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General enquiries on this form should be made to:Defra, Science Directorate, Management Support and Finance Team,Telephone No. 020 7238 1612E-mail: [email protected]
SID 5 Research Project Final Report
SID 5 (Rev. 3/06) Page 1 of 25
NoteIn line with the Freedom of Information Act 2000, Defra aims to place the results of its completed research projects in the public domain wherever possible. The SID 5 (Research Project Final Report) is designed to capture the information on the results and outputs of Defra-funded research in a format that is easily publishable through the Defra website. A SID 5 must be completed for all projects.
This form is in Word format and the boxes may be expanded or reduced, as appropriate.
ACCESS TO INFORMATIONThe information collected on this form will be stored electronically and may be sent to any part of Defra, or to individual researchers or organisations outside Defra for the purposes of reviewing the project. Defra may also disclose the information to any outside organisation acting as an agent authorised by Defra to process final research reports on its behalf. Defra intends to publish this form on its website, unless there are strong reasons not to, which fully comply with exemptions under the Environmental Information Regulations or the Freedom of Information Act 2000.Defra may be required to release information, including personal data and commercial information, on request under the Environmental Information Regulations or the Freedom of Information Act 2000. However, Defra will not permit any unwarranted breach of confidentiality or act in contravention of its obligations under the Data Protection Act 1998. Defra or its appointed agents may use the name, address or other details on your form to contact you in connection with occasional customer research aimed at improving the processes through which Defra works with its contractors.
Project identification
1. Defra Project code WR0302
2. Project title
Forced air flow and distribution in landfilled waste
3. Contractororganisation(s)
Waste Mangement Research GroupUniversity of SouthamptonSouthampton
54. Total Defra project costs £ 129800(agreed fixed price)
5. Project: start date................ 01 October 2006
end date................. 29 August 2008
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6. It is Defra’s intention to publish this form. Please confirm your agreement to do so...................................................................................YES NO (a) When preparing SID 5s contractors should bear in mind that Defra intends that they be made public. They
should be written in a clear and concise manner and represent a full account of the research project which someone not closely associated with the project can follow.Defra recognises that in a small minority of cases there may be information, such as intellectual property or commercially confidential data, used in or generated by the research project, which should not be disclosed. In these cases, such information should be detailed in a separate annex (not to be published) so that the SID 5 can be placed in the public domain. Where it is impossible to complete the Final Report without including references to any sensitive or confidential data, the information should be included and section (b) completed. NB: only in exceptional circumstances will Defra expect contractors to give a "No" answer.In all cases, reasons for withholding information must be fully in line with exemptions under the Environmental Information Regulations or the Freedom of Information Act 2000.
(b) If you have answered NO, please explain why the Final report should not be released into public domain
Executive Summary7. The executive summary must not exceed 2 sides in total of A4 and should be understandable to the
intelligent non-scientist. It should cover the main objectives, methods and findings of the research, together with any other significant events and options for new work.Creating aerobic conditions in landfilled waste has the potential to stabilise closed and operational landfills, offering considerable long-term environmental benefits. Aerobic stabilisation of landfill sites has been investigated in various field trials including projects in North America, Germany and Japan. The studies have found significant reductions and improvements in leachate quantity and quality and the virtual elimination of methane generation. Aerating landfills may have a very important role in improving the sustainability of landfills: dramatically reducing the timescales for aftercare and the long-term pollution risk to the environment.
Despite these benefits, there is little understanding of the mechanisms and controls that affect aeration in landfills. Although the biological and biochemical processes of aerobic and anaerobic decomposition of organic matter are reasonably well understood, the factors that control air movement and distribution in landfills are not. Following the introduction of air into a landfill, the actual spatial distribution of oxygen, and hence the potential for aerobic degradation to take place, will be very dependent on the flow paths and rates of flow of the introduced air.
There are no current guidelines for implementing an air injection scheme in landfill, or indeed any design standards. Greater knowledge of the distribution of air through a landfill and the effectiveness of air injection systems are crucial factors for the successful design and engineering of aeration treatment systems.
This research project investigated the movement and distribution of forced air injection in municipal waste. The overall aim of the project was to develop a better understanding of the theory and practical application of injecting air into waste materials.
The specific objectives of the research were as follows:
Review the theory of air/ gas flow in drained waste materials to identify a theoretical framework that takes into account waste heterogeneity and composition and appraise empirical data from published work on aerobic treatment of waste in field and full-scale conditions.
Incorporate the theoretical framework into the established University of Southampton model for landfill Manage a series of coordinated short-term field trials to generate key data on efficient air distribution
related to operational conditions on homogenised municipal waste Use the LDAT model to analyse experimental data generated from the trial to gain a better
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understanding of controlled air distribution through municipal waste.
The experimental part of the research was carried out within a newly built research facility, designed to demonstrate the feasibility of the Aerox (aerobic) process to treat fresh MSW. The facility has been designed to be compliant with the Animal By-Products Regulations (2003), and is based around 5 waste treatment bays with under-floor aeration, providing a well controlled environment which approaches the field scale.
A detailed instrumentation design was produced to measure 3D pressure head distributions, and uses temperature and air quality measurements to infer the movement of air to different parts of the waste in the treatment facility. The interpretation of the results from this aerobic waste treatment process has direct application to the aerobic stabilisation of wastes in landfill sites. Four separate trials were instrumented and data collected.
In order to assess the data within the context of a theoretical framework, the University of Southampton’s "Landfill Degradation and Transport" (LDAT) model was developed to incorporate air flow modelling capabilities. These included aerobic degradation and heat transfer components. During the development of the model the role of water in the aerobic treatment of waste (through the solubility of oxygen) was highlighted, both in terms of its effect on air and fluid flow, but also as a medium required to enable the breakdown of organic material. Recognition of the importance of gas solubility to the process of aerobic degradation was a major advance in LDAT’s modelling capability and its potential role as a rate limiting step.
LDAT was configured to replicate the general operating conditions of the field trials and many of the observations of the trials were produced in model runs. In particular the rapid temperature increase in the waste during treatment was reproduced by the model. A further encouraging result was that the measured gas permeabilities correlated well with the values calculated by LDAT, based on the relationship between saturated hydraulic conductivity and effective stress determined in previous work. These results may have significant value in the wider context of unsaturated gas flow in landfills.
The aeration of landfills to accelerate stabilisation is undergoing a considerable revival. There are indications that the process can considerably reduce the pollution potential of a site over a relatively short period of time. If these claims are realised, the process will have an important role to play in increasing the sustainability of landfilling.
