landfill gas production before, during and after landfill
LANDFILL GAS PRODUCTION BEFORE, DURING AND AFTER LANDFILL AERATION M. RITZKOWSKI* AND K. KUCHTA* * Institute of Environmental Technology and Energy Economics, Hamburg University of Technology, Harburger Schlossstr. 36, 21079 Hamburg, Germany
SUMMARY: Landfill aeration aims, beside others, to minimize current and future landfill gas (LFG) production. Due to both the reduction of methane generation and methane potential, aerated landfills are often considered for climate protection projects. However, data evaluation confirms that during aeration the specific methane load may (at least temporarily) increase which clearly underlines the importance of an appropriate off-gas treatment (thermal or biological methane oxidation). After aeration completion LFG production may re-start and reach methane loads exceeding the biological methane oxidation potential in engineered landfill top covers. Consequently, aeration periods may have to be prolonged or target values adjusted.
1. INTRODUCTION Several examples of aerated landfills demonstrate both a reduction in landfill gas generation rates as well as LFG generation potentials, associated with reduced emissions during and after the aeration period (Hrad and Huber-Humer, 2017; Raga et al., 2015; Heyer et al., 2005). The observed reductions in current and long term gas emissions are caused by a depression of anaerobic degradation processes as well as an extensive decomposition of biodegradable waste organic compounds under aerobic conditions, which can be followed by decreasing concentrations of total organic carbon (TOC) and gas formation potential (GP21). Towards the completion of aeration, TOC concentrations in the solid waste material typically reach values around 8% (mainly caused by persistent compounds such as plastics, leather and rubber) whereas GP21 values are as low as 0.5 to 1 lN/kg TS. However, in many cases data about the initial LFG generation rates from landfills to be aerated are missing or are just estimates based on model results. During the aeration process a reduction in LFG generation rates are often assumed on the basis of low methane concentrations in the extracted off-gas. These assumptions do not adequately consider the significantly increased gas fluxes during active aeration measures. After completion of aeration LFG generation rates are expected to be very limited due to the altered characteristic of the solid waste material. Verifications of the residual LFG generation rates after completion of aeration are rarely conducted. In order to verify the actual LFG production before, during and after landfill aeration comprehensive data from own lab scale and full scale experiments have been analysed and compared with literature data. LFG production rates were evaluated against the site specific biological methane oxidation potential to be realized during the gas passage through the landfill top cover.
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
2. TARGET VALUES FOR LANDFILL AERATION PROJECTS Beside target values for the bio-conversion of organic carbon from solid waste material and concentrations for organic and inorganic leachate pollutants, the success of landfill aeration can be assessed through determination of the residual methane production rates. The latter can be assessed either on field scale (through LFG extraction tests) or in the laboratory by means of the BMP test. Based on the results of previous landfill aeration projects specific target values for both parameters have been proposed in Germany and Austria. Table 1 contains these target values and indications on how the determination has to be conducted.
Tab. 1: Target values (gas phase) for the completion of landfill aeration projects in Germany and Austria. Parameter
unit
Gas formation potential in 21 days (GP21)
Gas generation by incubation test (GS21)
Methane (field)
production
lN/kgTS
lN/kgTS
Target value (D)
50,000 m², the acceptable methane production rate for the completion of aeration and termination of active landfill aftercare would be 0.017 [l CH4/(MgTS*h)] (see also Table 2). Results from SAL for the post aeration period therefore show that at least for a certain time period after aeration completion the target value would not be observed. 5.2 Case study small landfill K After completion of the 6-years aeration process for landfill K waste samples have been taken and investigated in the laboratory. A total of 5 simulated anaerobic landfills have been operated over a period of 17 months. The evaluation of the observed data for residual methane production is summarized in Figure 6.
Fig. 6. Methane production after completion of aeration; SAL filled with waste from landfill K.
