carla Asquer1, Efisio Antonio Scano1,2

1Biofuels and Biomass Laboratory, Sardegna Ricerche, Z.I. Macchiareddu 09010 Uta (CA), Italy.

2University of Cagliari, Department of Mechanical, Chemical and Materials Engineering, Via Marengo, 2 09123 Cagliari, Italy

Background

Potato industry by-products are typically produced in industrial processing such as freezing, drying, canning and frying. This kind of waste includes, for instance, skins and tubers with defects and mechanical damages that are rejected in the process of potato chips production.

These residues have a high perishability and their quick removal and disposal is therefore mandatory, with consequent high costs for the food companies. Since these materials have a high moisture content, they are an eligible feedstock for anaerobic digestion (AD). Nevertheless, because of their high soluble organic compounds content, the anaerobic digestion of potato by-products as a single substrate is considered very critical. Scientific literature contains some studies on anaerobic digestion of potato tuber and its industrial by-products (Kaparaju et al., 2005; Parawira et al., 2004, 2005).

Many authors have reported unstable process conditions when treating this kind of substrate in mesophilic conditions. In fact potato industrial by-products contain high amounts of soluble substances that can be easily degraded to volatile fatty acid and inhibit the methanogenic microorganisms in single stage anaerobic digesters, as shown by Kaparaju et al., 2005. To overcome this problem, these authors used pig manure as co-substrate. Another study reports the results of the batch anaerobic digestion of potato waste alone and in combination with sugar beet leaves (Parawira et al., 2004). In this case the authors obtained an improved methane yield when applying the co-digestion compared to the digestion of the single substrates.

They studied the anaerobic digestion of solid potato waste in two double-stage mesophilic anaerobic digestion systems. The first consisted of a solid-bed reactor connected to an upflow anaerobic sludge blanket reactor (UASB), while the second was a solid-bed reactor connected to a methanogenic reactor packed with a wheat straw biofilm. The configuration with the packed straw bed had a greater speed of degradation compared to an UASB system. The methane yield was the same for both systems (Parawira et al., 2005).

The anaerobic treatment of potato processing solid waste was also studied in completely stirred tank reactors (CSTR) under thermophilic conditions (55°C) and on lab-scale equipments. In particular a kinetic study to find simple model equations for the design of completely stirred tank reactors (CSTR) was carried out (Linke, 2006).

The study shows that an increase in the organic loading rate (OLR) causes a decrease in both biogas yield and CH4 content. To the authors’ knowledge, all the experiments described were performed on laboratory scale reactors, while experiments at large scale have not been performed yet.

Actually, anaerobic digestion of potato industrial by-products as a single substrate is a challenging process due to the high biodegradability of this material which can lead to the rapid and strong acidification inside the reactor with a consequent inhibition of the methanogenic bacteria activity.

In this paper an experimental assessment of biogas production from potato industrial wastes at thermophilic conditions is presented.

Author to whom all correspondence should be addressed: e-mail: pistis@sardegnaricerche.it

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In particular, the study is based on the experimental performance of a pilot scale anaerobic digestion reactor (1.13 m3 of volume) fed with potato industrial by-products as a single substrate derived from a Potato Chips Line (Terrantica Srl) located in Sardinia (Italy).

Scanning Electron Microscopy (SEM) was used to the preliminary study of the morphology of the microbial species involved in the anaerobic digestion process, as this technique was recently demonstrated to be suitable for this purpose (Molinuevo-Salces et al., 2012).

Methods

In this experiment, a pilot plant (RES Italia) was used. The plant includes: a cutter for substrate pre-treatment, a feeding hopper, a tubular horizontal reactor, a pneumatic feeding pump, a pneumatic digestate discharge pump, a biogas measuring and a treatment unit, a digestate storage tank, an air compression unit, a gas holder and a control and supervision system (PLC). After cutting, the potato wastes are delivered to the feeding hopper, where they are mixed by means of a vertical stirrer before being pumped inside the reactor. The feeding hopper is mounted on load cells, is thermally insulated and can be heated with an electrical heater. The tubular horizontal reactor has an overall volume of 1.13 m3, it is partially insulated with a polymeric layer and equipped with a radial stirrer and an electrical heating system which allows working at temperatures up to 60°C. The biogas composition, the total solid contents (TS%), the pH and the ratio between volatile organic acids and alkaline buffer capacity (FOS/TAC) were daily monitored.

