Landfilling to save the climate?

Well-run landfills can perform just as well as more expensive AWT facilities when it comes to greenhouse impacts, a life cycle assessment conducted by Hyder Consulting on Hanson’s Wollert landfill in Melbourne has found. In a report sure to cause plenty of excitement and debate throughout the industry, a variety of waste disposal scenarios were modeled and each – including landfill – was found to result in net savings of greenhouse gases.

The study, undertaken for the City of Whittlesea and Hanson Landfill Services, considered the 144,000 tonnes of household garbage and 96,000 tonnes of C&I waste sent to Wollert landfill in 2008. It assessed impacts from that base case compared to potential alternative scenarios, including aerobic and anaerobic treatment technologies.

The take-home message of the report, according to Hanson’s Sam Bateman, is that “it can no longer be assumed that diverting organics from landfill has any greater greenhouse benefit from sending them to a well designed and operated putrescible landfill with gas capture and electricity generation”.

Lead author Joe Pickin (who has since moved from Hyder to Blue Environment) has gone to considerable lengths to explain the uncertainties and assumptions in the report. He concedes it is difficult to produce a “static report on one piece of paper” when there are multiple variables and uncertainties involved. The report he has produced does, however, consider in detail how changes to key assumptions could affect overall results.

Using mid-range estimated values and a standard 100 year assessment timeframe, the base case landfill “produced similar outcomes” to anaerobic mechanical biological treatment (MBT), resulting in net savings of between 66,000 and 72,000 tonnes of CO2e. Aerobic MBT, however, resulted in significantly lower savings of 33,000 tonnes CO2e.

The report notes savings in terms of avoided emissions, “mostly arose from carbon storage (in the landfill and compost) and product offsets (metals, plastics and especially electricity)”.

“The avoided emissions outweighed the actual emissions, which were mostly associated with landfill methane and process inputs.”

While many researchers (and the IPCC) agree that some organic material never breaks down in landfill and therefore there is some storage of carbon, one of the more controversial aspects of Hyder’s study is that the impact of this storage is counted as an offset: carbon not emitted is considered equivalent to CO2 removal from the atmosphere.

Another aspect guaranteed to spark considerable debate is the assumed methane recovery (gas capture) rate, estimated for Wollert at 60-88% based upon a range of sources, including direct measurement. This is the whole of life gas capture range; emissions before collection begins are assumed to be negligible, and emissions after collection stop range from 5-10%. It assumes 1.5% are collected but not oxidized.

With methane recovery at the high end of the range, the base case (Wollert landfill) was the best performing scenario. At the low end of the methane recovery rate, however, the base case was worse than anaerobic MBT (although it still outperformed aerobic MBT).

In a very interesting finding, the report suggests the best way to generate renewable energy from waste is indeed sending it to landfill. The low-range estimate of net electricity output from Wollert – around 150kWh/tonne of waste –greatly exceeded even the high range estimate for anaerobic MBT, 68kWh/t, largely due to the energy required running AWT plants.

While Wollert was recently crowned Australia’s best landfill through the Waste Management Association’s National Excellence Awards, Bateman said when it comes to greenhouse performance, “I am confident that any major urban putrescible landfill in Australia with a well designed and operated gas capture and electricity generation system will perform as well”.

“I expect that some of these sites will be encouraged by our LCA study to do their own LCA and challenge the waste policy assumptions in their own state.”

The report concludes by considering why some of the findings it contains are “more favorable to landfill than those of many other Australian studies”.

“There is no single answer to this question but, importantly, our estimated rate of recovery of methane from landfill is site specific and higher than historical data. We apply a relatively low benefit from the use of compost and stabilised organics. Unlike some studies, we count carbon remaining in landfill as a benefit.”

It is important to recall the total environmental impacts of different waste disposal process are more complex than greenhouse emissions alone.

Carbon impacts have, however, been a key justification for many councils looking to invest in AWTs, and one of the main reasons landfill is considered bottom-of-the-heap in the waste hierarchy. While this latest study is sure to draw plenty of fire from critics, it will also undoubtedly give councils pause for thought when evaluating their waste options.

Download the full report by clicking here.

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