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Landfill aeration worldwide: Concepts, indications and findings
Waste Management 32 (2012) 1411–1419
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Landfill aeration worldwide: Concepts, indications and findings M. Ritzkowski a,⇑, R. Stegmann b a b
Institute of Environmental Technology and Energy Economics, Hamburg University of Technology, Harburger Schlossstr. 36, 21079 Hamburg, Germany Consultants for Waste Management, Prof. R. Stegmann and Partner, Schellerdamm 19–21, 21079 Hamburg, Germany
a r t i c l e
i n f o
Article history: Received 5 December 2011 Accepted 28 February 2012 Available online 28 March 2012 Keywords: Landfill aeration Sustainable landfills Biological stabilisation Aeration concepts Emission potential
a b s t r a c t The creation of sustainable landfills is a fundamental goal in waste management worldwide. In this connection landfill aeration contributes towards an accelerated, controlled and sustainable conversion of conventional anaerobic landfills into a biological stabilized state associated with a minimised emission potential. The technology has been successfully applied to landfills in Europe, North America and Asia, following different strategies depending on the geographical region, the specific legislation and the available financial resources. Furthermore, methodologies for the incorporation of landfill aeration into the carbon trade mechanisms have been developed in recent years. This manuscript gives an overview on existing concepts for landfill aeration; their application ranges and specifications. For all of the described concepts examples from different countries worldwide are provided, including details regarding their potentials and limitations. Some of the most important findings from these aeration projects are summarised and future research needs have been identified. It becomes apparent that there is a great demand for a systematisation of the available results and implications in order to further develop and optimise this very promising technology. The IWWG (International Waste Working Group) Task Group ‘‘Landfill Aeration’’ contributes towards the achievement of this goal. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The creation of sustainable landfills is a fundamental goal in waste management worldwide. In this case, sustainability is closely tied to the potential of terminating expensive perpetual landfill aftercare. However, objectives of many conventional landfill operations largely focus on the short term prevention of hazards and risks to the environment that are caused by polluted leachate and migrating landfill gas (LFG) as well as on a controlled utilisation of the energy provided by extracted LFG. Once this option is not feasible or at least not economically beneficial anymore, many landfills will still exhibit a significant emission potential, which tapers off with time to very low levels. At this stage, these conventional landfills are considered to be unsustainable. In this respect, landfill aeration is considered to be an indispensable tool for the controlled and sustainable conversion of conventional anaerobic landfills into a biological stabilised state associated with a significantly lowered or the near elimination of the landfill gas emission potential. Although landfill aeration is not a widely applied concept so far, it has already been successfully applied to several landfills in Europe, North America and Asia
⇑ Corresponding author. Tel.: +49 40 42878 2053; fax: +49 40 42878 2375 E-mail addresses: [email protected] (M. Ritzkowski), [email protected] (R. Stegmann). URL: http://www.tu-harburg.de/iue/ (M. Ritzkowski). 0956-053X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.wasman.2012.02.020
(Stegmann and Ritzkowski, 2007). Some of the most comprehensive investigations have been carried out in Germany and due to the high interest in this technology more investigations are underway in different countries at the present time. This is certainly essential as in situ aeration means more than just injecting air into a landfill. Aspects such as well design and spacing, selection of appropriate air volume and pressure, control of air distribution, temperature and moisture as well as the potential mobilisation of pollutants in both, the gas and liquid phases have to be considered. Moreover, the question of a particular point in time to terminate the aeration process has to be answered. The latter includes indications about the biological landfill stability achieved during aeration and is therefore related to the considerations of landfill sustainability. Finally, not only technical and ecological aspects have to be considered, but also economic issues. In order to get an answer to the questions raised above the following chapters provide an overview on landfill aeration concepts, implemented examples and most important findings so far. 2. Landfill aeration concepts 2.1. High pressure aeration (shock pressure concept) High pressure aeration is mostly associated with the Bio-PusterÒ concept. The aeration is realised by shock pressure releases (up to 6 bars) from lances, using air which might be enriched by additional oxygen (up to 20%) and potentially by nutrients. Supplied by a
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compressed air distribution network, each lance features a quick-release valve which is intermittently opened once a specified positive pressure has been built-up. The released aeration gas is capable of penetrating both, highly and weakly compacted waste materials. In order to minimise the uncontrolled release of off-gases, the shock pressure concept involves an off-gas extraction system consisting of suction lances. The system, operated in parallel to the aeration and with an increased extraction capacity of 30% (in comparison with the gas volumes used for aeration), collects the offgases before being treated by means of biofilters and/or activated carbon (ARGE Biopuster, 2011; Fig. 1a). The concept of high pressure aeration is mainly associated with the implementation of landfill mining projects. This is based on the fact that for a widely bio-stabilisation, treatment durations in the range of several years are required. Therefore, this energy and operating material (industrial gas) intensive method holds some drawbacks. 2.2. Low pressure aeration Low pressure concepts are applied in the majority of landfill aeration cases aiming at an accelerated biological waste stabilisation in situ. In contrast to the high pressure concept positive pressure differences do not exceed 0.3 bars; normally they are in a range of 20–80 mbars. Although the general conditions are similar, a number of different concepts and variants have been developed over the past 10–15 years. The most important concepts are briefly summarised in the following sub paragraphs. 2.2.1. Active aeration and off-gas extraction As for high pressure aeration, the majority of low pressure aeration concepts exhibit a simultaneous aeration and extraction operation. Currently most of the applications are based on the AEROflottÒ (Heyer et al., 2005), AIRFLOWÒ (Cossu et al., 2003) or Smell-WellÒ (IUT Group, 2011) concept. Through a system of vertical gas wells ambient air is continuously introduced into the landfilled waste. The air distributes by means of convection and diffusion and is further directed through the simultaneously operated off-gas extraction system. The latter consists of the same kind of gas wells as the ones used for air supply, being connected to an air compressing unit and the final off-gas purification stage. The simultaneously operated supply and extraction system offers significant advantages in terms of flexibility: air can be targeted to be introduced to zones featuring an oxygen deficit and the air flow inside the wastes can be manipulated through the selection of appropriately located aeration and gas extraction wells (Fig. 1b). Differences among the concepts for active low pressure aeration and off-gas extraction mainly exist with the selection of an appropriate off-gas treatment. With regard to an optimised reduction in greenhouse gas emissions the thermal oxidation of the residual methane load in the extracted off-gases is mandatory. Systems like the flameless non-catalytic thermal oxidation (regenerative thermal oxidation, RTO) are capable of completely avoiding methane emissions via the off-gases. At the same time the external energy demand is very low as the required oxidation temperature (about 1100 °C) is retained by the energy release during thermal methane oxidation. The RTO is an integral part of the AEROflottÒ system. Other systems apply a biological off-gas treatment concept, such as a biofilter or combinations of bio-scrubbers and biofilters. In most cases, these biological systems are capable of reducing odours to a wide extent; however, they show significant deficits in terms of methane oxidation due to the often insufficient retention times of the off-gas in the filters. 2.2.2. Active aeration without off-gas extraction Air supply into the landfilled waste can be executed without a need for operating an active extraction system for the off-gases
simultaneously. In this case, the landfill cover might function as a biological filter layer, either in its original condition or after enhancement of its biological methane oxidation capacity. If landfill aeration is realised without the integration of a simultaneous extraction system and treatment of the off-gases, a continuous operation is certainly easier to maintain, although it can result in a significantly lowered emission reduction (in comparison to combined systems). The aeration can be realised by both, a system of vertical gas wells in the waste body (Reiser et al., 2011) or through air injection into the unsaturated soil zone beneath the wastes (Kraiger, 2011; Fig. 1c). For the latter case, the soil functions as an air distribution layer aiming at an even aeration of the wastes from bottom to top. 2.2.3. Passive aeration (air venting) Passive aeration concepts follow the basic approach of air venting, (i.e., the introduction of ambient air into the landfill through its surface or eventually through open gas wells) driven by a negative pressure induced inside the landfill body. This concept is mainly recognised under the brand DEPO+Ò. The gas wells are perforated only in the deeper waste layers in order to increase the waste volume to be affected by the aeration and to avoid short circuits near the landfill surface. To ensure a gradual aeration, starting at the surface before shifting into the deeper layers, extracted gas volumes are significantly higher than the gas production rate of the landfilled waste. The extracted off-gas is treated by means of a biofilter (Bröcker et al., 2010; Fig. 1d). Regarding the stabilisation performance, two different stages have to be distinguished for air venting concepts. At the initial stage the methane flux in the extracted off-gas is increased in comparison to the previous flux under ordinary LFG extraction operation. This is due to both, the specific gas well design featuring a short perforated tail section in combination with significantly increased gas extraction flow rates (Bröcker et al., 2010). Through this approach, LFG is captured even from zones that have not been affected by the extraction system when operated prior to the start of air venting. Only during the later stages the gas composition changes towards reduced methane concentrations as a result of air infiltration via the landfill surface. 2.2.4. Energy self-sufficient long term aeration In contrast to the afore-mentioned active and passive aeration concepts, an energy self-sufficient system might be applied in particular for long term aeration. This system can be established during the transitional period between the end of ‘‘active’’ forced aeration and the subsequent installation of a final surface cover that includes a qualified methane oxidation layer. By means of an on-going low air supply the restart of LFG-generation can be widely avoided in the long term, thus significantly reducing the methane load to be biologically oxidised without relevant subjection to climatic changes. Energy self-sufficient aeration systems consist of wind driven aspirators to be mounted directly on some of the existing gas wells and pneumatic air pumps, powered by wind wheels. The compressed air is directed into existing gas wells, which leads to an on-going oxygen supply of the already widely stabilised landfill (Ritzkowski et al., 2009a). 2.3. Semi aerobic concept The concept of semi-aerobic landfills is actually the oldest with regard to landfill aeration. Back in 1975, the first semi-aerobic landfill was developed in Japan. In semi-aerobic landfills, the leachate collection system consists of a central perforated pipe (main collection pipe) with perforated branch pipes on either side of it laid at a suitable interval. The pipes are embedded in graded gravel
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Fig. 1. Schematic outlines of different aeration concepts (a: high pressure aeration; b: low pressure aeration with parallel off-gas extraction; c: low pressure aeration w/o off-gas extraction; d: low pressure aeration/air venting).