The uptake of aerobic landfilling technology in the UK will probably require further research to be undertaken. In particular there is a need for a full scale trial to be completed where the costs and benefits are assessed in a controlled and well instrumented manner. The instrumentation developed as part of this research should form an integral part of the monitoring of any such trial. LDAT, with its new functionality, would be an invaluable tool by both helping to design the trial and by analysing results.
Further information and detailed reports can be found on the project website at www.civil.soton.ac.uk/research/airflow .
Project Report to Defra8. As a guide this report should be no longer than 20 sides of A4. This report is to provide Defra with
details of the outputs of the research project for internal purposes; to meet the terms of the contract; and to allow Defra to publish details of the outputs to meet Environmental Information Regulation or Freedom of Information obligations. This short report to Defra does not preclude contractors from also seeking to publish a full, formal scientific report/paper in an appropriate scientific or other journal/publication. Indeed, Defra actively encourages such publications as part of the contract terms. The report to Defra should include: the scientific objectives as set out in the contract; the extent to which the objectives set out in the contract have been met; details of methods used and the results obtained, including statistical analysis (if appropriate); a discussion of the results and their reliability; the main implications of the findings; possible future work; and any action resulting from the research (e.g. IP, Knowledge Transfer).
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1. INTRODUCTION
1.1 BackgroundCreating aerobic conditions in landfilled waste has the potential to stabilise closed and operational landfills offering considerable long-term environmental benefits (Rich et al 2008). Aerobic stabilisation of landfill sites has been investigated in various field trials including projects in North America, Germany and Japan (e.g. Green et al, 2005; Ritzkowski and Stegmann, 2005 and Chong et al 2005). The studies have found significant reductions and improvements in leachate quantity and quality and the virtual elimination of methane generation. Aerating landfills may have a very important role in improving the sustainability of landfills: dramatically reducing the timescales for aftercare and the long-term pollution risk to the environment.
Despite these benefits, there is very little understanding of the mechanisms and controls that affect aeration in landfills. There are no current guidelines for implementing an air injection scheme, or indeed any design standards. Greater knowledge of the distribution of air through a landfill and the effectiveness of air injection systems are crucial factors for the successful design and engineering of aeration treatment systems.
1.2 Research ObjectivesThe overall aim of this research project was to develop a better understanding of the theory and practical application of injecting air into waste materials. Thus processes and control mechanisms that determine the effectiveness of air distribution through waste were investigated.
The specific objectives of the research (which were all met – see section 7.1) were as follows:
• Review the theory of air/ gas flow in drained waste materials to identify a theoretical framework that takes into account waste heterogeneity and composition and appraise empirical data from published work on aerobic treatment of waste in field and full-scale conditions.• Incorporate the theoretical framework into the established University of Southampton model for landfill degradation and transport (LDAT) • Manage a series of coordinated short-term field trials to generate key data on efficient air distribution related to operational conditions on homogenised municipal waste • Use the LDAT model to analyse experimental data generated from the trial to gain a better understanding of controlled air distribution through municipal waste.
The experimental part of the research was undertaken within a demonstration Aerox (aerobic) waste treatment facility, described below. This facility provided a well controlled environment which approaches the field scale. The interpretation of the results from this aerobic waste treatment process has direct application to the aerobic stabilisation of wastes in landfill sites.
1.3 Aerox research facilityAn Aerox demonstration facility was constructed at Pitsea landfill site, Essex by Purcell Limited with separate funding from the Landfill Tax Credit Scheme (LTCS). This defra funded research, involving the instrumentation of the facility to investigate air flow mechanisms, thus benefited from the infrastructure created by an existing research project. Construction started in late 2006 and the plant was fully commissioned in Summer 2007. The LTCS research project into the feasibility of using the Aerox process to treat MSW ran until March 2008.
The plant was designed to simulate near full-scale conditions to evaluate the engineering and key process control parameters for the aerobic stabilisation of municipal solid waste. The trial facility incorporates an enclosed waste reception and processing area with 2 treatment bays, and 3 outdoor treatment bays, all with under floor aeration (Figures 1, 2 and 4). The treatment bays have an area of between 180-220 m2, separated by concrete walls approximately 3.5 m high.
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Three blowers (fans) at the rear of the facility (Figure 3), inject air at a controlled rate into under-floor channels beneath each bay. The channels can be individually opened or closed during injection. Perforated steel plates line the floor of the bay above the injection channels onto which waste is loaded.
The three blowers have an output range of 0 to 12,000 m3/hour, which, at maximum output, equates to between 55 and 66 m3/hr/m2 of treatment bay floor.
The facility has been designed to be compliant with the Animal By-Products Regulations (2003).
Figure 1. Plan of Aerox waste treatment facility
Figure 2. Aerox waste treatment facility: Bays 3 & 4 loaded with waste
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Treatment Bay 5
Treatment Bay 3
Treatment Bay 4
Treatment Bay 1
Treatment Bay 2
BLOWER 3
BLOWER 1
BLOWER 2
SHREDDER
OUTDOORLOADING AREA &
TREATMENT BAYS
ENCLOSEDTREATMENT BAYS
28m
7m
40m
Figure 3. Blowers at rear of facility Figure 4. Bays 1 & 2 loaded with waste
Operation of Aerox facilityThe Aerox facility is operated on a batch process. Bagged MSW waste is brought into the enclosed facility building by lorry direct from the local waste transfer station. The waste is passed through a shredder, from which ferrous material is removed, and any large items (e.g. mattresses, tyres) are removed by hand. The shredded waste is then loaded into Bays 1 and 2 for treatment. When fully loaded, the blower at the rear of the two bays is started to begin the aeration process. Initially a high air injection rate is used. To increase the moisture content and to accelerate microbial activity, leachate (from an adjacent treatment facility) is sprayed onto the surface of the waste. The under floor injection channels incorporate a drainage system to collect excess leachate for recirculation. In practice, however, most of the leachate added is either adsorbed by the waste or driven off as steam.
Treatment temperatures (in excess of 60°C measured from thermocouples installed in the upper 1 m of waste) are generally reached within 24 hours of loading the waste, after which the waste temperature is controlled by the blowers to maintain the optimum temperature. A set temperature is programmed into a central computer which then automatically adjusts the output of the blowers to maintain this temperature.
To comply with the Animal By-Products Regulations (2003), during the first stage of the waste treatment process, it is necessary for the waste to reach and maintain a temperature in excess of 60 °C for at least 48 hours.
