Theoretical Modelling.

There are no theoretical models that ever equate to empirical data acquired from a well field. For this reason all the 'models' derived and created by Australian Biomass are all dependant on empirical data and are all site specific. Site models are only valid after the entire extraction system has been installed and tested. All models are nothing more than an exercise for theoreticians to examine how well their models can fit the almost infinite amount of variables that can be thought up in the domain of non-linear analysis and then by induction assume that such derived models are correct because of the 'symmetry of their equations'.

Models, though, are best used on the macroscopic scale only. To try to determine the production capacity from including all the possible amount of minutiae is not practical or possible. And will always be wrong. Any landfill that has had organic matter deposited within will, in time and with favourable factors, produce landfill gas. All organic matter will eventually succumb to decomposition either anaerobically or aerobically given time. Within a landfill that contains organic matter and has a reliable moisture through put; landfill gas will be produced. Initially rapidly and then smoothing out over time.

A landfill is not a controlled anaerobic reactor such as a biogas digester. Factors affecting the production of landfill gas within the landfill body proper are too variant and hence the rate of decomposition, too, is variant.

Macroscopic models show that with any sealed cell of refuse it takes up to 18 months for landfill gas to begin to be produced and that this process can last as long as 12 - 40 years. Peaking within the first 8 years. The macroscopic model would then assume 50% landfill gas production in the first 8 years and the remainder over whatever length the observer/scientist wishes to extend it. Generally I have found that 12 - 18 years for the second phase sufficiently reliable. Experience gradually assists in placing the various limits for the modelling process. Every landfill is different.

It is important to remember that all models are akin to the landfills' horoscope and though giving a rough guide, are invariably flawed.

Principal variables in determining the ability of a landfill to produce landfill gas are:

Age profile of the refuse and cell management.

Old landfills, despite their content, may not be reliable producers of landfill gas. This, though, is dependant on whether there has been sufficient moisture throughput through the landfill to facilitate the decomposition process. Also, an old landfill may have an enormous tonnage of refuse and even after 20 years may still be viable ( though this too is environmentally dependant). A large landfill ( over 3 million tonnes of refuse ) in Riyadh may not be a great producer of landfill gas irrespective of the tonnage of refuse and organics that it may contain because of the lack of rainfall and moisture throughput. The same sized landfill in a much wetter climate ( say Manchester, UK) would follow the standard curve for its landfill gas production.

The landfill in Riyadh would commence producing once water is added

Where refuse cells are small yet plentiful ( end of day cover as opposed to end of month cover); anaerobic decomposition commences more quickly and more frequently. The active boundary limit migrates more rapidly through the landfill body.

It is never too early to begin to extract landfill gas from the landfill. If the site is managed properly then extraction can commence as early as 2 years after a particular section of the landfill has been completed. Usually landfill gas extraction is commenced shortly after the whole landfill has been closed / capped. By keeping in step with the standard production curve tied to cell closure and installing the extraction system along with this then it is possible to maximize the amount of landfill gas that can be extracted from a landfill. So doing also minimizes leakage to atmosphere of the potentially harmful greenhouse gasses ( methane, carbon dioxide and VOC's) that emanate as a consequence of the decomposition process and inadequate capping of the closed sections of the landfill.

Investigation of numerous small old landfills ( less than 500,000 tonnes gross ) within South East Queensland found that after 20 years after closure there was no methane production to speak of. Traces as high as 15% were only recorded after the initial test bore was sunk. Subsequent testing revealed levels less than 2% to zero in 90% of cases.

Organic content of the refuse and Fill to organics ratio.

Hardfill landfills are not prolific producers of landfill gas. For a landfill to be an excellent commercial producer of landfill gas it needs to have a minimum critical mass of organic refuse. Generally a minimum of one million tonnes. Landfills of less than this amount will produce landfill gas but not of a commercially viable quantity.

Fill to organics ratio is generally a minimum of 50% domestic refuse to 50% fill is a normal mix when managing landfills. Hence a one million tonne landfill will have only half a million tonnes of domestic refuse and this is on average 50% potentially putrescible organic material. Hence in a one million tonne landfill there is 250,000 tonnes of putrescible organic material which is used to determine the landfill gas production potential of the landfill.

Carbon / Nitrogen ratios are not important indicators when it is known that there is a substantial tonnage of domestic refuse within a landfill. The carbon/nitrogen ratio is only important in the management of bio-reactors such as municipal digesters where it is manageable. There are very few manageable parameters within a landfill for altering the production of landfill gas.

Once the landfill is closed; controlled water ( leachate or condensate ) recirculation will assist in stimulating production. While the landfill is being filled: cell cover management, refuse composition, constituency and compaction are the only other parameters that can be manipulated for landfill gas production enhancement.

Indicators such as partial pressure percentile of the gas mixture in recovered samples and the depth gradient of carboxylic acid activity only confirm what we already know: and that is that landfills produce landfill gas. Roll on Blind Freddie!

Moisture throughput through the landfill.

The principal fuel for the anaerobic bacteria within a landfill is water. Without water the process dries up. With too much water the process is over quickly.

At the Hulett St., Landfill in Sunshine, Melbourne Victoria I was asked to investigate as to why the landfill was not meeting another consultants models' production prediction. As it turned out, of the 4 million tonnes within one of the landfills, 2 million was under the natural water table level. ( There were 2 landfills adjacent to each other. One was far older than the other and the older one contributed a small overall percentage of the total landfill gas extracted. It was not investigated as intensely as the more recently closed landfill). Groundwater temperatures never fell below 4oCelsius.