(5–15 cm) and installed with adequate slope. The main collection pipe ends in an open leachate collection pond. The pipes are designed in a way that only one-third of the section is filled with liquid. At each intersection of the main collection pipe with the branch pipes, and at the end of each branch pipe, vertical gas ventilation wells enclosed in graded gravel (eventually packed inside a wire netting) are erected. The air will be able to flow into the waste
layer through these pipes when the leachate head is low. Since the two piping systems are connected, ambient air and landfill gas flows through the leachate collection pipes and the gas ventilation wells, thus, enhancing the intrusion of the air into the inner part of the landfilled wastes occasionally (Fig. 2). Due to higher temperatures in the waste (compared to the ambient air), the gas inside the waste tends to rise and gets vented through the gas wells, thus,
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generating a negative pressure siphoning effect that draws more air into the leachate collection pipes (Matsufuji and Tachifuji, 2007). During recent years the semi-aerobic concept has been slightly expanded and adapted. Assuming that the leachate collection system is not functioning anymore, associated with an increased leachate head and the inability of ambient air to be introduced through the leachate pipes, passive aeration might be realised through the landfill surface. In principle, the same concept as the ‘‘air venting’’ applies here, whereas the venting effect is induced by temperature differences only (thus no external energy demand for the aeration exists). The passive aeration system might be further enhanced through the installation of additional gas venting wells at an appropriate spacing (Kim et al., 2010). 3. Implemented examples of landfill aeration 3.1. Examples from Europe In Europe, landfill aeration projects have been implemented in Germany, Austria, Italy, Switzerland and The Netherlands. The majority of projects are located in Germany and refer to the concept of low pressure aeration. The vast majority of these projects are aiming at an accelerated and sustainable bio-stabilisation of the deposited waste mass. By means of the active aeration and simultaneous off-gas extraction/treatment approach (i.e., AEROflottÒ technique), landfills in the German Federal States of Lower Saxony (Kuhstedt landfill), Bavaria (Amberg-Neumühle landfill) and Brandenburg (Milmersdorf landfill) have been biologically stabilised. Fig. 3 shows an example about the technical installations required for low pressure landfill aeration. In all three cases more than 90% of the residual biodegradable organic carbon was bioconverted and released, mainly in terms of carbon dioxide (CO2) via the gas phase. Application of this technology (i.e., aeration via the AEROflottÒ technique) at additional conventional (anaerobic) landfills in North Rhine-Westphalia (Doerentrup landfill and Halberbracht landfill), Lower Saxony (Suepplingen landfill) and Saarland (SchwalbachGriesborn) is currently underway to accelerate the biostabilization of the organic fraction of the wastes at these sites (Ritzkowski et al., 2009b; Heyer et al., 2007). By means of the passive aeration concept (including open air access wells), landfills in the German Federal State of Schleswig Holstein (Kiel-Drachensee landfill, Schenefeld landfill, Barsbüttel
landfill) have been biologically stabilised. These landfills exhibit an intensive after use, including many buildings (both, commercial and/or residential) that have been constructed after landfill closure. In order to avoid the perpetual formation of methane gas and to reduce current and future associated risks for the users, the landfills have been converted from anaerobic to aerobic conditions (Heerenklage and Stegmann, 2003). Additionally, a number of landfills have been aerated by means of the passive aeration/gas venting concept (i.e., DEPO+Ò technique) over the last two decades. This particular technology was applied at landfills in the German Federal States of Schleswig Holstein and North Rhine-Westphalia. However, information regarding the progression of stabilisation as well as indications regarding the sustainability of the aeration process have not been reported, hence, no pertinent literature on the application of this technology on these sites is currently available. In northern Italy, three landfills have been aerated by means of the active aeration and parallel off-gas extraction/treatment (biofilter) approach according to the AIRFLOWÒ system. This technology has been applied at the Modena landfill since 2002, in order to create conditions that allow for the subsequent mining of the landfill site. Two other landfills in northern Italy have been biologically stabilised within a period of 3 years since 2005 (Cossu et al., 2007a). In Austria, the concept of low pressure aeration was applied for the first time in a pilot-scale demonstration project at an old landfill in Mannersdorf (Lower Austria). In 2003 and 2006, respectively, a limited part of the landfill has been aerated for demonstration purposes. Here the aeration and the parallel off-gas extraction system was combined with the treatment of the exhausts through a biofilter. On the basis of the promising results of the demonstration project, the whole landfill in Mannersdorf is currently being biologically stabilised by means of the same aeration concept (Gamperling et al., 2011). Furthermore, a landfill in Tyrol is currently undergoing aeration since 2008 by means of the low pressure concept, but excluding the capture and treatment of the exhausts. In this particular case, air is introduced to the unsaturated soil beneath the landfilled waste whereas the landfill soil cover functions as a biological filter for the emitted off-gas (Kraiger, 2011). Further to the above mentioned landfill aeration projects, aiming at biological waste stabilisation, a number of European landfills (or parts of landfills) have been aerated in order to
Fig. 2. Schematic outline of the semi-aerobic landfill concept.
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Fig. 3. Master plan of the technical installations for in situ aeration at the Milmersdorf landfill in Germany (Heyer et al., 2007; modified).
minimise odours and methane concentrations prior to landfill mining activities. However, for a majority of these cases, either the high pressure aeration concept (Bio-PusterÒ) or the low pressure Smell-WellÒ concept has been applied. Since 1990 more than 12 landfills in Austria, three landfills in Germany, two landfills in Italy and Czech Republic, respectively and one landfill in The Netherlands have been treated by means of these technologies (ARGE Biopuster, 2011). 3.2. Examples from the US and Canada In the US, first studies on landfill aeration had been carried out in the early 1970’s. Pilot studies were conducted on cells on landfills that were filled with fresh MSW. However, these studies involved intermittent aeration without the addition or recirculation of water. Due to the increasing high temperatures within the landfill cells (up to 90 °C) resulting from the aeration process coupled with the associated problems of severe reduction in waste moisture content, further projects had been delayed until the early 1990s. That is when a number of lysimeter tests combining both, aeration and water addition, were conducted to assess the impact of moisture content on aeration. Towards the end of the 1990s, the first landfills underwent aeration in the course of large-scale tests in Georgia and South Carolina. To date, more than 20 landfills located within various American states have been aerated following the intention to minimise negative impacts on surface water and groundwater, reduce leachate volumes, eliminate the production of methane gas, rapidly stabilise the waste or redevelop the site for further utilisation. For the vast majority of cases, low pressure concept of aeration (without off-gas extraction and treatment) was implemented at these landfills. As a
rule, the injection of air was accompanied by the parallel injection of liquid (mostly through wells, in some cases sprayed over the surface) in order to control temperatures and to reduce leachate volumes (Berge et al., 2007). One of the oldest full-scale aerobic landfills in the world is located in Toronto, Canada. Since 1978 the landfill has been kept at a mainly aerobic stage by means of the passive aeration (air venting) concept. Air is introduced into the waste mass via the landfill surface if the gas extraction flow rates exceeding the gas generation rates. The intention is to widely avoid the formation of methane gas as to prevent hazards for a couple of buildings in the immediate vicinity of the landfill. Currently, it can be concluded that in addition to the landfill gas hazard prevention, a significant reduction in greenhouse gas emissions can be realised over the past decades (Beatty et al., 2009). 3.3. Examples from Asia The majority of landfills in Japan, over half of the number of the landfills in Korea and a few landfills in Malaysia have been constructed and operating according to the semi-aerobic concept (Matsufuji and Tachifuji, 2007). Semi-aerobic landfills are mainly characterised by their long term behaviour as they exhibit reduced methane gas generation rates and an enhanced leachate quality in comparison to anaerobic landfills. However, due to the limited intensity of aeration induced by natural ventilation and the remaining anaerobic zones inside the landfilled waste, the biological stabilisation occurs rather slowly in comparison to actively aerated landfills. During recent years, the semi-aerobic concept has been recognised for its potential towards landfill remediation. Through the
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installation of more than 70 passive gas extraction wells into an old landfill, an air venting effect (passive aeration) could be induced at the Nakazono landfill in Asahikawa, Japan (Yoshida, 2011). Air is sucked in through the landfill surface, following a chimney effect which is induced by temperature differences between the waste and the ambient air. During 3 years after the installation of passive gas extraction wells a significant temperature increase (up to 60 °C) could be observed in the proximity of some wells. At the same time methane concentrations decreased to low levels in the range of 5% (v/ v). However, other parts of the old landfill (in particular deeper waste layers) were hardly affected by the passive aeration because methane concentrations remained at high levels there. 4. Findings from landfill aeration projects 4.1. General aspects The term ‘‘landfill aeration’’ comprises a variety of different concepts and methodologies as demonstrated in Section 2 and summarised in Table 1. Over the past decades many of these concepts have proven to be applicable at full scale, even though in the majority of cases a clear proof of success has not been provided. The reasons for this are manyfold and include, beside others, a lack of general parameters and target values for the successful completion of aeration as well as the application of insufficient monitoring programs during aeration. Furthermore, the aeration of landfills has to follow a number of case specific objectives in accordance with the local situation and framework requirements. Landfill mining projects are often associated with short term aeration measures, aiming at a minimisation of both, odours and methane concentrations. For these purposes, high pressure concepts offer advantages due to their ability to ensure a fast and consistent aeration of the landfilled wastes whereas cost aspects and long term reliability of the system are of secondary importance. In order to prevent long term hazards like methane gas accumulation in and/or around buildings, passive aeration or air venting concepts offer certain advantages. These are mainly associated with a relatively simple operation mode (gas extraction only) where air is passively introduced (via the surface) into the landfill. However, challenges related to this concept exist in the form of potential accumulation of water (leachate) in the gas wells and/or the formation of preferential flow paths. The aerobic bioreactor concept can be applied for the fast recovery of landfilling capacity. This concept enables the combination of water addition and leachate recirculation coupled with the introduction of air into relatively reactive (biologically unstable) waste materials. Through the combined operation, the landfill can be used
as an in situ bioreactor, where processes like nitrification, denitrification and accelerated reductions in organic constituents, take place. Furthermore, water functions as a suitable measure to control and limit the temperatures inside the aerated landfill. Problems regarding this approach are associated with the ‘‘competition’’ between water and air for free void volume. As a consequence, the aeration effect (i.e., the biological stabilisation) might be limited or reduced to those zones exhibiting only moderate moisture contents (i.e., mainly the upper waste layers). Additionally, the concept of aerating fresh MSW (i.e., waste with a high organic content) exhibit an inferior energy balance compared to other strategies, where anaerobic degradation with the utilisation of captured methane (for electricity and/or heat generation) is carried out first prior to the beginning of the aeration process. In terms of an accelerated biological stabilisation of the organic fraction of waste, low pressure aeration concepts that combine air injection and off-gas extraction are most appropriate. These concepts are designed for a continuous operation over periods of 3– 10 years at both, moderate costs and operational efforts. Data regarding the initial amount of organic carbon and related conversion rates in the cause of aeration have been published for three completed full-scale projects in Germany. These data are summarised in Table 2. Major advantages of these concepts exist with their operational flexibility (i.e., dynamic systems which allow for an adaptation of flow rates and flow directions), simple but robust technology (i.e., vertical gas wells, blowers) and feasibility to include an effective off-gas treatment system. In order to reduce negative environmental impacts caused by landfills (mainly the uncontrolled emission of greenhouse gases and polluted leachate) under the situation of limited economical resources, the semi-aerobic landfill concept holds promise. A major advantage of applying this concept results in minimal operational efforts (i.e., only monitoring) and the complete avoidance of both, additional installations and energy demand. Although semi-aerobic landfills may improve the emission behaviour of a landfill, this concept prevents the utilisation of the energy contained in the methane gas (from anaerobic landfills) and does not reach the positive performance (regarding biological waste stabilisation, depression of methane formation, improvement of leachate quality) of an aerobic landfill under active aeration. 4.2. Impact of climatic conditions In order to select an appropriate aeration technology for a specific landfill, local climatic conditions (mainly temperature and precipitation levels) should be taken into account. These conditions are geographically dependant and can be roughly
Table 1 Overview of different landfill aeration concepts, application areas and related methods for off-gas treatment. Aeration concept
Sub-concept
Main area of application
Off-gas treatment method
High pressure
–
In preparation for landfill mining
Biofilter
Low pressure
Active aeration and off-gas extraction
Accelerated bio-stabilisation and GHG emissions minimisation Accelerated bio-stabilisation Increasing gas extraction rates/subsequent aerobic biostabilization Avoidance of residual LFG generation in the long term
Thermal oxidation (biofilter during further advanced stabilisation stages) Biocover (landfill surface) Biofilter
Long term reduction of GHG emissions and improvement of leachate quality
None
Landfill remediation/reduction of GHG emissions
None
Active aeration w/o off-gas extraction Passive aeration (air venting) Energy self-sufficient long term aeration Semi-aerobic
Connection between leachate collection pipes and gas vents as a part of the design layout Subsequent installation of passive gas vents (w/o connection to leachate pipes)
None/landfill surface
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M. Ritzkowski, R. Stegmann / Waste Management 32 (2012) 1411–1419 Table 2 Comparison between the total amounts of biodegradable organic carbon and the accordant discharged amounts, during the aeration process at three German landfills.
