HUNGARIAN JOURNAL OF INDUSTRY AND CHEMISTRY Vol. 49(1) pp. 31–35 (2021) hjic.mk.uni-pannon.hu DOI: 10.33927/hjic-2021-05 COMPARATIVE STUDY ON ANAEROBIC DEGRADATION PROCESSES OF PRESSED LIQUID FRACTION OF ORGANIC SOLID WASTE TAMÁS RÓZSENBERSZKI*1 , LÁSZLÓ KOÓK1 , PÉTER BAKONYI1 , NÁNDOR NEMESTÓTHY1 , AND KATALIN BÉLAFI-BAKÓ1 1Research Centre for Biochemical, Environmental and Chemical Engineering, University of Pannonia, Egyetem u. 10, Veszprém, 8200, HUNGARY Anaerobic degradation processes: anaerobic digestion (biogasification), biohydrogen fermentation (dark) and microbial fuel cells were applied to treat the organic fraction of a municipal solid waste. The processes were compared based on their ability of energy recovery and Chemical Oxygen Demand reduction. Keywords: organic waste, anaerobic biodegradation, microbial fuel cell, energy recovery, process comparison 1. Introduction, background 1.1 Waste challenges The world population has more than doubled over the last 60 years. Due to this growing tendency and urbaniza- tion the world’s energy consumption and value of waste generation present us with major challenges with sustain- able development in mind. Furthermore, it is obvious that waste treatment is one of the most critical global issue, because it has significant impacts for the health, local and global environment and economy [1]. According to the World Bank Group, 2.01 billion tonnes of municipal solid waste (MSW) around the world are generated annu- ally, and at least one third of that is not managed environ- mentally acceptable manner [2]. The average waste gen- erated per person per day is 0.74 kilogram, but there are significant differences between data by countries, from 0.11 to 4.54 kilograms. Actually, high-income countries only cover for 16 percent of world’s community, although generate around 34 percent of the world’s waste. Based on their estimation global waste will grow to 2.2 billion tonnes by 2025 and to 3.40 billion tonnes by 2050 [1, 2]. These facts make solid waste management (SWM) is a challenging task for decision-makers, who are required to provide essential waste collection and disposal ser- vices, generally under increasingly stringent budgetary pressures and regulatory requirements [3]. 1.2 Biowaste MSW typically consists of food waste, paper, glass, met- als, plastics, textiles, etc. In developed countries the *Correspondence: rozsenberszki.tamas@uni-pannon.hu amount of paper and plastics are relatively higher than the case of developing countries, where the main part of MSW is organic waste [4]. There are variations in the characteristics of MSW across the world, but remarkable part of the municipal solid waste is containing biodegrad- able organic components (world average: 46%) [1, 5]. There is a variety of treatment alternatives that provide not only disposal of this organic part but also energy recovery options. This section is going to present some anaerobic biodegradation processes so the following part of this section will focus on the organic waste. Based on the data of the Food and Agriculture Orga- nization roughly one-third of food produced for human consumption is lost or wasted globally, which amounts more than 1.2 billion tons per year [6, 7]. In the Euro- pean Union, more than 85 million tonnes of food waste are generated per year with associated costs estimated at around 143 billion euros [7, 8]. According to San Mar- tin et al. vegetable waste deposited as landfill could be reduced to 30% [9]. Some studies in this topic have in- dicated that vegetable waste has a remarkable potential for use as a raw material for animal feed. For example, Garcia at al. concluded that some part of various organic wastes (meat, fish, restaurant and household waste, fruit and vegetable) was possible to use in animal feed formu- lations [10]. 1.3 Treatment processes for the municipal solid waste It is important to notice the reduction of the waste prob- lem should be started at the prevention and reduce the level of the overconsumption. However, in our consumer https://doi.org/10.33927/hjic-2021-05 mailto:rozsenberszki.tamas@uni-pannon.hu 32 RÓZSENBERSZKI, KOÓK, BAKONYI, NEMESTÓTHY, AND BÉLAFI-BAKÓ Figure 1: Schematic illustration of an example how to in- tegrate bioprocesses in the MBT for the efficient MSW treatment society today the market sphere is not interested in the reduction of the consumption, because the drop of con- sumption means less profit. It is still common to dump the treated or not treated waste, instead of produce valuable products to sell commercially or for own use, possibly recover energy from them [11]. Waste dumping seems convenient and cheap solution but in the long term it is unprofitable and unsustainable technique. As long as this practice is followed, efforts should be made to continue the development such research that can minimize the neg- ative effects of excess use. In the case of society it is an important task to focus on how can expand the environ- mental friendly thinking already from the basic educa- tion. Fig. 1 presents the main treating processes of the MSW. In most cases the