After 10-20 days of treatment, the waste from bays 1 and 2 is transferred to Bay 3. During moving, the waste is turned and well mixed, but maintains much of its temperature and generally starts life in the bay at 40-60°C. The waste is then treated in Bay 3 for a further 1-2 weeks before being transferred to either bay 4 or 5. In the final bay, the blowers are increased to between 50-75 % of maximum to drive off moisture and help dry the waste.
1.4 LDAT as a research tool and role in this investigationThe landfill model LDAT has been developed over the past ten years by the Waste Management Research Group at the University of Southampton. During this time LDAT has proved useful in assessing the consistency of landfill datasets and extrapolating data. It also works as a powerful tool in exposing areas where there is a need to improve the understanding of landfill systems.
The waste geometry in LDAT is represented as an assembly of linked elements for which the solution of constitutive equations linking key parameters are solved using standard finite difference techniques. The constitutive equation used in the LDAT model is a development of the conventional spatially distributed, time-dependent, equation representing the transport of liquid and gas in a porous media. The source term in this equation is used to accommodate both the bio-chemical degradation conversions and phase changes.
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2. DEVELOPMENT OF INSTRUMENTATION
The purpose of the instrumentation of the Aerox trial was to provide information on air distribution and flow within waste materials. A detailed instrumentation design was produced. The monitoring data was correlated with the Aerox operational information that was collected by Purcell Limited for the Aerox research project. This included data on waste type, water content, additives for nutrient balance (e.g. higher carbon to nitrogen ratio activators including compost tea), and data on the delivery system, e.g. air flow and delivery pressure.
This created the opportunity to undertake short-term tests to measure pressure head distributions, and use temperature and air quality measurements to infer the flux of air to different parts of the waste
2.1 Operational constraints
The short waste processing time in each bay created significant instrumentation challenges, made worse by the inability to use anything other than hand held mechanical plant on top of the waste. Instrumentation had to be installed as quickly as possible after waste had been placed in a bay. Data was then collected and the instrumentation removed with as little damage as possible all within this narrow time frame.
On health and safety grounds, it was decided that only the three outdoor treatment bays would be used in the trials. All instrumentation therefore, had to be weatherproof and suitable for repeated outdoor use in all seasons.
2.2 Piezometer design
To measure air flow and distribution throughout the waste mass, piezometers fitted with thermistors and differential pressure sensors were designed that could be rapidly installed across the treatment bay in a 3D array that would allow both temperature and pressure to be monitored in discrete response zones. The piezometers were designed to be constructed and installed in the waste by hand, and include a sealed monitoring port at the base which is connected by internal tubing and cables to surface mounted temperature and pressure sensors.
The specification for the piezometers was that they needed to be:
Reusable and robust Capable of being installed vertically in waste to a depth of 3-4 m by hand and without pre-drilling Able to be installed and removed rapidly Of a sufficient diameter to allow the housing of sealed pressure sensors and thermistors
The piezometers were constructed from fixed length, thick-walled steel pipe, with the following specification
Outside diameter = 25.4 mmInternal bore = 12.7mmWall thickness = 6.3 mmFlush, 20 mm standard threads.
Pipe sections could be joined together to allow installation from 0 to 4 m depth below the surface of the waste. All threaded joints were flush with the outer diameter of the pipe.
To measure the gas pressure within the waste at specific depths, a monitoring port was fitted to the leading length of piezometer pipe. This was perforated to allow the free movement of gas into the end-piece, but sealed from the main body of the piezometer. A 6 mm flexible tube, which runs up inside the
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piezometer body, connects the monitoring port to a surface mounted pressure sensor. Waste temperature is measured from a thermistor that is sealed in the monitoring port and connected to the surface by cables running inside the piezometer. To prevent heat transfer along the steel piezometer pipe adversely affecting the thermistor output, a plastic (PTFE) buffer was installed between the piezometer and the monitoring port.
A nose-cone was fitted to the first length of piezometer to aid in the insertion of the pipe into the waste, and the top of the piezometer pipe was fitted with a protective cap to act as a driving head to aid in installation. The piezometer design is shown in Figure 5 and detailed drawings of all piezometer parts can be found in Appendix 2.
The following method was used to install the piezometers into the waste: a 50 mm over-size steel pipe was driven into the waste using a fence-post hammer, to approximately 0.5 m above the required piezometer depth. A ‘dummy’ piezometer, with no monitoring port or buffer was then inserted into the over-sized pipe, and driven to the approximate depth. The dummy probe was removed, and the monitoring piezometer installed down the cavity, and pushed a little further into the waste to ensure the port was well sealed. The over-sized pipe was then removed from around the piezometer. This method allowed piezometers to be installed up to 3 m deep in the waste mass. During treatment, any settlement in the waste or piezometers could be monitored by measuring the surface height and piezometer height from a fixed location above the top of the waste (usually the top right corner of the rear wall).
Figure 5 Piezometer tip design
2.3 Temperature
Two thermistors are used, one located inside the monitoring port to measure temperature in the waste, and another in the instrumentation box to provide temperature compensation data for the transducers.
The thermistors were sourced from RS components and are model number 151/215. They are rated at 3 kohms at 25 °C, have a temperature range of -80 to +150 °C and a resistance tolerance of 0.2. They have a negative temperature coefficient that is curve matched for calibration.
Laboratory testing has shown the thermistors to be accurate to within ± 1 °C.
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Nosecone
Monitoringport Temp.
buffer
Piezometer pipe
Drivinghead
Thermistor Internaltubing
Internalcable
2.4 Pressure
A differential pressure sensor was used with each piezometer to measure gas pressure within the waste body. The pressure sensors chosen for the trials (sensor model PX138 from Omega Engineering) have a full scale range of 2000 Pascals (Pa), and the ability to differentiate pressures of ~5 Pa (equivalent to 0.5 mm of water head). Laboratory testing has shown, however, that due to the effects of temperature, cable interference and zero-balancing, the resolution is more realistically ± 20 Pa.
The pressure sensors were housed in weather-proof boxes on the surface, and connected to the monitoring port via 6 mm tubing.
2.5 Humidity
Thermal conductivity humidity sensors (model HS-13, BFI Optilas), were used to measure the absolute humidity (the ratio of the mass of water vapour to the volume of air or gas, expressed in grams per cubic meter) of the atmosphere in the loading bay, the atmosphere outside the bay and the gas within the top 0.3 m of the waste.
The range of the sensors is 0 – 130 g/m3 (+/- 3 g/m3), with a temperature range of 0 – 200 °C.
A thermistor was located with each humidity sensor, to measure the temperature of the gas in the monitoring zone.
