Feature

A Touch of Gas

Though it smells and can be toxic, not to mention explosive, gas from landfills isn't necessarily all bad...

Mark Milke

Much of our rubbish biodegrades when buried in landfills. Garden waste, kitchen waste and various paper products all produce carbon dioxide and methane gas.

The methane, or natural gas, presents both a problem and an opportunity, and much research and development is underway worldwide to better manage it.

Two projects at Canterbury University's School of Engineering have improved techniques for assessing gas problems at the many old landfills, tips and dumps in New Zealand.

At bigger and newer landfills we can collect enough of the gas to use its energy. In Wellington, landfill gas is being used to help meet the energy needs of a hospital, and of a pool and recreation centre; in larger cities, like Los Angeles, landfill gas can be burnt to generate enough electricity for tens of thousands of people.

Not collecting and burning the gas can lead to all sorts of problems.

For one thing, the gases stink. If toxic wastes are dumped with more ordinary rubbish, the gases produced can also be toxic. Methane is a greenhouse gas and so contributes to global warming.

Most problematic is that landfill gas can lead to explosions that damage people or property. In a number of instances worldwide, landfill gas has seeped into basements, trenches or pits, where a spark has caused an explosion. Old dumps around the country still can produce enough gas to cause problems 20 or even 50 years after disposal.

One project undertaken to improve our ability to analyse old tips looked at using heat production from the tip as an indicator of gas production. The same biodegradation process that produces the gas also releases heat; so more heat will come off soil that is above old dumps. The temperature difference can be detected close-up with a temperature probe, or from far away with infrared detectors.

A Masters of Engineering student has used this techniques to scan the old Bexley landfill in Christchurch from a helicopter. This technique can be useful for estimating the gas production rate, and it can also help identify the boundaries of old tips.

Another Masters student has developed a software tool to aid in analysing gas migration and mitigation at old landfill sites. Given estimates of gas generation rates and soil properties, the program calculates the gas pressures in the soil. This in turn can be used to estimate gas flow out of the soil at various locations.

Users can add in impermeable covers, wells, or trenches to get a rough idea of what might help to control gas movement in the field. The program runs using Visual Basic macros written for the popular Excel spreadsheet program. The intention is to make the add-on program and its documentation available on the Web for free worldwide use.

Gas production from landfills is a "booming" business with great potential for research and development to improve our ability to turn a liability into an asset.

Mark Milke is with the Department of Civil Engineering at the University of Canterbury.