aim is to reduce the toxicity of the waste in addition energy generation and in the case of composting soil conditioners could be recovered. The most unpreferable technique of them is the waste dump- ing without gas collection or recovery [12]. Somewhat better choice is the landfilling which is currently the main technological facility applied to treat and dispose MSW worldwide. But this represents still low level based on the waste treatment hierarchy [13, 14]. Although landfill seems a cheap alternative, it can pollute the surround- ing area (air, soil and also the water). Over the years the collected landfill gas has limited use (no more than 60% methane content) [15]. Landfill gas with low CH4 content (low calorific gas) is difficult to directly burn so it does not seem to be the best solution [15]. The thermal pro- cesses can reduce significantly the volume of the waste but the cost of the plant installation and operation is rel- atively high. Moreover the flue gas and ash resulted need further treatments from environmental point of view [12]. 1.4 Biodegradation processes for waste treat- ment In the case of aerobic biological methods composting can stabilize the organic waste and could produce soil con- ditioners but the bound energy of the waste cannot be utilized. On the other hand it needs relatively large area and longer time to get valuable products [4]. Neverthe- less, due to the comparatively simple operation it is still a widely used technique for the treatment of organic rich fraction. Staying on the biological line the anaerobic diges- tion (AD) or biogasification is operating under anaero- bic conditions. Consequently, organic matter is degraded by a microbial community consisting of bacteria in the absence of oxygen and generating methane, carbon diox- ide, and useable residue without any exothermic heat. It seems advantageous to choose AD because the biogas (around 60 − 70% methane content) and biomethane are economically more valuable products than compost or the landfill gas. In addition the residue resulted by an anaero- bic process integrated with an aerobic stage has the same quality parameters like compost [16]. Before the installation of an AD system it is necessary to focus on the typical waste composition for the area be- cause it can show significant diversity. It is not a simple process but there are modelling possibilities. According to Cermiato et al. these combined bioprocesses includ- ing AD and digestate composting resulted higher perfor- mance than those applied pure composting [16]. In addi- tion AD usually causes lower environmental impact than composting because it can fulfill two levels of the waste hierarchy at the same time. Actually, the biodegradable waste (e.g., food loss, green waste) can be considered as a type of sustainable resources. In this view through AD process the energy is generated by a renewable source (biowaste) thus avoiding the energy which produced from conventional or fossil sources. Generally around 120 m3 of biogas can be produced with a total electricity yield of about 250 kWh and a net electricity yield of 204 kWh from one ton of biowaste [16]. Fei et al. carried out life cycle assessment on MSW treatment technologies. Results showed that the mechanical-biological treatment (MBT) had higher effi- ciency than landfill and incineration [17]. According to the life cycle assessment the worst option was the raw land filling. The incineration had a higher energy effi- ciency (20.5% energy recovery) but in this case the large amount of fly ash and exhaust treatment caused more en- vironmental impacts. It seemed the MBT had the highest energy efficiency (38.5%) when it combined with biogas purification method. Montejo et al. found similar results about connection MBT and AD [18]. In addition, MBT had less environmental impacts and relatively good sta- bility for the changing composition of MSW. On the other hand, MBT had weak economy performance and required economy support policy. Hungarian Journal of Industry and Chemistry COMPARATIVE STUDY ON ANAEROBIC DEGRADATION PROCESSES 33 Table 1: Main types of the municipal solid waste treatment [12] Treatment process Thermal treatment Biological treatment Landfilling Method Incineration Pyrolysis Gasification Refuse derived fuel Anaerobic digestion Composting Landfill with gas recovery Landfill without gas recovery Product Heat, Power Gas, Oil, Charcoal Syngas Heat, Power Biogas Compost Landfill gas - Energy recovery yes yes yes yes yes no yes no Table 2: Summary of the results coming from the sample utilization by various anaerobic degradation methods Process AD HDF MFC Amount of sample (cm3) 25 25 25 Volume of inoculum (cm3) 25 25 25 Valuable product methane∗(299 cm3) hydrogen∗(91 cm3) electricity Theoretical energy recovery∗∗ 11.7 kJ 1.14 kJ 0.031 kJ Particular energy recovery∗∗∗ 4.1 kJ 0.8 kJ 0.031 kJ Operation time (day) 40 2 30 Reduction of COD medium low high * Reffering to standard temperature and pressure ** Reffering to maximal utilization rate with no losses *** Reffering to utilization with losses: AD: biogas motor, HDF: fuel cell, MFC: direct