EPSRC Final Report

7/31/2019 EPSRC Final Report

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Development of sustainable landfill practices

and engineering landfill technology

Final report to the Engineering and Physical Sciences Research Council

(Grant reference GR/L 16149)

W Powrie

A P HudsonR P Beaven

Department of Civil & Environmental Engineering, University of Southampton, Highfield, SouthamptonSO17 1BJ

February, 2000

Note: the format of this report is specified by EPSRC. The main body of the text is limited in length to six

pages of typescript. Further details may be found in the papers listed in Appendix B, copies of which are

available on request from Professor W Powrie.


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Research Grant Title: Development of sustainable landfill practices and engineered landfill technology

(GR/L 16149)

Investigators and Institutions: W Powrie, A P Hudson, R P Beaven, C J Banks, D Montagnani and T W Tanton,

Department of Civil and Environmental Engineering, University of Southampton; and J P Robinson, Queen Mary

and Westfield College. Project carried out with additional funding from and in collaboration with Cleanaway Ltd.

Summary: In recent years, the driving principle of landfill management has been to prevent saturation of the waste

to minimize the likelihood of leachate leaking into the surrounding ground. This has resulted in very slow rates of

waste degradation, with projected stabilization times of the order of hundreds of years. Degradation could in

principle be accelerated by circulating fluids through the waste in a controlled manner, and operating the landfill as

an engineered wet bioreactor. This approach is more consistent with the aims of a sustainable waste management

policy than the entombment approach, which leaves landfilled wastes in a potentially polluting state for many

generations.

One of the main uncertainties concerning the practicality of operating a landfill as a controlled bioreactor is the

hydraulic conductivity of the waste, because this governs the ease with which fluids may be introduced into and

extracted from the landfill. The aim of the research described in this report was to develop an understanding of the

factors controlling the hydraulic conductivity of landfilled wastes, in the context of the design and operation of a

landfill as an engineered flushing bioreactor.The research consisted of three components:

large scale experiments on samples of wastes, carried out at Cleanaway Ltds Pitsea landfill site in a purpose-

built compression cell;

laboratory studies to investigate (a) the impact of leachate/liquid recirculation on waste degradation rates, (b)

moisture content suction relationships for certain types of waste, and (c) the feasibility of monitoring

changes in geotechnical and hydraulic properties, settlement and gas generation rates in samples of wastes

during leachate/liquid recirculation under constant applied stress; and

the development of simple models to enable the application of the results of the experimental work to the

sustainable operation of landfills.

A key factual output of the research has been the quantification of relationships between the drainable porosity

and vertical stress, and between hydraulic conductivity and vertical stress, for samples of processed, unprocessed

and aged household waste. On the basis of these data, and using simple analytical models developed to enable

their application to problems in landfill operation, the following conclusions can be made.

At depth, most wastes are likely to be approaching saturation even if they are free to drain under gravity.

Although some differences in hydraulic conductivity between processed, unprocessed and aged household

wastes were apparent, these are generally insignificant in comparison with the orders of magnitude change in

hydraulic conductivity that results from waste compression.

On unloading, very little of the deformation due to loading was recovered, and the hydraulic conductivity

remained substantially unchanged. Thus the hydraulic conductivity of a waste is governed by the maximum

equivalent vertical stress to which the waste has been subjected, so that the stress history or the density of the

waste must be considered when assessing its hydrogeological and geotechnical properties.

During compression, waste develops a layered structure. This results in a degree of anisotropy of hydraulic

conductivity (expressed as the ratio Kh/Kv) that increases with increasing applied stress.

Provided that the dependence of hydraulic conductivity on vertical stress and stress history is taken into

account, calculations suggest that vertical infiltration rates comparable with achieving stabilization of the wastewithin a timescale of one generation can be achieved. Precompaction of the waste to densities greater than 0.9 -

1.1 t/m3, or placement of loose waste unsaturated to a depth in excess of about 40 m prior to saturation, would

both prevent the required infiltration rate from being realised.

Analyses of pumped vertical wells using a stress dependent hydraulic conductivity suggest that there will be

little increase in flowrate for increases in drawdown in excess of about 33% of the initial saturated depth.

Preliminary tests have demonstrated the importance of the interaction between gassing and leachate flow in

terms of the pore volume available for liquid flow, and suggest that changes in gas production rate could affect

the apparent leachate level measured in the field. This is a subject requiring further research.

EPSRC grant and duration For further information please contact

231,226 over 36 months Professor W Powrie, Department of Civil & Environmental

plus project studentship Engineering, University of Southampton SO17 1BJ

(tel. 023 80593214; fax 023 80677519; e-mail [email protected])


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1. BACKGROUND AND OBJECTIVES

1.1 Background

In recent years, the driving principle of landfill management has been to prevent saturation of the waste to

minimize the likelihood of leachate leaking into the surrounding ground. This has resulted in very slow rates of

waste degradation, with projected stabilization times of the order of hundreds of years (e.g. Knox, 1990).

Degradation could in principle be accelerated by circulating fluids through the waste in a controlled manner, andoperating the landfill as an engineered wet bioreactor. This concept, which is espoused by Waste Management

Paper 26B (DoE, 1995), offers significant economic and environmental benefits and is more consistent with the

aims of a sustainable waste management policy than the entombment approach, which leaves landfilled wastes in

a potentially polluting state for many generations.

One of the main uncertainties concerning the practicality of operating a landfill as a controlled bioreactor is

the hydraulic conductivity of the waste, because this governs the ease with which fluids may be introduced into

and extracted from the landfill. This was the issue addressed by the research described in this report.