A hand-held humidity probe (model HygroPalm2, from Rotronic Instruments Ltd.), was periodically used to calibrate the sensors and to take spot readings of surface humidity.
A Campbell Scientific CS215 temperature and relative humidity probe was also used to measure ambient conditions.
2.6 Gas quality
Gas quality was measured manually using an infrared gas analyser (model GA2K from Geotechnical Instruments Ltd). To measure the gas, the 6 mm hose connecting the monitoring-port within the waste to the surface mounted sensor was disconnected from the sensor and attached to the gas analyser.
2.7 On site calibration techniques
To ensure that the pressure sensors remained accurate and consistent over time, on site calibration was carried out before each test, and periodically during longer tests to ensure that there was no drift in output. The calibration method was as follows:
- a pressure sensor is attached to a digital manometer and an in-line hand-pump,- a laptop computer is connected to the data-logger to allow monitoring of real time sensor outputs,- the pressure in the tubing connecting the instruments is vented and the manometer ‘zeroed’,- a pressure is applied to the sensor using the hand pump,- when the system has stabilised, the sensor output is recorded from the computer,- at least 6 different pressures are applied, to cover the full experimental range of the transducer,- the sensor output is plotted against the known pressure of the manometer to determine accuracy.
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2.8 Data-logging and additional instrumentation
A Campbell Scientific CR1000 data-logger, fitted with sufficient multiplexers to allow the simultaneous logging of up to 48 channels, was used for the monitoring trials. The logger was powered by battery, and located on the rear outside wall of treatment facility and was moveable to allow access to any of the treatment bays. The piezometer instruments were connected to the logger with surface laid Category 5, 8-core cable with 8P8C (RJ45) modular connectors. The data-logger, cable runs and general instrumentation are shown in Figures 6 to 9.
In addition to the monitoring equipment installed within the waste, pressure sensors were installed in the delivery pipe directly after the blower to measure the air injection pressure, and in the floor channels beneath the treatment bay, and as controls. Barometric pressure, ambient temperature and relative humidity were also monitored. Gas quality was measured manually using an infrared gas analyser connected directly to the piezometer monitoring ports.
Figure 6. Data-logger and cabling, Trial C Figure 7. Pressure sensor instrument box connected to piezometer, Trial D
Figure 8. Cables connecting piezometers installed treatment bay 3 during Trial C
Figure 9. Piezometers and instrument boxes installed in waste during Trial D
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3. FIELD SCALE TRIALS
The field scale trials were all undertaken in either Bay 3 or Bay 5 of the Aerox waste treatment facility (see Figure 1). Each trial involved the installation of a 3D array of monitoring piezometers, generally arranged in 2 cross-sections across the bays.
A typical array of monitoring piezometers (from Trial C) are shown in Figures 10 to 12. The two cross-sections (Figure 11 and 12) also show the location of the individual under-floor air delivery channels (1-10) used to introduce air at various rates and configurations into the waste.
Figure 10. Plan of Bay 3 showing piezometer locations and line of cross-sections (dimensions in metres)
A1
A2
A3
B1
B2
B3
C1
C2
C3
H1 H2
-1
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1
2
3
4
-1 0 1 2 3 4 5 6 7 8
Distance (m)
Elev
atio
n ab
ove
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ay (m
)
Waste level:startWaste level:endA1
A2
A3
B1
B2
B3
C1
C2
C3
H1
H2
Floor/walls
87654321 109
Figure 11. Cross-section 1, showing first line of piezometer monitoring zones, humidity sensors (H1 and H2), waste level and floor channels
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D1
D2
D3
E1
E2
E3
F1
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F3
H3 H4
-1
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-1 0 1 2 3 4 5 6 7 8
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atio
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ove
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ay (m
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D2
D3
E1
E2
E3
F1
F2
F3
H3
H4
Floor/walls
87654321 109
Figure 12. Cross-section 2, showing second line of piezometer monitoring zones, humidity sensors (H3 and H4), waste level and floor channels
3.1 Trial A (Appendix 3)
General description
The objectives of Trial A, the first field-scale trial, were to test a small number of piezometers and monitoring instruments in a fully commissioned Aerox bay (number 5) for suitability, to gather some initial data on air distribution, and to finalise the design in preparation for more extensive full-scale monitoring.
The main elements of the design that needed investigating were as follows:
• Methods of installing and removing piezometers• Accuracy and repeatability of pressure transducers, including calibration• Temperature measurements• Cabling and logistics• Data logging requirements / Data management
Key findings
Difficulties were encountered during the installation of the piezometers, due to large and impenetrable objects within the waste. Even using a post-hammer and a sledge-hammer to help install the probes, it was not possible to completely install the piezometers to the base of the bay. It was also found that the piezometers were prone to break above the monitoring port at the joint with the plastic spacers. Following this trial, the installation methods outlined in section 2.2 were adopted.
The piezometers and thermistors performed well (Figure 13), and an on-site calibration method was designed (see section 2.7) allowing for consistent results in future tests.
A general increase in waste temperature occurred during the monitoring period, with an average increase of ~5 °C. The probes also showed a delayed response to diurnal changes in ambient temperature, with a lag of ~6-12 hours. The temperature response, is assumed to be caused by to the cooling effects of the air that is injected into the bay, which was cooler at night by around 9 °C.
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The data-logger performed well throughout the trial, with a logging rate of 1 minute deemed suitable for recording changes within the waste. However, the high logging rate also resulted in large data-files (which were imported into Excel for analysis), which would become unmanageable as more instruments were used. A Visual Basic code was written to allow fast data selection within and between spread-sheets of records.
Ch 1-2
Ch 1-2
Ch 1-4
Ch 1-4
Ch 1-6
Ch 1-6
Ch 1-8
Ch 1-8
Various
Various
Ch 1-8
Ch 1-8
7-8 5-8
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3-8
3-8
1-8
1-87-8
-50
0
50
100
150
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02/07/2007 08:35 02/07/2007 09:03 02/07/2007 09:32 02/07/2007 10:01 02/07/2007 10:30 02/07/2007 10:59
Date / Time
Gas
pre
ssur
e (P
a)
0
20
40
60
80
100
120
Blo
wer
out
put (
%)
Probe C (1.8m) Probe D (0.85m) Probe F (0.95m) Channels Open Blower
Figure 13. Pressure response to changes in delivery channels and blower output
3.2 Trial B (Appendix 4)
General description
Trial B was carried out in bay 5 where the degradation processes in the waste have slowed down. The objectives of Trial B, were to provide general information on air flow and distribution within the most stabilised waste available from the Aerox process, and provide a degree of repeatability within the experiments. Accurately measuring surface humidity and gas quality were also trialled.
