5th Australian Landfill & Transfer Stations Conference
HOW TO AVOID A REPEAT OF
THE STEVENSON'S ROAD
DEBACLE
August 2013
Dr Robert Eden
Organics Ltd
Sovereign Court II, University of Warwick Science Park,
Coventry, England.
T: +44 24 7669 2141 F: +44 24 7669 2238
W: www.organics.com E:[email protected]
5th Australian Landfill & Transfer Stations Conference
HOW TO AVOID A REPEAT OF THE STEVENSON'S ROAD DEBACLE
Dr Robert Eden
Organics Ltd, Sovereign Court II, University of Warwick Science Park,
Coventry / England
ABSTRACT
The Stevenson’s Road landfill site is well known in the industry for the difficulties caused to
the stakeholders involved in the site by the migration of landfill gas into an adjacent property,
where a housing estate is located. The current paper will discuss the issue of landfill gas
migration and how it may be reliably halted.
Organics has worked on several hundred landfill sites to control landfill gas migration, mainly
in the UK but also in many other countries around the world. The first site the author was
involved with was the Loscoe landfill site in the UK, where an explosion in a domestic
property in 1986 was the key event which signalled the commencement of the UK landfill gas
control industry.1 In the current paper the basic rules for landfill gas migration control will be
presented, together with selected case studies.
Experience has taught that even in the most difficult of situations, landfill gas migration may
be reliably prevented. The nature of the geology and hydrogeology, as well as the hazards
generated in possible migration events, determine the extent to which various measures
should be implemented.
KEYWORDS
Landfill, landfill gas, migration, migration control, explosive gases
INTRODUCTION
The Stevenson’s Road landfill ceased taking waste on the 24th June 2005. After that it was put
into an aftercare phase of operation, with the installation of leachate and gas collection works.
In March 2006 it was confirmed that landfill gas was migrating from the site into the Brookland
Greens Estate. On the 31st August 2008 methane gas was found to a concentration of 63% in a
property on the housing estate. On the 9th September 2008 emergency management
arrangements were implemented based on advice from the EPA indicating imminent danger to
residents. The Country Fire Authority led the emergency response, which involved forty-five
relocations.2
The circumstances surrounding these events, and their consequences, remain the subject of legal
proceedings. It is, therefore, not the remit of this paper to enter into the territory of causes and
responsibility, which is probably considerably more challenging than the subject matter under
review here. The subject of landfill gas migration, and the accepted practices for controlling
such migration, will be discussed in the following text.
It is a simple fact that there are a great many housing estates around the world located adjacent
to landfill sites. The Loscoe landfill site in Derbyshire, England, actually had a property explode
as a result of landfill gas migration. However, in all such instances the migration of landfill gas
has effectively been halted. The ability to prevent migration was encapsulated in the UK by
legislation which penalised managers who failed to take the necessary steps to prevent
migration of landfill gas offsite.3 Monitoring regimes were instituted for detecting migration, by
means of monitoring boreholes, and emergency response plans prepared to counter occurrences
of landfill gas migration.4 Some particularly difficult situations needed to be addressed.
Saturated sites, sites with perched water tables, sites abutting deep, fractured rock faces with no
lining and houses located at the top of the rock face; all of the worst possible combinations had
to be attended to.
By way of contrast, the Stevenson’s road site is relatively straightforward. It has permeable and
fractured-rock strata abutting the waste. There are water tables passing through the site.
However, the principles referred to in the following text may readily be implemented into such
sites.
LANDFILL GAS MIGRATION
To understand the methods employed to control landfill gas migration it is useful to compare
the differences between landfill gas utilisation and landfill gas migration prevention. Whilst
there is a common gas and a common source, these two main design objectives of landfill gas
extraction systems are completely different. One is to secure the use of landfill gas for
commercial return, the other is to control landfill gas and ensure it does not migrate outside of
the landfill site. As will be seen, these varying objectives do not often provide for a singular
operating procedure. Landfill gas for commercial use must have a high-methane
concentration. Anything less than approximately 40% methane may be considered as
unusable. Landfill gas for migration control is often, though not always, low, or very low, in
methane concentration, making the latter generally unsuitable for use and often unsuitable for
even combustion. This latter is an important point which will be referred to in greater detail
later in the text.
