Taking the measure of landfill borehole gas flows
Dr Roger Riley, NPI Manager at Geotechnical Instruments (UK) Ltd, describes current practice in measuring and interpreting gas flows from landfill boreholes
Current landfill gas measurement practice for measuring the gas composition is well understood, and there are standard procedures and instruments available to analyse the gas.
It is also current recognised practice to measure the gas “flow” from boreholes to monitor the production of gas in a landfill site. If care is taken to make the measurement in the same way each time, changes in the “flow” can be a useful indicator that something has changed within the landfill and may indicate the need for further investigation.
However, using these “flow” readings as an absolute measure of the gas generation within the landfill will lead to misleading results, as explained below.
A capped landfill, with sealed boreholes, will build up a static pressure, Ps. Although this pressure is related to the rate of gas generation it is also heavily influenced by other parameters, such as the gas permeability and thickness of the cover, and of course any gas escape routes that may exist through the cover. Thus the pressure in the borehole (and thus landfill) cannot be used as an indicator of gas generation rate.
The equilibrium situation within a landfill is shown in Fig 1a. Here the gas generation within the landfill is balanced against the loss of gas. Under these conditions there will be no gas flow into or out of the borehole and the gas pressure in the borehole, PB, will equal the gas pressure in the landfill, PL.
When a flow measuring device is connected to the borehole and the tap opened the situation changes. If the flow device has no restriction to flow, as the tap is opened the pressure in the borehole will force the gas out until the borehole is at or near atmospheric pressure
In removing the gas we have upset the equilibrium of the landfill. With the tap open and the system open to atmosphere we have a borehole at near atmospheric pressure within a landfill at higher pressure. The low pressure borehole will act as a sink for the gas in the landfill which is still at the elevated pressure of the landfill. We will thus get a flow of gas into the borehole from the surrounding landfill, as shown in the fig 1b.
The measured flow will depend on many factors including the pressure in the landfill, the permeability of the waste, the existence of alternative gas pathways, the moisture content and the water level. It will not be an indicator of the gas generation within the landfill.
The exact area of the landfill that will contribute to the gas flow is known as the radius of influence, or ROI. The ROI is difficult to determine and will depend on the exact nature of the landfill. The permeability of the landfill, and the moisture content will both affect the ROI. However, typical values for the ROI can be tens or hundreds of metres and may well encompass other boreholes.
Since there is no restriction in the flow device, the restriction of the system will depend mainly on parameters such as the diameter and length of the tube, the size of the tap on the borehole etc. This could lead to unrepeatable measurements if these parameters are not controlled, so this method cannot be recommended for long term monitoring of flow rates over the life of the landfill.
Suppose now that the flow measurement device incorporates a restriction to gas flow. Because of the restriction less gas is removed from the borehole, and thus the pressure within the borehole is not reduced as much as before. Because of this the equilibrium state of the landfill is not disturbed as much, as shown in Fig 1c. The measured flow will be less than with an unrestricted flow. This “low flow” sampling method does not suffer as much from the gas sink effect of the open flow method.
One advantage of the restricted method is that the flow device is the limiting restrictor in the system, and the measured flows will be independent of other parameters such as pipe sizes and length. This will give a more repeatable flow reading and allows the operator to compare readings taken over a long time and look for changes over the life of the landfill.
However, even with this method it is not possible to infer a gas generation rate from the landfill for the reasons given above.
Flow rates obtained by any method will also vary with atmospheric pressure and whether it is rising or falling. It is therefore essential to record the atmospheric pressure with each reading if comparisons are to be made.
In the US, the code of federal regulations (40CFR Subtitle D) details the method to be used to determine the ROI and thus gas generation rate of a landfill. This requires measuring gas flows and pressures at predetermined distances from the borehole. However, it has been showni that even this much more complex method can give erroneous results for the gas generation rate by factors of two or three.
In the UK, the DOE waste management paper No 26A, Landfill Completion, states that borehole measurements must be made to achieve completion of a site. It also states flow rates that are acceptable. However, it does not say how these should be measured, although it does point out that they can vary with atmospheric pressure.
This guideline is under review and is due to be replaced soonii. The new guideline sensibly does not state specific measurement values for flow from boreholes. Indeed it states a number of parameters that should be monitored to establish completion of a landfill, of which the gas flow is only one. It emphasises the need for continuous monitoring of parameters so that the performance of the landfill can be understood throughout its life.
In summary, it is not possible to extrapolate from a measurement of borehole flow, however taken, to achieve a measure of the gas generation in a landfill. The readings should be used more as an indicator of changes within the landfill, which would then require further investigation to identify the cause.
It is therefore important that any method used for measuring flow gives consistent and reliable measurements over the life of the landfill, free from outside and operator influences.
i Walter, G W J Air & Waste Management Assoc 53:461-468
ii Guidance on Landfill Completion. A consultation by the Environment Agency 14 March 2003