12 piezometers were installed in the waste at the start of the trial, with 11 pressure sensors and 9 temperature sensors working successfully. Of the 12 piezometers installed, 11 were recovered intact, though the plastic temperature buffer used at the base of the piezometer was damaged in 9 cases and needed replacing for the next trial.
Key findings
Compaction and stabilization of the waste occurs in the first few hours of waste being placed in the treatment bay, and occurred before any monitoring piezometers were installed. During treatment, no further settlement or compaction occurred.
Differences in air flow distribution were noted between the two lines of piezometers, with significantly higher flows measured around line 2 than line 1 (Figures 14 and 15). The higher flow rates were also observable in the waste temperature, which was much lower around line 2 (presumably due to evaporative cooling), and in the gas quality with a faster response (O2 replacing CO2) around line 2.
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The moisture content of the waste decreased by around 10 %, from an initial 33 % (by weight) to 23 % at the end of the treatment process.
Off 25% 50% 75% 100% 25% 50% 75% 100% 25% 50% 75% 100%Off Off
-100
0
100
200
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700
0 10 20 30 40 50 60 70 80 90
Time since start (min)
Pres
sure
(Pa)
A2A3B2B3C2C3
Channels 1 to 3 open Channels 4 & 5 open Channels 6 to 8 open
Figure 14. Transient pressure response to changes in floor channel configuration in Line 1, Trial B
Off 25% 50% 75% 100% 25% 50% 75% 100% 25% 50% 75% 100%Off Off
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(Pa) D2
D3E2F2F3
Channels 1 to 3 open Channels 4 & 5 open Channels 6 to 8 open
Figure 15. Transient pressure response to changes in floor channel configuration in Line 2, Trial B
3.3 Trial C (Appendix 5)
General description
Trial C was carried out in treatment bay 3, the second stage of the Aerox process. The objectives of Trial C, were to provide additional information on air flow and distribution, but with a greater number of monitoring piezometers, and in waste that had a greater potential for degradation than the test in Trials
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Higher pressures recorded at the same waste depth as the piezometers shown in Line 1.
Lower pressures recorded than at the same waste depth in Line 2.
A and B. More emphasis was also placed on moisture content, which was increased during treatment by adding leachate to the surface of the waste. A more substantial analysis of humidity and gas quality was also made.
12 piezometers were installed in two lines comprising of 1, 2 and 3 m deep piezometers (Figures 10 to 12). In addition to the piezometers installed in the waste, pressure sensors were installed in the delivery pipe from the blower and in four of the delivery ports of the under-floor channels. Four absolute humidity sensors were also installed in the surface waste. Barometric pressure, air temperature and relative humidity were also logged.
Key findings
Adding leachate did not significantly affect the air-flow distribution through the waste, but temperatures either increased or decreased, depending on elevation. Moisture content analysis of the waste before, during and after the trial showed that the leachate added did not penetrate the waste mass, but was concentrated near the surface. A greater amount of drying occurred at the base of the waste, above the perforated floor. The absolute humidity of the air in the surface waste fell by up to 10 g/m3 when leachate was added, though this is presumed to be because of the cooling effect of the leachate, rather than a significant change in vapour concentration.
3.4 Trial D (Appendix 6)
General description
Trial D was carried out in treatment bay 3, the second stage of the Aerox process. The objectives of Trial D, were to provide additional information on air flow and distribution, but with a greater emphasis on the effects of moisture content, which would be increased during treatment by adding leachate to the surface of the waste in two stages.
9 piezometers were installed in clusters of three, in a single group towards the centre of the treatment bay. The piezometers were installed with monitoring zones approximately 1, 2 and 3 m below the surface of the waste.
Key findings
In the first stage of leachate irrigation, adding leachate did not significantly affect the air-flow distribution through the waste, but temperatures either increased or decreased, depending on elevation. Moisture content analysis of the waste before, and after leachate irrigation, showed that the leachate added did not initially penetrate the waste mass, but was concentrated at the surface.
The leachate added during the second irrigation stage was seen to be flowing out of the floor injection pipe-work. Moisture content analysis also demonstrated that the waste was fairly evenly covered.
Despite increasing the moisture content, the absolute humidity of the gas in the upper waste, did not increase during or after irrigation.
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Figure 16. Irrigating waste with leachate during Trial D
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4. LDAT DEVELOPMENT
4.1 LDAT aerobic degradation pathway (Annex A)
At the commencement of this project the source term of the LDAT model constitutive equation only represented anaerobic degradation processes. In order to represent the Aerox treatment process the LDAT code was reconstructed to accommodate the additional stoichiometric degradation pathways, and additional chemical compounds, required for modelling aerobic degradation. Provision was made for three waste types to be present, each having a different rate of aerobic degradation.
The aerobic pathways, which were developed by reference to the work of Iannelli et al (2005), Reichel et al (2005), and Polprasert (1996), are:
Aerobic degradation of waste322232.056.396 32.002.4623.6 NHOHCOONOHC
Fate of nitrogenOHHNOONH 2224 25.1
322 5.0 NOONO
Growth of bacteriaOHHNOHCNHNOHC 2275432.056.396 8.5964.45
Death of bacteria 461262275 6
53 NHOHCHOHNOHC
An explanation of how these pathways are derived is given in Annex A.
4.2 Development of the LDAT degradation algorithm (Annex B)
At the commencement of the project the LDAT code accommodated four primary anaerobic degradation pathways for four waste types, each having three different rates of anaerobic degradation. The addition of the aerobic pathways provided the opportunity to improve the flexibility of the way the LDAT code accommodates and processes multiple degradation pathways. This was done by implementing a matrix based approach and developing the work of Bryers (1984) and Reichel et al (2005). The LDAT model now has 23 primary waste degradation pathways, each with an associated biomass growth and decay pathway. The pathways connect a total of 52 chemical compounds each of which can exist in one or more of the solid, liquid and gas phases.
Details of the development of the improved LDAT degradation algorithm are given in Annex B. A listing of the key parameters and functions in LDAT, together with lists of the associated compounds and pathways are given in the Tables to Annex B.
4.3 LDAT leachate and gas flow algorithm (Annex C)
The LDAT leachate and gas transport algorithm has been reviewed and updated to include the effect of phase changes and capillarity.
Earlier versions of LDAT assumed that the phase change from liquid to gas took place immediately and the excess pressures were dissipated to ambient levels by venting. In the updated model it is assumed that there is no immediate change in the mass of the gas phase as the result of degradation reactions.
