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ScienceDaily (Dec. 10, 2009) — Cities can progress from consuming energy and emitting greenhouse gasses (GHG) to actually producing energy while saving on GHG emissions, due to the substitution of fossil fuels elsewhere. These findings are based on research in the city of Aalborg in Northern Denmark, published this week in Waste Management & Research, published by SAGE.

Cities following similar waste management strategies are already having a far-reaching impact on GHG emissions, in some regions of Europe.

Given the global interest in GHG emissions, it is perhaps surprising that to date few scientists have produced studies that measure the impact of waste treatment system changes over the longer-term. Tjalfe Poulsen and Jens Aage Hansen from Aalborg University in Denmark used historical data from their own municipality of Aalborg to gain a broader, longer-term overview of how a 'joined-up' approach to waste impacts GHG emissions. The assessment included sewage sludge, food waste, yard waste and other organic waste (including paper and plastic).

Aalborg's citizens have already implemented a package of measures to take on waste that benefits the environment. In 1970 Aalborg's municipal organic waste management system resulted in net GHG emissions with methane from landfill accounting for almost 100%. But between 1970 and 2005, the city changed its waste treatment strategy to include yard waste composting, with the city's remaining organic waste incinerated for combined heat and power production. Of this, waste incineration contributed 80% to net energy production and GHG turnover, wastewater treatment (including sludge digestion) contributed another 10%, while other waste treatment processes used (composting, transport, and land application of treated waste) had minor impacts.

"Generally incineration with or without energy production and biogas production with energy extraction are the two most important processes for the overall energy balance mainly due to the substitution of fossil fuel-based energy," says Poulsen.

Poulsen and Hansen calculate that the energy potential tied up in municipal organic waste in Denmark is equivalent to 5% of the country's total energy consumption including transport. The Aalborg municipality represents about four percent of the Danish population.

The researchers also looked forward to 2020, and predict that further improvements are possible by reducing energy consumed by wastewater treatment (for aeration), increasing anaerobic digestion and incineration process efficiency and source separating food waste for anaerobic co-digestion.
Aalborg's progress shows how far reaching waste management can be in reaching energy and GHG goals, and should offer encouragement to other cities embarking on greener waste management strategies for the future.

Within the European Union (EU), municipal waste management has already reduced GHG emissions significantly, from 64 to 28 million tons CO2 per year between 1990 and 2007, which is equivalent to a drop from 130 to 60 kg CO2 each year per capita. The EU municipal waste sector will achieve 18 percent of the reduction target set for Europe before 2012 according to the Kyoto agreement. The International Solid Waste Association (ISWA) discusses these findings in its Waste & Climate White Paper, due for publication in December. Looking forward, between 2012 and 2020 the EU municipal waste sector will become a net saver of GHG emissions according to current predictions. Aalborg is not alone among Northern European cities where citizens are already reducing overall GHG emissions thanks to optimized waste management.