Total amount of biodegradable organic carbon Total discharge (via off-gas) of biodegradable organic carbon Monthly discharge (via off-gas) of biodegradable organic carbon Percentage of discharged biodegradable organic carbon (in relation to total biodegradable organic carbon) a b
(mg) (mg) (mg) [%]
Landfill Aa
Landfill Bb
Landfill Cb
1890 1730 9–46 (mean: 22) 92
4300 3900 80 91
530 500 10 94
Aeration time: 6 years. Aeration time: 4 years.
subdivided between ‘‘temperate climate’’, ‘‘humid/tropical climate’’ and ‘‘climate with extreme seasonal temperature variations’’. The latter condition applies to countries located in the mid-high latitudes and mostly situated in the northern hemisphere but may also be of importance for locations at higher elevations (mountainous regions). Landfills located in a temperate climate might be actively aerated by means of the low or high pressure concept. Due to moderate temperature and precipitation levels operational problems occur infrequently. A humid/tropical climate, on the other hand, might lead to operational problems since water might accumulate in the landfill and the off-gas extraction system, thus significantly reducing the operation efficiency. Landfills under the latter climate conditions might therefore be subjected to the semi-aerobic concept, provided that the temperature differences between the ambient and the waste (i.e., the major driver for the passive aeration effect) remains sufficient. A location with extreme seasonal temperature variations, however, represents the most challenging situation. In particular under very low temperatures problems related to an increased formation of condensate in the off-gas extraction system may occur. Also, active aeration concepts without off-gas extraction, may operate sufficiently under these conditions but might fall short as to an efficient biological methane oxidation in the landfill surface. 4.3. Monitoring concepts The unavailability of sufficient data regarding full scale aeration projects is, at least partly, associated with the choice of inadequate monitoring. In most of the cases, data are collected only for the gas phase during operation. In contrast, changes in leachate quality or in the solid waste material are measured quite rarely. Furthermore, evolutions in temperatures as well as in settlements are seldom recorded. In particular for the low pressure aeration concepts aiming at an accelerated biological stabilisation of the organic fraction of waste, mandatory monitoring activities should include parameters associated with the bioconversion of organic carbon. In the off-gases, methane and carbon dioxide content have to be determined (in the ideal case continuously) whereas in the leachate, the concentration of total organic carbon (TOC) and/or the chemical oxygen demand (COD) should be analysed frequently. In combination with the assessment of biodegradable organic carbon in the solid waste material (following waste sampling and laboratory analysis), a carbon balance can be set-up on the basis of these data, thus, enable the effective monitoring of the stabilisation process. A new approach is also provided by the application of FT-IR (Fourier Transformation Infrared Spectroscopy) to assess the aeration effect on the organic waste matter (Tesar et al., 2007). Another option exists with the set-up of a nitrogen balance. However, the potential nitrogen conversion processes that may occur during landfill aeration are manifold and include severe challenges in terms of an appropriate analysis (e.g. the determination of nitrous oxides or ammonia in the off-gas or the calculation of elementary nitrogen originating from de-nitrification processes).
4.4. Completion of aeration: when and how? During recent years a couple of proposals have been made and discussed regarding parameters and target values which might be used to determine the completion of an aeration project. As for the monitoring concepts, these proposals are mainly related to landfill aeration projects aiming at an accelerated biological stabilisation of the waste organic fraction. It is evident that particularly those parameters describing the state of biological stabilisation of the waste organic fraction are useful for determining the success of an aeration project. In this regard, solid waste samples have to be investigated for their specific respiration activity and/or methane-gas generation potential. In combination with the monitoring data (in particular the amount of emitted carbon) it is possible to assess the residual emission potential of an aerated landfill. Proposals for stabilisation parameters and target values have been made in the context of two comprehensive R&D projects in both, Austria and Germany (Tesar et al., 2007; Ritzkowski et al., 2006). One of the general approaches used to determine the biological stability achieved during the aeration project is a requirement whereby more than 90% of the biodegradable organic carbon has to be bioconverted (mainly to CO2) (Ritzkowski et al., 2006). Once this limit value has been reached, it can be assumed that the residual methane gas generation rates are very low, settlements are widely completed and leachate quality has significantly improved (in terms of reduced organic loads). 4.5. Transfer from lab-scale to full-scale In literature there are numerous examples on laboratory scale investigations of different waste materials under aerated landfill conditions (Kim et al., 2006; Giannis et al., 2008; Hao et al., 2010; He et al., 2011). Generally, many of the reported results show a rapid biodegradation of the organic fraction of waste, an almost complete avoidance of methane gas emissions and significantly elevated carbon dioxide emissions simultaneously. With regard to leachate pollution, reductions of dissolved organic carbon (DOC) and chemical oxygen demand (COD) as well as ammoniumnitrogen have been observed. In this connection, the transferability of the results from lab scale to full scale has to be investigated further. Therefore, the question to be answered is: do the lab scale experiments actually represent the conditions that occur in old landfills during full-scale aeration? Apparently the results from full-scale aeration projects often indicate that the answer is negative. Although in the majority of cases considerable residual methane emissions have not been detected after completion of aeration, partly increased organic loads in the leachate have been observed and in particular the reduction in ammonium-nitrogen was smaller than predicted by the lab tests (Bachofner et al., 2010; Cossu et al., 2007b). So what are the fundamental differences between the laboratory and the field approach? One aspect to be considered is the fairly homogeneous oxygen supply at sufficient rates in lab-scale experiments, whereas under full scale conditions this is very difficult to achieve. However, the
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major difference to be observed is related to the significant temperature increase inside the aerated waste material. Laboratory scale investigations have been, as a rule, carried out under constant temperatures in the mesophilic range (20–35 °C). Most of the heat produced by the microbial activity under aerobic conditions is balanced with the ambience, resulting in an only moderate temperature increase inside the test vessels. Increasing temperatures, however, are the major factors to determine the nitrogen dynamics and the moisture losses during aeration. Temperatures above 40 °C are intensifying both, hydrolysis and ammonification processes (Scheffer and Schachtschabel, 2009) while at the same time inhibiting nitrification (Robinson and Olufsen, 2004). As a consequence, the pH increases and free ammonia is formed which, at certain concentrations, is a strong inhibitor for nitrite and ammonia oxidisers (Onay and Pohland, 1998). Temperatures in a range of 40 °C and above are observed in the majority of landfill aeration projects over long time periods and often for many months and/ or for several years (Heyer et al., 2005). One of the consequences of these observations is that laboratory scale investigations should be conducted in a way respecting the temperature dynamic of an aerated landfill in order to get representative results. 4.6. The importance of off-gas treatment Despite the semi-aerobic landfill approach and some examples from low pressure aeration, the majority of aeration concepts include the treatment of the extracted off-gases. However, in many cases this treatment is carried out by means of biological processes, either in an external biofilter (container) or in the landfill surface cover, wherein the methanotrophic activity of certain microorganisms is utilised. These treatment approaches are robust and easy to maintain, but at the same time they may be relatively ineffective regarding the reduction in methane emissions due to often insufficient retention times and climatic influences (extreme temperatures and moisture contents). Furthermore it has to be taken into account that, in particular during the early phases of aeration, the methane flux (i.e., the amount of methane extracted during a defined period of time) might nearly equal (sometimes even exceed) the one to be present under anaerobic conditions (Ritzkowski and Stegmann, 2007). Reasons for this behaviour are to be seen in both, increased flow rates resulting in the collection of gas from previously unaffected landfill areas as well as increased bio-conversion rates under the prevailing increased temperatures. This methane flux might significantly exceed the methane oxidation capacity of a biological off-gas treatment system (biofilter). Therefore, if an aeration project aims at a massive reduction in methane emissions, the inclusion of a thermal off-gas treatment is mandatory. This is very important in particular approaches where landfill aeration is considered for climate projects. 4.7. Costs and cost savings Information regarding the costs of landfill aeration is rarely provided in literature. For the low pressure aeration concept (parallel aeration and off-gas extraction and/or treatment) costs are indicated in a range from 0.45 € up to 7 € per ton of waste material to be aerated (Heyer et al., 2007; van Vossen et al., 2009; HuberHumer, 2011). These costs include the investment and operation over a period of up to 8 years and are related to landfill aeration projects in Europe. For aerobic landfills in the US, cost estimates have been reported in a range of US$3–5 per ton of waste (Read et al., 2001). These costs include piping requirements, air and leachate injection equipment, gas monitoring and operation. However, it has to be noted that these figures have been estimated
