integrated ozonation and anaerobic digestion for the
INTEGRATED OZONATION AND ANAEROBIC DIGESTION FOR THE RECOVERY OF ORGANIC SOLID WASTE A. CESARO AND V. BELGIORNO Sanitary Environmental Engineering Division (SEED), Department of Civil Engineering, University of Salerno, via Giovanni Paolo II, 84084 Fisciano (SA), Italy
SUMMARY: This work focuses on the ozonation of the digestate originating from the anaerobic treatment of the organic fraction of municipal solid waste. The experimental activity was carried out to verifiy the effects of the application of ozone, a powerful oxidant, on the anaerobic biodegradability of the digestate as well as on the presence of organic contaminants. Such pollutants could, indeed, enter the anaerobic treatment along with waste and end up in the digestate due to their recalcitrance to biological degradation processes. Experimental results pointed out the effectiveness of the proposed approach, suggesting that the integrated ozonation and anaerobic digestion process could become a proper strategy to ensure the sustainable recovery of organic solid waste.
1. INTRODUCTION Anaerobic digestion is a well-established process for the treatment of the organic fraction of municipal solid waste, with an installed treatment capacity continuously growing over time in Europe (De Baere and Mattheeuws, 2015). The main strength of this technology lays in the possibility of converting residual organic substrates into a methane-rich gas, which can be recovered for energy purposes (Woon et al., 2016; Riggio et al., 2017). The optimization of anaerobic digestion yields in terms of methane production represents, thus, an aspect of great technical and scientific interest. It has been extensively recognised that the increase in methane production can be pursued through proper pretreatments, relying on chemical, physical, biological techniques and aiming at promoting the solubilisation of complex organic substrates, thus enhancing the amount of organic matter readily available for its biological conversion into methane (Ariunbaatar et al., 2014; Cesaro et al., 2014). The most frequently studied chemical pretreatments rely on the of use of alkali reagents, often in combination with thermal processes (Abudi et al., 2016; Solé-Bundó et al., 2017). More recent trends are directed towards the use of organic solvents, including alcohols (Amiri et al., 2014; Li et al., 2012; Zhu et al., 2015) as well as organic acids (Wang et al., 2011; Qin et al., 2012). They have been mainly investigated to process lignocellulosic substrates, with the aim of separating high-purity cellulose from both the lignin and hemicellulose fractions (Zhang et al., 2016), which can be differently recovered and reintroduced in the material cycle, in accordance with the principles of the circular economy approach (Cossu, 2013; McArthur, 2013; Ghisellini et
Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
al., 2016). Ozonation, relying on the application of ozone, a powerful oxidant, is a further chemical pretreatment method, which has been successfully applied at full-scale to process sewage sludge destined to anaerobic stabilization (Sievers et al., 2004). It is an attractive technique, whose main advantage lays in the absence of residues left in the ozonated organic substrate (Carrére et al., 2010). The application of ozone doses up to 0.16 gO3/gTS was found to effectively improve the solubilisation of organic solid waste and its anaerobic biodegradability. However, the larger scale application of this technology seems to be not competitive with other pretreatments, due to the need of operating with tightly low ozone doses (Cesaro and Belgiorno, 2013). Conversely, the application of such low ozone doses to both sewage and anaerobically digested sludge has been proved to be effective in reducing the concentration of selected organic contaminants (Bernal Martinez et al., 2009). The removal yield was observed to be dependent on the ozonation conditions as well as on the kind of contaminant. The presence of persistent organic contaminants in the organic solid waste destined to anaerobic digestion has been raising great concern: these pollutants are not effectively degraded during biological treatments (Brändli et al., 2007), ending up in the final products of waste recovery processes. This condition poses a severe issue related to the potential sanitary and environmental risk associated to the application of waste-based products on soil as well as the need for the identification of proper technical solutions. The aim of this work is in investigating the effects of ozonation on the digestate originating from the anaerobic treatment of the organic fraction of municipal solid waste. To this end, digestate samples were collected at a full-scale plant, treating the separately collected organic fraction of solid waste by a wet anaerobic digestion process, characterized by their chemicalphysical properties and contaminated with benzo[a]pyrene, as target compound for a polycyclic aromatic hydrocarbon contamination. These samples were used to verify the influence of ozone, both in terms of organic contaminant removal and digestate anaerobic biodegradability enhancement.
