Slovak Society of Chemical Engineering Institute of Chemical and Environmental Engineering Slovak University of Technology in Bratislava PROCEEDINGS 37 th International Conference of Slovak Society of Chemical Engineering Hotel Hutník Tatranské Matliare, Slovakia May 24 28, 2010 Editor: J. Markoš ISBN 978-80-227-3290-1 Kubaská, M., Sedláček, S., Bodík, I., Kissová, B.: Food waste as biodegradable substrates for biogas production, Editor: Markoš, J., In Proceedings of the 37th International Conference of Slovak Society of Chemical Engineering, Tatranské Matliare, Slovakia, 1413 1418, 2010.
Food Waste as Biodegradable Substrates for Biogas Production M.KUBASKÁ*, S.SEDLÁČEK, I.BODÍK, B.KISSOVÁ Institute of Chemical and Environmental Engineering, Slovak University of Technology, Radlinského 9, 812 37 Bratislava, Slovak Republic (*E-mail: miroslava.kubaska@stuba.sk) Keywords Anaerobic digestion, biogas, food waste, organic biodegradable substrates The presented contribution deals with laboratory testing of biodegradable municipal organic substrates for biogas production. The anaerobic fermentation and biogas production from biodegradable organic substrates such as expired wine, beer, bread, meat and dairy products, food oils and fats, were tested in the laboratory anaerobic models. INTRODUCTION According to Green Paper on the management of bio-waste in the European Union SEC(2008)2936 bio-waste is defined as biodegradable garden and park waste, food and kitchen waste from households, restaurants, caterers and retail premises, and comparable waste from food processing plants. Anaerobic digestion is especially suitable for treating kitchen waste. It produces a gas mixture (mainly methane 50 to 75 % - and carbon dioxide) in controlled reactors. Biogas can reduce greenhouse gas (GHG) emissions most significantly if used as biofuel for transport or directly injected into the gas distribution grid. The residue from the process, the digestate, can be composed and use for similar purpose as compost [1]. In the European Union, 2000 million tonnes of waste per year are generated and municipal solid waste (MSW) makes up about 14 % of total wastes. The management of MSW is a priority because improper treatment and disposal have a negative impact on the environment and also on human health. In this sense the following MSW management hierarchy was established: Prevention or minimization in generation Material recovery Recycling Incineration Disposal in controlled landfills [2]. Besides agricultural and industrial wastes the most relevant part of MSW is the organic fraction of municipal solid waste (OFMSW), with a daily production in Europe of about 400 000 tonnes. Among biological treatments, anaerobic digestion of biodegradable organic wastes is frequently the most popular and cost effective, due to the high energy recovery and low environmental impact [3]. Biogas production throughout Europe could reach over 15 million m 3 /d of methane [4]. In 2006, organic waste recycling in the EU had changed and with the exceptions of few countries, all the old European countries had started to collect organic waste separately to recycle it. Across Europe 1800 commercial composting sites were identified, treating 17 million tonnes of organic waste, however, disposal of food waste by landfill occupies large-scale areas. An important part of composted material (food waste, restaurant waste) is biologically easily degradable with high biogas production potential. 1413
Treatment of food waste by anaerobic digestion is also often reported with an estimated total capacity of 3.5 million tons [5]. During past decades, anaerobic digestion (AD) of organic matter has been used as a suitable method for treatment of organic biodegradable waste and production of energy from combustion of biogas. Also the anaerobic co-digestion of sewage sludge with OFMSW was examined and seems to be attractive. The benefits of this co-digestion include: dilution of potential toxic compounds, improved balance of nutrients, synergistic effects of microorganisms, increased load of biodegradable organic matter and better biogas yield [6]. Food wastes also have potential to be converted into methane by anaerobic digestion, and some studies have been carried out worldwide [7]. The thermophilic AD process could be applied to the kitchen wastes and it could increase the methane recovery rate of yield biogas [8]. The design and performance of anaerobic digestion processes are affected by many factors. Some of them are related to feedstock characteristics, reactor design and operation conditions. The physical and chemical characteristics of the organic waste are important information for designing and operating anaerobic digesters, because they affect biogas production and process stability during anaerobic digestion. They include moisture content, volatile solids content, nutrient content, particle size, and biodegradability. The biodegradability of a feedstock is indicated by biogas or methane yield and percentage of solids (total solids or volatile solids) that are destroyed in the anaerobic digestion. The biogas or methane yield