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CHAPTER
4EXPERIMENTAL SET-UP
4.1 Experimental Cell Design and Construction
4.2 Instrumentation andMonitoring Program
4.3 Leachate Recirculation Strategy
The engineering design, construction, supervision and instrumentation of the full-scale
experimental cell constituted a significant part of this thesis, which laid down the required
groundwork for the subsequent investigations. This chapter provides a detailed
description of the experimental cell including its instrumentation and the related
monitoring program. It also outlines and justifies the strategy of leachate recirculation
employed in the test.
4-1
Chapter 4
4.1 EXPERIMENTAL CELL DESIGN AND CONSTRUCTION
4.1.1 General
The experimental cell is located in Browning-Ferris Industries Inc. (BFI) ’s Lyndhurst
Sanitary Landfill site which is about 35km south-east of Melbourne city centre as
shown in the location plan (Figure 4.1).
The existing pit used for landfilling, in common with many landfill sites in the same
south-eastern sand belt region of Melbourne, 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,
which is at approximately 12m AHD (Australian Height Datum). The regional
hydraulic gradient is falling towards the west.
The site is at a latitude of 38.02o S. Historic climatic data from a meteorological station
located 15km north of the landfill reveal that the mean annual Class-A pan evaporation
(1227 mm) exceeds the mean annual rainfall (874 mm).
The cell commissioned for the full-scale experiment is Cell 3 at the north-western
corner of the site as shown in the aerial photograph (Plate 4.1 – Appendix A). Table 4.1
summarises the progress of the experiment up to December 1997.
4-2
Experimental Set-up
4-3
Chapter 4
Table 4.1 - Progress of Experiment
4.1.2 Size of Experimental Cell
The cell covers a footprint area of approximately 180m x 75m (about 1.4 hectares) as
shown in the as-constructed survey plan (Figure 4.2). This cell size is regarded as
typical compared with other operational cells in the same landfill.
The final capping level rises from +15m AHD at the north-western corner up to +20m
AHD at the south-eastern corner as the landscape slopes up south-easterly (Figure 4.2).
The surface of the base liner is at +4m AHD. The thickness of fill thus varies from 10m
to 15m (excluding the 1m final capping).
Based on survey data, the as-constructed volume of the experimental cell is 180,400 m3.
Total tonnage of MSW as recorded by weighbridge is 100,800 tonnes. The filling of the
cell took about two years to complete (Table 4.1).
In plan, the cell is divided into two sections of roughly equal areas. The western half
has been designated as the control section for dry landfilling and the eastern half as the
test section for leachate recirculation (Figure 4.2).
Plate 4.2 (Appendix A) shows a general view of the completed experimental cell taken
from the north-western corner.
4-4
Experimental Set-up
4-5
Chapter 4
4.1.3 Waste Composition
As the composition of waste is one of the most vital factors that influence
biodegradation and hence the enhancement strategy, it is important to be able to
quantify and qualify the types of waste being investigated in the experiment. This
information would also be essential to allow any findings to be cross-referenced with
other similar studies.
Although the Lyndhurst Landfill is also licensed to accept prescribed wastes (as defined
in SEPP 1991) and contaminated soil, for the purpose of this bioreactor experiment,
only domestic garbage and non-hazardous/ non-toxic waste from the commercial/
industrial waste stream were used in the experimental cell.
A record of all the wastes placed in the experimental cell was kept according to their
waste streams. It is shown graphically in Figure 4.3. The ratio of domestic to
commercial/ industrial waste stream is 1:1.6. The filling thus constitutes a higher
proportion of commercial/ industrial waste as this forms the bulk of the waste source of
Lyndhurst Landfill.
Figure 4.3 – Record of Waste Disposed According to Waste Streams
4-6
0
10,000
20,000
30,000
40,000
50,000
60,000
70,000
Dec-93 Mar-94 Jun-94 Sep-94 Dec-94 Mar-95 Jun-95 Sep-95 Dec-95
Cum
mul
ativ
e To
nnag
e
Domestc Waste Stream
Commercial & IndustrialWaste Stream
Experimental Set-up
In terms of composition, both domestic waste and industrial/ commercial waste streams
from metropolitan Melbourne have been studied recently (Waste Management Council,
1995). Their compositions according to the survey are listed in Table 4.2. These figures
should provide a close representation of the wastes disposed in the experimental cell.
After being weighted according to the above 1:1.6 ratio, the overall waste composition
was estimated. It is shown in Figure 4.4. Note that the percentages are expressed in
terms of wet mass.