about a decade ago. A more recent assessment of qualitative bioreactor costs and benefits is presented by Berge et al. (2009). The presented model results indicate that aerobic bioreactor landfills are less costly than traditional anaerobic landfills. The differences in cost estimations (cost ranges) are mainly associated with specific local conditions: small landfills without existing infrastructure (e.g. gas wells, blowers, etc.) exhibit higher investment costs in comparison to larger landfills (which have been formerly operated anaerobically) with existing gas extraction systems. Furthermore, the complexity of the accompanying monitoring program has to be considered when determining the overall costs. The costs for landfill aeration aiming at an accelerated biological waste stabilisation (European approach, by application of the low pressure concept) might be offset by considerable cost savings during both, landfill closure and aftercare. These potential cost savings are mainly related to the required provision of capital for: Replacement of a cost-intensive standard surface cover by an alternative long-life surface cover, adjusted to the minimised emission potential of the bio-stabilised wastes (lower investment costs and particularly lower maintenance costs). Lower costs in connection with ground water decontamination (in particular at old landfills without a base liner and no leachate collection system in place) due to reduced leachate pollutants. Lower operating costs resulting from reduced leachate treatment complexity and shorter treatment durations in landfills with a base liner and a leachate collection system. Reduction of the aftercare period (earlier release from aftercare) by at least several decades; potentially associated with an earlier revitalisation (for possible transformation to a green space) and after-use (might be of importance particularly in congested urban areas). When balancing these potential cost savings with the costs for aeration, cost reductions are mainly expected in the medium and long term. Calculations show that landfill aeration implies a total cost reduction potential of 10–25% (maybe even higher rates) during landfill closure and aftercare in comparison with the costs associated with anaerobic landfills (Heyer et al., 2007). 5. Conclusions Landfill aeration is a methodology for the fast, controlled and sustainable conversion of landfills into a biological stabilised state, which attracts increasing attention worldwide. In many cases, landfill aeration can be considered the only technically and economically feasible method to significantly reduce the aftercare both, by time and complexity. Numerous studies in laboratory scale have been conducted over the past two decades and full-scale examples have increased considerably within the last 10 years. Although comprehensive data have been published to date, there is a great need for a systematisation of the results in order to gain generally admitted indications. In this connection data regarding carbon conversion rates, settlements, temperature pattern as well as changes in both, solid waste characteristic and leachate quality are of particular interest. Against this background, the International Waste Working Group (IWWG) has set up a task group on the topic of landfill aeration to bring together international perspectives and expertise in the area in order to better define the technology and to disseminate experiences to the public. Currently the group is focussing on the compilation of an overview on the status of landfill aeration worldwide, definitions of landfill aeration methods, stabilization and quality criteria for landfill aeration as well as the set-up of a database that documents landfill aeration
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projects and experiences. In future, the long term behaviour of aerobically bio-stabilised waste regarding both, its emissions and emission potential, will be an important issue of the group activities. In connection with the determination of target values for the aeration process, further research in this direction is considered necessary and vital. Furthermore, it becomes apparent that there is still a lack of understanding regarding specific processes that may occur during aeration (e.g., the nitrogen and temperature dynamic under full scale conditions) and a deficit in transparency regarding overall costs. Obviously the great variety of landfill aeration concepts and the increasing number of realised aeration projects should be used as an opportunity and basis for a broader discussion and sharing of data among experts in this field to refine this technology as a potential sustainable waste management strategy. Based on the existing experiences, potential combinations of concepts may be further developed, such as the inclusion of wind driven aspirators in the semi-aerobic landfill concept, the application of deepfiltered gas wells for active aeration, intermittent operation in low pressure concepts as well as the adjustment of temperatures by a controlled addition of water or leachate. Acknowledgements This manuscript is an outcome of the Task Group on Landfill Aeration of the International Waste Working Group (IWWG). The authors acknowledge the contributions of individual task group members regarding country specific issues in connection with landfill aeration projects. References ARGE Biopuster, 2011. The original Biopuster process. Available from: www. biopuster.at. Bachofner, A., Meier, W., Düring, A., 2010. Aerobisierung von Deponien in der Schweiz – Erkenntnisse aus dem Anlagenbetrieb. In: Lorber, K.E., Adam, J., Arnberger, A., Bezama, A., Kreindl, G., Müller, P., Sager, D., Sarc, R., Wruss, K. (Eds.), DepoTech 2010, Tagungsband zur 10. DepoTech-Konferenz in Leoben, 3–5.11.2010. IAE Eigenverlag, pp. 537–542, ISBN: 978-3-200-02018-4. Beatty, B., Benda, E., Jadeja, M., Lee, S., Mohamed, A., 2009. Canada’s 30 year old aerobic landfill. In: Cossu, R., Diaz, L.F., Stegmann, R. (Eds.), Proceedings Sardinia 2009, Twelfth International Waste Management and Landfill Symposium, Session B11. CISA, pp. 217–218. Berge, N.D., Reinhart, D.R., Hudgins, M., 2007. The status of aerobic landfills in the United States. In: Stegmann, R., Ritzkowski, M. (Eds.), Landfill Aeration. IWWG Monograph. CISA, ISBN: 978-88-6265-002-1. Berge, N.D., Reinhart, D.R., Batarseh, E.S., 2009. An assessment of bioreactor landfill costs and benefits. Waste Management 29, 1558–1567. Bröcker, C., Klos, U., Hübl, F., 2010. DEPO+VerfahrenÓ – Energieausbeute und Langzeitverhalten verbessern bei HM-Deponien. In: Lorber, K.E., Adam, J., Arnberger, A., Bezama, A., Kreindl, G., Müller, P., Sager, D., Sarc, R., Wruss, K. (Eds.), DepoTech 2010, Tagungsband zur 10. DepoTech-Konferenz in Leoben, 3–5.11.2010. IAE Eigenverlag, pp. 483–488, ISBN: 978-3-200-02018-4. Cossu, R., Raga, R., Rosetti, D., Cestaro, S., 2003. Full scale application of in situ aerobic stabilization of old landfills. In: Christensen, T.H., Cossu, R., Stegmann, R. (Eds.), Proceedings of SARDINIA 2003 – Ninth International Waste Management and Landfill Symposium, Session B13. CISA, pp. 180–181. Cossu, R., Raga, R., Rosetti, D., Cestaro, S., 2007a. Case study of application of the in situ aeration on an old landfill: results and perspectives. In: Cossu, R., Diaz, L.F., Stegmann, R. (Eds.), Proceedings of SARDINIA 2007 – Eleventh International Waste Management and Landfill Symposium, Session B14. CISA, pp. 239–240. Cossu, R., Raga, R., Rosetti, D., Cestaro, S., Zane, M., 2007b. Preliminary tests and full scale applications of in situ aeration. In: Cossu, R., Diaz, L.F., Stegmann, R. (Eds.), Proceedings of SARDINIA 2007 – Eleventh International Waste Management and Landfill Symposium, Session B14. CISA, pp. 885–886. Giannis, A., Makripodis, G., Simantiraki, M., Somara, M., Gidarakos, E., 2008. Monitoring operational and leachate characteristics of an aerobic simulated landfill bioreactor. Waste Management 28, 1346–1354. Gamperling, O., Hrad, M., Huber-Humer, M., 2011. Lessons learned during a three year full-scale application of in-situ landfill aeration in Austria. In: Cossu, R., He, P., Kjeldsen, P., Matsufuji, Y., Reinhart, D., Stegmann, R. (Eds.), Proceedings of SARDINIA 2011 – Thirteenth International Waste Management and Landfill Symposium, Session B14. CISA, pp. 237–238.