is measured by the amount of biogas or methane that can be produced per unit of volatile solids contained in the feedstock after subjecting it to anaerobic digestion for a sufficient amount of time under a given temperature [9]. In order to develop an anaerobic digestion process for Korean food wastes containing 15-30% total solids the biochemical methane potentials of their components and mixer food waste were measured. The methane yields of cooked meat, boiled rice, fresh cabbage and mixed food wastes were 482, 294, 277 and 472mL/g VS added and anaerobic biodegradability based on stechiometric methane yield were 82%, 72%, 73% and 86%, respectively at 37 C and 28 days of digestion time [10]. In another study was evaluated the biodegradability of a traditional Korean food consisted of boiled rice (10-15%), vegetables (65-70%), and meat and eggs (15-20%) and reported that after 40 days a methane yield of 489mL/g VS could be obtained at 35 C [11]. The effectiveness of methane fermentation treatment used in food waste processing is currently limited by solubilization and acidogenesis. Temperature is an important factor in microbial activity, but the effect of temperature on the anaerobic solubilization and acidogenesis of food waste is unclear yet. Since solubilization is a potential limiting step in anaerobic digestion, it is crucial to determine the optimal temperature for anaerobic solubilization [12]. The objectives of this study were to describe the food waste digestion in the laboratory conditions with the aim of characterization of the basic technological parameters such a specific biogas production and biodegradability of substrates. The results of this study indicate that the biodegradable municipal solid waste is a highly desirable substrate for anaerobic digesters with regards to its high biodegradability and biogas yield. MATERIALS AND METHODS Characterization of food waste The fifteen different kinds of food waste used in this study were evaluated as potential substrate for anaerobic digestion. For each type of food waste, samples were obtained from 1414
shops, restaurants and households. All collected biodegradable food waste samples were analyzed for COD, total suspended solids (TSS) and volatile suspended solids (VSS). TSS was determined by drying the sample to a constant weight in an oven at 105 C and VSS was determined by calcination at 550 C to a constant weight. Anaerobic digestion tests The biodegradability and biogas yield of food waste were determined at 37 C (Schneider electric digital thermostat) using five batches anaerobic digestion tests (producer ASIO Bytča, Slovakia) with the total volume of each reactor 1.8 L. The reactors were stirred by IKA RW 14 basic. The evolve speed was 100 in minute, the stirring started every thirty minutes and the duration of stirring was 15minutes. Anaerobic microbial sludge, used as inoculums for the anaerobic reactors, was collected from an anaerobic digester of WWTP Devínska Nová Ves (Slovakia). The laboratory models were filled with the 1.4 L of new sludge before every dosage of new food waste sample. The initial values of the each sludge were measured. The dosage of substrate was adjusted by VSS content to avoid overloading of reactors. Daily biogas production from each digester was measured by using biogas flow meter. Chemical analysis of COD, TSS, VSS, nutrient contents and ph values was performed at the beginning, during the test and at the end of each biodegradability test. Figure. 1 Laboratory anaerobic digestion reactors RESULTS AND DISCUSSION Chemical composition of waste As a consequence of different nature of substrates the food waste samples are defined by different parameters. Liquid substrates such as sunflower oil, vine, beer and soft drink are characterized only by COD value, composition of solid substrates as bread, rolls, ham and salami is specified as VSS (g/kg). Apple, acidophilus milk and mixed food waste are defined both by COD values and VSS (g/l). The characteristics of food waste used in anaerobic digestion experiments are shown in Table 1. Anaerobic digestion tests Right before every dosage of food waste reactors were filled with new anaerobic sludge or with a mixture of sludge used in the previous digestion tests. The initial values of each sludge inoculums were determined and average were: ph 7.55, COD 14.86 g/l, TSS 18.41 g/l, VSS 10.99 g/l and VSS/TSS 60%. The range of values was wide and this was caused by reusing 1415