Table 4.2 - Compositions of Waste from Metropolitan
Melbourne (WMC, 1995)
Components Domestic Waste (%) Commercial/ Industrial Waste (%)Residual 20% 10%Plastics 6% 12%Garden 16% 5%Kitchen/Food 25% 17%Paper 19% 22%Metals 3% 5%Textiles 2% 10%Inert 3% 4%Glass 6% -Domestic - 2%Timber/Wood - 11%Others - 2%Total 100% 100%
Figure 4.4 Figure 4.5
4-7
Inert (building /daily covers /
residual)47%
Paper12%
Plastic9%
Garden/food23%
Glass1%
Metal3%Textlle
5%
Waste Composition Based on Test Cell Samples
(By Dry Mass)
Inert + Residual
18%
Paper21%
Plastic10%
Garden / Food / Timber
36%
Textile7%
Metal4%
Glass4%
Waste Composition Based on Waste streams & WMC 1995 Data (By Wet Mass)
Chapter 4
Additional waste composition information was also determined by collecting continuous
waste samples from seven augered holes immediately after final capping. These holes
were drilled to install access tubes for subsequent moisture monitoring (refer Section
4.2.1 for locations and details). The samples were dried to determine their gravimetric
moisture (refer Section 4.1.4) prior to sorting. The composition as sorted is presented in
Figure 4.5. In contrast to Figure 4.4, it is expressed in terms of dry mass.
It is impossible to make a direct comparison between the compositions in Figure 4.4
and Figure 4.5, as the moisture contents of individual components are not available.
Nevertheless, by assuming a reasonable moisture content for each component, the two
compositions tend to agree reasonably well. The dry mass composition is always pre-
ferred as it is independent of moisture content.
As the control and test sections were filled up simultaneously and the ratio of domestic
waste to commercial/ industrial waste was maintained fairly consistently (Figure 4.3),
the composition of waste in the experimental cell (in macroscopic scale) can be
considered to be reasonably uniform. For the same reason, the wastes in the control and
test sections can be treated as identical, at least within the context of this experiment.
4.1.4 Waste Moisture Content
The variations of moisture content (immediately after final capping) with depth for the
seven sampling holes mentioned above are plotted in Figure 4.6. The moisture content
range and distribution of all the samples are plotted in Figure 4.7. The volumetric
values in the figures were calculated based on a bulk density of 0.83 tonne/ m3 (refer
Section 4.1.6).
The results of the moisture content analysis are summarised in Table 4.3. Thus, for the
purpose of the water balance analysis in Chapter 6, the mean value of 27% (volumetric)
is taken to be the overall as-capped moisture content of the waste.
4-8
Experimental Set-up
Figure 4.6 – Variation of Moisture Content with Depth in the Seven Sampling Holes:(a) By Dry Mass; (b) By Wet Mass; (c) By Volume
4-9
(a) As-capped Waste Moisture Content with Depth - by dry mass
0%
50%
100%
150%
200%1
to 2
m
2 to
3 m
3 to
4 m
4 to
5 m
5 to
6 m
6 to
7 m
7 to
8 m
8 to
9 m
9 to
10
m
10 to
11
m
11 to
12
m
Depth
Moi
stur
e C
onte
nt AT1
AT2
AT3
AT4
AT5
AC1
AC2
(b) As-capped Waste Moisture Content with Depth - by wet mass
0%
10%
20%
30%
40%
50%
60%
70%
1 to
2 m
2 to
3 m
3 to
4 m
4 to
5 m
5 to
6 m
6 to
7 m
7 to
8 m
8 to
9 m
9 to
10
m
10 to
11
m
11 to
12
m
Depth
Moi
stur
e C
onte
nt AT1
AT2
AT3
AT4
AT5
AC1
AC2
(c) As-capped Waste Moisture Content with Depth - by volume
0%
10%
20%
30%
40%
50%
60%
1 to 2
m
2 to 3
m
3 to 4
m
4 to 5
m
5 to 6
m
6 to 7
m
7 to 8
m
8 to 9
m
9 to 1
0 m
10 to
11 m
11 to
12 m
Depth
Moi
stur
e C
onte
nt AT1
AT2
AT3
AT4
AT5
AC1
AC2
Chapter 4
Figure 4.7 – Moisture Content Range and Distribution of MSW:(a) By Dry Mass; (b) By Wet Mass; (c) By Volume
4-10
(b) Moisture Content Distribution- by wet mass
0%
10%
20%
30%
40%
10% -2
0%
20%-30
%
30%-40
%
40%-50
%
50%-60
%
60% to
70%
Moisture Content Range
Rela
tive
frequ
ency
(c) Moisture Content Distribution- by volume
0%
10%
20%
30%
40%
50%
< 15%
15%-25
%
25%-35
%
35%-45
%>45
%
Moisture Content Range
Rela
tive
frequ
ency
(a) Moisture Content Distribution- by dry mass
0%
10%
20%
30%
40%
50%
0 - 20
%
20 - 4
0%
40 - 6
0%
60 - 8
0%
80 - 1
00%
100 -
120%
120 -
140%
140 -
160%
160 -
180%
Moisture Content Range
Rela
tive
frequ
ency
Experimental Set-up
Table 4.3 - Results of Waste Moisture Content AnalysisMoisture Content (%)
By dry mass By wet mass By volumeMaximum 173 63 53Minimum 15 13 11Mean 55 32 27Std. Deviation 38 13 11
4.1.5 Daily/ Interim Covers
Due to licensing requirements as well as operational needs to control litter, birds and
odour during filling, daily cover was used in a manner similar to other operational cells.
