Combined Landfill Gas and Leachate Extraction Systems

There is currently a shift of emphasis in the approach to treatment of landfill gas and leachate. Until recently, there have been two disparate schools of thought concerning the collection and disposal of these two by-products of organic waste degradation within the same bio-reactor. The tendency in the past has been to study gas and leachate as separate problems each of which requiring a separate solution. The reality is that both gas and leachate are consequences of the same processes within landfill sites, and are merely different phases from a common source. The optimum approach would logically appear to be treatment of both gas and leachate within the same system and attended to during the same equipment installation phase. Indeed, following the tenets provided for by the BATNEEC (Best Available Technology Not Entailing Excessive Cost) notes this would appear to be the preferred option, in terms of ease of installation, long term maintenance and, perhaps most importantly in the present climate, cost. As far as prediction of potential production and end treatment solutions is concerned, leachate and landfill gas can be treated as discrete by-products of degradation. However, the testing of theoretical models of production rates, as well as spatial location within a collection network, can be treated together. Technological improvement, and the continued downward trend in material costs allow those who specify systems, and those who buy them, a wealth of options for the safe combined disposal of both landfill gas and leachate. 1.0 INTRODUCTION One of the main impetuses for the installation of landfill gas collection and disposal systems is the already considerable weight of environmental legislation, driven by an increasingly aware public opinion. For leachate, however, although much has been written on the subject, legislation has, as yet, failed to produce any concrete guidelines for treatment. There remains confusion concerning environmentally acceptable leachate strengths and its concomitant polluting potential, and consequently the cost of treatment remains unrealistically biased towards off-site methods. The situation is unlikely to remain static; change will come, and the well prepared operator will be looking for efficient and cost effective collection and disposal systems which will allow him to fulfil his/her environmental obligations into the twenty-first century. This paper briefly reviews methods of evaluating production of both landfill gas and leachate and focuses on available technology for a method of co-collection and disposal of both landfill gas and leachate. 2.0 ASSESSMENT OF GAS AND LEACHATE PRODUCTION Important in the decision pathway as to which method of collection and disposal is most suitable for any particular site, is the evaluation of whether the proposed design will meet its objectives in terms of capacity. It is unfortunately too often the case that specifiers leave this crucial evaluation exercise until the competitive tendering stage, relying on the integrity of contractors to judge the necessity or otherwise of active site investigations. This can potentially result in costly errors of judgement if the winning contractor deems it unnecessary to conduct such trials to achieve the prime commercial objective of winning the work. There are basically two methods for determining the production of toxic emissions from a landfill site. Both of these can be subject to considerable margins of error relying essentially on the experience of the investigator. However, they do provide an empirical basis for specification which reduces the likelihood of error. The first method is by using mathematical models, ideally done as part of a desk study of the site; while the second is field based pumping trials which are designed to determine as much as possible about the behaviour of the aqueous and gaseous phases within the landfill. In the case of landfill gas, pumping trials are especially important when utilisation is being considered. Data gathered from the trials can be entered into a cumulative spreadsheet and the real-time production rate compared with predicted values. This also allows for extrapolation of gas production rates into the future. 2.1 Pre-installation studies It is critical at this stage to learn as much as possible about the site. In many cases this work will have been undertaken by the client. The objective is to ensure a clear understanding as to the nature of the problem. Studies centre around collating as much information about a site as possible. In order to construct a realistic model of gas and leachate production from the site, it is necessary to gather the following information: a) Period, method and rate of landfilling. b) Mass of waste in place. c) Site dimensions d) Waste types infilled ie domestic, industrial, commercial, inert and hazardous. e) Waste configuration ie baled, shredded, compacted. f) Packing density and moisture content of waste in place. g) Internal temperature and pH of the waste. h) Site geology including any borehole logs available. i) Type of capping and potential gas recovery effectiveness. j) Gas and leachate monitoring results. 1) Available assays from leachate samples m) Meteorological conditions and rainfall data. n) Site hydrogeology and hydrological data, including: i) surface water run-off ii) rainfall and evapotranspiration iii) local water utilisation 2.1.1 Modelling landfill gas production In modelling landfill gas production the amount of gas that can potentially be generated by a unit of waste is an important parameter, yet difficult to define. Previous studies1'2 have arrived at an average figure for gas yield of about 150m3 for every wet tonne, although this figure varies depending on source. Of this it is stated that about 70 80% would be recoverable. A rule of thumb estimate is that between 6 to 10m3 of landfill gas will be produced per tonne per year for ten to fifteen years from placement. Inputting the raw data concerning waste fractions, amounts deposited and periods of deposition into a computer programme results in exponential decay curves. These curves can also model varying concentrations of methane within the gas stream and an input of maximum potential collection efficiency will allow an initial assessment as to the size of system required. 