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Hao, Y.-J., Ji, M., Chen, Y.-X., Wu, W.-X., Zhang, S.-G., Liu, H.-Q., 2010. The pathway of in-situ ammonium removal from aerated municipal solid waste bioreactor: nitrification/denitrification or air stripping? Waste Management and Research 28, 1057–1064. He, P., Yang, N., Gu, H., Zhang, H., Shao, L., 2011. N2O and NH3 emissions from bioreactor landfill operated under limited aerobic degradation conditions. Journal of Environmental Sciences 23 (6), 1011–1019. Heerenklage, J., Stegmann, R., 2003. Design and operation of the gas extraction system for the aeration of an old landfill used as industrial area. In: Christensen, T.H., Cossu, R., Stegmann, R. (Eds.), Proceedings of SARDINIA 2003 – Ninth International Waste Management and Landfill Symposium, Session D15. CISA, p. 431. Heyer, K.-U., Hupe, K., Ritzkowski, M., Stegmann, R., 2005. Pollutant release and pollutant reduction – impact of the aeration of landfills. Waste Management 25 (4), 353–359. Heyer, K.-U., Hupe, K., Koop, A., Stegmann, R., 2007. Aerobic in situ stabilization of landfills: long term experience and new developments. In: Cossu, R., Diaz, L.F., Stegmann, R. (Eds.), Proceedings of SARDINIA 2007 – Eleventh International Waste Management and Landfill Symposium, Session B14. CISA, pp. 241–242. Huber-Humer, M., 2011. Grundlagen der in-situ-Aerobisierung. In: Dokumentation des 3. ÖVA-Technologie Workshops 04-2011, 28/29, April 2011, Innsbruck. IUT Group, 2011. Landfill Remediation. Available from: www.theiutgroup.com. Kim, D.J., Lee, D.I., Keller, J., 2006. Effect of temperature and free ammonia in nitrification and nitrite accumulation in landfill leachate and analysis of its nitrifying bacterial community by FISH. Bioresource Technology 97, 459–468. Kim, H.-J., Yoshida, H., Matsuto, T., Tojo, Y., Matsuto, T., 2010. Air and landfill gas movement through passive gas vents installed in closed landfills. Waste Management 30, 465–472. Kraiger, H., 2011. Rotteballendeponie Pill – Umsetzung und erste Ergebnisse. In: Dokumentation des 3. ÖVA-Technologie Workshops 04-2011, 28/29, April 2011, Innsbruck. Matsufuji, Y., Tachifuji, A., 2007. The history and status of semi-aerobic landfills in Japan and Malaysia. In: Stegmann, R., Ritzkowski, M. (Eds.), Landfill Aeration. IWWG Monograph. CISA, ISBN: 978-88-6265-002-1. Onay, T.T., Pohland, F.G., 1998. In situ nitrogen management in controlled bioreactor landfills. Water Research 32, 1383–1392. Read, A.D., Hudgins, M., Phillips, P., 2001. Perpetual landfilling through aeration of the waste mass; lessons from test cells in Georgia (USA). Waste Management 21 (2001), 617–629. Reiser, M., Laux, D., Lhotzky, K., Kranert, M., 2011. Pilotprojekt zur Deponiebelüftung und Methanoxidation an der Deponie KonstanzDorfweiher, Ergebnisse nach 1 Jahr Betriebszeit. In: Rettenberger, G., Stegmann, R. (Eds.), Stilllegung und Nachsorge von Deponien – Schwerpunkt Deponiegas 2011, Trierer Berichte zur Abfallwirtschaft, Bd. 20. Verlag Abfall aktuell, pp. 63–74, ISBN 978-3-9812867-5-5. Ritzkowski, M., Heyer, K.-U., Stegmann, R., 2006. Fundamental processes and implications during in situ aeration of old landfills. Waste Management 4, 356– 372, Elsevier Ltd., ISSN 0956-053X. Ritzkowski, M., Stegmann, R., 2007. Controlling greenhouse gas emissions through landfill in situ aeration. International Journal of Greenhouse Gas Control. http:// dx.doi.org/10.1016/S1750-5836(07)00029-1, pp. 281–288. Ritzkowski, M., Heyer, K.-U., Hupe, K., Stegmann, R., 2009a. Long term landfill aeration by means of a wind driven air venting and aeration system. In: Cossu, R., Diaz, L.F., Stegmann, R. (Eds.), Proceedings SARDINIA 2009, Twelfth International Waste Management and Landfill Symposium, Session G15. CISA, pp. 879–880. Ritzkowski, M., Heyer, K.-U., Stegmann, R., 2009b. Practical application of landfill aeration in Germany. In: Cossu, R., Diaz, L.F., Stegmann, R. (Eds.), Proceedings SARDINIA 2009, Twelfth International Waste Management and Landfill Symposium, Session B11. CISA, pp. 219–220. Robinson, H., Olufsen, J., 2004. Design and operation of cost-effective leachate treatment schemes at UK landfills: recent case studies. In: Proceedings of the CIWM Conference and Exhibition, 15–18th June 2004, Paignton, Torbay, UK. Scheffer, F., Schachtschabel, P., 2009. Lehrbuch der Bodenkunde. Spektrum Akademischer Verlag, ISBN: 978-3827413246. Stegmann, R., Ritzkowski, M. (Eds.), 2007. Landfill Aeration. IWWG Monograph. CISA, ISBN: 978-88-6265-002-1. Tesar, M., Prantl, R., Lechner, P., 2007. Application of FT-IR for assessment of the biological stability of landfilled municipal solid waste (MSW) during in situ aeration. Journal of Environmental Monitoring 9 (1), 111–120. van Vossen, W., Heyer, K.-U., Woelders, H., 2009. 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Contents lists available at SciVerse ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Landfill aeration worldwide: Concepts, indications and findings M. Ritzkowski a,⇑, R. Stegmann b a b
Institute of Environmental Technology and Energy Economics, Hamburg University of Technology, Harburger Schlossstr. 36, 21079 Hamburg, Germany Consultants for Waste Management, Prof. R. Stegmann and Partner, Schellerdamm 19–21, 21079 Hamburg, Germany
a r t i c l e
i n f o
Article history: Received 5 December 2011 Accepted 28 February 2012 Available online 28 March 2012 Keywords: Landfill aeration Sustainable landfills Biological stabilisation Aeration concepts Emission potential
a b s t r a c t The creation of sustainable landfills is a fundamental goal in waste management worldwide. In this connection landfill aeration contributes towards an accelerated, controlled and sustainable conversion of conventional anaerobic landfills into a biological stabilized state associated with a minimised emission potential. The technology has been successfully applied to landfills in Europe, North America and Asia, following different strategies depending on the geographical region, the specific legislation and the available financial resources. Furthermore, methodologies for the incorporation of landfill aeration into the carbon trade mechanisms have been developed in recent years. This manuscript gives an overview on existing concepts for landfill aeration; their application ranges and specifications. For all of the described concepts examples from different countries worldwide are provided, including details regarding their potentials and limitations. Some of the most important findings from these aeration projects are summarised and future research needs have been identified. It becomes apparent that there is a great demand for a systematisation of the available results and implications in order to further develop and optimise this very promising technology. The IWWG (International Waste Working Group) Task Group ‘‘Landfill Aeration’’ contributes towards the achievement of this goal. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The creation of sustainable landfills is a fundamental goal in waste management worldwide. In this case, sustainability is closely tied to the potential of terminating expensive perpetual landfill aftercare. However, objectives of many conventional landfill operations largely focus on the short term prevention of hazards and risks to the environment that are caused by polluted leachate and migrating landfill gas (LFG) as well as on a controlled utilisation of the energy provided by extracted LFG. Once this option is not feasible or at least not economically beneficial anymore, many landfills will still exhibit a significant emission potential, which tapers off with time to very low levels. At this stage, these conventional landfills are considered to be unsustainable. In this respect, landfill aeration is considered to be an indispensable tool for the controlled and sustainable conversion of conventional anaerobic landfills into a biological stabilised state associated with a significantly lowered or the near elimination of the landfill gas emission potential. Although landfill aeration is not a widely applied concept so far, it has already been successfully applied to several landfills in Europe, North America and Asia
⇑ Corresponding author. Tel.: +49 40 42878 2053; fax: +49 40 42878 2375 E-mail addresses: [email protected] (M. Ritzkowski), [email protected] (R. Stegmann). URL: http://www.tu-harburg.de/iue/ (M. Ritzkowski). 0956-053X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.wasman.2012.02.020
(Stegmann and Ritzkowski, 2007). Some of the most comprehensive investigations have been carried out in Germany and due to the high interest in this technology more investigations are underway in different countries at the present time. This is certainly essential as in situ aeration means more than just injecting air into a landfill. Aspects such as well design and spacing, selection of appropriate air volume and pressure, control of air distribution, temperature and moisture as well as the potential mobilisation of pollutants in both, the gas and liquid phases have to be considered. Moreover, the question of a particular point in time to terminate the aeration process has to be answered. The latter includes indications about the biological landfill stability achieved during aeration and is therefore related to the considerations of landfill sustainability. Finally, not only technical and ecological aspects have to be considered, but also economic issues. In order to get an answer to the questions raised above the following chapters provide an overview on landfill aeration concepts, implemented examples and most important findings so far. 2. Landfill aeration concepts 2.1. High pressure aeration (shock pressure concept) High pressure aeration is mostly associated with the Bio-PusterÒ concept. The aeration is realised by shock pressure releases (up to 6 bars) from lances, using air which might be enriched by additional oxygen (up to 20%) and potentially by nutrients. Supplied by a
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compressed air distribution network, each lance features a quick-release valve which is intermittently opened once a specified positive pressure has been built-up. The released aeration gas is capable of penetrating both, highly and weakly compacted waste materials. In order to minimise the uncontrolled release of off-gases, the shock pressure concept involves an off-gas extraction system consisting of suction lances. The system, operated in parallel to the aeration and with an increased extraction capacity of 30% (in comparison with the gas volumes used for aeration), collects the offgases before being treated by means of biofilters and/or activated carbon (ARGE Biopuster, 2011; Fig. 1a). The concept of high pressure aeration is mainly associated with the implementation of landfill mining projects. This is based on the fact that for a widely bio-stabilisation, treatment durations in the range of several years are required. Therefore, this energy and operating material (industrial gas) intensive method holds some drawbacks. 2.2. Low pressure aeration Low pressure concepts are applied in the majority of landfill aeration cases aiming at an accelerated biological waste stabilisation in situ. In contrast to the high pressure concept positive pressure differences do not exceed 0.3 bars; normally they are in a range of 20–80 mbars. Although the general conditions are similar, a number of different concepts and variants have been developed over the past 10–15 years. The most important concepts are briefly summarised in the following sub paragraphs. 2.2.1. Active aeration and off-gas extraction As for high pressure aeration, the majority of low pressure aeration concepts exhibit a simultaneous aeration and extraction operation. Currently most of the applications are based on the AEROflottÒ (Heyer et al., 2005), AIRFLOWÒ (Cossu et al., 2003) or Smell-WellÒ (IUT Group, 2011) concept. Through a system of vertical gas wells ambient air is continuously introduced into the landfilled waste. The air distributes by means of convection and diffusion and is further directed through the simultaneously operated off-gas extraction system. The latter consists of the same kind of gas wells as the ones used for air supply, being connected to an air compressing unit and the final off-gas purification stage. The simultaneously operated supply and extraction system offers significant advantages in terms of flexibility: air can be targeted to be introduced to zones featuring an oxygen deficit and the air flow inside the wastes can be manipulated through the selection of appropriately located aeration and gas extraction wells (Fig. 1b). Differences among the concepts for active low pressure aeration and off-gas extraction mainly exist with the selection of an appropriate off-gas treatment. With regard to an optimised reduction in greenhouse gas emissions the thermal oxidation of the residual methane load in the extracted off-gases is mandatory. Systems like the flameless non-catalytic thermal oxidation (regenerative thermal oxidation, RTO) are capable of completely avoiding methane emissions via the off-gases. At the same time the external energy demand is very low as the required oxidation temperature (about 1100 °C) is retained by the energy release during thermal methane oxidation. The RTO is an integral part of the AEROflottÒ system. Other systems apply a biological off-gas treatment concept, such as a biofilter or combinations of bio-scrubbers and biofilters. In most cases, these biological systems are capable of reducing odours to a wide extent; however, they show significant deficits in terms of methane oxidation due to the often insufficient retention times of the off-gas in the filters. 2.2.2. Active aeration without off-gas extraction Air supply into the landfilled waste can be executed without a need for operating an active extraction system for the off-gases
simultaneously. In this case, the landfill cover might function as a biological filter layer, either in its original condition or after enhancement of its biological methane oxidation capacity. If landfill aeration is realised without the integration of a simultaneous extraction system and treatment of the off-gases, a continuous operation is certainly easier to maintain, although it can result in a significantly lowered emission reduction (in comparison to combined systems). The aeration can be realised by both, a system of vertical gas wells in the waste body (Reiser et al., 2011) or through air injection into the unsaturated soil zone beneath the wastes (Kraiger, 2011; Fig. 1c). For the latter case, the soil functions as an air distribution layer aiming at an even aeration of the wastes from bottom to top. 2.2.3. Passive aeration (air venting) Passive aeration concepts follow the basic approach of air venting, (i.e., the introduction of ambient air into the landfill through its surface or eventually through open gas wells) driven by a negative pressure induced inside the landfill body. This concept is mainly recognised under the brand DEPO+Ò. The gas wells are perforated only in the deeper waste layers in order to increase the waste volume to be affected by the aeration and to avoid short circuits near the landfill surface. To ensure a gradual aeration, starting at the surface before shifting into the deeper layers, extracted gas volumes are significantly higher than the gas production rate of the landfilled waste. The extracted off-gas is treated by means of a biofilter (Bröcker et al., 2010; Fig. 1d). Regarding the stabilisation performance, two different stages have to be distinguished for air venting concepts. At the initial stage the methane flux in the extracted off-gas is increased in comparison to the previous flux under ordinary LFG extraction operation. This is due to both, the specific gas well design featuring a short perforated tail section in combination with significantly increased gas extraction flow rates (Bröcker et al., 2010). Through this approach, LFG is captured even from zones that have not been affected by the extraction system when operated prior to the start of air venting. Only during the later stages the gas composition changes towards reduced methane concentrations as a result of air infiltration via the landfill surface. 2.2.4. Energy self-sufficient long term aeration In contrast to the afore-mentioned active and passive aeration concepts, an energy self-sufficient system might be applied in particular for long term aeration. This system can be established during the transitional period between the end of ‘‘active’’ forced aeration and the subsequent installation of a final surface cover that includes a qualified methane oxidation layer. By means of an on-going low air supply the restart of LFG-generation can be widely avoided in the long term, thus significantly reducing the methane load to be biologically oxidised without relevant subjection to climatic changes. Energy self-sufficient aeration systems consist of wind driven aspirators to be mounted directly on some of the existing gas wells and pneumatic air pumps, powered by wind wheels. The compressed air is directed into existing gas wells, which leads to an on-going oxygen supply of the already widely stabilised landfill (Ritzkowski et al., 2009a). 2.3. Semi aerobic concept The concept of semi-aerobic landfills is actually the oldest with regard to landfill aeration. Back in 1975, the first semi-aerobic landfill was developed in Japan. In semi-aerobic landfills, the leachate collection system consists of a central perforated pipe (main collection pipe) with perforated branch pipes on either side of it laid at a suitable interval. The pipes are embedded in graded gravel
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Fig. 1. Schematic outlines of different aeration concepts (a: high pressure aeration; b: low pressure aeration with parallel off-gas extraction; c: low pressure aeration w/o off-gas extraction; d: low pressure aeration/air venting).