From Figure 6 it becomes obvious that during 6 months after the completion of aeration elevated methane production rates could be observed. The highest value (month 2) exceeded 0.16 l CH4/(MgTS*h). Following months 6 methane production rates declined and reached a final value of 0.07 at months 17 of the post aeration period. In contrast to landfill T, landfill K is considered a small and shallow landfill. Therefore the acceptable methane production rate for the completion of aeration and the termination of active landfill aftercare is calculated to 0.107 [l CH4/(MgTS*h)]. Landfill K exceeded this target value only for a short period and remained well below the limit towards the finalization of the
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
experimental time frame.
6. DISCUSSION AND CONCLUSIONS Landfill aeration aims, beside others, at the sustainable reduction of current and future methane emissions. Since methane is considered a potent greenhouse gas (GHG), landfill aeration projects may make a significant contribution towards the desired reduction in GHG emissions. In the majority of cases aeration is applied to landfills when the utilisation of the extracted LFG becomes less attractive from the economic point of view. However, as a general rule between 5 and 15% of the LFG production potential is still available at this point in time and could eventually emit into the atmosphere. This residual LFG production potential is considered for the potential emission savings to be realized during landfill aeration. On the basis of data derived from two case studies it could be shown that during the ongoing aeration process the methane load extracted with the off-gas exceeds the methane load extracted by the LFG during previous anaerobic landfill operation. The reasons can be mainly seen in an enhanced radius of influence of the gas extraction system due to the increased flow rates during aeration. Since any additional GHG emissions should be avoided during aeration, the accurate and complete collection of the off-gas is required and suitable off-gas treatment methods have to be applied. In this connection thermal off-gas treatment by means of regenerative thermal oxidation (RTO) hold some advantages since they are capable to reliably minimize the emitted methane loads. Alternatively biological oxidation methods may be applied. In this regards methane oxidation may be realized in the landfill bio-cover or during the off-gas passage through external bio-filter (eventually in combination with bio-scrubbers). After completion of aeration the residual methane productions rates should be as low that a complete oxidation, also in non-engineered landfill covers and independent of seasonal influences, is possible. In this connection the target values for the termination of the aeration process do correspond to proposed values for the termination of active landfill post closure care. Based on the results of the two considered case studies it could be demonstrated that the currently proposed target values for the residual methane production can be observed for small and shallow landfills. In contrast, larger landfills may have to apply different target values which are unlikely to be observed during reasonable aeration period. The question to be answered in this connection is whether the proposed target values have to be adjusted for this kind of landfills (which may be associated with a maceration of the climate protection goals) or if stabilisation periods have to be significantly prolonged. The latter may be associated with a remarkable increase in the overall costs of landfill aeration projects. The third option, which is the preferred one in the opinion of the authors, is to put great efforts in the optimization of the aeration process in terms of optimized air distribution inside the landfill, provision of optimal waste moisture contents during the entire aeration project, temperature control and regulation as well as the conduction of a comprehensive and regular monitoring approach. Since a number of larger landfill are currently being aerated in different European countries, future results will show which can be the preferred option for the successful completion of landfill in situ stabilisation.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
REFERENCES Heyer, K.-U., Hupe, K., Ritzkowski, M., Stegmann, R. (2005): Pollutant release and pollutant reduction - Impact of the aeration of landfills. In: Waste Management 25 (4 - Special issue), 353359. Hrad, M., Huber-Humer, M. (2017): Performance and completion assessment of an in-situ aerated municipal solid waste landfill – Final scientific documentation of an Austrian case study. In: Waste Management 63, 397-409. Raga, R., Cossu, R., Heerenklage, J., Pivato, A., Ritzkowski, M. (2015): Landfill aeration for emission control before and during landfill mining. In: Waste Management 46, 420-429. Stegmann, R., Heyer, K.-U., Hupe, K., Willand, A. (2006): Deponienachsorge – Handlungsoptionen, Dauer, Kosten und quantitative Kriterien für die Entlassung aus der Nachsorge. Umweltforschungsplan des Bundesministeriums für Umwelt, Naturschutz und Reaktorsicherheit. Abfallwirtschaft, Förderkennzeichen (UFOPLAN) 204 34 327, im Auftrag des Umweltbundesamtes.
