"Biogas from municipal organic waste: Trondheim’s environmental holy grail?"

Five scenarios for organic household waste treatment from the municipality of Trondheim were studied.  Combustion and anaerobic digestion of the waste in three potential locations were considered.  The reference case, representing the current treatment approach for the waste, involves the combustion of the waste at the Heimdal district heating plant.  The first scenario investigates transport and combustion of the waste to a combined heat and power plant (CHP) in Sundsvall, Sweden.  The remaining scenarios explore the use of the organic waste as a feedstock for biogas plants in Trondheim, Verdal and Sundsvall.

As all of the scenarios assess facilities in different locations and have different final products, system expansion was used to define the functional definition of the system.  The combination of all potential end products deliverable from the waste was considered to be the functional unit. This corresponds to district heating delivered in Trondheim, district heating and electricity produced in Sundsvall, local bus fuelling in Trondheim and organic fertilizer product in Trondheim.

The results show a negligible climate benefit from biogas production in these scenarios, although the benefit is much more pronounced in other environmental impact categories such as photochemical oxidation (smog production) and fossil resource depletion.  A sensitivity analysis demonstrates a relatively strong dependence on assumed values for lower heating value (LHV) of the organic waste, methane yield in the anaerobic digestion process, and fuel efficiency of the buses.

Results from this study are compared to those from another biogas study wherein manure is used as a feedstock.  The comparison shows that the environmental benefit of biogas facilities are strongly contextual and that generalisations of the environmental advantages of biogas should not be made from a single study.

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