(5–15 cm) and installed with adequate slope. The main collection pipe ends in an open leachate collection pond. The pipes are designed in a way that only one-third of the section is filled with liquid. At each intersection of the main collection pipe with the branch pipes, and at the end of each branch pipe, vertical gas ventilation wells enclosed in graded gravel (eventually packed inside a wire netting) are erected. The air will be able to flow into the waste
layer through these pipes when the leachate head is low. Since the two piping systems are connected, ambient air and landfill gas flows through the leachate collection pipes and the gas ventilation wells, thus, enhancing the intrusion of the air into the inner part of the landfilled wastes occasionally (Fig. 2). Due to higher temperatures in the waste (compared to the ambient air), the gas inside the waste tends to rise and gets vented through the gas wells, thus,
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generating a negative pressure siphoning effect that draws more air into the leachate collection pipes (Matsufuji and Tachifuji, 2007). During recent years the semi-aerobic concept has been slightly expanded and adapted. Assuming that the leachate collection system is not functioning anymore, associated with an increased leachate head and the inability of ambient air to be introduced through the leachate pipes, passive aeration might be realised through the landfill surface. In principle, the same concept as the ‘‘air venting’’ applies here, whereas the venting effect is induced by temperature differences only (thus no external energy demand for the aeration exists). The passive aeration system might be further enhanced through the installation of additional gas venting wells at an appropriate spacing (Kim et al., 2010). 3. Implemented examples of landfill aeration 3.1. Examples from Europe In Europe, landfill aeration projects have been implemented in Germany, Austria, Italy, Switzerland and The Netherlands. The majority of projects are located in Germany and refer to the concept of low pressure aeration. The vast majority of these projects are aiming at an accelerated and sustainable bio-stabilisation of the deposited waste mass. By means of the active aeration and simultaneous off-gas extraction/treatment approach (i.e., AEROflottÒ technique), landfills in the German Federal States of Lower Saxony (Kuhstedt landfill), Bavaria (Amberg-Neumühle landfill) and Brandenburg (Milmersdorf landfill) have been biologically stabilised. Fig. 3 shows an example about the technical installations required for low pressure landfill aeration. In all three cases more than 90% of the residual biodegradable organic carbon was bioconverted and released, mainly in terms of carbon dioxide (CO2) via the gas phase. Application of this technology (i.e., aeration via the AEROflottÒ technique) at additional conventional (anaerobic) landfills in North Rhine-Westphalia (Doerentrup landfill and Halberbracht landfill), Lower Saxony (Suepplingen landfill) and Saarland (SchwalbachGriesborn) is currently underway to accelerate the biostabilization of the organic fraction of the wastes at these sites (Ritzkowski et al., 2009b; Heyer et al., 2007). By means of the passive aeration concept (including open air access wells), landfills in the German Federal State of Schleswig Holstein (Kiel-Drachensee landfill, Schenefeld landfill, Barsbüttel
landfill) have been biologically stabilised. These landfills exhibit an intensive after use, including many buildings (both, commercial and/or residential) that have been constructed after landfill closure. In order to avoid the perpetual formation of methane gas and to reduce current and future associated risks for the users, the landfills have been converted from anaerobic to aerobic conditions (Heerenklage and Stegmann, 2003). Additionally, a number of landfills have been aerated by means of the passive aeration/gas venting concept (i.e., DEPO+Ò technique) over the last two decades. This particular technology was applied at landfills in the German Federal States of Schleswig Holstein and North Rhine-Westphalia. However, information regarding the progression of stabilisation as well as indications regarding the sustainability of the aeration process have not been reported, hence, no pertinent literature on the application of this technology on these sites is currently available. In northern Italy, three landfills have been aerated by means of the active aeration and parallel off-gas extraction/treatment (biofilter) approach according to the AIRFLOWÒ system. This technology has been applied at the Modena landfill since 2002, in order to create conditions that allow for the subsequent mining of the landfill site. Two other landfills in northern Italy have been biologically stabilised within a period of 3 years since 2005 (Cossu et al., 2007a). In Austria, the concept of low pressure aeration was applied for the first time in a pilot-scale demonstration project at an old landfill in Mannersdorf (Lower Austria). In 2003 and 2006, respectively, a limited part of the landfill has been aerated for demonstration purposes. Here the aeration and the parallel off-gas extraction system was combined with the treatment of the exhausts through a biofilter. On the basis of the promising results of the demonstration project, the whole landfill in Mannersdorf is currently being biologically stabilised by means of the same aeration concept (Gamperling et al., 2011). Furthermore, a landfill in Tyrol is currently undergoing aeration since 2008 by means of the low pressure concept, but excluding the capture and treatment of the exhausts. In this particular case, air is introduced to the unsaturated soil beneath the landfilled waste whereas the landfill soil cover functions as a biological filter for the emitted off-gas (Kraiger, 2011). Further to the above mentioned landfill aeration projects, aiming at biological waste stabilisation, a number of European landfills (or parts of landfills) have been aerated in order to
Fig. 2. Schematic outline of the semi-aerobic landfill concept.
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Fig. 3. Master plan of the technical installations for in situ aeration at the Milmersdorf landfill in Germany (Heyer et al., 2007; modified).