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Thus gaseous compounds arising from solid or liquid degradation are assumed to remain dissolved in the liquid phase pending a solubility calculation. Following the degradation calculation a solubility calculation is carried out to determine the mass and volumetric changes to maintain solubility equilibrium. This is discussed further in Section 4.4.
Capillarity affects the effective densities of the liquid and gas phases, the relative permeability (primarily of the liquid phase), and the capillary pressure, which relates pressure in the gas phase to pressure in the liquid phase. The functional relationships linking these parameters with degree of saturation have been identified and incorporated in the LDAT code. Further information is contained in Annex C which gives full details of the flow theory used for LDAT, and of the flow algorithm used to calculate the liquid and gas flow fields.
4.4 Development of a phase change algorithm in LDAT (Annex D)
The phase change algorithm based on Henry’s Law has been developed and is described in Annex D, together with the theory being used to estimate the water vapour phase. This has been introduced into LDAT for all compounds and implemented and tested for the liquid/gas phase exchanges of carbon dioxide, methane, oxygen, ammonia and water vapour. Its impact may be seen in Annexes F and G. The temperature dependence of Henry’s Law ‘constants’ has also been implemented in the LDAT code.
4.5 Calculation of Heat Generation (Annex E)
The calculation of heat generation has been developed on the basis attaching an enthalpy of formation to each of the compounds in LDAT following the approach of El-Fadel et al (1996). Values for the enthalpy of formation have been assumed to be related the molecular weight of the compound and a functional relationship has been calibrated using known values from the literature. This has enabled values to be attached to waste materials and other compounds for which the enthalpy of formation is unavailable or uncertain. Whilst this approach has not been experimentally validated directly, the results obtained from the modelling runs described in Sections 5.1 and 5.2 indicate that the approach gives a reasonable representation of heat generation levels in aerobic degradation.
5. CONFIGURATION OF LDAT TO THE AEROX PROCESS
5.1 LDAT code testing – single cell, single pathway cases
The initial LDAT code changes were tested by running single cell applications of LDAT for both anaerobic and aerobic degradation pathways, see Annex F. These tests exposed the requirement to introduce into the transport algorithms of the code a functional relationship between relative liquid permeability and the degree of saturation. In addition the tests highlighted that it is necessary to sustain high air flows through aerobic systems and that is unlikely that the diffusion of gas will be particularly relevant to this type of process. As a result the adaptation of LDAT to include gas diffusion was not given a high priority in this project.
5.2 LDAT code testing – multi-cell aerobic cases
A two-dimensional application of LDAT has been developed to demonstrate how LDAT may be used to simulate the aerobic waste treatment process in the Aerox facility at Pitsea. This application is described in Annex G.
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The results from a three element single stack model which incorporated all of the changes made to LDAT during this project are discussed below and in Beaven et al (2008) and Rees-White et al (2008).
6. DISCUSSION
6.1 Comparison of LDAT output to field results
Beaven et al (2008) gives details of a one dimensional LDAT model of the Aerox process and compares the results to field data from the process in general terms. A reasonable correlation of air flow was obtained between the model and the field monitoring although it is recognised that some of the assumptions within LDAT on the relationship between effective stress and waste permeability are stretched at the very low stresses used in this experiment. Very rapid rises in temperatures within the waste mass over a period of one day are seen in both the model and field data, and an overall drying of the waste during the treatment process is also seen. Evidence that the overall treatment process is being reasonably well represented by LDAT is seen by general waste settlement and variations in CO2
and O2 concentrations.
More detailed comparisons between the LDAT model and the field results are to be found in T. Rees-White et al (2008). The gas pressure data, collected from the three dimensional array of piezometers installed in the waste, have been used to derive air permeability values for partially saturated waste. These permeabilities have been compared to the Powrie and Beaven (1999) relationship of pore pressure measurements (extended in the context of this work to unsaturated waste), and the results suggest that, even extrapolated beyond the range of effective stresses considered by Powrie and Beaven (1999), a good estimate of the permeability of partially saturated waste to air can be derived. These results are very encouraging, and may have significant value in the wider context of unsaturated gas flow in landfills. For example, the results are relevant to the design of landfill gas extraction systems and in situ aerobic treatment programmes. Outputs from the LDAT model compare favourably with the range of pressure and temperature measured in the field.
The indications are that the aerobic processes occurring in the waste treatment process are well represented by LDAT and the field data will be an invaluable source of information that will allow the LDAT model to be refined and improve our knowledge and understanding of waste processes.
6.2 Relevance of research to aerobic landfill
Role of water content and water vapourA detailed account on how the liquid phase influences bio-chemical processes in landfills may be found in White and Beaven (2008). This paper also includes output material from this project.
It is important to realise that the liquid phase accommodates the chemical reactions that take place in a degrading waste material, and that it can provide both a hostile and benevolent environment for these reactions. Water in the liquid phase also acts as a solvent and provides the pathway for the solid phase to dissolve into the liquid phase and thus become available to take part in the chemical reactions. Through its involvement in the bio-degradation process the liquid phase influences the production of gas in a landfill. Whilst water in the liquid phase is an important reactant in anaerobic degradation it does not have this role in aerobic chemical reactions – see Section 4.1. In fact it is a product.
A further consideration is that, through its presence in the pore spaces of the waste material, the liquid phase has an impact on the transport of gas that has to take place through the same pore spaces.
It is therefore important to be able to account for the liquid phase reasonably accurately in any modelling task associated with the design and operation of waste aeration treatment systems. This involves a correct representation of both transport and the phase changes between the liquid and
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vapour phases. Development of this methodology, as described in Annexes C and D, and demonstrated in Beaven et al (2008), has been advanced in this project.
Gas solubilityA major modeling development in this project has been the introduction into LDAT of liquid/gas phase changes based on Henry’s Law as described in Annex D, and the development of trials to calibrate the relevant parameter values as described in Annex E and White et al (2008).
The phase change of air borne oxygen from the gas phase into solution in the liquid phase is an essential precursor to enabling the aerobic reaction identified in Annex A and Section 4.1 to take place. If this does not take place rapidly enough it can become a rate limiting process in the stabilization of the waste by aerobic treatment.
The formalisation of this process into a model which enables estimates to be made of the appropriate rate at which air should be supplied to waste being aerobically treated is an important contribution from the project.
Gas and liquid flow – roles of parameters, dry density/porosity, hydraulic conductivity/permeability to gasDry density and porosity are directly related. They influence both the direct stress imposed by self weight on waste material and its degree of saturation. These parameters in turn influence both the permeability of the waste material to liquid, hydraulic conductivity, and the permeability of the waste to gas, which may be derived from the hydraulic conductivity. These linkages are explored in detail in Annex C.