1.2 Aims and objectives

The aim of the research was to develop an understanding of the factors controlling the hydraulic conductivity of

landfilled wastes, in the context of the design and operation of a landfill as an engineered flushing bioreactor. As

stated in the original case for support, these included the effects of waste composition, overburden pressure,microbial activity, degradation and two phase flow (gassing). However, a budget reduction of 100,000 from the

329,927 initially requested necessitated a reduction in the scope of the research. In the light of guidance

received from the Waste and Pollution Management Programme Management Committee, the work on the effect

of waste composition was deleted from the programme, and the work on the effects of waste degradation and

gassing (two -phase flow) scaled down considerably. The objectives of the proposed research were revised from

those given in the original case for support. The revised objectives were

to quantify the effects of overburden pressure on the mechanical and hydraulic properties of different types

of waste, and

to carry out a preliminary assessment of the effects of microbial activity on the mechanical and hydraulic

properties of wastes

in the context of the operation of a landfill as an engineered wet bioreactor, as stated in a letter from Professor W

Powrie (UoS) to Dr M Partridge (EPSRC) dated 18 April 1996.

2. PROGRAMME MANAGEMENT

In addition to the EPSRC grant, the project was funded by a donation of 150,000 from Cleanaway Ltd under the

Landfill Tax Credit Scheme. The research may conveniently be divided into three components:

large scale experiments on samples of wastes, carried out at Cleanaway Ltds Pitsea landfill site in a purpose-

built compression cell;

laboratory studies to investigate (a) the impact of leachate/liquid recirculation on waste degradation rates, (b)

moisture content suction relationships for certain types of waste, and (c) the feasibility of monitoring

changes in geotechnical and hydraulic properties, settlement and gas generation rates in samples of wastes

during leachate/liquid recirculation under constant applied stress; and

the development of simple models to enable the application of the results of the experimental work to the

sustainable operation of landfills.

The work using the Pitsea compression cell was carried out by Andrew Hudson, a Research Assistant funded by

the EPSRC grant, under the supervision of Dr R P Beaven (Senior Research Fellow funded from the grant) and

Professor W Powrie (Principal Investigator). The laboratory studies were carried out by (a) Daniele Montagnani,

an EPSRC-funded project research student associated with this grant (supervised by Dr C J Banks); (b) Mansoor

Imam, a research student funded by the Faculty of Engineering and Applied Science and the Department of Civil

and Environmental Engineering at the University of Southampton (supervised by Dr D J Richards); and (c) Lewis

Parker, a research student at QMW (supervised by Professor J K White and Dr J P Robinson). The development

of models for the application of the results to sustainable landfill practice was carried out by Professor W Powrie

and Dr R P Beaven.

The project was overseen by a steering group comprising:

Professor W Powrie, University of Southampton; Dr R P Beaven, University of Southampton;

Dr C J Banks, University of Southampton; Dr J P Robinson, QMW;

Dr L de Rome, ETSU (for EPSRC); and Mr M J Dyer, Cleanaway Ltd.


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Additional input and advice was provided on an ad hoc basis by Professors T W Tanton and J B Joseph.

Early dissemination of the results has been achieved by means of

papers published in the Sardinia International Landfill Conference, 1999 and the Proceedings of the

Institution of Civil Engineers special issue on Landfill Engineering (October 1999);

an article in Waste Management, (November 1998); presentations at seminars at the Nottingham Trent University (September 1998), University College London

(January 1999), the Institution of Civil Engineers (ICE), London (November 1999), the ICE South Wales

Geotechnical Group, Swansea (November 1999), the University of Southampton (December 1999), and WRc

leachate management workshops; and

Dr Beaven and Professor Powries membership of the Institution of Wastes Management Working Group on

Sustainable Landfill, which published its report in Spring 1999

A full list of papers and reports published to date is given in Appendix B.

3. DESCRIPTION OF THE RESEARCH

3.1 The Pitsea compression cell: modifications, waste testing and new experimental techniques

Most of the experimental research was carried out using the Pitsea compression cell. This is a purpose built

apparatus for determining the hydrogeological and geotechnical properties of 2 m dia. samples of waste atstresses up to 600 kPa (Figure 1). The cell was originally used in research sponsored by the Waste Technical

Division of the Department of the Environment and Cleanaway Ltd. To enhance the testing capabilities and

improve the quality of the data, a number of major modifications were made to the cell and new testing protocols

were developed as part of this research. New techniques for analysing experimental data from the Pitsea

compression cell have also been developed, and applied to the results of both this research and earlier tests

(Section 3.1).

Two new wastes were tested in the Pitsea compression cell. Waste AG2 was a 20 year old (predominantly

household) waste excavated from a landfill (Table 1). Waste DN1 was household waste that had been processed

using the DANO technique (Table 2); it was selected as an example of a processed waste that had previously

been used in field scale research in the Mid-Auchencarroch landfill (Wingfield-Hayes et al, 1997). Initially, it had

been hoped that a t least one other waste type would have been tested in the Pitsea compression cell. However, it

was decided during the course of the research to investigate the properties of the wastes in unloading as well as

loading, which increased considerably the duration of the testing programme for each waste. Data from earlier

tests on crude household waste (DM2 and DM3 -Table 3), pulverised waste (PV1 and PV2 -Table 4) and an aged

waste (AG1 -Table 5) were re-analysed and used together with the new results to develop theories and models

concerning sustainable landfill (Section 3.3).