minimise odours and methane concentrations prior to landfill mining activities. However, for a majority of these cases, either the high pressure aeration concept (Bio-PusterÒ) or the low pressure Smell-WellÒ concept has been applied. Since 1990 more than 12 landfills in Austria, three landfills in Germany, two landfills in Italy and Czech Republic, respectively and one landfill in The Netherlands have been treated by means of these technologies (ARGE Biopuster, 2011). 3.2. Examples from the US and Canada In the US, first studies on landfill aeration had been carried out in the early 1970’s. Pilot studies were conducted on cells on landfills that were filled with fresh MSW. However, these studies involved intermittent aeration without the addition or recirculation of water. Due to the increasing high temperatures within the landfill cells (up to 90 °C) resulting from the aeration process coupled with the associated problems of severe reduction in waste moisture content, further projects had been delayed until the early 1990s. That is when a number of lysimeter tests combining both, aeration and water addition, were conducted to assess the impact of moisture content on aeration. Towards the end of the 1990s, the first landfills underwent aeration in the course of large-scale tests in Georgia and South Carolina. To date, more than 20 landfills located within various American states have been aerated following the intention to minimise negative impacts on surface water and groundwater, reduce leachate volumes, eliminate the production of methane gas, rapidly stabilise the waste or redevelop the site for further utilisation. For the vast majority of cases, low pressure concept of aeration (without off-gas extraction and treatment) was implemented at these landfills. As a
rule, the injection of air was accompanied by the parallel injection of liquid (mostly through wells, in some cases sprayed over the surface) in order to control temperatures and to reduce leachate volumes (Berge et al., 2007). One of the oldest full-scale aerobic landfills in the world is located in Toronto, Canada. Since 1978 the landfill has been kept at a mainly aerobic stage by means of the passive aeration (air venting) concept. Air is introduced into the waste mass via the landfill surface if the gas extraction flow rates exceeding the gas generation rates. The intention is to widely avoid the formation of methane gas as to prevent hazards for a couple of buildings in the immediate vicinity of the landfill. Currently, it can be concluded that in addition to the landfill gas hazard prevention, a significant reduction in greenhouse gas emissions can be realised over the past decades (Beatty et al., 2009). 3.3. Examples from Asia The majority of landfills in Japan, over half of the number of the landfills in Korea and a few landfills in Malaysia have been constructed and operating according to the semi-aerobic concept (Matsufuji and Tachifuji, 2007). Semi-aerobic landfills are mainly characterised by their long term behaviour as they exhibit reduced methane gas generation rates and an enhanced leachate quality in comparison to anaerobic landfills. However, due to the limited intensity of aeration induced by natural ventilation and the remaining anaerobic zones inside the landfilled waste, the biological stabilisation occurs rather slowly in comparison to actively aerated landfills. During recent years, the semi-aerobic concept has been recognised for its potential towards landfill remediation. Through the
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installation of more than 70 passive gas extraction wells into an old landfill, an air venting effect (passive aeration) could be induced at the Nakazono landfill in Asahikawa, Japan (Yoshida, 2011). Air is sucked in through the landfill surface, following a chimney effect which is induced by temperature differences between the waste and the ambient air. During 3 years after the installation of passive gas extraction wells a significant temperature increase (up to 60 °C) could be observed in the proximity of some wells. At the same time methane concentrations decreased to low levels in the range of 5% (v/ v). However, other parts of the old landfill (in particular deeper waste layers) were hardly affected by the passive aeration because methane concentrations remained at high levels there. 4. Findings from landfill aeration projects 4.1. General aspects The term ‘‘landfill aeration’’ comprises a variety of different concepts and methodologies as demonstrated in Section 2 and summarised in Table 1. Over the past decades many of these concepts have proven to be applicable at full scale, even though in the majority of cases a clear proof of success has not been provided. The reasons for this are manyfold and include, beside others, a lack of general parameters and target values for the successful completion of aeration as well as the application of insufficient monitoring programs during aeration. Furthermore, the aeration of landfills has to follow a number of case specific objectives in accordance with the local situation and framework requirements. Landfill mining projects are often associated with short term aeration measures, aiming at a minimisation of both, odours and methane concentrations. For these purposes, high pressure concepts offer advantages due to their ability to ensure a fast and consistent aeration of the landfilled wastes whereas cost aspects and long term reliability of the system are of secondary importance. In order to prevent long term hazards like methane gas accumulation in and/or around buildings, passive aeration or air venting concepts offer certain advantages. These are mainly associated with a relatively simple operation mode (gas extraction only) where air is passively introduced (via the surface) into the landfill. However, challenges related to this concept exist in the form of potential accumulation of water (leachate) in the gas wells and/or the formation of preferential flow paths. The aerobic bioreactor concept can be applied for the fast recovery of landfilling capacity. This concept enables the combination of water addition and leachate recirculation coupled with the introduction of air into relatively reactive (biologically unstable) waste materials. Through the combined operation, the landfill can be used
as an in situ bioreactor, where processes like nitrification, denitrification and accelerated reductions in organic constituents, take place. Furthermore, water functions as a suitable measure to control and limit the temperatures inside the aerated landfill. Problems regarding this approach are associated with the ‘‘competition’’ between water and air for free void volume. As a consequence, the aeration effect (i.e., the biological stabilisation) might be limited or reduced to those zones exhibiting only moderate moisture contents (i.e., mainly the upper waste layers). Additionally, the concept of aerating fresh MSW (i.e., waste with a high organic content) exhibit an inferior energy balance compared to other strategies, where anaerobic degradation with the utilisation of captured methane (for electricity and/or heat generation) is carried out first prior to the beginning of the aeration process. In terms of an accelerated biological stabilisation of the organic fraction of waste, low pressure aeration concepts that combine air injection and off-gas extraction are most appropriate. These concepts are designed for a continuous operation over periods of 3– 10 years at both, moderate costs and operational efforts. Data regarding the initial amount of organic carbon and related conversion rates in the cause of aeration have been published for three completed full-scale projects in Germany. These data are summarised in Table 2. Major advantages of these concepts exist with their operational flexibility (i.e., dynamic systems which allow for an adaptation of flow rates and flow directions), simple but robust technology (i.e., vertical gas wells, blowers) and feasibility to include an effective off-gas treatment system. In order to reduce negative environmental impacts caused by landfills (mainly the uncontrolled emission of greenhouse gases and polluted leachate) under the situation of limited economical resources, the semi-aerobic landfill concept holds promise. A major advantage of applying this concept results in minimal operational efforts (i.e., only monitoring) and the complete avoidance of both, additional installations and energy demand. Although semi-aerobic landfills may improve the emission behaviour of a landfill, this concept prevents the utilisation of the energy contained in the methane gas (from anaerobic landfills) and does not reach the positive performance (regarding biological waste stabilisation, depression of methane formation, improvement of leachate quality) of an aerobic landfill under active aeration. 4.2. Impact of climatic conditions In order to select an appropriate aeration technology for a specific landfill, local climatic conditions (mainly temperature and precipitation levels) should be taken into account. These conditions are geographically dependant and can be roughly
Table 1 Overview of different landfill aeration concepts, application areas and related methods for off-gas treatment. Aeration concept
Sub-concept
Main area of application
Off-gas treatment method
High pressure
–
In preparation for landfill mining
Biofilter
Low pressure
Active aeration and off-gas extraction
Accelerated bio-stabilisation and GHG emissions minimisation Accelerated bio-stabilisation Increasing gas extraction rates/subsequent aerobic biostabilization Avoidance of residual LFG generation in the long term
Thermal oxidation (biofilter during further advanced stabilisation stages) Biocover (landfill surface) Biofilter
Long term reduction of GHG emissions and improvement of leachate quality
None
Landfill remediation/reduction of GHG emissions
None
Active aeration w/o off-gas extraction Passive aeration (air venting) Energy self-sufficient long term aeration Semi-aerobic
Connection between leachate collection pipes and gas vents as a part of the design layout Subsequent installation of passive gas vents (w/o connection to leachate pipes)
None/landfill surface
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M. Ritzkowski, R. Stegmann / Waste Management 32 (2012) 1411–1419 Table 2 Comparison between the total amounts of biodegradable organic carbon and the accordant discharged amounts, during the aeration process at three German landfills.
Total amount of biodegradable organic carbon Total discharge (via off-gas) of biodegradable organic carbon Monthly discharge (via off-gas) of biodegradable organic carbon Percentage of discharged biodegradable organic carbon (in relation to total biodegradable organic carbon) a b
(mg) (mg) (mg) [%]
Landfill Aa
Landfill Bb
Landfill Cb
1890 1730 9–46 (mean: 22) 92
4300 3900 80 91
530 500 10 94
Aeration time: 6 years. Aeration time: 4 years.
subdivided between ‘‘temperate climate’’, ‘‘humid/tropical climate’’ and ‘‘climate with extreme seasonal temperature variations’’. The latter condition applies to countries located in the mid-high latitudes and mostly situated in the northern hemisphere but may also be of importance for locations at higher elevations (mountainous regions). Landfills located in a temperate climate might be actively aerated by means of the low or high pressure concept. Due to moderate temperature and precipitation levels operational problems occur infrequently. A humid/tropical climate, on the other hand, might lead to operational problems since water might accumulate in the landfill and the off-gas extraction system, thus significantly reducing the operation efficiency. Landfills under the latter climate conditions might therefore be subjected to the semi-aerobic concept, provided that the temperature differences between the ambient and the waste (i.e., the major driver for the passive aeration effect) remains sufficient. A location with extreme seasonal temperature variations, however, represents the most challenging situation. In particular under very low temperatures problems related to an increased formation of condensate in the off-gas extraction system may occur. Also, active aeration concepts without off-gas extraction, may operate sufficiently under these conditions but might fall short as to an efficient biological methane oxidation in the landfill surface. 4.3. Monitoring concepts The unavailability of sufficient data regarding full scale aeration projects is, at least partly, associated with the choice of inadequate monitoring. In most of the cases, data are collected only for the gas phase during operation. In contrast, changes in leachate quality or in the solid waste material are measured quite rarely. Furthermore, evolutions in temperatures as well as in settlements are seldom recorded. In particular for the low pressure aeration concepts aiming at an accelerated biological stabilisation of the organic fraction of waste, mandatory monitoring activities should include parameters associated with the bioconversion of organic carbon. In the off-gases, methane and carbon dioxide content have to be determined (in the ideal case continuously) whereas in the leachate, the concentration of total organic carbon (TOC) and/or the chemical oxygen demand (COD) should be analysed frequently. In combination with the assessment of biodegradable organic carbon in the solid waste material (following waste sampling and laboratory analysis), a carbon balance can be set-up on the basis of these data, thus, enable the effective monitoring of the stabilisation process. A new approach is also provided by the application of FT-IR (Fourier Transformation Infrared Spectroscopy) to assess the aeration effect on the organic waste matter (Tesar et al., 2007). Another option exists with the set-up of a nitrogen balance. However, the potential nitrogen conversion processes that may occur during landfill aeration are manifold and include severe challenges in terms of an appropriate analysis (e.g. the determination of nitrous oxides or ammonia in the off-gas or the calculation of elementary nitrogen originating from de-nitrification processes).
