The design, construction and instrumentation of a full-scale bioreactor landfill
S.T.S.Yuen, J.R.Styles, Q.J.Wang & T.A. McMahon
Department of Civil & Environmental Engineering, University of Melbourne, Melbourne, Australia
ABSTRACT : This paper outlines the design and construction of a full-scale experimental bioreactor landfill
test cell located at the Lyndhurst Sanitary Landfill, Victoria. It also describes the instrumentation employed to
obtain the required experimental data.
1. INTRODUCTION
Due to advances in the knowledge of landfill
behaviour and decomposition processes in recent
years, there has been a strong thrust to upgrade
existing landfill technology from a permanent
storage (dry cell) concept to a bioreactor or processbased (wet cell) approach (Maurer 1994 & Krol et
al. 1994). A bioreactor landfill allows a more active
landfill
management
that
recognises
the
understanding of the biological, chemical and
physical processes involved. In contrast to the
conventional permanent storage approach to
preserve the waste, it focuses on enhancing the
degradation processes to degrade and stabilise the
waste and thus minimise potential long term
environmental impacts that may follow a
containment system failure. By far, the most
promising bioreactor landfill management option is
by leachate recirculation (Yuen et al. 1994).
Although there is little doubt that the bioreactor
technology is to be preferred as part of an integrated
approach to achieve the best practice for solid waste
management, further research particularly with a
view to the local situation is required (Waste
Management Council 1995a).
A full-scale leachate recirculation bioreactor
landfill research project is being undertaken by the
Department of Civil and Environmental Engineering
Department, University of Melbourne in BrowningFerris Industries Inc. (BFI)’s Lyndhurst Landfill,
Victoria. The project objectives are :
(i) To investigate full-scale bioreactor landfill
behaviour promoted by leachate recirculation and
quantify the decomposition process in terms of :
 leachate production and quality,
 gas generation and composition, and
 waste stabilisation and settlement.
(ii) To study the hydrological aspects of
bioreactor landfills in terms of :
 in-situ moisture measurement of municipal solid
waste (MSW),
 saturated/unsaturated flow in a MSW medium for
the prediction of moisture movement and the
design of leachate recirculation systems, and
 water balance of containment cells for the
purpose of estimating leachate production,
The research methodology is described
elsewhere (Yuen et al. 1997a & 1997b). This paper
aims to provide a summary of the design and
construction of the test cell and discusses the
instrumentation employed.
2. TEST CELL DESIGN & CONSTRUCTION
2.1 General Site Description
The test site is located at BFI ‘s Lyndhurst Sanitary
Landfill which is about 35 km south-east of
Melbourne. The existing pit used by landfilling, as
common to many landfill sites in the same southeastern sand belt region, was created by previous
sand mining operations.
The geology comprises a sequence of Tertiary
age sands and clays of 15 to 35m depth underlain by
granite rock. The natural groundwater level is
shallow at about 6m below original ground level
with the regional hydraulic gradient falling towards
the west. Historic climatic data at a nearby
meteorological station reveal that the mean annual
pan evaporation (1227 mm) exceeds the mean
annual rainfall (854 mm).
GeoEnvironment 97, 1st Australia - New Zealand Conference on Environmental Geotechnics, Melbourne, November 1997.
2.2 Size of Test Cell
The cell commissioned for the full-scale experiment
is Cell 3 at the north-western corner of the site which
covers a footprint area of approximately 180m x
75m (about 1.5 hectares).
The thickness of MSW fill varies from 10m to
15m according to the surface landscape. Based on
survey data, the as-constructed volume of the test
cell is 180,365 m3 (excluding liner and cap). Total
tonnage of municipal solid waste (MSW) as
recorded at the weighbridge is 100,824 tonnes.
In plan the cell is divided into two sections of
roughly equal area (Figures 1& 2). The western half
has been designated as the control section (i.e. dry
landfilling) and the eastern half as the test section
(i.e. wet landfilling by leachate recirculation).
2.3 Waste Composition
As the composition of waste is one of the most vital
factors that influences the bio-degradation and hence
the enhancement strategy, it is important to be able
to quantify and qualify the types of waste being
investigated in the experiment.