The licence requires a 150mm layer of earth material as daily cover during waste
disposal. In addition, each completed vertical lift (of 2m) should be covered by an
interim cover of 300mm earth material. This requirement was applied to the
experimental cell.
In common with other sand-pit landfills in the region, semi-dry to dry slimes were used
for daily and interim covers in the experimental cell. The slimes were generally a
clayey sandy silt material left behind from previous sand washing (Yuen and Styles,
1995). They 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. This would allow a reasonable permeability, which is desirable for
moisture movement induced by later leachate recirculation.
To reduce the barrier effects of the daily and interim covers for later recirculation,
permeability was improved by stripping and mixing the earth material with waste
before placing the next lift.
Figure 4.8 provides a record of all cover material (excluding final capping) placed in
the experimental cell. The cumulative waste volume is also plotted. From the figure, it
can be deduced that the cover material occupies about 15% of the total waste volume.
4-11
Chapter 4
This figure compares well with the amount as stipulated by the licence requirement
which is 17% as calculated based on the above configuration.
Figure 4.8 – Record of Cover Material against Total waste Volume
4.1.6 Density and Porosity of Waste
The waste was compacted in vertical layers by a Caterpillar 826C landfill compactor
with an operating weight of 32 tonnes. It is the same compaction method employed in
other operational cells.
Table 4.4 lists all the weight and volume components and the procedure used to
calculate the in-situ density of waste. The bulk density, accounting also for the
daily/interim earth covers, is 0.83 tonne/m3.
Based on this density and the bulk moisture content (55% by dry mass) determined in
Section 4.1.4, the dry density and porosity of the MSW was calculated, which are 0.54
tonne/m3 and 0.55 respectively. The literature suggests a porosity range between 0.5 to
0.6 (e.g. Korfiatis et al., 1984; Oweis et al., 1990; Zeiss and Major, 1992). In this case
0.55 reflects that the MSW in the experimental cell is reasonably well-compacted.Table 4.4 - Calculation of in-situ waste densityItem Description Volume Weight Density
4-12
0
20,000
40,000
60,000
80,000
100,000
120,000
140,000
160,000
Dec-93 Mar-94 Jun-94 Sep-94 Dec-94 Mar-95 Jun-95 Sep-95 Dec-95
Cum
mul
ativ
e Vo
lum
e (m
3) MSW
Daily / Interim Covers
Experimental Set-up
(m3) (tonne) (tonne/m3)a Volume between top of liner and top of capping 180,400
b Volume of 1m thick final cap 13,500
c Volume of daily/interim covers 24,750
d Volume of leachate collection drainage layer 4,160
e Net volume of MSW ( = a - b - c - d ) 137,950
f Total MSW weight as recorded by weighbridge 100,820
g Total weight of daily interim covers as recorded 34,650
h As placed in-situ density of MSW ( = f / e)(excluding daily/interim covers, drainage & cap) 0.73
i As placed in-situ density of combined MSW & cover material (= [f + g] / [a-b-d] ) 0.83
4.1.7 Cell Containment System
In common with all other operational cells, the experimental cell is lined with a 1m
thick side and base clay liner. The compacted clay has a specified hydraulic
conductivity of less than 10-9 m/s. The construction of the liner was subject to a quality
assurance program as required by the licence.
Figure 4.9 – Details of Final Capping
4-13
Top soil
Sand
Waste
Compacted Clay
Grass
300mm200mm500mm
Chapter 4
Immediately upon completion of filling, a final capping was laid, the details of which
are shown in Figure 4.9. The capping falls gently on a 1 on 7.5 gradient towards its
north-western corner (refer Figure 4.2).