What is a gas extraction system?
A gas extraction system is an arrangement of wells and pipework installed into a landfill site
and designed either to draw gas consisting of a large proportion of methane into a gas-use
facility, or to prevent such gases from migrating outside the perimeter of the landfill site.
The gas extraction system, otherwise known as a gas field, may vary enormously in size from
a few simple wells located around the sensitive perimeter of a small landfill site to hundreds
of wells arranged around both the perimeter and through the central body of waste contained
within the landfill site. Wells are drilled from the surface through the waste to either the
bottom of the previous void or to within a few meters of its maximum depth. The objective is
to place the maximum amount of suction possible on a radius around the well whilst at the
same time minimising the amount of air that is drawn in from the surface. The wells are
connected in series or parallel through to a gas compressor, or gas booster, which applies a
negative pressure, or vacuum, throughout the network of pipes and wells. The gas compressor
may either be of the positive-displacement type or, more commonly, it may be a centrifugal
fan unit. Whilst it is normal to use vertical wells, systems have been developed based upon
horizontal wells.
Sizing a gas field
There are many methods of sizing a gas extraction system. The fact of the matter is that the
precise contents of a landfill site remain often unknown and indeterminable. In these
circumstances it is virtually impossible to provide an estimate of gas production potential to a
high-degree of accuracy. The use of mathematical models, pumping trials and other
associated techniques will reduce the level of uncertainty but until the gas extraction system is
actually installed and gas is being withdrawn from the site, the exact size of the resource will
be unknown.
Estimating landfill gas production rates is a large and mature subject in its own right with a
pedigree running for many years.5 The optimum method of sizing a gas field is, however, to
install and operate it. By this means alone will it be possible to be precise about gas quantities.
Prior to final connection of the wells with interconnecting pipework, a pumping trial,
employing an appropriately sized mobile flaring unit drawing landfill gas from segments of
the gas field, will enable an operator to determine the flow rates necessary for either gas use
or migration control. Short of being able to achieve such a complete installation at trial-stage,
it is necessary to use various other methods to estimate gas production potential.
Where such estimating methods are employed, the only protection against project failure in
these circumstances is to employ healthy factors of safety, of up to ±50%. Where utilisation is
intended a deduction of 50% of the mean gas production estimate should be made; in the case
of migration control an addition of 50% of the necessary extraction flow rate should be
assumed for design purposes. Whilst some may consider these factors of safety excessively
aggressive, they will help ensure project success. Failure in commercial terms means there
will not be adequate gas to recover capital costs. Failure in terms of migration control will
mean that gas is escaping off-site.
The system objective
There are two clear and separate strategies employed in running a gas extraction system. The
first is designed to minimise gas migration off-site from the landfill. The second is to
maximise the production of gas for a specific landfill gas use facility. These objectives cannot
be confused. In the case of the former, migration control, it is adequate to maintain methane
levels in the extracted gas of approximately 20 to 30% without causing undue concern to the
overall optimum operation of the system. Where necessary to prevent migration, extracted gas
may sometimes contain close to zero methane. On the other hand, where a gas-use facility is
intended, should the methane percentage drop much below 40% there should be serious
concerns about the system viability. In the latter ease, it is usual to try and maintain
equilibrium gas production rates at around 50% methane by volume
Lateral emissions of landfill gas
Emissions of landfill gas through the surface boundaries of a landfill site are driven by
concentration differentials (diffusive flow according to Fick’s Law), or pressure differentials
(advective flow), or both. Advective flows are promoted by pressure differentials between
locations. Slower, diffusional flow will still exist in these situations, but flow will be
predominantly advective. However, the impact of the slow diffusional flow of landfill gas into
confined spaces should not be underestimated.
It may take many years for the gas diffusing into a confined space to achieve the same
concentration as that achieved by an advective plume in a few days. However, both are
equally significant, as the build-up of gases can lead to potentially harmful situations.