of sludge from previous experiments. Half an hour after the addition of substrate the samples of digestion mixture were taken and analyzed for ph, COD, TSS and VSS. In the filtered samples ph, COD, N tot, P tot, NH4-N and volatile fatty acids (VFA)values were measured (Tab. 2). Table 1. Chemical composition of food waste SUBSTRATE COD (g/l) SUBSTRATE COD (g/l) Apple 74.7 Sunflower oil used 3452.3 Acidophilus milk 74.6 Soft drink Kofola 71.0 Mixed food waste 252.0 White vine 64.6 Sunflower oil unused 2991.2 Beer 63.5 SUBSTRATE TSS (g/kg) VSS (g/kg) VSS/TSS (%) Ham 222.6 184.9 83 Salami 729.5 687.3 93.6 Bread 875.0 850.0 97 Rolls 733.8 715.8 96 SUBSTRATE TSS (g/l) VSS (g/l) VSS/TSS (%) Apple 88.9 86.6 97 Acidophilus milk 103.2 97.5 97 Mixed food waste 155 142.7 92 Table 2. Basic parameters of sludge UNFILTERED SAMPLE FILTERED SAMPLE VSS/TSS ph N tot NH4-N P tot VFA SUBSTRATE TSS (g/l) (%) (-) (mg/l) (mg/l) (mg/l) (mg/l) Apple 11 69 8.62 605 450 35 481 Acidophilus milk 12 65 8.63 765 696 28 471 Mixed food waste 12 63 8.31 743 698 25 498 Sunflower oil unused 11 65 8.38 858 508 27 598 Sunflower oil used 10 68 8.42 928 520 26 645 Soft drink Kofola 17 66 7.27 648 580 15 550 White vine 10 60 6.46 643 344 29 489 Beer 8 62 8.22 580 460 14 520 Ham 21 51 8.42 967 800 20 498 Salami 28 48 8.37 775 690 18 470 Bread 13 65 8.39 953 512 25 631 Rolls 12 72 8.34 740 516 28 730 Composition of sludge was monitored during whole digestion tests to assure right development of experiment and to observe degradation of organic matter. Biodegradability of food waste could be characterized by decrease of COD value as it shown at fig.2 (a) and 2 (b). From this figures it can be concluded that progress of COD removal is different for every substrate and this process have various development. As it is shown at fig. 2 (a) COD of vine was decreased by 120 mg/l after 15 days but on the other hand 3000 mg/l COD of apple was removed after two days which suggests that apple is high-speed degradable substrate for AD than vine. This fact leads to conclusion that high-speed degradable substrate is required in more frequent doses. From fig. 2 (b) is obvious that degradation time of substrates as ham and 1416
salami was more than 30 days but progress was continuous. Digestion time was set by biogas production not by COD removal. a b Figure 2 (a), 2 (b). Degradation of COD value in anaerobic digestion process. Biogas production Measurement of biogas production was provided daily and it is introduced in figure 3 (a) and 3 (b). An important part was observing of process development due to which can be concluded that biogas yield from substrates as salami, ham, bread and oils have continuous progress. It is also obvious that biogas production from ham 1.3 L/g VSS is highest but development is not stabile. Biogas yields from salami, rolls, mixed food waste and acidophilus milk is at same level above 0.4 L/gVSS. From liquid substrates: vine, beer and Kofola it is clear that production of biogas is rapid at the beginning but then the process is stopped. The lowest biogas yield was reach from apple 0.1 L/gVSS and unused sunflower oil 0.15 L/gCOD, but sunflower oil also has high value of COD 2991g/L. a b Figure 3 (a), 3(b) Development of biogas production. 1417
CONCLUSIONS The results from research in Faculty of Chemical and Food Technology Bratislava reveal that most of the food waste studied had a high potential for utilization as biodegradable substrate for biogas production. However, in the order to use these waste fraction as substrate for anaerobic digestion it is necessary to take into account character, composition and availability of each waste. ACKNOWLEDGEMENT: The presented contribution was supported by the Slovak Research and Development Agency under the contract No. LPP-0019-09. REFERENCES 1. European Commission, 2008. Green Paper On the management of bio-waste in the Europian Union {SEC (2008)2936}. 2. García, A.J., Esteban, M.B., Márquez, M.C., Ramos, P., 2005. Biodegradable municipal solid waste: Characterization and potential use as animal feedstuff, Waste Management 25, 780-787. 3. Mata-Alvarez, J., Macé, S., Llabrés, P., 2000. Anaerobic digestion of solid wastes. An overview of research achievements and perspectives, Bioresource Technology 74, 3-16. 4. Tichle, A., Malaspina, F., 1998. Biogas production in Europe. Paper presented at the 10 th European Conference Biomass for Energy and Industry, Wurzburg, Germany, 8-11 June. 5. Kidby, D., 2009. Food, glorious food. On-line journal Waste Management World, www.waste-management-world.com. 6. Poggi-Varaldo, H.M., Oleszkiewicz, J.A., 1992. Anaerobic co-composting of municipal solid waste and waste sludge at high total solid level. Environmental Technology 13, 409-421. 7. Lai, CM., Ke, GR., Chung, MY., 2009. Potentials of food wastes for power generation and energy conservation in Taiwan, Renewable Energy 34, 1913-1915. 8. Lai, CM, Chen, SW, Chen, KH, Lee CC, Liu KI, Wei CB., 2006. The coposting of household food waste in Teipai city. Biomass Energy Soc China 25, 53-64. 9. Zhang, R., El-Mashad, H.M., Hartman, K., 2007. Characterization of food waste as feedstock for anaerobic digestion, Bioresource Technology 98, 929-935. 10. Cho, J.K., Park, S.C., Chang, H.N., 1995. Biochemical methane potential and solid state anaerobic digestion of Korean food wastes, Bioresource Technology 52, 245-253. 11. Heo, N.H., Park, S.C., Kang, H., 2004. Effects of mixture ratio and hydraulic retention time on single-stage anaerobic co-digestion of food waste and waste activated sludge. Int. J. Hydrogen Energy 29 (15), 1607-1616. 12. Komemoto, K., Lim, Y.G., Nagao, N., Onoue, Y., Niwa, C., Toda, T., 2009. Effect of temperature on VFA s and biogas production in anaerobic solubilization of food waste, Waste Management 29, 2950-2955. 1418