4.1.8 Leachate Collection System
A more efficient leachate drainage system different from other operational cells is used.
It aims to provide a better control of the hydraulic head on the base liner during
leachate recirculation. The system comprises a 300mm thick drainage layer of 20/40mm
basalt gravel immediately above the liner, with 90mm diameter slotted collector pipes
installed at 15m centres. These then drain to a 150mm diameter header pipe falling at a
0.16% gradient into a leachate collection sump (Figure 4.10).
To enable both leachate quantity and quality from the control and test sections to be
monitored separately, each of the two sections has its own collection system. The two
systems are isolated by a compacted clay bund wall constructed on top of the base liner
as shown in Figure 4.10.
Figure 4.10 – Schematic Plan showing Leachate Collection System
4-14
Collector drain
15 m
CONTROL AREA RECIRCULATION AREA
Leachatesump
Header pipe
Compacted clay bermN
( N.T.S. )
Experimental Set-up
4.1.9 Gas Extraction System
An active extraction system employing a small suction is used to collect gas. To enable
both flow rate and composition in the control and test sections to be monitored
independently, two isolated gas fields (Figure 4.11) are employed. Each field comprises
nine collection wells spaced approximately 25m apart connecting to a manifold station
(Plate 4.3 in Appendix A). As the amount of gas that can be extracted from each well
may vary, each well has a separate control valve for vacuum adjustment at the manifold
station.
The details of the gas wells in the control section are shown in Figure 4.12. For the test
section, the wells are designed also for leachate injection (see also Section 4.1.10).
Construction details are shown in Figure 4.13.
Further discussion on the gas collection system is provided in Section 8.2.
Figure 4.11 – Schematic Plan Showing Gas Extraction Wells and Combined Leachate injection/ Gas Extraction Wells
4-15
Legends :
Combined Leachate Sub-surface injection/ Gas Collection Well Infiltration Trench
for RecirculationGas Collection Well
057
053
Control Area Recirculation AreaN
(N.T.S.)Plan
050 049 048
051052053
054055056
057 059058
062061060
065064063
Chapter 4
Figure 4.12 – Details of Gas Extraction Wells
Figure 4.13 – Details of Combined Gas Extraction/
4-16
G.L.
To gas manifold
63mm dia.HDPE pipe
450 mm dia.borehole
45mmgravel pack
3.5m
plu
g(c
ompa
cted
cla
y)
4m to top of liner
Perf
orat
ed p
ipe
sect
ion
0.5m
Suction
G.L.
Perf
orat
ed p
ipe
sect
ion
0.5m
To gas manifold
63mm dia.HDPE pipe
450 mm dia.borehole
45mmgravel pack
3.5m
plu
g(c
ompa
cted
cla
y)
4m to top of liner
Suction
From header tank Leachate
Experimental Set-up
Leachate Injection Well4.1.10 Leachate Recirculation System
A wide range of recirculation options was considered. They included surface irrigation,
surface ponding, buried drip irrigation tubing, in-ground piping grid, sub-surface
infiltration trench/field, and vertical recharge well.
With potential problems such as odour control and wind-blown misting, surface
application of leachate by spray irrigation or ponding was ruled out at an early stage.
The use of buried drip irrigation tubing or an in-ground piping grid was also excluded.
This decision was based on previous trials which indicated that there are numerous
difficulties in maintaining the connection of the system as the landfill subsides.
A combination of sub-surface horizontal infiltration trenches and deep vertical injection
wells was finally selected for this experiment. Both devices were constructed after final
capping to avoid interruption to waste filling. The integrated system is shown schemat-
ically in Figure 4.14. The design, including the sizing and spacing of wells and
trenches, was based on a numerical model simulation as discussed later in Section 7.2.1.
The details of a sub-surface horizontal infiltration trench are shown in Figure 4.15.
There are eight infiltration trenches (RT1 to RT8) strategically placed between each
pair of wells (Figure 4.14).
As described previously in Section 4.9 (Figure 4.13), the nine leachate injection wells
(RW1 to RW9 in Figure 4.14) also serve to extract gas in the test section.
Leachate is pumped from the collection sump (Plate 4.4 in Appendix A) into three
storage/header tanks (Plate 4.5) 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
(Plate 4.6). The system has been designed to allow flexibility to inject either an
individual well/trench or a group of selected wells/trenches.