Only domestic garbage and non-hazardous /nontoxic waste from the industrial & commercial waste
stream were allowed to fill the test cell. Records
were kept according to the waste streams which
revealed a 1:1.6 ratio of domestic to commercial &
industrial waste. The composition of both waste
streams from metropolitan Melbourne was published
recently by the Waste Management Council (1995b).
Based on these figures and the above records, a close
representation of the waste composition in the test
cell was deduced. In addition, waste samples were
collected and sorted as part of the MSW moisture
measurement study (Yuen et al. 1997c).
As both control and test sections were filled up
simultaneously and the ratio of domestic waste to
industrial & commercial waste stream was
maintained fairly consistently during filling, the
composition of waste in the test cell (in macroscopic
scale) can be considered to be reasonably uniform.
Due to the same reason, the waste in the control and
test sections can be treated as identical, at least
within the context of this experiment.
2.4 Daily cover
It is part of the Lyndhurst landfill licensing
requirement that a 150 mm layer of earth material
should be provided as daily cover during waste
disposal. In addition, each completed vertical lift (of
2m) shall be covered by an interim cover of 300mm
earth material. This requirement also applied to the
test cell.
In common with other sand-pit landfills in the
region, semi-dried to dry slimes (a material of clayey
sandy silt left behind from previous sand washing,
Yuen & Styles 1995) were used for daily and interim
covers in the test cell. The materials spread
reasonably well at “spadable dryness”. While still
moist, they exhibit low permeability to both gas and
odour. As they dry out, they crack to form
agglomerates that would allow a reasonable
permeability that is desirable for moisture movement
induced by later leachate recirculation.
To reduce the barrier effects of the daily and
interim covers for recirculation, permeability in the
test cell was improved by stripping and mixing the
earth material with waste before placing the next lift.
According to weighbridge records and survey
data, the cover material takes up 15.2 % of the total
volume of cell (excluding final cap, liner & drainage
layer). This compares well with the above licensing
requirement which implies a 17.7 % proportion.
2.5 Density of Waste
Compaction of waste was carried out as for other
operational cells. The waste was compacted in
vertical layers by a Caterpillar 826C landfill
compactor with an operating mass of 32 tonnes.
Based on weighbridge records and survey data, the
as-placed in-situ density of MSW, excluding cover
material, drainage and cap, was 0.73 tonne/m3 (or
0.83 tonne/m3 combined with cover material).
2.6 Cell Containment System
In common with all other operational cells, the test
cell comprises a 1m thick base and side liner of
compacted clay with a specified maximum hydraulic
conductivity of 1x10-9 m/s.
Upon completion of filling, a 1m thick final
capping was laid which was made up of 300 mm top
soil on compacted clay. The surface is grassed to
prevent erosion and to promote evapotranspiration.
The capping falls gently on a 1 on 7.5 gradient
towards its north-western corner (Figure 2).
2.7 Leachate Collection Drains
To enable both leachate quantity and quality from
the control and test sections to be monitored
separately, each section has its own collection
system isolated by a compacted clay bund wall. Each
system comprises a 300mm thick gravel drainage
layer immediately above the base liner with 90mm
diameter slotted collector pipes installed at 15m
centres. These then drain to a 150mm diameter
GeoEnvironment 97, 1st Australia - New Zealand Conference on Environmental Geotechnics, Melbourne, November 1997.
header pipe falling at a 0.16% gradient into a
leachate collection sump.
2.8 Gas Extraction System
An active system (employing suction created by a
blower) is used to extract gas. To enable landfill gas
(both flow rate and composition) in the control and
test sections to be monitored independently, gas is
being collected separately by two isolated gas fields.
Each gas field comprises nine collection wells
arranged in a 3x3 grid spaced at 25m apart.
As each well is expected to have a different
characteristic with respect to the amount of gas that
can be extracted, each has a separate control valve
(for suction adjustment) and a sampling point. The
flow rate of an individual well can be measured
through a pre-calibrated orifice plate. Landfill gas
composition in terms of percentage of methane,
carbon dioxide and oxygen is routinely monitored by
a portable non-dispersive infra-red absorption
landfill gas analyser. A portable gas chromatograph
(GC) with a micro-thermal conductivity detector is
used to detect other gases and to calibrate the landfill
gas analyser.