3.1.1 Building Enclosure

An improved working environment and more controlled test conditions have been created by the construction of

an enclosure to the building (Figure 2). This was financed by a contribution from Cleanaway Ltd through the

landfill tax credit system.

3.1.2 Modifications to allow horizontal hydraulic conductivity to be measured

To measure horizontal hydraulic conductivity in the compression cell, it was necessary to induce horizontal flow

across the samples. This required the addition of eleven inlet ports and eleven diametrically opposite outlet portsto the compression cell wall. Extra piezometer monitoring ports were also added. To monitor the individual flow

rates through each of the inlet ports, eleven small header tanks were constructed and mounted on the header tank

scaffold tower (Figure 3). Inflatable seals were added around the perimeter of the top platen to prevent leakage

through the clearance gap against the cylinder wall.

3.1.3 Modifications to monitor effects of degradation

The construction of landfill gas collection facilities on the Pitsea compression cell and the provision of a sensitive

load cell weighing system has allowed a preliminary assessment to be made of the effect of gas on the

hydrogeological properties of waste, in particular on the drainable porosity and hydraulic conductivity. The

displacement of gas from the waste by liquid flow was measured for a range of flow rates in upward, downward

and horizontal flow.

3.1.4 Measurement of differential waste compression within the cell

Load is applied to the upper surface of waste samples in the compression cell through a hydraulically operated

platen. Sidewall friction between the cylinder walls and the waste causes a reduction with depth in the vertical


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stress transmitted to the waste. This may result in differential compression of the sample, and hence a variation in

hydrogeological properties with depth. A magnetic ring extensometer has been successfully installed and used to

measure the pattern of waste compression within the cell (Figures 4 & 5).

3.1.5 Load Cells

Accurate measurement of the weight of samples is necessary for density calculations, and for monitoring water

and gas content. The existing equipment was unsatisfactory and had to be replaced.

3.1.6 Data interpretation and analysis of results from Pitsea compression cell

A correction for the effect of sidewall friction has been developed (Powrie and Beaven, 1999), and data are now

reported as functions of the average transmitted vertical stress.

As direct measurement of the horizontal hydraulic conductivity of waste samples is not possible owing to the

geometry of the compression cell, the USGS three dimensional groundwater flow model MODFLOW (in

combination with Groundwater Vistas) was used to model the flow regime (e.g. Figure 6). Measured flow rates

and leachate pressure heads within the waste were matched with computer analyses to obtain the horizontal

hydraulic conductivity of samples at different vertical stresses (Hudson, Beaven and Powrie, 1999).

3.2 Laboratory based tests

3.2.1 Unsaturated suction curve characterisationWaste moisture characteristic curves were determined for samples of Dano processed waste using a filter paper

contact method (ASTM D5298-92).

3.2.2 Effect of flushing rate on waste degradation

The benefits of leachate treatment, either within or external to the landfill, and the effect of leachate recirculation

or flushing rate on optimizing waste degradation were assessed in 1-litre, 21 day batch laboratory experiments on

the organic fraction of municipal solid waste (OFMSW). The initial biological methane potential (BMP) was used

as a benchmark against which to assess the degradation of the waste. The effects of hydraulic retention time

(experimental range 80 to 240 hours) and flushing medium (tap water, aerobically treated leachate and

anaerobically treated leachate) on degradation were assessed by monitoring the change in total and volatile

solids, the organic content (TOC, TON, VFA) of the flushing liquor and the quantity and composition of gas

production. Larger scale experiments, to confirm the findings of the batch experiments, were carried out in 30-litre

static bed upflow lysimeters packed with OFMSW and inert pore rings, which increased the porosity of the waste

material allowing shorter hydraulic retention times (Figure 7).

3.2.3 Effect of degradation on properties of degrading solid waste

A prototype laboratory scale anaerobic refuse digester has b een built to measure the effects of waste degradation

on the physical and hydrogeological properties of refuse whilst subjected to vertical effective stresses typical of

those encountered in a landfill (Figure 8).

The reactor chamber is an acrylic cylinder, 476mm in diameter and 900mm high providing a capacity of 160

litres. The chamber is placed within an Eland Engineering T413/2 loading frame, where it is subjected to a constant

vertical stress while liquid is circulated through the waste. Instrumentation is incorporated to measure the

following:

pore water pressures and leachate recirculation rates reactor chamber temperature

influent/effluent leachate chemistry gas production and composition waste density and settlement hydraulic conductivity

drainable porosity

The successful functioning of the cell and monitoring systems, and the feasibility of assessing the effects of

degradation in this way, have been demonstrated using a 20 year old sample of waste (AG2).

3.3 Development and application of models for flow in landfills

To start applying the results of the research to landfill operations, a number of analytical and numerical flow

models taking into account the variation of hydraulic conductivity with effective stress have been developed:

one dimensional vertical flow or infiltration was modelled using a finite difference method, with density and

hydraulic conductivity dependent on the effective stress (Powrie and Beaven, 1999),

closed form analytical solutions were derived relating the discharge from a well to the drawdown in

unconfined and confined aquifers where the hydraulic conductivity depends on the effective stress (Figure 9:

Powrie and Beaven, 1999), and


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the USGS three dimensional ground water flow model MODFLOW was modified to incorporate hydraulic

conductivities that vary with effective stress. The model was verified against the simpler analytical and finite

difference solutions above. It was then used to simulate more complex leachate flow problems, including the

performance of a grid of leachate injection and abstraction wells (Figure 10: Beaven and Powrie, 1999).