2. MATERIALS AND METHODS The experimental activity was carried out on digestate samples collected at a full-scale treatment plant, treating the source sorted organic fraction of mumicipal solid waste through a wet anaerobic digestion process. The digestate samples, characterized by their content in organic pollutants including policycle aromatic hydrocarbons (PAHs), were spiked with a benzo[a]pyrene solution and ozonated. The effects of the ozone oxidation were referred to both the reduction in b[a]p concentration and the anaerobic biodegradability of the digestate. 2.1 The substrate The experimental acidity was performed using the digestate originating from the full-scale anaerobic treatment of the organic fraction of municipal solid waste. The samples were collected from the digester, ensuring a retention time of 11 days, and completely characterized by its main chemical and physical parameters (Table 1). The analysis of polycicle aromatic hydrocarbons (PAHs) was performed to assess the need for artificial contamination.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Table 1. Characterization of the digestate samples. Parameter
Digestate
TS [%]
4,59 ± 0,47
VS [%TS]
58,04 ± 6,93
COD [mg/L]
89.200 ± 20.903
sCOD [mg/L]
56.149 ± 8.610
BOD5 [mg/L]
45.960 ± 5.600
PAH [mg/kgSS]
-
2.2 The preparation of the spiked digestate samples Benzo[a]pyrene was assumed as the target compound for a polycyclic aromatic hydrocarbon contamination. The digestate samples were spiked with a 5 mg/L benzo[a]pyrene solution, so as to obtain a concentration of 100 mg/kgSS. To this end, the solution was added to the digestate samples, that were stirred for 24 hours, so as to promote the controlled volatilization of the solvent used to prepare the b[a]p solution. 2.3 The ozone treatment Ozone production was obtained from a UV generator (model Ozone - Procom srl), using air from a compressor. The air stream containing ozone was then introduced at the bottom of a glass reactor containing the samples; exhaust gases were extracted on the top of this reactor and forced to pass through a Drechsel trap filled with 200 mL of 2% KI solution, in order to capture residual ozone and determine the ozone demand. Ozone doses ranging between 0 and 0,16 gO3/gTS were investigated. 2.4 Analytical set up Chemical Oxygen Demand (COD), total solid (TS) and volatile solid (VS) were evaluated according to Standard Methods (AWWA-APHA-WEF, 1998). The soluble COD (sCOD) was measured on each sample after centrifugation and filtration (
SUMMARY: This work focuses on the ozonation of the digestate originating from the anaerobic treatment of the organic fraction of municipal solid waste. The experimental activity was carried out to verifiy the effects of the application of ozone, a powerful oxidant, on the anaerobic biodegradability of the digestate as well as on the presence of organic contaminants. Such pollutants could, indeed, enter the anaerobic treatment along with waste and end up in the digestate due to their recalcitrance to biological degradation processes. Experimental results pointed out the effectiveness of the proposed approach, suggesting that the integrated ozonation and anaerobic digestion process could become a proper strategy to ensure the sustainable recovery of organic solid waste.
1. INTRODUCTION Anaerobic digestion is a well-established process for the treatment of the organic fraction of municipal solid waste, with an installed treatment capacity continuously growing over time in Europe (De Baere and Mattheeuws, 2015). The main strength of this technology lays in the possibility of converting residual organic substrates into a methane-rich gas, which can be recovered for energy purposes (Woon et al., 2016; Riggio et al., 2017). The optimization of anaerobic digestion yields in terms of methane production represents, thus, an aspect of great technical and scientific interest. It has been extensively recognised that the increase in methane production can be pursued through proper pretreatments, relying on chemical, physical, biological techniques and aiming at promoting the solubilisation of complex organic substrates, thus enhancing the amount of organic matter readily available for its biological conversion into methane (Ariunbaatar et al., 2014; Cesaro et al., 2014). The most frequently studied chemical pretreatments rely on the of use of alkali reagents, often in combination with thermal processes (Abudi et al., 2016; Solé-Bundó et al., 2017). More recent trends are directed towards the use of organic solvents, including alcohols (Amiri et al., 2014; Li et al., 2012; Zhu et al., 2015) as well as organic acids (Wang et al., 2011; Qin et al., 2012). They have been mainly investigated to process lignocellulosic substrates, with the aim of separating high-purity cellulose from both the lignin and hemicellulose fractions (Zhang et al., 2016), which can be differently recovered and reintroduced in the material cycle, in accordance with the principles of the circular economy approach (Cossu, 2013; McArthur, 2013; Ghisellini et
Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
al., 2016). Ozonation, relying on the application of ozone, a powerful oxidant, is a further chemical pretreatment method, which has been successfully applied at full-scale to process sewage sludge destined to anaerobic stabilization (Sievers et al., 2004). It is an attractive technique, whose main advantage lays in the absence of residues left in the ozonated organic substrate (Carrére et al., 2010). The application of ozone doses up to 0.16 gO3/gTS was found to effectively improve the solubilisation of organic solid waste and its anaerobic biodegradability. However, the larger scale application of this technology seems to be not competitive with other pretreatments, due to the need of operating with tightly low ozone doses (Cesaro and Belgiorno, 2013). Conversely, the application of such low ozone doses to both sewage and anaerobically digested sludge has been proved to be effective in reducing the concentration of selected organic contaminants (Bernal Martinez et al., 2009). The removal yield was observed to be dependent on the ozonation conditions as well as on the kind of contaminant. The presence of persistent organic contaminants in the organic solid waste destined to anaerobic digestion has been raising great concern: these pollutants are not effectively degraded during biological treatments (Brändli et al., 2007), ending up in the final products of waste recovery processes. This condition poses a severe issue related to the potential sanitary and environmental risk associated to the application of waste-based products on soil as well as the need for the identification of proper technical solutions. The aim of this work is in investigating the effects of ozonation on the digestate originating from the anaerobic treatment of the organic fraction of municipal solid waste. To this end, digestate samples were collected at a full-scale plant, treating the separately collected organic fraction of solid waste by a wet anaerobic digestion process, characterized by their chemicalphysical properties and contaminated with benzo[a]pyrene, as target compound for a polycyclic aromatic hydrocarbon contamination. These samples were used to verify the influence of ozone, both in terms of organic contaminant removal and digestate anaerobic biodegradability enhancement.
2. MATERIALS AND METHODS The experimental activity was carried out on digestate samples collected at a full-scale treatment plant, treating the source sorted organic fraction of mumicipal solid waste through a wet anaerobic digestion process. The digestate samples, characterized by their content in organic pollutants including policycle aromatic hydrocarbons (PAHs), were spiked with a benzo[a]pyrene solution and ozonated. The effects of the ozone oxidation were referred to both the reduction in b[a]p concentration and the anaerobic biodegradability of the digestate. 2.1 The substrate The experimental acidity was performed using the digestate originating from the full-scale anaerobic treatment of the organic fraction of municipal solid waste. The samples were collected from the digester, ensuring a retention time of 11 days, and completely characterized by its main chemical and physical parameters (Table 1). The analysis of polycicle aromatic hydrocarbons (PAHs) was performed to assess the need for artificial contamination.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Table 1. Characterization of the digestate samples. Parameter
Digestate
TS [%]
4,59 ± 0,47
VS [%TS]
58,04 ± 6,93
COD [mg/L]
89.200 ± 20.903
sCOD [mg/L]
56.149 ± 8.610
BOD5 [mg/L]
45.960 ± 5.600
PAH [mg/kgSS]
-
2.2 The preparation of the spiked digestate samples Benzo[a]pyrene was assumed as the target compound for a polycyclic aromatic hydrocarbon contamination. The digestate samples were spiked with a 5 mg/L benzo[a]pyrene solution, so as to obtain a concentration of 100 mg/kgSS. To this end, the solution was added to the digestate samples, that were stirred for 24 hours, so as to promote the controlled volatilization of the solvent used to prepare the b[a]p solution. 2.3 The ozone treatment Ozone production was obtained from a UV generator (model Ozone - Procom srl), using air from a compressor. The air stream containing ozone was then introduced at the bottom of a glass reactor containing the samples; exhaust gases were extracted on the top of this reactor and forced to pass through a Drechsel trap filled with 200 mL of 2% KI solution, in order to capture residual ozone and determine the ozone demand. Ozone doses ranging between 0 and 0,16 gO3/gTS were investigated. 2.4 Analytical set up Chemical Oxygen Demand (COD), total solid (TS) and volatile solid (VS) were evaluated according to Standard Methods (AWWA-APHA-WEF, 1998). The soluble COD (sCOD) was measured on each sample after centrifugation and filtration (