2.9 Leachate Recirculation System
A combination of sub-surface infiltration trenches
and deep injection wells was selected for this
experiment. The design of this experimental
integrated system (including sizing and spacing of
the wells and trenches) was based on a numerical
simulation of a saturated/unsaturated porous flow
model (Yuen et al. 1997b). Figure 1 provides a
schematic plan of the integrated system.
Leachate is pumped from the collection sump
into three storage/header tanks with a total capacity
of 27,000 litres. From there the leachate feeds the
wells and trenches by gravity via a system of
pipework and valves . The system has been designed
to allow flexibility to inject either an individual
well/trench or a group of selected wells/trenches.
3. MONITORING PROGRAM
Table 1 lists out all the items that are being
monitored. The method employed and the frequency
of monitoring for each item are also shown.
4. INSTRUMENTATION
In designing the test cell instrumentation, the
following factors and constraints have been taken
into consideration: costs (both capital and running
costs), compatibility with the landfill environment,
reliability, and simplicity.
4.1 In-situ Neutron Probe Access Tubes for MSW
Moisture Monitoring
Based on a separate study (Yuen et al. 1997c)
neutron probe was identified to be a feasible and
practicable tool for monitoring moisture of in-situ
MSW in a landfill. Seven in-situ access tubes each
of 12m long have been installed as shown in Figure
2. They are used to monitor seasonal moisture
change in the control section and to monitor
moisture change adjacent to a recirculation well and
a trench in the test section.
Table 1- Test Cell Monitoring Program
Items Required Monitoring
Waste Moisture Distribution
Profile
Climatic Data
Surface Runoff
Landfill Settlement
Waste Temperature
Leachate Level
Leachate Volume
Leachate Quality
Landfill Gas Composition
Landfill Gas Flow Rate
Groundwater Quality
Method Employed
Using neutron probe to measure moisture changes
via in-situ access tubes
Automatic weather station
Collection by surface channels & measurement by
flume with water level auto-logger
Level survey on settlement plates
Stainless steel sheathed thermocouples
Measure leachate levels in the sumps and open
wells by a water level sensor
Combining the use of a ultrasonic flowmeter and
tank measurement
Collect and test leachate samples from all leachate
sumps and open wells
Portable non-dispersive infra-red absorption
landfill gas analyser.
Portable gas chromatograph (GC)
Pre-calibrated orifice plate
Collect and analyse groundwater samples from
adjacent monitoring bores
The material and size of the access tubes used in the
test cell are identical to that used in the laboratory
Frequency
Test Cell : during recirculation
Control Cell : quarterly intervals
Continuous
Continuous
Monthly
Monthly
Monthly
Daily
Monthly
Bi-monthly
As required
Bi-monthly
Quarterly
experiment, i.e. drawn aluminium tubes of 40mm
internal diameter and 2mm thickness. Aluminium
GeoEnvironment 97, 1st Australia - New Zealand Conference on Environmental Geotechnics, Melbourne, November 1997.
was used as it is virtually transparent to neutrons. In
both laboratory and full-scale tests, the same neutron
moisture depth meter (CPN Corporation model 503
DR) with a 38mm diameter probe was used.
In selecting the size of the tube, it is desirable to
minimise the air gap between probe and tube as the
gap constitutes a discontinuity in the system being
measured. This is also important from the point of
view of reproducibility of probe location and
avoidance of possible asymmetry effects.
Considerable difficulties were encountered in the
installation of the access tube. First, each tube had to
penetrate 12m down into the landfill. Hence at least
one joint was required (6m length sections were
used). As the internal diameter of the tube (40mm) is
only marginally larger than the diameter of the
neutron probe (38mm), welding the tubes inevitably
reduces the effective internal diameter at the joint
and would not allow the probe through. After some
trials, a “pin and socket” joint was successfully
employed and glued with Araldite (a two-pack
epoxy glue). The joint was pre-made in the
workshop by machine turning to remove half of the
thickness of the tubes, one end externally and the
other end internally.