4. MAJOR RESULTS AND FINDINGS

4.1 Waste density and water contentFigure 11 shows the dry density as a function of effective stress for the different wastes tested. The sample with

the highest dry density was the 20 year old aged waste excavated from a landfill site. This is consistent with the

large proportion of soil type material (represented by the


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load, or from increases in the pore water pressure. However, no significant changes in drainable porosity or

hydraulic conductivity were detected when the effective stress was reduced. The implication of this is that fluid

flow in landfills will be predominantly controlled by the maximum previous effective stress to which the waste has

been subjected. Any loading process, such as placement of the waste in an unsaturated state or dewatering by

pumping from a vertical well, will probably cause an irreversible reduction in hydraulic conductivity.

4.5 Waste particle density and effective stress theoryThe results of the tests undertaken in the compression cell suggest that the waste particles undergo significant

changes in density as the overburden stress is increased (Table 8). This is in contrast to some of the theories

used in conventional soil mechanics, in which the particles are assumed to be incompressible. On the basis of the

data in Table 8 for raw household refuse at 120 kPa and above, the effective stress for calculations of volume

change would be ' = - A.uw, where A lies in the range 0.19 to 0.57 (Powrie, Beaven and Harkness, 1999). In

general terms, the applicability of the principle of effective stress to landfilled wastes remains a subject requiring

further research.

4.6 Unsaturated waste characteristics

Figure 19 shows the relationship between suction and water content for the Dano processed waste at an average

bulk density of 700 kg/m3. These data will be essential to any future modelling of flow in the unsaturated zone.

4.7 Effect of flushing rate on waste degradation

The batch scale experiments on the degradation of OFMSW showed that solids destruction increased as the

flushing rate increased and HRT reduced (Figure 20). The greatest amount of solid destruction were achieved

with a tap water flush, and the lowest with anaerobically treated leachate. The formation of volatile fatty acid

(VFA) was stimulated by higher flushing rates and is a reflection of the greater solids destruction. Gas production

from the batch reactors depended on the available soluble substrate concentration, and is shown for the various

recirculation systems in Figure 21 (a-d). Gas production in reactors operating at reduced flushing rates (HRTs of

240 and 504 hours) was impeded due to the inhibition of substrate hydrolysis as a result of an accumulation of

VFA and/or a lowered pH. Gas production was also impeded in reactors operating at high flushing rates with

deionised water, due to the washing out of essential ions and poor buffering.

The larger scale experiments generally confirmed the findings of the smaller experiments. A summary of typical

results is given in Table 9, which indicates a substantial increase in degradation on flushing with tap water,

although there was no additional benefit from increasing the flushing rate to give a hydraulic retention time of

less than 4 days. Flushing with untreated leachate significantly reduced the amount of solids destruction.

The work has increased our understanding of how the flushing medium influences degradation, and has

shown quite clearly the beneficial effects of VFA removal in a leachate recirculation system.

5. SIGNIFICANCE OF RESULTS FOR ENGINEERING PRACTICE

5.1 Maximum vertical infiltration rates through landfills

A key constraint on the viability of the flushing bioreactor as a sustainable landfill is the rate at which flushing

can take place. A one dimensional flow model incorporating empirical relations between waste density and

effective stress (e.g. Figure 13) and hydraulic conductivity and effective stress (e.g. Figure 17) was developed to

examine maximum infiltration rates through landfills of various depths with different initial waste densities (Powrie

and Beaven, 1999). These flow rates were related to the minimum required to flush the pollution load from landfills

over a 30 year period, which is consistent with the definition of a sustainable landfill as one that is approachingequilibrium with the surrounding environment within a period of one generation from cessation of landfilling

activities (e.g. IWM, 1999). Greater flushing rates are achieved through saturated than unsaturated waste. Landfill

depths would need to be less than approximately 20 m to achieve the necessary flushing rates through

unsaturated wastes and less than approximately 40 m through saturated waste, if it is assumed that hydraulic

conductivity is governed by the effective stresses applied during placement of the waste in an unsaturated state.

If the waste is placed saturated, there is virtually no limit on landfill depth to achieve the required flushing rate

(Figure 22). Pre-compaction of waste at the tipping face can also affect possible flushing rates (Figure 23): in

general waste should not be compacted to densities greater than between 0.9 and 1.1 t/m3.

5.2 Flow to a pumped leachate well

Flowrate-drawdown curves for pumped wells in confined and unconfined aquifers whose hydraulic conductivity

varies with effective stress are shown in Figure 24. The unconfined aquifer analysis (Figure 24b) is more

representative of field conditions at many landfills. The specific capacity (i.e. the flow rate per unit drawdown)

decreases with increasing drawdown. The flowrate increases with drawdown, but there is little increase in flowrate


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for increases in drawdown in excess of about 33% of the initial saturated depth. This is an important result that

has significant operational implications for the pumping of leachate from wells on landfills.

5.3 Leachate flushing using a well field

MODFLOW was used to investigate the feasibility of using vertical wells to flush waste horizontally in a 30 metre

deep landfill with a 20 metre confined saturated zone. With a grid spacing of 20 metres, the model (Figure 25)

calculated a steady state pumping and injection rate of 4.3 m3/day based on a fixed hydraulic conductivity profilerelated to the stress distribution in unsaturated waste. This pumping rate is theoretically sufficient to flush and

remove contaminants from all areas of the waste between the wells in approximately 30 years. The flow or flushing

rate at the top of the saturated layer is approximately 20 times the rate at the bottom (Figure 26). If K h is increased

relative to Kv (Figure 18) then the pumping rate is increased and the ratio of the flowrates at the top and bottom is

reduced to 8. Nevertheless, flushing strategies involving vertical wells will probably require the targeting by

discrete well response zones of different horizons in the waste.