4.4. Completion of aeration: when and how? During recent years a couple of proposals have been made and discussed regarding parameters and target values which might be used to determine the completion of an aeration project. As for the monitoring concepts, these proposals are mainly related to landfill aeration projects aiming at an accelerated biological stabilisation of the waste organic fraction. It is evident that particularly those parameters describing the state of biological stabilisation of the waste organic fraction are useful for determining the success of an aeration project. In this regard, solid waste samples have to be investigated for their specific respiration activity and/or methane-gas generation potential. In combination with the monitoring data (in particular the amount of emitted carbon) it is possible to assess the residual emission potential of an aerated landfill. Proposals for stabilisation parameters and target values have been made in the context of two comprehensive R&D projects in both, Austria and Germany (Tesar et al., 2007; Ritzkowski et al., 2006). One of the general approaches used to determine the biological stability achieved during the aeration project is a requirement whereby more than 90% of the biodegradable organic carbon has to be bioconverted (mainly to CO2) (Ritzkowski et al., 2006). Once this limit value has been reached, it can be assumed that the residual methane gas generation rates are very low, settlements are widely completed and leachate quality has significantly improved (in terms of reduced organic loads). 4.5. Transfer from lab-scale to full-scale In literature there are numerous examples on laboratory scale investigations of different waste materials under aerated landfill conditions (Kim et al., 2006; Giannis et al., 2008; Hao et al., 2010; He et al., 2011). Generally, many of the reported results show a rapid biodegradation of the organic fraction of waste, an almost complete avoidance of methane gas emissions and significantly elevated carbon dioxide emissions simultaneously. With regard to leachate pollution, reductions of dissolved organic carbon (DOC) and chemical oxygen demand (COD) as well as ammoniumnitrogen have been observed. In this connection, the transferability of the results from lab scale to full scale has to be investigated further. Therefore, the question to be answered is: do the lab scale experiments actually represent the conditions that occur in old landfills during full-scale aeration? Apparently the results from full-scale aeration projects often indicate that the answer is negative. Although in the majority of cases considerable residual methane emissions have not been detected after completion of aeration, partly increased organic loads in the leachate have been observed and in particular the reduction in ammonium-nitrogen was smaller than predicted by the lab tests (Bachofner et al., 2010; Cossu et al., 2007b). So what are the fundamental differences between the laboratory and the field approach? One aspect to be considered is the fairly homogeneous oxygen supply at sufficient rates in lab-scale experiments, whereas under full scale conditions this is very difficult to achieve. However, the
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major difference to be observed is related to the significant temperature increase inside the aerated waste material. Laboratory scale investigations have been, as a rule, carried out under constant temperatures in the mesophilic range (20–35 °C). Most of the heat produced by the microbial activity under aerobic conditions is balanced with the ambience, resulting in an only moderate temperature increase inside the test vessels. Increasing temperatures, however, are the major factors to determine the nitrogen dynamics and the moisture losses during aeration. Temperatures above 40 °C are intensifying both, hydrolysis and ammonification processes (Scheffer and Schachtschabel, 2009) while at the same time inhibiting nitrification (Robinson and Olufsen, 2004). As a consequence, the pH increases and free ammonia is formed which, at certain concentrations, is a strong inhibitor for nitrite and ammonia oxidisers (Onay and Pohland, 1998). Temperatures in a range of 40 °C and above are observed in the majority of landfill aeration projects over long time periods and often for many months and/ or for several years (Heyer et al., 2005). One of the consequences of these observations is that laboratory scale investigations should be conducted in a way respecting the temperature dynamic of an aerated landfill in order to get representative results. 4.6. The importance of off-gas treatment Despite the semi-aerobic landfill approach and some examples from low pressure aeration, the majority of aeration concepts include the treatment of the extracted off-gases. However, in many cases this treatment is carried out by means of biological processes, either in an external biofilter (container) or in the landfill surface cover, wherein the methanotrophic activity of certain microorganisms is utilised. These treatment approaches are robust and easy to maintain, but at the same time they may be relatively ineffective regarding the reduction in methane emissions due to often insufficient retention times and climatic influences (extreme temperatures and moisture contents). Furthermore it has to be taken into account that, in particular during the early phases of aeration, the methane flux (i.e., the amount of methane extracted during a defined period of time) might nearly equal (sometimes even exceed) the one to be present under anaerobic conditions (Ritzkowski and Stegmann, 2007). Reasons for this behaviour are to be seen in both, increased flow rates resulting in the collection of gas from previously unaffected landfill areas as well as increased bio-conversion rates under the prevailing increased temperatures. This methane flux might significantly exceed the methane oxidation capacity of a biological off-gas treatment system (biofilter). Therefore, if an aeration project aims at a massive reduction in methane emissions, the inclusion of a thermal off-gas treatment is mandatory. This is very important in particular approaches where landfill aeration is considered for climate projects. 4.7. Costs and cost savings Information regarding the costs of landfill aeration is rarely provided in literature. For the low pressure aeration concept (parallel aeration and off-gas extraction and/or treatment) costs are indicated in a range from 0.45 € up to 7 € per ton of waste material to be aerated (Heyer et al., 2007; van Vossen et al., 2009; HuberHumer, 2011). These costs include the investment and operation over a period of up to 8 years and are related to landfill aeration projects in Europe. For aerobic landfills in the US, cost estimates have been reported in a range of US$3–5 per ton of waste (Read et al., 2001). These costs include piping requirements, air and leachate injection equipment, gas monitoring and operation. However, it has to be noted that these figures have been estimated
about a decade ago. A more recent assessment of qualitative bioreactor costs and benefits is presented by Berge et al. (2009). The presented model results indicate that aerobic bioreactor landfills are less costly than traditional anaerobic landfills. The differences in cost estimations (cost ranges) are mainly associated with specific local conditions: small landfills without existing infrastructure (e.g. gas wells, blowers, etc.) exhibit higher investment costs in comparison to larger landfills (which have been formerly operated anaerobically) with existing gas extraction systems. Furthermore, the complexity of the accompanying monitoring program has to be considered when determining the overall costs. The costs for landfill aeration aiming at an accelerated biological waste stabilisation (European approach, by application of the low pressure concept) might be offset by considerable cost savings during both, landfill closure and aftercare. These potential cost savings are mainly related to the required provision of capital for: Replacement of a cost-intensive standard surface cover by an alternative long-life surface cover, adjusted to the minimised emission potential of the bio-stabilised wastes (lower investment costs and particularly lower maintenance costs). Lower costs in connection with ground water decontamination (in particular at old landfills without a base liner and no leachate collection system in place) due to reduced leachate pollutants. Lower operating costs resulting from reduced leachate treatment complexity and shorter treatment durations in landfills with a base liner and a leachate collection system. Reduction of the aftercare period (earlier release from aftercare) by at least several decades; potentially associated with an earlier revitalisation (for possible transformation to a green space) and after-use (might be of importance particularly in congested urban areas). When balancing these potential cost savings with the costs for aeration, cost reductions are mainly expected in the medium and long term. Calculations show that landfill aeration implies a total cost reduction potential of 10–25% (maybe even higher rates) during landfill closure and aftercare in comparison with the costs associated with anaerobic landfills (Heyer et al., 2007). 5. Conclusions Landfill aeration is a methodology for the fast, controlled and sustainable conversion of landfills into a biological stabilised state, which attracts increasing attention worldwide. In many cases, landfill aeration can be considered the only technically and economically feasible method to significantly reduce the aftercare both, by time and complexity. Numerous studies in laboratory scale have been conducted over the past two decades and full-scale examples have increased considerably within the last 10 years. Although comprehensive data have been published to date, there is a great need for a systematisation of the results in order to gain generally admitted indications. In this connection data regarding carbon conversion rates, settlements, temperature pattern as well as changes in both, solid waste characteristic and leachate quality are of particular interest. Against this background, the International Waste Working Group (IWWG) has set up a task group on the topic of landfill aeration to bring together international perspectives and expertise in the area in order to better define the technology and to disseminate experiences to the public. Currently the group is focussing on the compilation of an overview on the status of landfill aeration worldwide, definitions of landfill aeration methods, stabilization and quality criteria for landfill aeration as well as the set-up of a database that documents landfill aeration
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projects and experiences. In future, the long term behaviour of aerobically bio-stabilised waste regarding both, its emissions and emission potential, will be an important issue of the group activities. In connection with the determination of target values for the aeration process, further research in this direction is considered necessary and vital. Furthermore, it becomes apparent that there is still a lack of understanding regarding specific processes that may occur during aeration (e.g., the nitrogen and temperature dynamic under full scale conditions) and a deficit in transparency regarding overall costs. Obviously the great variety of landfill aeration concepts and the increasing number of realised aeration projects should be used as an opportunity and basis for a broader discussion and sharing of data among experts in this field to refine this technology as a potential sustainable waste management strategy. Based on the existing experiences, potential combinations of concepts may be further developed, such as the inclusion of wind driven aspirators in the semi-aerobic landfill concept, the application of deepfiltered gas wells for active aeration, intermittent operation in low pressure concepts as well as the adjustment of temperatures by a controlled addition of water or leachate. Acknowledgements This manuscript is an outcome of the Task Group on Landfill Aeration of the International Waste Working Group (IWWG). The authors acknowledge the contributions of individual task group members regarding country specific issues in connection with landfill aeration projects. References ARGE Biopuster, 2011. The original Biopuster process. Available from: www. biopuster.at. Bachofner, A., Meier, W., Düring, A., 2010. Aerobisierung von Deponien in der Schweiz – Erkenntnisse aus dem Anlagenbetrieb. In: Lorber, K.E., Adam, J., Arnberger, A., Bezama, A., Kreindl, G., Müller, P., Sager, D., Sarc, R., Wruss, K. (Eds.), DepoTech 2010, Tagungsband zur 10. DepoTech-Konferenz in Leoben, 3–5.11.2010. IAE Eigenverlag, pp. 537–542, ISBN: 978-3-200-02018-4. Beatty, B., Benda, E., Jadeja, M., Lee, S., Mohamed, A., 2009. Canada’s 30 year old aerobic landfill. In: Cossu, R., Diaz, L.F., Stegmann, R. (Eds.), Proceedings Sardinia 2009, Twelfth International Waste Management and Landfill Symposium, Session B11. CISA, pp. 217–218. Berge, N.D., Reinhart, D.R., Hudgins, M., 2007. The status of aerobic landfills in the United States. In: Stegmann, R., Ritzkowski, M. (Eds.), Landfill Aeration. IWWG Monograph. CISA, ISBN: 978-88-6265-002-1. Berge, N.D., Reinhart, D.R., Batarseh, E.S., 2009. An assessment of bioreactor landfill costs and benefits. Waste Management 29, 1558–1567. Bröcker, C., Klos, U., Hübl, F., 2010. DEPO+VerfahrenÓ – Energieausbeute und Langzeitverhalten verbessern bei HM-Deponien. In: Lorber, K.E., Adam, J., Arnberger, A., Bezama, A., Kreindl, G., Müller, P., Sager, D., Sarc, R., Wruss, K. (Eds.), DepoTech 2010, Tagungsband zur 10. DepoTech-Konferenz in Leoben, 3–5.11.2010. IAE Eigenverlag, pp. 483–488, ISBN: 978-3-200-02018-4. Cossu, R., Raga, R., Rosetti, D., Cestaro, S., 2003. Full scale application of in situ aerobic stabilization of old landfills. In: Christensen, T.H., Cossu, R., Stegmann, R. (Eds.), Proceedings of SARDINIA 2003 – Ninth International Waste Management and Landfill Symposium, Session B13. CISA, pp. 180–181. Cossu, R., Raga, R., Rosetti, D., Cestaro, S., 2007a. Case study of application of the in situ aeration on an old landfill: results and perspectives. In: Cossu, R., Diaz, L.F., Stegmann, R. (Eds.), Proceedings of SARDINIA 2007 – Eleventh International Waste Management and Landfill Symposium, Session B14. CISA, pp. 239–240. Cossu, R., Raga, R., Rosetti, D., Cestaro, S., Zane, M., 2007b. Preliminary tests and full scale applications of in situ aeration. In: Cossu, R., Diaz, L.F., Stegmann, R. (Eds.), Proceedings of SARDINIA 2007 – Eleventh International Waste Management and Landfill Symposium, Session B14. CISA, pp. 885–886. Giannis, A., Makripodis, G., Simantiraki, M., Somara, M., Gidarakos, E., 2008. Monitoring operational and leachate characteristics of an aerobic simulated landfill bioreactor. Waste Management 28, 1346–1354. Gamperling, O., Hrad, M., Huber-Humer, M., 2011. Lessons learned during a three year full-scale application of in-situ landfill aeration in Austria. In: Cossu, R., He, P., Kjeldsen, P., Matsufuji, Y., Reinhart, D., Stegmann, R. (Eds.), Proceedings of SARDINIA 2011 – Thirteenth International Waste Management and Landfill Symposium, Session B14. CISA, pp. 237–238.
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