use 2. MBT with other anaerobic processes In this subsection a particular example for an inte- grated anaerobic treatment is presented. A special sam- ple coming from the organic fraction of a munici- pal solid waste was studied [19–21]. Actually it was concentrated organic rich wastewater produced from mixed collected solid waste by pressing in a MBT plant (Királyszentistván). During the MBT separation technol- ogy a biodegradable fraction generated called biofraction utilized by the Plant’s biological stabilizing hall (compo- station) to treat it before the dumping (Fig. 1). The aim was to utilize the sample (before the composting process) with different anaerobic biodegradation methods to re- duce the organic content and produce energy or valuable products (hydrogen, methane). Thus the volume of waste will decrease (from the aspect of environmental protec- tion) whilst the energy content of the waste can be ex- ploited. In the first stage of experimental work the sample was characterized by analytical methods. It has high Chem- ical Oxygen Demand (COD) and Biochemical Oxygen Demand (BOD) content 111 g L−1 and 61 g L−1 respec- tively, which parameters are promising for the biological treatment used. The various methods were the anaerobic digestion (AD), biohydrogen dark fermentation (HDF) and a kind of bioelectrochemical system (BES): the mi- crobial fuel cell (MFC). As an inoculum mesophilic sludge from a biogas plant was used in each cases. The details about the materials and methods used were de- scribed in our previous studies [19–21]. Based on the experiments presented in Table 2, AD seems to be the most preferable method to integrate in the MBT process. It resulted high cumulative energy recov- ery (11.7 kJ) and medium COD removal, but it needs long time for the degradation mechanism (methanogenic path- ways). HDF lasted a few days and during the process 1.14 kJ cumulative energy was generated however the COD removal was in low level, thus the effluent needed further treatment. There were successful experiments where AD and MFC were combined to treat the COD of the effluent from HDF. On the other hand HDF is a promising method if the desired final product is the hydrogen which is oth- erwise an encouraging energy sources for the future [22]. During the two chambered MFC process direct electrical energy was generated, but it had lot of limitation factors including type and structure of the system, electrode ma- terials used, type of membrane, external and internal re- sistant, operation and adaptation period, biofouling, etc. Our results showed that if MFC system was integrated to HDF or AD the system’s energy recovery (coulombic efficiency) and COD removal could be higher. 3. Conclusion In many countries the waste management still does not get enough attention. The technologies of the biowaste treatment are already known just need to optimize for the characteristics of the waste streams in that area. Decision makers should choose the sustainable and low risk ways for the environment. The results of our and other exper- imental works showed that MBT combined with anaero- bic degradation processes could be an acceptable way to the clean and economical treatment in the case of signif- icant amount of mixed collected MSW. However, selec- tively collected biowaste has even more potential to max- imize the recovery of their energy content. Depending on the composition of the waste it may be advantageous to 49(1) pp. 31–35 (2021) 34 RÓZSENBERSZKI, KOÓK, BAKONYI, NEMESTÓTHY, AND BÉLAFI-BAKÓ integrate the different treatment methods to improve for an appropriate level of the effectiveness. Acknowledgements The authors thank for the financial support provided by the Széchenyi 2020 Programme under the project EFOP- 3.6.1-16-2016-00015, and by the Excellence of Strate- gic R+D Workshops under the project GINOP-2.3.2- 15-2016-00016 entitled “Development of modular, mo- bile water treatment systems and wastewater treatment technologies based at the University of Pannonia to en- hance growing dynamic exportation from Hungary be- tween 2016 and 2020”. REFERENCES [1] Hoornweg, D.; Bhada-Tata, P.: What a Waste: A Global Review of Solid Waste Manage- ment. Urban Development Series. Knowl- edge Papers No. 15, World Bank, 2012, 1. https://openknowledge.worldbank.org [2] Kaza, S.; Yao, L.C.; Bhada-Tata, P.; Van Woerden, F.: What a Waste 2.0: A Global Snapshot of Solid Waste Management to 2050. World Bank Publica- tions, 2018. 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Energy Rev., 2016, 57, 850– 866 DOI: 10.1016/j.rser.2015.12.112 49(1) pp. 31–35 (2021) https://doi.org/10.1007/s12649-015-9379-y https://doi.org/10.1007/s12649-015-9379-y https://doi.org/10.1016/j.chemosphere.2016.12.114 https://doi.org/10.1016/j.chemosphere.2016.12.114 https://doi.org/10.1016/j.jclepro.2015.06.116 https://doi.org/10.1016/j.jclepro.2015.06.116 https://doi.org/10.1016/j.rser.2015.12.112 Introduction, background Waste challenges Biowaste Treatment processes for the municipal solid waste Biodegradation processes for waste treatment MBT with other anaerobic processes Conclusion