6. PRINCIPAL CONCLUSIONS

Relationships between the drainable porosity and vertical stress have been quantified for a number of

wastes. At depth, most wastes are likely to be approaching saturation even if free to drain under gravity.

Relationships between hydraulic conductivity and vertical stress in first compression have also been

determined. Although some differences in hydraulic conductivity between processed, unprocessed and agedhousehold wastes were apparent, these are generally insignificant in comparison with the orders of

magnitude change in hydraulic conductivity that results from waste compression.

On unloading, very little of the deformation caused by loading was recovered, and the hydraulic conductivity

and drainable porosity remained substantially unchanged. This suggests that the hydraulic conductivity of a

waste is governed by the maximum equivalent vertical stress to which the waste has been subjected, so that

the stress history or the density of the waste must be considered when assessing its hydrogeological and

geotechnical properties.

During compression, waste develops a layered structure which results in an anisotropy of hydraulic

conductivity. The degree of anisotropy, expressed as the ratio Kh/Kv, increases with increasing applied

stress.

Provided that the dependence of hydraulic conductivity on vertical stress and stress history is taken into

account, calculations suggest that vertical infiltration rates comparable with achieving stabilization of the

waste within a timescale of one generation can be achieved. Precompaction of the waste to densities greater

than 0.9 - 1.1 t/m3, or placement of loose waste unsaturated to a depth in excess of about 40 m prior to

saturation, would both prevent the required infiltration rate from being achieved.

Analyses of pumped vertical wells using a stress dependent hydraulic conductivity suggest t hat there will be

little increase in flowrate for increases in drawdown in excess of about 33% of the initial saturated depth. If

arrays of vertical wells are used for horizontal flushing, the tendency for preferential flow through the upper

saturated layers will need to be addressed.

Preliminary tests have demonstrated the potential importance of the interaction between gassing and

leachate flow in terms of the pore volume available for liquid flow, and suggest that changes in gas

production rate could affect the apparent leachate level measured in the field. This is a subject requiring

further research.

7. ACKNOWLEDGEMENTSThe research summarized in this report was carried out with the support of the Engineering and Physical Sciences

Research Council (EPSRC), with an additional financial contribution from Cleanaway Ltd.


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Figure 1 Pitsea compression cell prior to modifications Figure 2 New enclosure to Pitsea compression cell


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Figure 3 Configuration of horizontal hydraulic conductivity tests

Supply Tank

Header tanks

Cylinder

Gravel layer

Isolated ports

Seal

Bottom platen

Inlet Ports

Inlet Valves

Gravel layerFlow

outlets (isolated)

Outlets

Piezometer tubesinserted in sampleat various positions

Piezometrichead

New Seals

Top platen

Top platenoutlets

(isolated)

Tomeasuring

cylinder

Waste Sample

Figure 4 Magnet and extensometer method of measuring sample movement

3 way valve mounted on top for either:

* Outflow to reservoir tank

* Outflow to gas collection tank

* Inflow from header tanks

* Isolated

Pointer fixed to framework

Plastic tube - 3m long

Bottom of tube sealed and

resting on bottom platen

Water & gas tight seal

Unclamped to allow

platen movement

Gravel layer - for water

distribution or removal

Dividing Ring - 1400mm dia x 150mm deep

Creates an inner and outer core in the

sample for assessing preferential flow paths

Valve stems

Part of strengthening framework

Extensometer - lowered into

tube. Magnet position indicated

with buzzer and warning light

Magnet & plate

Sample movement causes

magnet to slide on tube

Waste sample

- forced downwards

by applied load

Waste cylinder (ID2000mm)

- fixed position

Inflatable seals(3 off)

Clearance Gap (5mm nominal)

rings for seals

Graduated tape

12 holes through top platen

with flanged extension tubes

Top Platen - Dia 1990mm

Connected to hydraulic rams

for application of load to waste

Fabricated retaining


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Figure 5 Magnet displacement in waste DN1 between 0 and 87kPa illustrating uniform compression

0

100

200

300

400

500

600

700

800

0 200 400 600 800 1000 1200 1400 1600

Elevation above base of waste (mm)

Downnrddisplacementrelativetopositionat0kPa(mm)

Magnet positions end of 40kPa Magnet positions - end of 87kPa

Uniform compression (40kPa) Uniform compression (87kPa)

Figure 6 Typical MODFLOW simulation to determine KhWest EastCross-Section along Row 29

470

470

490

490

510

510

510

510

530

53

0

530

550

550

570

570

590

590

610

6306

50

670

Top Gravel

layer

Outlet Port

constant head

cells

Pressure head contours (cm AD):

asymmetry reflecting lower

hydraulic conductivity at the top

of the waste

Bottom Gravel

layer

No flow squares

(light grey) denoting

outside wall of

cylinder

Constant head squares

simulating outflow

through the

bottom platen.

Inlet Port

constant head

cells

Constant head squares

simulating outflow

through the

upper platen.