The importance of liquid flow, and therefore the assessment of hydraulic conductivity, has already been discussed above. In the context of landfill processes gas flow is also important and is the dominant transport process in aerobic treatment systems. Thus understanding how the permeability of waste to gas should be evaluated and applied is vital.
In this project access the Aerox facility in effect made available a very large scale gas permeameter together with the resources to instrument it and carry out closely controlled tests which provided a wealth of information about permeability and porosity. Particularly valuable results were obtained confirming the reliability of the Powrie and Beaven (1999) approach to estimating liquid and gas permeabilities and the feasibility of adopting the van Genuchten (1980) approach to determining the liquid relative permeability in partially saturated conditions. The application of the Powrie and Beaven (1999) and van Genuchten (1980) approaches are described in Annex C and the analysis of the experimental data obtained from the project is described in Rees-White (2008).
Heat generation and temperature controlA feature of aerobic waste treatment is the high rate at which heat is generated and the way in which temperature changes can be controlled by utilizing the introduction of relatively cool air as a heat sink. The project has provided an opportunity to develop models of the heat generation – see section 4.5 – and to test the existing heat transfer algorithms in LDAT. It has also stimulated the identification and implementation of functions that model the temperature dependence of many of the parameters that control landfill processes.
Performance indicators – fate of carbon/ammoniumWhilst it is important to understand how the degradation of waste by the forced introduction of air can be initiated, it is also important to be able to monitor the process and ensure that it is operating as effectively and efficiently as possible.
The monitoring techniques that have been developed in this project gather data on pressure and temperature distributions, and on gas emissions all of which can be used as indicators of performance.
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Simply put, the temperature and pressure distributions indicate the extent to which the aeration is distributed through the body of the waste, and the emissions indicate the extent to which the degradation processes represented by the stoichiometry given in Section 4.1 are being completed.
7. CONCLUSIONS AND RECOMMENDATION
7.1 Extent to which research objectives have been met
All the objectives set out in section 1.2 have been met in full.
A theoretical framework to incorporate aerobic treatment mechanisms into LDAT was established and is summarised in Annexes A to E.
This theoretical framework has been incorporated into LDAT.
Four short-term field trials were undertaken on homogenised municipal waste within the Aerox waste treatment demonstration facility and an extensive data set containing information relevant to the performance of efficient air distribution and treatment collected (Appendices 3 to 6).
LDAT has been configured to replicate the operating conditions of the Aerox facility and has accurately modelled a number of trends seen in the experimental data sets. This work is reported in Annexes E and F, as well as in a number of conference papers (White and Beaven, 2008; Beaven et al 2008; Rees-White et al 2008).
7.2 Key findings
A suite of instrumentation suitable for monitoring air flow distribution in waste materials has been developed, and could easily be moved into the field setting. The system is based around relatively cheap and reliable pressure sensors linked to a data logging system, and appears to give good levels of sensitivity and accuracy.
The greatest difficulty encountered related to the need to thermally isolate the piezometer tip at the point of measurement. This involved the use of a PTFE spacer which introduced a point of weakness into the design. This aspect would need to be considered further in any field application of the monitoring system.
The use of absolute humidity monitors did not appear to produce very reliable or reproducible data. However, this is a key variable that needs to be monitored and it is recommended that attempts are made to refine the technique further to achieve better results.
The evidence from all the tests was that air flow was very well distributed, although there was some evidence of spatial variability in flow and presumably permeability to gas as evidenced by higher gradients (by a factor of 3 to 4 times) and slower rates of cooling in response to air injection in one monitoring line compared to the other in Test B.
The measured gas permeabilities correlated well with the values calculated by LDAT, based on the relationship between saturated hydraulic conductivity and effective stress determined in previous work. These results are very encouraging, and may have significant value in the wider context of unsaturated gas flow in landfills.
There was no evidence of any preferential horizontal air flow in the bays at the low waste densities associated with the Aerox treatment process. Furthermore there was no evidence that there was any preferential flow at the boundaries of the cell (i.e. rapid flow up the interface between the waste and the
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Bay walls). However, waste temperatures were generally cooler nearer the bay walls than in the centre.
The addition of (cold) water to the waste had a large short term effect on temperature at the waste near the top of the cell. However, despite raising the average water content by approximately 15% there was no discernible effect on waste permeability. This was a surprising result, as the general response to an increase in water content of a partially saturated material is a reduction in gas permeability. The theoretical link between gas and water phases and its impact on both treatment processes and flow has been incorporated into LDAT.
The response of temperature and changes to gas quality in response to variations in the injected flow rate was as expected. Injecting high fluxes of air into the base of the treatment bays resulted in waste cooling (the flow of air was far in excess of that needed to support the aerobic processes, with oxygen concentrations of 17-19% and low levels of CO2) and waste drying. Reducing the flow rate would generally result in an increase in waste temperature and higher concentration of CO2 in the gas phase. The incorporation of heat generation functionality in LDAT appears to be working well and is able to predict typical increases in temperature seen in the Aerox trial.
A major advance in LDAT’s modelling capability was recognising the importance of gas solubility to the process of aerobic degradation, and its potential role as a rate limiting step. Henry’s Law was used as the basis for modelling liquid/gas phase changes, and some of the parameters incorporated into LDAT have been calibrated using results from this study.
The formalisation of this process into a model which enables estimates to be made of the appropriate rate at which air should be supplied to waste being aerobically treated is an important contribution from the project.
7.3 Future Work
Within the UK, the application of aerobic landfill treatment process as a way to accelerate the biochemical stabilisation of landfills has not been undertaken at the field scale. Aerobic landfill is a technology used more widely in other countries: in the United States the USEPA has supported a bioreactor research programme which includes the investigation of aerobic landfill. Within Germany, the use of aerobic landfill technology has been used very successfully to provide a final polishing stage to old landfill sites. However, to our knowledge, none of these trials have been designed or results analysed using a fully coupled degradation and gas transport model with capabilities similar to LDAT.
There is a need for a UK based field scale aerobic landfill trial to demonstrate the applicability of using aerobic landfill in the UK context. The instrumentation developed as part of this research should form an integral part of the monitoring of any such trial. LDAT, with its new functionality, would be invaluable by both helping to design the trial and by analysing results.