2.2 Landfill Gas Pumping Trials A landfill gas pumping trial is generally conducted after all the known data concerning the site has been gathered and a theoretical exercise such as described above has been carried out. One of the main objectives of the trial is to ensure that the design under consideration is of the correct order of magnitude in terms of capacity, and provides a test for the calculations carried out previously. If conducted correctly this relatively inexpensive measure will result in avoidance of costly remedial actions. It may allow savings to be made at the beginning of the project by reducing the capacity specification. It may also offset many of the direct expenses, such as the drilling of boreholes which can be used again, from the final cost. During the pumping trial data is gathered concerning the amounts and quality of gas being produced. The objectives of conducting a pumping trial on a landfill site can be summarised as follows: a) Investigation the radius of the zone (or cone) of influence being effected by individual extraction wells. This is obtained using an array of piezometers around individual wells; from this the optimum spacing of wells can be deduced. Alternatively, existing monitoring boreholes can be used, with the 3-D resolution being increased if multi-level piezometers are used. b) Defining well-head gas flow characteristics. These describe the behaviour of individual wells under passive and active conditions. c) Quantifying probable landfill gas production rates. These are obtained by active abstraction over a period of time, increasing flow rate until air is introduced into the system, and the quality of the recovered gas falls. This test can, however, be dependent on the quality of the capping. It is also critical to allow for seasonal variation as gas production may be higher in the summer months. d) Defining required abstraction pressures, which in turn enables correct sizing of the permanent abstraction equipment. The final sizing of the equipment will be a function of gas production rate versus flow rate, which should be extrapolated to include the whole site at the post-trial stage. e) Investigating the effect of active extraction on the incidence of far-field migration from the boundaries of a landfill site. This is of particular significance where the landfill site is located within or close to a sensitive area. Tests may be designed to gather data from local or restricted parts of the site or from individual gas wells, and can be expanded to cover the entire site by connecting to networks of pipe and wells. The most common method is to test a small part of the site and then to extrapolate the results to incorporate the whole site. 2.2.1 Strategy for the trials Trials consist of installing extraction wells within a representative section of the site, and connecting these to an extraction rig. Ideally, at least two wells should be drilled in each worked phase of the site in order to allow for possible blinding of extraction wells and in order to gather representative data from the whole site. Extraction wells will ideally be drilled to the base of the site and be lined with a perforated liner and suitable filter medium. 110 mm liner is generally sufficient for short term trials, although the diameter will depend on the use to which the well will be put after the pumping trial is finished. If the test is to be run in tandem with a pumping trial for leachate then it will be necessary to install larger bore liner in order to allow enough room for the installation of the pumping mechanism as detailed below. Once conditions within the site have stabilised following well drilling, and before connecting the wells to an active abstraction system, it is advisable to conduct static tests on each of the wells to establish the following: a) static pressure in milli-bars to monitor whether internal pressures exist; b) average gas temperature at the top, middle and bottom of the well; c) percentage by volume of CH4, CO2, O2; (At this stage some samples should be taken for gas chromatographic analysis.) d) atmospheric pressure, again in milli-bars. (Over time this often shows a mirror-type relationship with gas pressures, although a definitive relationship has yet to be shown.) After connecting to the extraction rig, the dynamic phase may commence. Tests during this stage are designed to investigate the physical characteristics of the wells and of the surrounding waste when under active pressure. During the tests gas quality should be frequently monitored to note the relative change as the test proceeds. Dynamic tests may include: Flow against suction pressure: The test has a duration of about five minutes and the results are plotted as a graph of gas flow (m3/hr) versus applied suction. The test is designed to show how easily gas can be extracted from the vicinity of the well. In general it is expected that wastes with low permeabilities will require greater rates of suction than those with high permeability. Flow rate is usually measured either at the extraction wells or at the flare stack, although it is important to ensure that there is laminar flow within the gas stream at the point of measurement. Pumping test: This can be subdivided as follows: single and multiple well testing. Single well testing is designed to calculate the zone of influence of an individual extraction well. This is obtained by measuring pressures within the sample borehole and comparing these with readings taken from a series of observation wells located at pre-set distances from the active well. The sphere of influence is defined when relative pressure within the observation well is measured to be the same as the static pressure established earlier. The zone of influence can be further defined by installing an array of piezometers radially around the well. These may also be installed vertically to obtain a 3-D representation of the zone of influence within the waste . This test defines the extent of influence within a particular body of waste and may have a direct bearing on the final number of extraction wells which will be installed for the permanent system. Multiple-well testing is carried out by connecting a network of wells and is used to determine the production rate within the site. In this test the emphasis is on determining the maximum abstraction pressure which the site can sustain before significant air is pulled into the system. The test is carried out over a number of weeks (six weeks is usual) and the total gas flow rate is recorded. As the methane concentration falls away the flow rate is stabilised to fulfil the equation: Flow rate = Generation rate This will be evidenced by constant methane concentrations in the region of 50% by volume methane.