Scanning electron microscope (SEM) images of the micro-organisms inside the feeding hopper and the reactor were also obtained by using a FEI Helios Nanolab 600 dualbeam (SEM/FIB).

The substrate materials used for the experimental assessment were sampled weekly at the potato chips line of Terrantica Srl (Italy) for the full duration of the experiment.

Once received in the laboratory, every sample was ground and homogenized by means of the cutter. If not immediately processed, the samples were stored in a refrigerator at 4 °C for a few days or frozen and stored at -18°C for later use. All samples were characterized in terms of chemical and physical properties. In particular, the Total Solid (TS) content, the Volatile Solid (VS) content, the ash content and the ultimate composition (C, H, N, and S contents) were determined. The TS and VS contents of the substrate were determined with a LECO TGA701 Thermo Gravimetric Analyzer according to the ASTM D5142 Moisture Volatile Ash method. The same instrument was used for the determination of TS and VS contents in the digestate. The Higher Heating Value (HHV) of both substrate and digestate was measured according to the UNI EN 14918:2009 method with a LECO AC500 Isoperibolic Calorimeter. The contents of C, H, N and S were determined with a LECO TRUSPEC CHN according to the ASTM D5373 method. The biogas composition (amount of methane, carbon dioxide, Oxygen, hydrogen sulphide) was determined with a Geotech GA2000 Gas Analyser. Experimental

The experiment was started by inoculating the reactor with 935 kg of digestate from anaerobic digestion of fruit and vegetable wastes to provide the starting population of microorganisms. The temperature of the reactor was then gradually raised up to thermophilic conditions (50 °C ± 0.5 °C) and this temperature was kept constant for the full duration of the experiment. Overall, the monitoring period lasted 72 days. It included a start-up period, which lasted 48 days and during which the complete substitution of the initial fruits and vegetables digestate with the potato wastes was achieved, and a stationary phase, called phase 1 in the following sections, which lasted 24 days .

The goal of the experimental study was to optimise the AD process operating parameters, to achieve an efficient conversion of potato wastes into biogas. The control of the process was carried out by monitoring the reactor mass content and the most important operating parameters, such as: TS and VS of both substrate and digestate, Organic Loading Rate (OLR), FOS/TAC ratio, pH, biogas composition, Gas Production Rate (GPR), Hydraulic Residence Time (HRT).

Results and discussion

The average analytical composition of potato industrial by-products shows a moisture content ranging from 75% to 85% depending on the particular type of waste (whole tubers, skins), a C/N ratio around 30 and a volatile solids content on dry basis ranging from 82% to 90%.

Fig. 1a-d summarizes the most important results of the study in terms of biogas composition (Fig. 1a), FOS/TAC ratio and organic loading rate (Fig. 1b), mass and VS daily loading rate (Fig. 1c), biogas production rate and methane content (Fig. 1d). As shown in Fig. 1a, the mass flow rate was increased constantly during the whole start-up phase, in order to achieve a complete substitution of the initial inoculum. In the following phase fluctuations in the daily loading rate can be observed. They were caused by corrective actions required to control the organic loading rate (OLR). As shown in Fig. 1b, in the start-up phase the OLR was increased up to about 2 kgVS/d·m3, with a corresponding increase of the FOS/TAC ratio. The FOS/TAC ratio of anaerobic digestion processes should be in the range of 0.3-0.4 and that FOS/TAC ratios above 0.4 could indicate instability process conditions (Lossie et al.,

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2008). This normally leads to a decrease in biogas production. For this reason, once the FOS/TAC ratio reached values above 0.4, the OLR was reduced to about 1.5-2.0 kgVS/d·m3. However, our experimental results demonstrate that even for FOS/TAC ratios up to 0.5, the AD process still maintained high yields (in terms of biogas production and methane content). Fig. 1c shows the biogas composition trend. As can be noted, the CH4 content ranged between 43.4% and 63.1 % with an average value of 54.7 % in the phase 1, while CO2 ranged between 37.4% and 56.8%. During the whole experimental study the hydrogen sulphide (H2S) content in the biogas reached a maximum value of 363 ppm, that is lower than the limit values (about 1000 ppm) suggested for the direct use of biogas in internal combustion engines. Fig. 1d shows the trend of the Gas Production Rate (GPR). In particular, it demonstrates that in good process conditions (FOS/TAC ratio around 0.4 and OLR not above 2.0 kgVS/d·m3), the GPR varied from 1.1 to 1.3 Nm³biogas/(m³reactor·d), while in suboptimal conditions the GPR was below 1.0 Nm³biogas/(m³reactor·d).