Advective gas migration can be caused by rapid differential pressure changes such as the
passing of a low pressure weather system over the landfill site, coupled with a highly
permeable migration pathway (e.g. a fracture or conduit). It can also be caused by changing
liquid levels in the site or by the rapid relief of pressure, which has built up behind a gas
barrier (e.g. a clay liner).4
Migration control
To achieve migration control the fundamental requirement is to place a curtain of negative
pressure along the perimeter through which migration must be stopped. There are many
factors affecting the degree to which migration occurs along any one boundary, and these all
impact upon the quality of the methane obtained for flaring. It is often the case that a system
installed purely for migration control may not, in fact, be able to produce adequate methane
for flaring whilst at the same time maintaining effective perimeter sealing to migrating gases.
In order to flare landfill gas it is normal to set a minimum methane concentration of 20% for a
standard landfill gas flare. However, other technologies, such as regenerative thermal
oxidisers, may facilitate combustion at much lower methane concentrations.
Where it is desired that higher levels of methane, i.e. in the range 25-30%, should be available
to facilitate combustion of extracted gases, it may be necessary to have one or more lines of
gas wells located within the body of the waste and away from the edge of the site. Even with
this precaution, it may often not be possible to obtain gas of sufficient quality for flaring.
Where there are highly-permeable rock strata abutting the landfill, it is quite possible that a
large proportion of the full extraction potential will be used to draw air into the system to
prevent the loss of migration control.
In order to overcome the problem of excessive air induction from permeable strata, there are a
number of options. The installation of additional wells along the sensitive boundary will allow
for a smaller sphere of influence. For example, the well spacing may be reduced to 5 meters,
or less, as opposed to the more standard 20 to 40 metres. Alternatively, or in addition, the use
of a programmable timer, to control the periods of operation of extraction equipment, will
allow a higher rate of extraction and suction on an intermittent basis and in a manner which
helps to achieve a methane concentration adequate for flaring, whilst at the same time
preventing migration.
The use of increased suction will in all probability mean that larger quantities of air are drawn
in, as well as methane bearing landfill gases, from around the perimeter of the well itself.
With the passage of time, the volumes of air being drawn into the vicinity of well will turn the
waste aerobic. In such a situation, all methane production would cease and be replaced by the
formation of carbon dioxide from wastes through which air passes.
Landfill gas use
The requirement to balance a gas field for landfill gas use is quite the reverse of that for
migration control. The objective is to maximise methane production, neglecting any concern
for the movement of gases not caught in the negative pressure curtain. Therefore, if a well is
showing a reduction in methane concentration it is necessary to reduce flow rate from the well
by closing down the well-head control valve. The assumption is that the gas extraction taking
place is greater than the equilibrium production for the sphere of influence of that particular
well. Where gas extraction is greater than the sphere of influence for any particular well, the
result will be an increase in air drawn in through the perimeter of the site and a consequent
gradual deterioration of the whole of the site. It is, therefore, essential in a gas-use scenario
not to over-extract any one well. It has been observed, for example, that where a gas
extraction system is over-extracted for an extended period of time the gas quality, in terms of
methane concentration, will eventually rapidly decline and will take some time to recover.
This is because large sections of the site have turned aerobic and the anaerobic bacteria, being
relatively slow growing, take some time to recover.
The inherent conflict
There is, therefore, an inherent conflict between landfill gas migration control and landfill gas
use. It has often been noted that operators of migration control systems will erroneously adjust
well-head valve settings to maximise methane production. Whilst this will ensure that the
landfill gas flare stack will remain alight, it may not, in actuality, control migration down to
the levels required. The real test of the effectiveness of a migration control system is to
monitor off-site migration in monitoring boreholes, regardless of the methane percentage in
the extraction system. Where off-site migration dictates an increased flow rate, this may need
to be implemented without regard to the presence or absence of a flame. There is, of course, a
consequential penalty associated with the flame being extinguished, which may come in the
form of odour problems or concerns for greenhouse gas formation.
