Assessment of biogas production from MBT waste under different operating conditions
Introduction
Landfills still represent the dominant option for waste disposal in many parts of the world (Laner et al., 2012). However, as known, this option may pose a threat to groundwater pollution, soil contamination and global warming effects due to the potential emission of leachate and landfill gas to the surrounding environment (Pantini et al., 2014, Scaglia et al., 2010, Thomsen et al., 2012, White and Beaven, 2013). Indeed, landfill has been recognised as one of the main source of anthropogenic methane emission and a significant contributor to global warming (Bogner et al., 2008). Gas emissions from landfills are mainly dominated by methane and carbon dioxide that are generated from the anaerobic conversion of organic matter contained in waste as a result of biological processes naturally occurring in landfill sites. Moreover, due to the generally high nitrogen content in wastes, there is also a considerable potential for nitrous oxide emissions from municipal solid waste (MSW) landfills that can further enhance the global warming effects (Harborth et al., 2013).
In view of these concerns, throughout the world, new regulations in waste management and treatment strategies of municipal solid waste (MSW) have been introduced. For instance, in Europe, the Landfill European Directive 1999/31/EC imposes member states only landfill wastes that have been preliminary subjected to treatment or incineration. The directive aims at limiting the amount of biodegradable waste in landfills while encouraging alternative strategies in order to move towards more sustainable waste management system, according to the waste hierarchy approach (De Gioannis et al., 2009, Sormunen et al., 2008). To meet the European targets, member states have adopted different options, such as separate collection and recycling of organic waste stream, MSW incineration with energy recovery, biological treatments of source separated organic wastes or Mechanical Biological Treatment (MBT) plants of residual MSW (Lornage et al., 2007, Pantini et al., 2015, Scaglia et al., 2010). Among these, the MBT technology is playing a key role in the waste management system of unsorted MSW wastes (Adani et al., 2004, Farrell and Jones, 2009, Pantini et al., 2015, Siddiqui et al., 2013). All over Europe, MBT facilities can apply different combinations of mechanical sorting, bio-drying, and biological processes depending on the specific target, that may be a pre-treatment before incineration or a pre-treatment to produce a bio-stabilized product that has a lower impact when disposed of in landfills (Adani et al., 2004, Di Maria et al., 2013, Farrell and Jones, 2009, Montejo et al., 2013). In the latter case, the MBT plant consists of a mechanical pre-processing stage including crushing, sieving and recovering of recyclable materials (such as metals, glass and plastics). This stage leads to two distinct flows: the oversize fraction, which is further processed to produce refuse-derived fuel, and the undersize fraction, rich in organic putrescible matter, which is biologically treated using an anaerobic/aerobic process in order to stabilize it. The main distinction between different MBT systems concerns the sequence of process steps and the type and duration of the biological treatment (Pan and Voulvoulis, 2007, Pantini et al., 2015). The specific technology and process applied may strongly affect the long-term behaviour of MBT wastes in landfills in terms of both liquid composition and gas generation (Boldrin et al., 2011, Siddiqui et al., 2013). However, gas emissions from MBT waste have been rarely measured on full scale MBT landfills (Harborth et al., 2013). Hence, the current state of knowledge on biogas emissions is based either on laboratory tests or on large scale experiments such as lysimeters (Sormunen et al., 2008). Depending on the specific aim of the test, lab scale studies on gas emissions from MBT wastes and solid organic wastes are usually carried out using different procedures and operative conditions (see Table 1). As highlighted by Lornage et al. (2007), the differences in the experimental procedure adopted may modify the biogas yield and kinetics, thus leading to results that are not always comparable. The anaerobic process is indeed sensitive to several factors such as pH, water content, temperature, particle size, as well as by the presence of inhibitors such as of volatile fatty acids (VFAs), ammonia and heavy metals (Cabbai et al., 2013, Elbeshbishy et al., 2012, Labatut et al., 2011, Lornage et al., 2007, Raposo et al., 2011). Among these, pH is recognised as the key parameter to be maintained in an appropriate range (6.4–7.5) in order to enhance the methane yield (Adani et al., 2004, Argun et al., 2008, Lo et al., 2010). High pH values would result in increased toxicity due to the shift to higher concentrations of ammonia, which is identified as one of the most toxic agent for methanogenic bacteria (Chen et al., 2008, Vigneron et al., 2007). In contrast, low pH values are indicative of the accumulation of VFAs within the system (Bouallagui et al., 2005, Li et al., 2011). VFAs represent the main intermediate products during the initial acidogenic stage of the anaerobic process that are successively converted into methane and carbon dioxide. However, VFAs concentrations at high level may result in an inhibition of the methanogenic activity, as observed by several authors (Argun et al., 2008, Borzacconi et al., 1997, Cabbai et al., 2013). Regarding the other operative conditions, an increase of temperature has a positive effect on the microbial growth and activity (Chen et al., 2008) thus leading to a faster gas generation process. Similarly, increasing the water content of incubated waste is beneficial for methane yield since it enhances the solute transport of nutrient, the organic matter solubilisation and the microorganism mobilisation within micro-environments, as well as dilutes the concentration of inhibitors (Donovan et al., 2010, Mora-Naranjo et al., 2004). Finally, the particle size of materials exerts a relevant influence on the process kinetic; it is well accepted that particle size reduction results in higher methane generation rate (Esposito et al., 2012, Lesteur et al., 2010, Mata-Alvarez et al., 2000), whereas its effect on biogas yield is still not completely elucidated (Mshandete et al., 2006, Nopharatana et al., 2007).
The objective of this work was to evaluate the effects of temperature, water content and inoculum addition on biogas generation from mechanically–biologically treated waste by performing anaerobic batch tests at different operating conditions. Furthermore, in order to determine the potential gas generation capacity under optimal conditions, biomethane potential tests (BMP) were carried out. All these tests were then compared in terms of cumulative biogas yield and rates. Besides, where applicable, a first-order kinetic model was used to compute the biogas rate constants from the cumulative gas generation curves observed in each experiment. Finally, the obtained results were addressed to assess the possible implications resulting from the different environmental conditions expected in the field.
Section snippets
MBT waste material
Mechanically–biologically treated waste samples were collected at the belt discharge point of the secondary refinement unit of a full-scale MBT plant operating in Italy. This MBT plant receives residual municipal solid waste (226,000 ton/y in 2013), with the average composition shown in Table 2.
In this plant the incoming wastes are subjected to a mechanical pre-processing consisting of pre-sorting of bulky materials, shredding and size separation. From these processes two flows are obtained: the
MBT waste characterisation
Results of the characterisation analysis performed on the MBT waste are reported in Table 4. Moisture content (W), as well as water field capacity, were slightly lower than the values usually measured for this type of waste (Di Lonardo et al., 2014, Pantini et al., 2015, Zach et al., 2000). Despite the waste underwent an aerobic treatment process in the MBT plant, the organic matter of waste is still quite high, as confirmed by VS, TOC and COD contents. As shown in Table 4, the pH was almost
Conclusions and perspectives
The gas production from MBT wastes was analysed by performing anaerobic batch tests under different operating conditions. In order to characterise the MBT material regarding its long-term gas emission in different landfill disposal scenarios, a wide range of water contents (26–43% w/w up to 75% w/w) and temperatures (20–25 °C, 37 °C and 55 °C) were investigated. The obtained results suggest that the analysed MBT material still contains a large amount of readily degradable organic matter, as
Acknowledgement
The authors wish to thank Hector Garcia and Hector Diaz, Laboratory of Technical University of Denmark, for the valuable cooperation to the chemical–physical analysis.
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