[Chinese Farm gas digesters ( of which there are numerous within China ) produce biogas all year 'round. Despite the freezing temperatures of the chinese winter, the anaerobic process within these small digesters never falls to freezing. The process slows but never stops. A series of batch digesters in the Philippines showed that a single batch of digestible organic matter will produce 90% of its potential landfill gas within the first 4 - 6 months while resident within the sealed digester.]

The 2 million tonnes at the Hulett St. landfill had been under the groundwater level for well over 2 years prior to 'mining' the resource.

A municipal digester which has a throughput of fluid and solids of 12 - 18 days manages to reduce its BOD by over 65% and in many instances as high as 95%. This, though, is done in a managed environment where the temperature and biochemistry of the digester is continuously monitored and controlled.

These parameters, apparently, escaped the prior consultants attention. The 2 million tonnes under the groundwater level were ( on testing by myself 1990-93 ) anaerobically dead. They produced no landfill gas at all. The BOD had long since been washed away both by the continuous transit of the groundwater through this base section and by the 'digester effect' of the organic refuse being continuously bathed in an ideal environment for the organics to happily digest away. The landfill being in an old Basalt quarry was an ideal 'crucible' for the stimulation of a virtual digester environment.

The top 2 million tonnes were exceptionally dry with only the transition zone between the wet and dry bands producing a reliable amount of landfill gas. And nowhere near the previous consultants' 'model'. The dry band 'kicks-in' shortly after rain for a time period proportional to the quantity of rain and rate of percolation of the water through to the wet band. Then it falls down to its normal level.

The capping and permeability of the landfill were such that on testing, the previous consultant determined that there were enormous reserves of landfill gas waiting to be mined. This obvious lack of experience missed the fact that what was perceived as mineable was actually being held back by virtue of the low permeability of the landfill body proper and the landfill cap. It was the 'reserve' and not the production rate.

Landfills in moist environments produce their landfill gas on a more reliable and rapid basis. Always influenced by rain and draw rate.

Tonnage of refuse.

Landfills should be kept BIG. There is no shortage of space for landfills. Nor, in my opinion, do landfills pose environmental hazards if constructed and managed properly. Landfills are the treasure houses of future archaeologists. If we don't throw it away and bury it they will never know how we lived. Not that we should abandon recycling or waste minimization for some dubious romanticism for future archaeologists.

Landfills do not have to be near populated centres as rail transport supported with suitable refuse transfer stations can easily transport all refuse to the large remote waste-management sites.

The ultimate waste- management facility would receive refuse into a sorting apparatus. All recyclables would be extracted and reprocessed on the site. The balance would be landfilled. If designed properly up to 80% of refuse could be recycled at such a facility. This requires the correct political will. Such a facility would occupy up to 1000 hectares and comfortably cope with refuse disposal from a population base from 10 - 20 million people for a century or more.

If there is a commitment to minimize the effect of environmental pollution from a landfill then the tonnage of refuse should be as large as possible. Keeping landfill sites to a minimum. In excess of 10 million tonnes and higher and with a high content of organic matter. This high tonnage is necessary in order to make the extraction of landfill gas economically easy and to allow a long time scale for the use of the site. Landfills properly designed as sanitary landfills have virtually zero impact on their environment.

Gas production capacity.

This is the 'black-art' of modelling. As in my introduction: there are no reliable models for the determination of the landfill gas production capacity from a landfill. Only macroscopic assumptions.

From the Philippines study on batch digestion and from research done at the Mosgiel Research centre in Dunedin NZ in the late 1970's and early 1980's it can safely be concluded that the maximum amount of landfill gas that can be extracted from one kilogram of solid organic matter is in the vicinity of 800 - 1400 litres. It is impossible to extract the total amount. A continuous feed anaerobic digester will only get up to less than half this amount while a landfill environment ( which can best be likened to a batch digester ) will get more. As high as 600 - 900 litres per kilo of VFA's.

Using this premise, ( and an inaccurate one it is ) given a landfill with one million tonnes of domestic refuse @ 40% putrescible organic matter then 1,000,000,000Kg x 600l/kg x 0.4%= 240,000,000 cubic metres of gas over the life of the landfill will be produced ( ideally ). Over an 18 year life this equates to 1500 cubic metres per hour. Actual recoverable quantity is more in the order of 750 cubic metres per hour. And possibly less. ( Speculation!).

Bear in mind that there is a 2 year dead band ( band 1 ) before any landfill gas is produced from the initial deposit ( the time scale is in years ) then there is a 7 - 11 year period ( band 2 ) where the production peaks and then begins its slow decline; then there is a 11 - 40 year period ( band 3 ) where the production declines to negligible levels. Band two is equivalent in production quantity to band three.

Factoring in rainfall; leachate recirculation, moisture migration, well and landfill suction centroid boundary, capping permeability, landfill permeability and porosity, interwell influences, rate of conversion of VFA's to landfill gas and aerobic/anaerobic boundary influences are all part of the fun of non-linear analysis and model fabrication. Fiddling with these variables does not equate to knowing how much landfill gas is going to be produced let alone recovered. We're talking horoscopes here folks! But it helps.

Production prediction pitfalls.

Any consultant, expert or specialist who says that they know exactly how much landfill gas can be produced from a landfill by virtue of modelling only is a liar.