The scope of this project did not allow for comprehensive analysis of all the data collected from the aerox trial. A more detailed modelling and analysis exercise using this data would improve the calibration of models and provide better constraints on model parameters. Furthermore, there are data available from some of the overseas aerobic landfill research projects, and these data would benefit from being analysed by LDAT. It is anticipated that such analyses would reveal insights not appreciated so far, and help to focus on what further information needs to be collected or researched.
The energy balance of aerobic landfill treatment technology needs to be investigated, so that a better understanding of the costs associated with this technology can be developed. The work would need to be linked to how the “treatment” of the waste accelerates the stabilisation process in a landfill, and how close to full stabilisation the technology can achieve.
The research undertaken on airflow in this project has concentrated on wastes with a very low density. There needs to be further research on wastes at a higher density more associated with landfills. The fact that increasing the water content of the waste in this project did not reduce the gas permeability needs to be investigated further, and may well link to the issue of low waste density mentioned above.
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It was anticipated that waste heterogeneity would have had a bigger impact on airflow and distribution in this project than it did. However, heterogeneity is related to scale and there is a need to investigate this aspect further. The observation that the Aerox process relies to a certain extent on mechanical mixing of the waste to achieve maximum rates of aerobic degradation indicates that on a micro-scale at least airflow was not uniform.
7.4 Policy ImplicationsThe aeration of landfills to accelerate stabilisation is undergoing a considerable revival. There are indications that the process can considerably reduce the pollution potential of a site over a relatively short period of time. If these claims are realised, the process will have an important role to play in increasing the sustainability of landfilling.
This research has produced both an instrumentation strategy and a modelling tool that can be applied to aerobic landfilling, both for design and data analysis purposes. The uptake of aerobic landfilling technology in the UK will probably require further research to be undertaken. In particular there is a need for a full scale trial to be completed where the costs and benefits are assessed in a controlled and well instrumented manner.
7.5 Dissemination and Knowledge Transfer
At the start of the research a project website was established and has been regularly maintained throughout the duration of the work. This website includes all the Appendices and Annexes to this report that contains the instrumentation and modelling detail of the project.
Three conference papers have been written and will be presented at the:
1) Global Waste Management Symposium, Colorado USA, September 2008;
2) Intercontinental Landfill Research Symposium, Colorado. September 2008; and
3) Waste 2008, Stratford-upon-Avon, UK. September 2008
Limited modelling results are also contained in a paper presented at the:
1st Middle European Conference on Landfill Technology, Budapest Hungary, February 2008
References to published material9. This section should be used to record links (hypertext links where possible) or references to other
published material generated by, or relating to this project.
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Project website http://www.civil.soton.ac.uk/research/airflow
Appendix 1 INSTRUMENTATION TESTING http://www.civil.soton.ac.uk/research/airflow/Appendix_1.pdf Appendix 2 INSTRUMENTATION DESIGNS http://www.civil.soton.ac.uk/research/airflow/Appendix_2.pdfAppendix 3 INSTRUMENTATION TRIAL A http://www.civil.soton.ac.uk/research/airflow/Appendix_3.pdfAppendix 4 INSTRUMENTATION TRIAL B http://www.civil.soton.ac.uk/research/airflow/Appendix_4.pdfAppendix 5 INSTRUMENTATION TRIAL C http://www.civil.soton.ac.uk/research/airflow/Appendix_5.pdfAppendix 6 INSTRUMENTATION TRIAL D http://www.civil.soton.ac.uk/research/airflow/Appendix_6.pdfAnnex A Development of an aerobic degradation pathway
http://www.civil.soton.ac.uk/research/airflow/Annex_A.pdfAnnex B Development of the LDAT degradation algorithm
http://www.civil.soton.ac.uk/research/airflow/Annex_B.pdfAnnex C The LDAT gas and leachate flow algorithm
http://www.civil.soton.ac.uk/research/airflow/Annex_C.pdfAnnex D Development of the LDAT phase change algorithm
http://www.civil.soton.ac.uk/research/airflow/Annex_D.pdfAnnex E Calculation of heat generation http://www.civil.soton.ac.uk/research/airflow/Annex_E.pdfAnnex F LDAT code testing – single cell, single pathway cases
http://www.civil.soton.ac.uk/research/airflow/Annex_F.pdfAnnex G LDAT code testing – multi-cell aerobic cases
http://www.civil.soton.ac.uk/research/airflow/Annex_G.pdf
Full reference List http://www.civil.soton.ac.uk/research/airflow/References.pdf
Bryers (1984). “Structured modelling of the anaerobic digestion of biomass particulates”.Biotechnology and Bioengineering, Vol. XXVII, 638-649. John Wiley and Sons.
Chong, T. L., Y. Matsufuji, et al. (2005). "Implementation of the semi-aerobic landfill system (Fukuoka method) in developing countries: A Malaysia cost analysis." Waste Management 25(7): 702-711.
El-Fadel, M., Findikakis, A.N. & Leckie, J.O. (1996) “Numerical modelling of generation and transport of gas and heat in landfills”. Waste Management and Research 14, 483-504.
Green, R.B.,Hater, G.R., Goldsmith, C.D., Kramer, F. and Tolaymat, T. (2005) Commercial-scale aerobic-anaerobic bioreactor landfill operations. Proc. Sardinia 2005, Tenth International Waste Management and Landfill Symposium, Cagliari, Italy, October 2005.
Iannelli, R., Giraldi, D., Pollini, M. and Russomanno, F. (2005) “Effect of pure oxygen injection as an alternative to air and oxygen-enriched air in the composting processes”. Proc. Sardinia 2005, Tenth International Waste Management and Landfill Symposium, S. Margherita di Pula, Cagliari, Italy, October 2005.
Polprasert, C. (1996) Organic waste recycling - Technology and management. 2nd Ed. John Wiley & Sons Ltd, 1996. ISBN 0471964824
Powrie W., Beaven R. P., (1999). "Hydraulic properties of household waste and their implications for fluid flow in landfills." Proceedings of the Institution of Civil Engineers (Geotechnical Engineering), 137 (4), 235-247
Reichel, T., Haarstrick, A. and Hempel, D.C. (2005) “Modeling long-term landfill emission - a segregated landfill model”. Proc. Sardinia 2005 Tenth International Waste Management and Landfill Symposium, Cagliari, Italy, October 2005
Rich, C., J. Gronow, et al. (2008). "The potential for aeration of MSW landfills to accelerate completion." Waste Management 28(6): 1039-1048.
Ritzkowski, M. and Stegmann, R. (2007) “Biostabilising a MSW landfill by means of in situ aeration - results of a 8-year project ”. Proc. Sardinia 2005 Tenth International Waste Management and Landfill Symposium, Cagliari, Italy, October 2005
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