One particular migration control system in Kent, England, was running as a vent with very
low levels of methane. It was set to run during the day, when the impact of noise would be
minimal, and shut down at night. The unit was located in the countryside close to domestic
properties. After several months of operation, this regime was reversed, with the unit running
at night and being shut down during the day. The reason for this turnaround was that the noise
levels were imperceptible, whereas the odour became a serious problem. The unit
subsequently ran between midnight and 6 o’clock in the morning, when the difficulties
associated with odour were minimised. In this particular case there was no option for
combustion as the methane concentration was approximately 1% by volume. Without the
operation of the gas extraction system, methane levels of 20% and upwards were reported in
properties adjacent to the fill.
Set against the requirement for minimising off-site migration, a classical utilisation scheme is
generally not designed with this objective in mind. On large sites it is, therefore, often
necessary to consider the possibility of two separate systems, one to control migration along
sensitive boundaries and the other to maximise gas yields for utilisation. Where a single
system is installed the overriding criteria has to be safety for properties adjacent to the site.
Where migration is found to be occurring, output from the utilisation scheme will need to be
sacrificed until adequate measures are taken to separate the two systems.
The dynamic relationship
Offsite migration
Where gas migration control is the objective, the natural control cycle is from off-site
monitoring bores to the compressor drawing gas from the site. Automatic systems have been
installed around the world which provide this positive feedback loop. When migrating gas
concentrations exceed certain thresholds, the gas extraction unit will adjust its relative flow
rates from wells to which it is connected by means of automatically opening or closing wellhead gas valves. It should be noted that as one well is closed off to reduce the extraction flow
rate, the degree of suction placed on all the remaining wells is increased and it is necessary to,
therefore, adjust all the other wells slightly to allow for this increase in suction pressure. This
feedback loop may be automatic or it may be manual. If the latter, the recording of offsite
migration levels versus exacted gas flow rates and concentrations is essential, to be able to
monitor overall system performance.
Equilibrium gas production
The anaerobic bacteria which exist within any one particular part of the landfill will produce
gas from the organic material in that location at a steady and recognisable rate. This is
referred to as the equilibrium gas production rate. Should gas be drawn at a rate greater than
the equilibrium, the deficit is made up by drawing in the gases from outside the sphere of
influence of the well. The sphere of influence in other words is increased. By creating a
vacuum at one point in the site greater than that satisfied by the equilibrium production rate,
gas may be drawn from any of the perimeters of the site. The appropriate feedback loop in a
gas use facility is, therefore, methane percentage versus flow rate. As the methane percentage
decreases the flow rate will need to be reduced in order to maintain a specific methane level.
The flow rate is reduced by reducing the vacuum applied to the extraction system.
Gas quality
In a migration control system, gas quality may only realistically be maintained by balancing
the flow from the body of the landfill site with that from the perimeter of the landfill site. This
may be accomplished either on a continuous basis or a timed-operation basis. Where attempts
are made at maintaining gas quality by balancing the perimeter wells as if they were part of a
gas-use facility, there will be the possibility that gas migration may occur. The objective in
balancing the system would be to maintain low levels of methane within the off-site
monitoring bores with the minimum flow rate possible. The balance of gas drawn into the
flare stack should be a high-quality gas, thereby lifting the migration gas percentages to a
more substantial methane percentage and, where possible, permitting combustion of such
gases.
Atmospheric pressure
One of the key factors, which remain beyond the control of the operator of a landfill site, is
atmospheric pressure. There is a complex set of consequences that result from changing
atmospheric pressures. These are not always as predictable as one might wish. In an idealised
situation as the atmospheric pressure increases the amount of migration off-site decreases.
The reason for this is atmospheric pressure tends to hold gas back in the body of the landfill
site. On the other hand, as atmospheric pressure decreases the pressure of gas within the
landfill site promotes off-site migration.
It has been noted that when the property adjacent to the Loscoe Landfill Site in Derbyshire
exploded, the incident followed one of the lowest barometric pressures recorded. This enabled
migrating landfill gas to move through pathways and in volumes not previously encountered.