Model calibrated against measured input andoutput flow rates and hea


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Figure 7 Process diagram for ... dan's work


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Figure 8 Anaerobic consolidation reactor


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Figure 9 Ideal (a) confined and (b) unconfined aquifer analyses of flow to a well where K=f(')

h w zh

H , D ( = 2 0 m )

Flow to wel l

b ) U n c o n f i n e d

a ) C o n f i n e d

h w

zh D ( = 1 0 m )

D (=20m)

F low to we l l

Con f in ing laye r

r w r0

In bo th ana lyses ln ( ro /r w) =8 an d t he un it we ig ht of s at ur at ed an d un s at ur at ed

w a s t e = 1 1 k N / m 3

1

2

1

2

(1+)D (=30m)

H ( = 2 0 m )

Figure 10 MODFLOW grid design to simulate operation of injection and abstraction wells

5 1 0 1 5 2 0 2 5 3 0 3 5

35

30

25

20

15

10

5

Elevat ion

P lan

C o n s t a n t h e a d b o u n d a r y c e l l s

( a t e a c h c o r n e r o f t h e g r i d )

r e p r e s e n t h e a d i n 1 / 4 o f e i t h e r

p u m p e d o r i n j e c t i o n w e l l

N o f l o w b o u n d a r y c e l l s1

2

1 1

2 1

West East

Layer

Nu mb e r s

W e l l 2

W e l l 1

W e l l 3

W e l l 4

C o l u m n s

R o w s

20m

10m

20.2m

20.2m


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Figure 11 Dry density vs effective stress

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500

Average effective stress (kPa)

Drydensity(t/m3)

Household waste DM3 Household waste DM2 Processed waste PV1

Processed waste PV2 Aged waste AG1 Aged waste AG2

Dano waste DN1

Figure 12 Water content at field capacity of crude household waste vs effective stress

40

50

60

70

80

90

100

110

0 100 200 300 400

Av. Stress (kPa)

WCdry(%)

Household waste 3 at Field Capacity Original Water Content

Absorptive

Capacity

(between lines)


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Figure 13 Average density of Household Waste vs effective stress

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

0 100 200 300 400 500 600

Av. vertical stress in waste (kPa)

Density(t/m3)

Dry density of DM2 and DM3 Density at Field CapacitySaturated density Density at original water content

Power (Density at Field Capacity)Power (Density at original water content)

dry = 0.1554.(') 0.248

FC= 0.448.(') 0.1563

sat = 0.6691.(') 0.0899

= 0.236.(') 0.248

B u l k d e n s i t y r a n g e o f

w a s t e s

unsatura ted

Figure 14 Average drainable porosity vs effective stress

0

5

10

15

20

25

0 100 200 300 400 500

Average effective stress (kPa)

Drainab

leporosity(%)

Household waste DM3 Household waste DM2 Processed waste PV1

Processed waste PV2 Aged waste AG1 Aged waste AG2

Dano waste DN1


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15

Figure 15 Effect of waste degradation and landfill gas production on the proportion of leachate occupying

drainable porosity voids

Figure 16 Effect of leachate movement on leachate and gas occupation of drainable voids in waste DN1 at 40

kPa: upward flushing

0

1 00

2 00

3 00

4 00

5 00

6 00

7 00

8 00

0 2 4 6 8 10 12

Time(days )

Volumeofgasinwaste(litres)

0

1000

2000

3000

4000

5000

6000

7000

Accumulatedgasvolume(litres)

Volume of gas in waste (based on load ce l l readings and vo lume of leachate d isp laced)

Volume of gas vented to a tmosphere by sample

Leachate occup ied

drainable porosity = 5%

Leachate occup ieddrainable porosity = 0%

Leachate occup ied

drainable porosity =10%

Leachate occup ied

drainable porosity = 15%

Leachate f lushing

event at Q=1.5 l /s

Leachate f lushing

event at Q=1.5 l /s

Leachate f lushing

event at Q=1.5 l /s

0

2

4

6

8

10

12

14

16

0 0 . 0 005 0 . 001 0 . 00 15 0 . 002 0 . 0 025 0 . 00 3 0 . 0 035 0 . 00 4 0 . 004 5

A v e r a g e l i n e a r f l o w v e l o c i ty b a s e d o n l i q u i d f i l l e d v o i d s (m / s )

Leachateoccupieddrainablevoids(%)

Le ac hat e oc cupi ed dr ai nabl e v oi ds D ra ina bl e po ro si ty (10 0% lea ch at e s at ur at io n)

Increas ing t ime

of f lush ing

Increas ing t ime

of f lush ing

Increas ing t ime

of f lush ing


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16

Figure 17 Hydraulic conductivity vs stress

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

10.0 100.0 1000.0

Hydraulicconductivity(m/s)

D M 3 P V 1 A G 1 A G 2

D N 1 Ser i es2 Ser i es4

Av. vertical stress (kPa)

K (m/s) = 10(') -3.1

(approx best fit line)

K (m/s) = 80(')-3.63

(approx fit based on

worst case hydraulic

conductivity)

Figure 18 Anisotropy in hydraulic conductivity

1

2

3

4

5

0 100 200 300 400 500 600Applied Stress (kPa)

Kh

/Kv


19/29

17

Figure 19 Dano waste moisture characteristic curve

Figure 20 OFMSW total solids destruction in 1 litre anaerobic digestion vessels operating variable

hydraulic retention times and flushing with different liquors, operated over a 21 day time period.