4-17
Chapter 4
Figure 4.14 – Integrated Leachate Recirculation System
4-18
A c c e s s R o a d
B a s e l i n e r
C a p p i n g
L e a c h a t e I n j e c t i o n /G a s C o l l e c t i o n w e l l
S u b - s u r f a c e I n f i l t r a t i o n T r e n c h
F e e d e r p i p e
S e c t i o n A - A( N . T . S . )
C O N T R O L A R E A R E C I R C U L A T I O N A R E A
N
( N . T . S . )
L e g e n d s :
L e a c h a t e I n j e c t i o n /G a s C o l l e c t i o n W e l l
S u b - s u r f a c eI n f i l t r a t i o n T r e n c h
F e e d e r P i p e
A
AP l a n
R W 1
R W 9R W 8R W 7
R W 6R W 5R W 4
R W 3R W 2
R T 1
R T 2
RT
3R
T5
RT
6R
T4
RT
7
RT
8
Experimental Set-up
Figure 4.15 – Details of Sub-Surface Horizontal Infiltration Trench
4-19
G.L.
Final Capping
Feeder pipe
Leachet from header tank
90mm dia. slottedPVC pipe
600mm
600m
m
45mm gravel pack
(N.T.S.)
Waste
Chapter 4
4.2 INSTRUMENTATION AND MONITORING PROGRAM
The design of the right instrumentation is crucial to enable the collection of reliable
data for later analysis. The following factors and constraints have been taken into
consideration in the design: costs (both capital and running costs), compatibility with
the landfill environment, reliability, and simplicity.
Table 4.5 lists all the items that are being monitored in the study with the corresponding
method/ instrumentation employed. Figure 4.16 shows the location of instrumentation
schematically in plan. It can be divided into four monitoring groups: Control A, Control
B, Test A and Test B.
Table 4.5 - Instrumentation/ Method Employed To Collect Data
Items Required Monitoring
Instrumentation/ Method
Waste Moisture Distribution Profile
Using neutron probe to measure moisture changes via in-situ access tubes
Climatic Data Automatic weather station
Surface Runoff Collection by surface channels & measurement by flume with water level auto-logger
Landfill Settlement Level survey on settlement plates
Waste Temperature Stainless steel sheathed thermocouples
Leachate Level Measure leachate levels in the sumps and open wells by a water level
sensor
Leachate Volume Combining the use of an ultrasonic flowmeter and tank measurement
Leachate Quality Collect and test leachate samples from both leachate sumps and open
wells
Landfill Gas Composition Portable non-dispersive infra-red absorption landfill gas analyser. Portable gas chromatograph (GC)
Landfill Gas Flow Rate Pre-calibrated orifice plate
Groundwater Quality Collect and analyse groundwater samples from adjacent monitoring bores
4-20
Experimental Set-up
Figure 4.16 – Location Plan Showing Instrumentation of Experimental Cell
4.2.1 In-situ Municipal Solid Waste Moisture Monitoring
Based on a parallel investigation as described later in Chapter 5, the combined use of a
neutron probe and in-situ access tubes has been identified as a practicable tool for mon-
itoring moisture of in-situ MSW. Seven in-situ access tubes each of 12m long have
been installed as shown in Figure 4.17. They are used to monitor: seasonal moisture
change in the control section (AC1 and AC2); moisture change adjacent to a recircula-
tion well (AT1 to AT3); and moisture change adjacent to a trench in the test section
(AT4 and AT5). The installation details of the access tubes are described in Section
5.5.1.
4-21
(N.T.S.)PlanLegends :
Neutron Probe Access Tube Open Well Flume
Temperature Probe Leachate Collection Sump Catch Pit
Settlement Plate Surface Channel Automatic Weather Station
Control Section Recirculation SectionN
FallFall
To stormwaterdischarge
Control A Control BTest A
Test B
Chapter 4
4-22
Experimental Set-up
Figure 4.17 – In-situ Access Tubes for Monitoring Moisture Changes
4-23
C o n t r o l A r e a R e c i r c u l a t i o n A r e a
N
( N . T . S . )P l a nL e g e n d s :
C o m b i n e d L e a c h a t e S u b - S u r f a c e I n f i l t a r t i o n T r e n c h i n j e c t i o n / G a s C o l l e c t i o n W e l lG a s C o l l e c t i o n W e l l N e u t r o n P r o b e A c c e s s T u b e
A A
B BA C 1 A C 2
A T 1A T 2A T 3
A T 5 A T 4
Neutron Probe Access Tubes
SECTION A - A (N.T.S)
AT1AT2
3 m
Liner
Final CappingAT3
3 m
Deep Injection WellRW5
3 m
Chapter 4
4.2.2 Climatic Data
An automatic weather station is installed on top of the experimental cell (Figure 4.16
and Plate 4.7). It is equipped with a data logger and the following sensors (with meas-
urement height and instrument accuracy in bracket):
Rain gauge (0.3m; 0.2mm tipping bucket)
Air temperature sensor (1.4m; +0.2o )
Relative humidity sensor (1.4m; +5%)
Anemometer (2.0m; +0.1 km/hr)
Solar radiation sensor (2.0m; +0.1 kJ/m2)
4.2.3 Surface Runoff
All runoff from the experimental cell catchment is collected by two surface channels
running along the north and north-western edges as show in Figure 4.16. Through two
catch pits, all flow is then diverted to a single main channel. A flume equipped with
two water level probes (one as backup) and a real-time logger is installed in-line with
the main channel to measure flow rate (Plate 4.8).