Atmospheric pressure has a less dramatic effect upon gas use facilities and is of far less
significance. As the atmospheric pressure increases the suction on the gas field will likewise
need to be increased marginally, and vice versa.
It has been noted that the relationship between atmospheric pressure and off-site migration is
not always as simple as that presented above. Various lags and follow-on effects may distort
the overall picture. Anaerobic bacteria are notoriously sensitive to any changes in their local
environment. The effect of ambient pressure variations is not always quite as simple as one
might wish.
Pressure drop through a system
When balancing a gas field, the available vacuum is distributed in a predictable manner
throughout the various entry points to a gas extraction system. As the flow rate increases the
pressure drop in the pipework likewise increases. As the pressure drop in the pipework
increases the proportion of available suction recorded at each wellhead will be found to
decrease. Likewise, as the flow rate decreases the contribution of pipeline pressure loss
becomes less and less significant and a greater proportion of the available head is transferred
directly to the wellhead.
It is advisable to ensure that even in the maximum flow situation only a very small proportion
of the available head is used in overcoming pipeline friction losses and that the bulk of the
available vacuum is used for extracting gas from the well. Where a pipe is restricted with
water condensing and lying in dips in the pipe, the overall system will be effected. In extreme
cases, where a pipe becomes flooded, gas extraction will cease.
Balancing a gas field is essentially an iterative procedure. As one well is closed to reduce the
flow rate from that particular well, the flow rate from all of the other wells is increased. Once
all the other wells are closed off, to reduce the flow rates from each well back to their original
levels, it will be found that gas extraction from the well of concern has now increased. This
process could theoretically be infinite.
In order to avoid such a difficulty, it is also possible to control flow rate from the system as a
whole at the gas extraction unit itself. Where it is desired to reduce overall flow rates, this
may more effectively be achieved at the flare stack.
A similar knock on effect occurs when it is desired to increase flow rate from any one
particular well. As a well is opened and the flow rate is thereby increased, the amount of
suction available on other wells is reduced. Once again it would be necessary to open all the
other wells to bring them back to their original level. This process in turn decreases the gas
being drawn from the well originally of interest. Once again it is theoretically possible to
continue this process of adjustment forever.
To compound the complexity of this situation, the contribution of atmospheric pressure will
be influencing separate wells in different ways. Also, as time passes the amount of gas
available from any one particular well may either increase or decrease, depending on such
factors as rainfall, temperature and available carbon for digestion by the bacteria.
Another major influence in the process of balancing the system is the lag involved in any
particular flow rate settling out in the area in the site from which landfill gas is being drawn.
For example, the flow rate may be increased on one well and adequate methane levels may be
observed for a period of time. However, a return to that well after several days may show that
the methane level is dropping rapidly because the well is drawing landfill gas above the
equilibrium production rate of its sphere of influence. It has simply taken several days for this
factor to work through to an observable reduction in methane percentage
The iterative approach
There are no simple answers to these difficulties. The necessary regular adjustment required
to wellheads may be made somewhat easier by installing centrally located manifold systems,
but the balancing of the gas field will remain a continual requirement. A simple form for
recording all available data from any one particular system is essential. By regularly
monitoring and reviewing the gas levels in off-site monitoring boreholes and the gas levels in
individual wells, it is possible to build up a picture of the typical operating ranges of any one
particular part of the site
Once a gas field has been “settled in” it becomes a more practical proposition to make small
adjustments to any particular section of the gas field. Such adjustments, once made, should be
noted. Upon a return to the site, the effect of these adjustments will become clear. Over a
period of time an operator will become familiar with the site that he/she is running and will
know approximately the positions in which well-head valves should be set up to achieve the
desired objectives.
The initial balancing is an iterative procedure and it is simply a matter of starting at the
beginning and going round the whole system recording the flow rates, suction pressures,
oxygen and methane levels. Once complete the operator must return to the beginning and start
again. In the first instance it is probably realistic to balance the system with three or four
sequential rotations around a single gas field. After this initial set-up the iteration should be
carried out over greater periods of time. Once the system is properly set up it may be adequate
to balance the system once a week or even, in certain circumstances, once a month.