The process stability was reached with an organic loading rate (OLR) of about 2.0 (kgVS/m3)/d. This corresponded to a loading rate of 40 kg/d with a hydraulic retention time of 24 days. The maximum GPR was 1.3 Nm3/d with a specific methane production of 0.429 Nm3/kgVS. In stable process conditions, about 70% of the volatile solid content of the feeding substrate was converted into biogas.

Fig. 2 shows SEM pictures of the material in the feeding hopper (A), at the head (B) and at the end of the anaerobic reactor (C). For each of the three samples two images at different magnifications are presented. In the hopper bacteria of the cocci and rod-shaped types were predominant. At the reactor head, long rod-shaped bacteria seemed to be predominant. At the reactor end long rod-shaped bacteria and other shape bacteria were present. However an in-depth study is necessary for a correct identification of the various kinds of bacteria involved.

Fig. 1. Main experimental results of the pilot-scale system. a) Loading rate and VS Loading rate. b) Organic

loading rate trend. c) Biogas composition. d) Biogas production rate and CH4 % content

Fig. 2. SEM images ad two different magnifications of the material in the feeding hopper (A), at the reactor head (B) and at the reactor

end (C)

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Table 1 summarizes the average values of the specific gas production and the specific methane production (i.e. the volume of biogas and methane produced per kg of volatile solids fed to the pilot plant) during the two experimental phases. It is interesting to observe that the reported values are high if compared to the ones obtained with other kinds of wastes such as fruits and vegetables. For this reason the thermophilic anaerobic digestion of potatoes industrial by-products as a single substrate can be a useful and advantageous technology from both environmental and the energetic point of view.

Scanning electron microscopy was then been used to perform a rough identification of the kinds of bacteria that are present in different parts of the anaerobic digestion plant.

Table 1. Average values of the specific gas production and the specific methane production

Experimental period Specific gas production Specific methane yield

Start- up 0.91 Nm3/kgVS 0.50 Nm3/kgVS Phase 1 0.68 Nm3/kgVS 0.37 Nm3/kgVS

Concluding remarks

In this paper we demonstrated that potato industrial by-products are suitable materials for biogas production

by means of single substrate AD. However the result of the experiment showed that it is essential to perform a tight control of the process parameters.

The maximum daily loading rate of wastes was 40 kg/d, with a corresponding hydraulic residence time of 24 days. Higher values of the daily loading can cause process instability. The optimum organic loading rate was around 2.0 kgVS/d·m3 and the average specific biogas production was about 0.68 Nm3/kgVS, with a specific methane yield of about 0.37 Nm3/kgVS. Moreover, a first rough identification of certain microbial forms was obtained.

Keywords: anaerobic digestion, biogas production, potatoes industrial by-products, scanning electron microscopy, thermophilic digestion

Acknowledgements The authors wish to thank Dr. Elodia Musu and Eng. Simona Podda of Sardegna Ricerche Telemicroscopy Laboratory. This work was carried out in the framework of an agreement protocol between Terrantica Srl and Sardegna Ricerche –Technology Park of Sardinia.

References Kaparaju P., Rintala J., (2005), Anaerobic co-digestion of potato tuber and its industrial by-products with pig manure, Resources

Conservation & Recycling, 43, 175-188. Linke B., (2006), Kinetic study of thermophilic anaerobic digestion of solid wastes from potato processing, Biomass and

Bioenergy, 30, 892-896. Lossie U., Pütz P., (2008), Targeted Control of biogas plants with the help of FOS/TAC, Practice Report, Hach Lange, On line at:

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Molinuevo-Salces B., González –Fernández C., Gómez X., Cruz García- González M., Morán A., (2012), Vegetable processing wastes addition to improve swine manure anaerobic digestion: Evaluation in terms of methane yield and SEM characterization, Applied Energy, 91, 36-42.

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