The flume belongs to the RBC (Replogle, Bos and Clemmens) family of long throated
flumes (Bos et al., 1984) which are designed for measurement of flow in open channels
with minimum head loss and reasonable sediment and debris passage without affecting
performance. They allow accurate measurement of head loss at the sill, ensure that crit-
ical flow occurs in the throat, and give an accuracy of about 2% over the range of meas-
ured discharge. They require only one measurement of flow depth at a point upstream
of the sill, which simplifies the operation compared to other types of flume. Their full
hydraulic design details are given by Bos et al. (1984).
4-24
Experimental Set-up
The flume is of a trapezoidal cross-section with dimensions as shown in Figure 4.18. It
has a maximum flow capacity of 80 litres/sec. It has been calibrated in a hydraulic
laboratory channel (Plate 4.9) and the calibrated curve in Figure 4.19 is used to relate
measured flow depth data to flow rate. The water level is scanned every 30 seconds and
recorded if it varies by more than 3mm from the previous measurement.
Figure 4.18 – Details of RBC Long Throated Flume Employed in Experimental Cell
4-25
700mm 300mm 300mm 300mm
Approach Transition Throat Exit
225mm100mm225mm
350mm
100mm
A
ALongitudinal Section Section A-A
Flow
0
50
100
150
200
250
300
0 10 20 30 40 50 60 70 80 90
Discharge (l/s)
Stag
e He
ight
(m
m)
First Calibration Points
Second Calibration Points
Chapter 4
Figure 4.19 – Flume Calibration Curve Relating Stage Height and Flow Rate
4.2.4 Landfill Settlement
Figure 4.20 shows in plan three groups of settlement monitoring plates, two in the
control section (Control A and Control B) and one in the test section (Test A). Each
group is composed of a series of five settlement plates installed approximately at +4m
AHD (i.e. surface of liner), +6m AHD, +8m AHD, +10m AHD, and +18m AHD (i.e.
ground level) respectively. Instead of just monitoring the subsidence at surface level,
the additional settlement data collected at various levels would provide some additional
information on subsidence behaviour along a vertical profile.
Construction details of a typical in-ground settlement plate are given in Figure 4.21.
The plates were pre-placed on the nominated levels during filling. Upon completion of
the cell, they were relocated in plan by a positioning survey. Boreholes were then sunk
to provide access for the installation of connection rod and sleeve. The level of each
plate is monitored by surveying the top level of the connection rod.
Originally two groups of settlement plates were pre-placed in the test section.
Unfortunately one group was placed too close to the south-eastern corner. Following
cell completion they are located underneath a slope (refer to Figure 4.2), and it is
virtually impossible to mobilise a machine there to bore the required holes.
Subsequently this group of plates was abandoned, which resulted in only one group in
the test section.
4.2.5 In-situ Waste Temperature
Four groups of temperature probes were installed as shown in Figure 4.22 (Control A
and Control B, Test A and Test B). Each group comprises three probes aligned along a
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Experimental Set-up
vertical profile (at 3m below ground surface, at mid-depth, and at 3m above liner
surface) in order to delineate any temperature variation with depth.
Figure 4.20 – Location Plan Showing Settlement Plates
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( N . T . S . )P l a n
C o n t r o l A r e a R e c i r c u l a t i o n A r e aN
S P 1 S P 2 S P 3 S P 4 S M 1 S P 5 S P 6 S P 7 S P 8 S M 2
S P 9 S P 1 0 S P 1 1 S P 1 2 S M 3
C o n t r o l A : C o n t r o l B :
T e s t A :
L e v e l C o n t r o l A C o n t r o l B T e s t A ( a p p r o x i m a t e )S P 1 S P 5 S P 9 4 m A H D ( l i n e r s u r f a c e )S P 2 S P 6 S P 1 0 6 m A H DS P 3 S P 7 S P 1 1 8 m A H DS P 4 S P 8 S P 1 2 1 0 m A H DS M 1 S M 2 S M 3 1 8 m A H D ( g r o u n d s u r f a c e )
S e t t l e m e n t P l a t e G r o u p
Chapter 4
Figure 4.21 – Details of Settlement Plate
Figure 4.22 – Location Plan Showing Temperature Probes
Each probe is composed of a stainless steel sheathed type K thermocouple. The buried
thermocouple is connected to a surface terminal plug via a special compensating cable
protected by a sealed duct. As each probe is subject to a potentially corrosive
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A A
1200 mm
1200
mm
Plan(N.T.S.)