For both migration control and gas use, the feedback loop involved must also be monitored.
With a migration control system, the feedback loop is to offsite migration monitoring wells.
With a gas use system, the feedback loop is with methane concentrations at the point of use.
LEACHATE AND WATER TABLES
In order for a gas extraction regime to function correctly it is necessary for leachate to be
substantially removed from a landfill site. Suction of gases will not occur, and a negative
pressure curtain will not be possible, where water is present in large quantities within a site. As
with landfill gas extraction, leachate extraction and control is also an established engineering
practice. There are several methods of removing leachate from a site, all of which may be
appropriate and applicable to any given location. The corollary of this requirement may well be
that large quantities of leachate need to be treated and removed off site. If this is the case, it will
be an inevitable consequence of a specific site design and installation. Short of removing the
site completely, leachate extraction will be required to enable landfill gas to be extracted and
controlled.
HAZARDOUS GAS CONCENTRATIONS
As is noted above, it may be necessary to run a migration control system with methane levels
below 20%. This takes the landfill gas towards the explosive range, classically defined as 5% to
15% methane in air. This relationship is modified by the presence of carbon dioxide and the
absence of oxygen to atmospheric oxygen concentrations, but it remains that as methane levels
drop the gas may be ignited and may explode. Figure 1 below gives a graphical presentation of
the range in which landfill gas air mixtures become flammable. As can be readily appreciated, it
is actually very difficult to ignite low-methane landfill gases, but not impossible.
As an operator of such a system the issue is to define priorities. Should landfill gas be allowed
to migrate into properties in potentially explosive concentrations, or should landfill gas be
extracted into an engineered system, designed to safely manage potentially explosive gases?
What is the purpose of engineering if the decision taken with the above choice is to allow gas to
migrate off-site and into domestic properties, rather than collect and dispose of it in a safe and
controlled manner? This may seem like a rather obvious and common-sense matter, but it is
surprising how many systems are operated to prevent the possibility of explosive gas formation
in plant and equipment, at the expense of risking migration into peoples’ homes.
For the avoidance of any doubt, engineered plant and equipment can be built to internationally
accepted standards to take even the most hazardous of gas combinations and deal with them
safely. It is not necessary to run a migration control system having concern for the explosive
range.
As highlighted in the above text, one concern with low methane gases is that it may not be
possible to combust with standard combustion equipment. This is a minor price to pay to ensure
that migration of landfill gas is not occurring. However, options do exist even here, ranging
from assisted combustion, to regenerative thermal oxidisers and low calorific-value gas
combustion systems. If there is concern about the release of low-methane concentration gases
into the atmosphere, this may be addressed. However, this concern is of a much lower priority
than ensuring that people’s lives are not put at risk from migrating landfill gas.
SUMMARY
Whilst it is acknowledged that hindsight gives a clear view of a situation, in the case of
Brookland Greens housing estate, it would have been possible to completely prevent migration
by clearing the site of leachate, installing a negative pressure curtain along the boundaries and
monitoring performance with suitably designed and installed monitoring wells. The fact that
this did not happen is a matter for historians and lawyers to understand. The fact remains,
however, that a situation like this does not need to happen again.
Figure 1 Flammability of landfill gas mixtures6
REFERENCES
1. http://en.wikipedia.org/wiki/Loscoe
2. Victoria Ombudsman’s Report, Brookland Greens Estate – investigation into methane gas
leaks, October 2009.
3. Environmental Protection Act 1990
4. UK Environment Agency. Guidance on the management of landfill gas, Landfill
Technical Guidance Note 03, 2004
5. Farquhar, G.J. and Rovers F.A. Gas Production during Refuse Decomposition. In: Water,
Air and Soil Pollution, 2, 483-495. 1973
6. Cooper, G., Gregory, R., Manley, B.J.W. and Naylor, E. (1993) Guidelines for the safe
control and utilisation of landfill gas. Published in seven volumes (1, 2, 3, 4A, 4B, 5, 6).
Reports ETSU B 1296 P1–P6 (CWM067A/B/C/D1/D2/E/F/92). ETSU, Harwell.