1 0 0 0

1 0 0 0 0

1 0 0 0 0 0

0 1 0 2 0 3 0 4 0 5 0 6 0

Moisture content (w) %

Dry Weight basis

Suction(kPa)

0.00

10.00

20.00

30.00

40.00

50.00

60.00

0 100 200 300 400 500 600

Hydrau l ic Retent ion T ime (hrs)

Solids

destruction(%)

tap water aerob ic trea tment anaerob ic trea tment de- ion ized water


20/29

Figure 21 Cumulative biogas production for anaerobic bioreactors with hydraulic retention time of

a) 80 hrs; b) 120 hrs; c) 240 hrs; and 504 hrs. Flush liquors: tap water; de-ionized water; leachate after secondary anaerobic treatment; leachate after

secondary aerobic treatment

a) b)

0

1000

2000

3000

4000

5000

6000

0.00 100.00 200.00 300.00 400.00 500.00 600.00

time (hrs)

biogasproduced(ml

Anaerobic Secondary Treatment Deionized Water Flush Aerobic Secondary Treatment Tap Water Flush

c) d)

0

1000

2000

3000

4000

5000

6000

0.00 100.00 200.00 300.00 400.00 500.00 600.00

time (hrs)

biogas(ml)


0

500

1000

1500

2000

2500

3000

3500

4000

4500

0.00 100.00 200.00 300.00 400.00 500.00 600.00

time (hrs)

biogas(ml)


0

500

1000

1500

2000

2500

3000

3500

0.00 100.00 200.00 300.00 400.00 500.00 600.00

t ime (hrs)

biogas(ml)

Anaerob ic Secondary Treatment De ionized Water Flush Aerob ic Secondary Treatment Tap Water F lush


21/29

19

Figure 22 Maximum vertical infiltration rate against landfill depth

0.1

1

10

100

1000

0 10 20 30 40 50 60

Landfill depth (m)

Max.Infiltrationrate(

m/a)

Variable KConstant K based on stress at base of landfillMinimum required flushing rate


22/29

20

Figure 23 Infiltration rate for a) K=2.1(')

-2.71(best fit line on Fig. 15 for waste DM3); and

b) K=17(')-3.26

(worst case line on Fig. 15 for waste DM3) against landfill depth for various

precompacted waste densities

0.0001

0.001

0.01

0.1

1

10

0 10 20 30 40 50 60

Landfill depth (m)

Max.infiltrationrate(m/d)

Min. flushing rate No pre-compaction 0.5 Mg/m3 0.75 Mg/m3

0.9 Mg/m3 1.0 Mg/m3 1.1 Mg/m3 1.2 Mg/m3

0.0001

0.001

0.01

0.1

1

10

0 10 20 30 40 50 60

Landfill depth (m)

Max.infiltrationrate(m/d)

Min. flushing rate No pre-compaction 0.5 Mg/m3 0.75 Mg/m3

0.9 Mg/m3 1.0 Mg/m3 1.1 Mg/m3 1.2 Mg/m3

a)

b)

All waste densities at a water content (WCdry) of 51.5% (assumed WC of waste as deposited)


23/29

21

Figure 24 Relationship between discharge rate and drawdown for approximate ideal (a) confined and (b)

unconfined aquifer analyses in which the hydraulic conductivity varies with drawdown according

to K=2.1(')-2.71

(best fit line on Fig. 15 for waste DM3).

0.0E+00

5.0E-05

1.0E-04

1.5E-04

2.0E-04

2.5E-04

3.0E-04

3.5E-04

0 2 4 6 8 10

Drawdown (m)

Flowrate(m3/s)

Variable K Constant K ( 4.09x10-6 m/s)

a)

0.0E+00

1.0E-04

2.0E-04

3.0E-04

4.0E-04

5.0E-04

6.0E-04

7.0E-04

0 5 10 15 20

Drawdown (m)

Flowrate(m3/s)

Variable K Constant K (4.09x10-6 m/s)

b)


24/29

22

Figure 25 Head distribution for injection wells and abstraction wells on parallel grid, with hydraulic

conductivity based on unsaturated stress distribution.

Figure 26 Variation of well discharge / injection rate with depth

10

12

14

16

18

20

22

24

26

28

30

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

Flow rate per linear metre of well (m3/day)

Depth(m)

Pumped / Injection well: Kh=KvPumped / Injection well: Kh=2-5Kv

Average pumped/ injection

rate = 4.3 m3/day per well

Average pumped/ injection rate

= 68.1 m3/day per well


25/29

23

Table 1. Size and category analysis of waste AG2

Size mm Weight

%

Pa/Cd PF Dp Tx Mc Mnc Gl Fe nFe Soil


26/29

24

Table 5. Size and category analysis of waste AG1

Size mm Weight % Pa/Cd PF Dp Tx Mc Mnc Gl Put Fe nFe


27/29

25

Table 8 Variation in particle density, particle compressibility and waste compressibility with applied stress

Average stress at end of stage, kPa 34 65 120 241 463

Average particle density, t/m3 0.88 0.97 1.02 1.17 1.30

Average particle compressibility, MPa-1

- 5.38* 1.62 1.85 0.81

Overall compressibility of waste, MPa-1 7.45 3.75 3.76 2.30 1.07

a) Raw household waste DM3


Average particle density, t/m3 0.59 0.68 0.72 0.78 0.93

Average particle compressibility, MPa-1

- 6.60* 1.65 1.11 1.22*

Overall compressibility of waste, MPa-1 7.29 6.18 4.12 1.45 0.87

b) Pulverized household waste PV1


Average particle density, t/m3

1.64 1.62 1.64 1.69 1.86

Average particle compressibility, MPa-1 - -0.67* 0.36 0.49 0.73*

Overall compressibility of waste, MPa-1

7.38 3.85 2.91 1.57 0.66

c) Aged household waste AG1

* It is not possible for the average compressibility of the particles to be greater than the overall compressibility of

the waste, and unlikely that it is negative. These values must therefore be in error to some extent.

Table 9 Summary of total solids and total volatile solids destroyed in 30 litre SBAFB (Static Bed

Anaerobic Flushing Bioreactor) system.