Waste
SECTION A-A
G.L.
12 mm steel plate (pre-placed during filling) Base Level
50mm PVCsleeve
20mm galvanisedsteel pipe
(N.T.S.)
450mm dia.drillhole
Gravel backfill
Cap
C o n t r o l A r e a R e c i r c u l a t i o n A r e aN
T P 7 T P 8 T P 9
T P 1 0 T P 1 1 T P 1 2
T P 1 T P 2 T P 3 T P 4 T P 5 T P 6
( N . T . S . )P l a n
C o n t r o l A : C o n t r o l B :
T e s t A :
T e s t B :
L e v e l C o n t r o l A C o n t r o l B T e s t A T e s t BT P 1 T P 4 T P 7 T P 1 0 A t 3 m b e l o w g r o u n d s u r f a c eT P 2 T P 5 T P 8 T P 1 1 A t m i d - d e p t hT P 3 T P 6 T P 9 T P 1 2 A t 3 m a b o v e l i n e r s u r f a c e
T e m p e r a t u r e P r o b e G r o u p
Experimental Set-up
environment, only the sensor tip of the thermocouple is exposed in the waste (Plate
4.10).
Using a compatible portable digital thermometer (Fluke model 51), temperatures are
read from the thermocouples manually by connecting to the surface terminal plugs.
4.2.6 Saturated Leachate Level/ Leachate Sampling
There are two open wells and a leachate collection sump in each of the two sections
where the saturated leachate level can be monitored (Figure 4.23). From these wells and
sumps, leachate samples are also collected for monthly analysis.
Each open well was installed down to the liner surface and is constructed of a 50mm
continuously perforated PVC pipe surrounded with gravel packing. Leachate levels in
the sumps and wells are measured manually by using a portable water level sensor
probe which has an instrument accuracy of + 10mm.
Figure 4.23 - Location Plan Showing Leachate Sumps and Open Wells
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Control Area Recirculation Area
N
(N.T.S.)Plan
Sump 3A Sump 3B
Open Well OW1
Open Well OW4
Open Well OW3
Open WellOW2
Chapter 4
4.2.7 Volume of Leachate Collected and Recirculated
A clamp-on type portable transit-time ultrasonic flowmeter (Plate 4.11) is used to
measure both flow volume and flow rate during recirculation trials of individual wells
and trenches. 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.
4.2.8 Landfill Gas Composition and Flow Rate
The flow rate of an individual well is measured through a pre-calibrated orifice plate
located at the manifold station (see Plate 4.12). Methane and oxygen concentrations are
routinely monitored by a portable non-dispersive infra-red absorption gas analyser
(ADC model LFG 20, see Plate 4.13). A portable gas chromatograph (GC) with a
micro-thermal conductivity detector is also used to provide more accurate
measurements and to detect other gases including nitrogen (refer Plate 4.14). Gas
sampling points at individual well heads are also provided.
4.2.9 Groundwater Quality
It is important in this study to demonstrate that the groundwater system is not signific-
antly affected by the recirculation operation. As part of the overall landfill monitoring
program, routine groundwater quality analysis is being conducted by an independent ac-
credited laboratory.
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Experimental Set-up
A total of eight observation bores were installed in and around the periphery of the
landfill site. They are shown in Figure 4.24. BH8 and BH9 are the two closest
observation bores most relevant to the experimental cell. As the groundwater hydraulic
gradient is falling gently in a westerly direction, BH9 can be taken to represent the up-
gradient groundwater and BH8 the down-gradient.
Up to the end of 1997, the experimental cell was the only landfilling activity conducted
at the north-western corner of the site. Hence the groundwater data obtained from both
BH8 and BH9 can be considered to be unbiased of any adjacent landfilling effects.
Groundwater samples have been taken from both bores at three-monthly intervals since
January 1995. The water level and in-situ groundwater temperature are also measured.