SUMMARY: Landfill aeration aims, beside others, to minimize current and future landfill gas (LFG) production. Due to both the reduction of methane generation and methane potential, aerated landfills are often considered for climate protection projects. However, data evaluation confirms that during aeration the specific methane load may (at least temporarily) increase which clearly underlines the importance of an appropriate off-gas treatment (thermal or biological methane oxidation). After aeration completion LFG production may re-start and reach methane loads exceeding the biological methane oxidation potential in engineered landfill top covers. Consequently, aeration periods may have to be prolonged or target values adjusted.
1. INTRODUCTION Several examples of aerated landfills demonstrate both a reduction in landfill gas generation rates as well as LFG generation potentials, associated with reduced emissions during and after the aeration period (Hrad and Huber-Humer, 2017; Raga et al., 2015; Heyer et al., 2005). The observed reductions in current and long term gas emissions are caused by a depression of anaerobic degradation processes as well as an extensive decomposition of biodegradable waste organic compounds under aerobic conditions, which can be followed by decreasing concentrations of total organic carbon (TOC) and gas formation potential (GP21). Towards the completion of aeration, TOC concentrations in the solid waste material typically reach values around 8% (mainly caused by persistent compounds such as plastics, leather and rubber) whereas GP21 values are as low as 0.5 to 1 lN/kg TS. However, in many cases data about the initial LFG generation rates from landfills to be aerated are missing or are just estimates based on model results. During the aeration process a reduction in LFG generation rates are often assumed on the basis of low methane concentrations in the extracted off-gas. These assumptions do not adequately consider the significantly increased gas fluxes during active aeration measures. After completion of aeration LFG generation rates are expected to be very limited due to the altered characteristic of the solid waste material. Verifications of the residual LFG generation rates after completion of aeration are rarely conducted. In order to verify the actual LFG production before, during and after landfill aeration comprehensive data from own lab scale and full scale experiments have been analysed and compared with literature data. LFG production rates were evaluated against the site specific biological methane oxidation potential to be realized during the gas passage through the landfill top cover.
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
2. TARGET VALUES FOR LANDFILL AERATION PROJECTS Beside target values for the bio-conversion of organic carbon from solid waste material and concentrations for organic and inorganic leachate pollutants, the success of landfill aeration can be assessed through determination of the residual methane production rates. The latter can be assessed either on field scale (through LFG extraction tests) or in the laboratory by means of the BMP test. Based on the results of previous landfill aeration projects specific target values for both parameters have been proposed in Germany and Austria. Table 1 contains these target values and indications on how the determination has to be conducted.
Tab. 1: Target values (gas phase) for the completion of landfill aeration projects in Germany and Austria. Parameter
unit
Gas formation potential in 21 days (GP21)
Gas generation by incubation test (GS21)
Methane (field)
production
lN/kgTS
lN/kgTS
Target value (D)
50,000 m², the acceptable methane production rate for the completion of aeration and termination of active landfill aftercare would be 0.017 [l CH4/(MgTS*h)] (see also Table 2). Results from SAL for the post aeration period therefore show that at least for a certain time period after aeration completion the target value would not be observed. 5.2 Case study small landfill K After completion of the 6-years aeration process for landfill K waste samples have been taken and investigated in the laboratory. A total of 5 simulated anaerobic landfills have been operated over a period of 17 months. The evaluation of the observed data for residual methane production is summarized in Figure 6.
Fig. 6. Methane production after completion of aeration; SAL filled with waste from landfill K.