Substrate: 60 : 40 mixed paper : food waste BMP: 0.546 m3

CH4

/ Kg TS destroyed

Reactor mode of operation % TS destroyed % TVS destroyed

HRT 24 hrs - untreated leachate 6.49 3.92

HRT 24 hrs - clean tap water 43.92 42.58



HRT - hydraulic retention time BMP - biological methane potential

TS - total solids TVS - total volatile solids


28/29

26

APPENDIX A

References

ASTM D 5298-92 Standard test method for measurement of soil potential (suction) using filter paper

Committee D-18 on soil and rock. Approved September 15 1992, Published November 1992, Designated as D5298-92

DoE (1994)National Household Waste Analysis Project - Phase 2 Volume 1 Report on composition and weight

data Report No CWM 082/94 produced for the Waste Technical Division of the Department of the Environment

under Research Contract PECD 7/10/288 by Warren Spring Laboratory and Aspinwall & Co.

DoE (1995)Landfill design construction and operational practice Waste Management Paper 26B. HMSO,

London 289pp

Knox, K. (1990) The relationship between leachate and gas Proc. of the International conference on Landfill

Gas: Energy and Environment. ISBN 0-7058-1628-1.

Knox, K. (1996)A review of the Brogborough and Landfill 2000 test cells monitoring data. Final report for the

Environment Agency R&D Technical Report P231. 113pp.

Wingfield-Hayes, C., Fleming, G. and Gronow, J. (1997) Field trials of waste manipulation techniques: the

Mid-Auchencarroch experimental landfill Proc. 6th International Sardinia Landfill Symposium. S. Margherita di

Pula, Cagliari, Italy. Vol I pp 311-322. October 1997

APPENDIX B

Publications arising directly from this research

Beaven, R.P. (1997). Hydraulic and Engineering Properties of Household Waste Proceedings of the IBC

conference on "Designing and Managing Sustainable Landfill". 26-27 February. Scientific Societies Lecture

Theatre. London

Beaven, R.P. (1997). Is accelerated s tabilisation achievable? Paper presented at the IWM 1997 annual

conference - Torbay 10 June.

Beaven, R.P. (2000). The hydrogeological and geotechnical properties of household waste in relation to

sustainable landfilling PhD Dissertation, University of London.

Beaven, R.P. and Powrie, W. (1996). Determination of the Hydrogeological and Geotechnical Properties ofRefuse in relation to Sustainable Landfilling. Paper presented at the Nineteenth International Madison Waste

Conference, September 25-26, 1996, Department of Engineering Professional Development, University of

Wisconsin-Madison. USA.

Beaven, R.P. and Powrie, W. (1999)Analysis of waste flushing and flow to wells using MODFLOW and an

effective stress dependent hydraulic conductivity Proceedings Sardinia 99, Seventh International Waste

Management and Landfill Symposium, S. Margherita di Pula, Cagliari, Italy; 4-8 October 1999. Vol II pp 33-41.

Hudson, A.P., Beaven, R.P. and Powrie, W. (1999)Measurement of the hydraulic conductivity of household

waste in a large scale compression cell Proceedings Sardinia 99, Seventh International Waste Management

and Landfill Symposium, S. Margherita di Pula, Cagliari, Italy; 4-8 October 1999. Vol III pp 461-468.

IWM (1999) The role and operation of the flushing bioreactorReport of the Institute of Wastes Management

Sustainable Landfill Working Group. Pub. IWM Business Services Ltd.


29/29

Knox, K. (1996)A review of the Brogborough and Landfill 2000 test cells monitoring data. Final report for the

Environment Agency Technical Report P231

Parker, L.P., White, J.K. and Powrie, W (1999) The measurement of the geotechnical and hydrogeological

properties of degrading solid waste Proceedings Sardinia 99, Seventh International Waste Management and

Landfill Symposium, S. Margherita di Pula, Cagliari, Italy; 4-8 October 1999. Vol III pp 469-478.

Powrie, W. and Beaven, R.P. (1998) Hydraulic conductivity of waste - current research and implications for

leachate management Waste Management November 1998 pp22-23.

Powrie, W. and Beaven, R.P. (1999) The hydraulic properties of household waste and implications for

landfills Proceedings of the Institution of Civil Engineers, Geotechnical Engineering, Vol 137, Oct 1999, pp235-

247.

Powrie, W., Beaven, R.P. and Harkness. (1999).Applicabili ty of soil mechanics principles to household waste

Proceedings Sardinia 99, Seventh International Waste Management and Landfill Symposium, S. Margherita di

Pula, Cagliari, Italy; 4-8 October 1999. Vol III, pp429-436.

Powrie, W., Richards, D. and Beaven, R.P. (1998). Compression of waste and implications for practice. Proc of

the Symposium on Geotechnical Engineering of landfills. pp 3-18. Held at Nottingham Trent University 24

September 1998 by the East Midlands Geotechnical Group of the ICE. Ed. Dixon, N. et al; Pub. Thomas Telford

Ltd. ISBN 0 7277 2708 7.

Walker, A.N., Beaven, R.P. and Powrie, W. (1997). Overcoming problems in the development of a high rate

flushing bioreactor Proc. 6th International Sardinia Landfill Symposium. S. Margherita di Pula, Cagliari, Italy.

Vol I pp 397-408. October 1997.

Walker, A.N., Beaven, R.P. and Powrie, W. (1998). A conceptual design for sustainable landfill Proceedings of

the 18th annual IAH groundwater seminar, Portlaoise, Ireland; April 1998. pp1-13.

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