Another installation constraint was the
requirement to keep the air gap between access tube
and MSW to a minimum due to the discontinuity
concern as discussed above, and to keep any MSW
distortion around the hole to a minimum.
Subsequent to some earlier unsuccessful trials,
installation was finally achieved by first pre-drilling
to the required depth with a slightly oversized
continuous flight auger. A steel casing marginally
larger than the external diameter of the aluminium
tube, was then pushed through the pre-drilled holes.
The tube was then inserted inside the casing prior to
withdrawal of the casing. While it is impossible to
install any tube without causing some MSW
distortion, this method managed to minimise it.
Pre-drilling the holes with an auger also
provided the opportunity to collect in-situ MSW
samples for waste composition and field calibration
purpose.
The finished tubes are bottom sealed and top
capped to prevent ingress of moisture. Additional
access tubes will be installed as necessary based on
the results obtained from the existing set.
4.2 Temperature Probes
Four sets of temperature probes (two sets in each of
the test and control sections) were installed as shown
in Figure 2. Each set comprises 3 probes aligned
along a vertical profile (at 3m below surface, at middepth and at 3m above base liner) in order to
delineate any temperature variation with depth.
Each probe is composed of a stainless steel
sheathed type K thermocouple and a sealed duct
which protects a special compensating cable
connecting the thermocouple to a surface terminal
plug. As the probe is subject to a potentially
corrosive environment, only the sensor tip of the
thermocouple is exposed in the waste.
A portable battery operated digital thermometer
is used to record temperature by connecting to the
surface terminal plugs.
4.3 Settlement Plates
Figure 2 shows in plan three clusters of settlement
monitoring points, two in the control section and one
in the test section. Each cluster is composed of a
series of five settlement plates installed
approximately on R.L.4m (top of liner), R.L.6m,
R.L.8m, R.L.10m, and R.L.18m (capping surface)
respectively . Instead of just monitoring the cell
surface subsidence, the settlement data collected at
various levels would provide some additional
information on subsidence behaviour along a vertical
profile.
All the settlement plates were pre-placed at the
nominated levels during filling. Upon completion of
the cell, they were located again in plan by a
positioning survey. Boreholes were then sunk to
provide access for the installation of a connection
rod and sleeve. The level of each plate can be
monitored by surveying the top level of the
connection rod.
4.4 Runoff Measurement
Surface runoff is measured as a component of the
cell water balance (Yuen et al.1997b). Runoff from
the whole cell is collected by two surface channels
running along the north and north-western edges as
show in Figure 2. Through two catch pits, all flow is
then diverted to a main channel. A flume equipped
with a water level probe and a real-time logger is
installed in-line with the main channel to measure
flow rate.
The flume installed in the test cell is a RBC
(Replogle, Bos and Clemmens; Bos et al.,1984) long
throated flume of a trapezoidal cross-section. It can
measure a maximum flow rate of over 80 litres/sec.
It was calibrated in a hydraulic laboratory channel
and the calibration curve is used to relate measured
flow depth to flow rate.
4.5 Climatic Data
Climatic
data including precipitation,
air
temperature, relative humidity, wind speed and
GeoEnvironment 97, 1st Australia - New Zealand Conference on Environmental Geotechnics, Melbourne, November 1997.
radiation are recorded continuously by an automatic
weather station installed on top of the test cell
(Figure 2). These data are required for the
containment cell water balance.
The authors gratefully acknowledge the enthusiastic
commitment of BFI (Browning-Ferris Industries
Inc.) in the Lyndhurst full-scale research project with
an aim to achieve a better landfill management.
4.6 Saturated Leachate Level
In each of the test section and control section, there
are two open wells and a leachate collection sump
where the saturated leachate level can be monitored
(Figure 2). The levels are measured manually by
using a portable water level sensor probe. From
these wells and sumps, leachate samples are also
collected for monthly analysis.