The water samples are tested for the following parameters:
(a) Physical indicators – pH, alkalinity, conductivity, TDS, redox potential
(b) Organic indicators – COD, TOC
(c) Nitrogen – nitrate, total Kjeldahl nitrogen, ammonia
(d) Sulphate
(e) Chloride
(f) Phosphate
(g) Cyanide
(h) Phenolic substances
(i) Azure A
(j) Metal and heavy metals – Ca, Fe, Mg, Cu, K, Na, Pb, Hg, Cr, Ca, Se.
The results of the independent groundwater monitoring from January 1995 to December
1997 are presented in Appendix K.
4.2.10 Monitoring Program
The scheduled frequency of monitoring for each item is shown in Table 4.6.
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Chapter 4
Table 4.6 - Monitoring ScheduleItems Required Monitoring Frequency
Waste Moisture Distribution Profile Test section: during recirculation trials
Control section: quarterlyClimatic Data Continuous
Surface Runoff Continuous
Landfill Settlement Monthly
Waste Temperature Monthly
Leachate Level Monthly
Leachate Volume Daily
Leachate Quality Monthly
Landfill Gas Composition Bi-monthly initially then monthly
Landfill Gas Flow Rate Bi-monthly initially then monthly
Groundwater Quality and Level Quarterly
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Experimental Set-up
Figure 4.24 – Location of Groundwater Monitoring Bores
4.3 LEACHATE RECIRCULATION STRATEGY
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Chapter 4
The leachate recirculation strategy adopted in this experiment has taken the following
factors and limitations into consideration:
(i) The leachate produced in other operational cells, which also accept
contaminated soils and hazardous industrial wastes, commonly contains high
concentrations of heavy metals and toxic chemicals. This source is considered
not suitable for recirculation as it may inhibit microbial growth. For this reason,
only leachate collected from within the experimental cell was used for
recirculation. Hence, there was a limited supply of suitable leachate for
recirculation. As the leachate from within the cell dried up in the recirculation,
stormwater was then injected to provide the required moisture.
(ii) This experiment did not consider alternative concepts such as the sequential
batch bioreactor employing the exchange of leachate between an acidogenic cell
and a methanogenic cell (e.g. Chugh 1996), and the high rate flushing bioreactor
to remove nitrogen, inorganic salts and the residual organic carbon (e.g. Walker
et al. 1997).
(iii) While small-scale experiments have suggested that it is desirable to have a high
moisture content and flow to encourage leachate contact (Section 2.3.3 (ix)), a
full-scale recirculation of this kind may create practical problems. As leachate
may accumulate and the pooling of leachate within the containment system
would impose an excessive head on the liner if the leachate is not removed
quickly enough. This sort of hydrological condition is undesirable. Hence in a
real landfill, there is a balance to be drawn between biodegradation requirements
and the hydrological capacity of the cell.
(iv) It is well recognised that excessive moisture at the initial stage of landfill
development would intensify acid production that may “sour” the system and
hinder the onset of methanogenesis (Section 2.3.3(i)). Because no external pH
buffer was used in the experiment (as explained in Section 3.1), any excessive
moisture introduced to the experimental cell could be undesirable.
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Experimental Set-up
In order to avoid any potential inhibitory effects on the overall process, and to prevent
overloading the hydrological capacity of the containment system, this experiment has
adopted the strategy of recirculating leachate gradually.
The leachate intake rate of each injection device varies substantially due largely to the
heterogeneity of waste (refer Section 7.2.2 (iii)). To avoid an uneven delivery and
distribution, the feeding of a group of devices simultaneously from the header tanks
was discouraged. Instead, every trench and well was fed separately in turn. Thus a
routine recirculation involved firstly the pumping of leachate from the sump to fill up
the header tanks, and then the adjustment of the valve system to deliver leachate via
gravity to feed a single trench or well until the tanks were drained. In this way, the
volume injected into each device could be measured.
This practice of intermittent injection to each device also provides another advantage. It
allows sufficient time for both saturated and unsaturated flow to propagate from the
device into the surrounding waste to wet the potential zone of influence without
overloading the leachate collection system and liner. As there are eight trenches and
nine wells, this strategy allows sufficient time for each individual device to “recover”
before the next injection round.
There were three main stages of recirculation. Initially, leachate was collected from
within the test section sump and recirculated. After drying up the test sump, leachate
was then pumped from the control section into the test section. When all leachate from
both sections was exhausted, external storm water was then introduced. A total of 2.5
million litres of leachate was injected through the recirculation system during the period
from July 1996 to October 1997. The progress and volume of recirculation are
discussed further under the water balance study in Section 6.3.4.
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