From Figure 6 it becomes obvious that during 6 months after the completion of aeration elevated methane production rates could be observed. The highest value (month 2) exceeded 0.16 l CH4/(MgTS*h). Following months 6 methane production rates declined and reached a final value of 0.07 at months 17 of the post aeration period. In contrast to landfill T, landfill K is considered a small and shallow landfill. Therefore the acceptable methane production rate for the completion of aeration and the termination of active landfill aftercare is calculated to 0.107 [l CH4/(MgTS*h)]. Landfill K exceeded this target value only for a short period and remained well below the limit towards the finalization of the
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
experimental time frame.
6. DISCUSSION AND CONCLUSIONS Landfill aeration aims, beside others, at the sustainable reduction of current and future methane emissions. Since methane is considered a potent greenhouse gas (GHG), landfill aeration projects may make a significant contribution towards the desired reduction in GHG emissions. In the majority of cases aeration is applied to landfills when the utilisation of the extracted LFG becomes less attractive from the economic point of view. However, as a general rule between 5 and 15% of the LFG production potential is still available at this point in time and could eventually emit into the atmosphere. This residual LFG production potential is considered for the potential emission savings to be realized during landfill aeration. On the basis of data derived from two case studies it could be shown that during the ongoing aeration process the methane load extracted with the off-gas exceeds the methane load extracted by the LFG during previous anaerobic landfill operation. The reasons can be mainly seen in an enhanced radius of influence of the gas extraction system due to the increased flow rates during aeration. Since any additional GHG emissions should be avoided during aeration, the accurate and complete collection of the off-gas is required and suitable off-gas treatment methods have to be applied. In this connection thermal off-gas treatment by means of regenerative thermal oxidation (RTO) hold some advantages since they are capable to reliably minimize the emitted methane loads. Alternatively biological oxidation methods may be applied. In this regards methane oxidation may be realized in the landfill bio-cover or during the off-gas passage through external bio-filter (eventually in combination with bio-scrubbers). After completion of aeration the residual methane productions rates should be as low that a complete oxidation, also in non-engineered landfill covers and independent of seasonal influences, is possible. In this connection the target values for the termination of the aeration process do correspond to proposed values for the termination of active landfill post closure care. Based on the results of the two considered case studies it could be demonstrated that the currently proposed target values for the residual methane production can be observed for small and shallow landfills. In contrast, larger landfills may have to apply different target values which are unlikely to be observed during reasonable aeration period. The question to be answered in this connection is whether the proposed target values have to be adjusted for this kind of landfills (which may be associated with a maceration of the climate protection goals) or if stabilisation periods have to be significantly prolonged. The latter may be associated with a remarkable increase in the overall costs of landfill aeration projects. The third option, which is the preferred one in the opinion of the authors, is to put great efforts in the optimization of the aeration process in terms of optimized air distribution inside the landfill, provision of optimal waste moisture contents during the entire aeration project, temperature control and regulation as well as the conduction of a comprehensive and regular monitoring approach. Since a number of larger landfill are currently being aerated in different European countries, future results will show which can be the preferred option for the successful completion of landfill in situ stabilisation.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
REFERENCES Heyer, K.-U., Hupe, K., Ritzkowski, M., Stegmann, R. (2005): Pollutant release and pollutant reduction - Impact of the aeration of landfills. In: Waste Management 25 (4 - Special issue), 353359. Hrad, M., Huber-Humer, M. (2017): Performance and completion assessment of an in-situ aerated municipal solid waste landfill – Final scientific documentation of an Austrian case study. In: Waste Management 63, 397-409. Raga, R., Cossu, R., Heerenklage, J., Pivato, A., Ritzkowski, M. (2015): Landfill aeration for emission control before and during landfill mining. In: Waste Management 46, 420-429. Stegmann, R., Heyer, K.-U., Hupe, K., Willand, A. (2006): Deponienachsorge – Handlungsoptionen, Dauer, Kosten und quantitative Kriterien für die Entlassung aus der Nachsorge. Umweltforschungsplan des Bundesministeriums für Umwelt, Naturschutz und Reaktorsicherheit. Abfallwirtschaft, Förderkennzeichen (UFOPLAN) 204 34 327, im Auftrag des Umweltbundesamtes.