4.7 Volume of Leachate collected and Recycled
A clamp-on type portable transit-time ultrasonic
flowmeter is used to measure both flow volume and
flow rate during recirculation trials of each
individual well and trench. It is also used to calibrate
the volume of each of the header/storage tanks to
allow an accurate record of daily leachate
recirculation obtained by volumetric tank
measurement.
5. PROGRESS
The progress of this bioreactor experiment can be
summarised by the following significant events :
Date
Nov 93
Dec 93
Dec 95
Jan 96
Feb 96
Jul 96
Events
Completion of cell containment system
Commencement of MSW filling
Filling completed
Completion of final capping
Installation of Instrumentation
Leachate recirculation commenced
While the cell instrumentation is working as
planned, it is still too early to comment on its longterm performance in terms of reliability and quality
of data collected.
For the bioreactor landfill behaviour, only
limited data are available from the relatively short
monitoring period since recirculation started in July
1996. The collection of data will continue. For the
hydrological study, monitoring will be continued to
allow at least one seasonal cycle of data to be
collected. Results will be published as sufficient data
are collected and analysed.
REFERENCES
Bos, M. G., Replogle, J. A., & Clemens, A. J. (1984).
Flow Measuring Flumes For Open Channel Systems:
John Wiley and sons, New York, NY, USA.
Krol, A., Rudolph, V., & Swarbrick, G. (1994). Landfill :
A containment Facility or a Process Operation.
Paper presented at the 2nd National Hazard & Solid
Waste Convention, Melbourne.
Maurer, R. W. (1994). A Paradigm Shift from Storage to
Bioreactors. Paper presented at the Landfill
Tomorrow - Bioreactors or Storage, Centre for
Environmental Control & Waste Management,
Imperial College, London.
Waste Management Council (1995a). Main Report:
Waste Disposal Strategy for the Greater Melbourne
Area, Victoria, Australia.
Waste Management Council (1995b). Main Report:
Waste Minimisation Strategy for Metropolitan
Melbourne, Victoria, Australia.
Yuen, S. T. S., Styles, J. R., & McMahon, T. A. (1994).
Process-Based Landfills Achieved By Leachate
Recirculation - A Critical Review and Summary
Centre for Environmental Applied Hydrology
Report. University of Melbourne.
Yuen, S. T. S., & Styles, J. R. (1995). Use of Sand
Washing Slimes as a Landfill Liner Material.
Australian Civil Engineering Transactions, The
Institution of Engineers, Australia, Vol. CE37(No.3).
Yuen, S. T. S., Styles, J. R., McMahon, T. A., & Wang,
Q. J. (1997a). A Full-Scale Bioreactor Landfill Study
- Report on Test Cell Design & Instrumentation .
Centre for Environmental Applied Hydrology,
University of Melbourne.
Yuen, S. T. S., Wang, Q. J., Styles, J. R., & McMahon,
T. A. (1997b). The Role of Water in Landfills: A
Full-Scale Hydrological Study. Paper presented at
the AWWA 17th Federal Convention - Water in the
Balance, March 1997, Melbourne.
Yuen, S. T. S., Wang, Q. J., Styles, J. R., & McMahon,
T. A. (1997c). A Practical Approach to Monitoring
In-situ Moisture of Municipal Solid Waste in
Landfills. Paper to be presented at the Sardinia 97,
Sixth International Landfill Symposium, October
1997, Cagliari, Italy.
ACKNOWLEDGMENT
GeoEnvironment 97, 1st Australia - New Zealand Conference on Environmental Geotechnics, Melbourne, November 1997.
Plan
N
(N.T.S.)
Recirculation Section
Control Section
Legends :
Leachate Injection Well
Sub-surface
Infiltration Trench
Figure 1 - Leachate Recirculation System
To stormwater
discharge
Plan
(N.T.S.)
N
Control Section
Recirculation Section
Fall
Fall
Legends :
Neutron Probe Access Tube
Open Well
Flume
Temperature Probe
Leachate Collection Sump
Catch Pit
Settlement Plate
Surface Channel
Automatic Weather Station
Figure 2 - Test Cell Instrumentation
GeoEnvironment 97, 1st Australia - New Zealand Conference on Environmental Geotechnics, Melbourne, November 1997.