2. System description and methods
2.1. Goal and scope
This study focuses on organic waste handling scenarios in the municipality of Trondheim in Norway.
Particular focus is placed on the anaerobic digestion of source separated organic household waste to
produce biogas upgraded for use as a vehicle fuel. The collection of the waste and the emissions
attributable to the spreading of the solid digestate used as an organic fertilizer are considered outside of
the system boundaries for this study. The waste collection process is common for all cases investigated,
and therefore the relative ranking of these cases will not be affected by the inclusion of this process. On
the other hand, the emissions resulting from digestate spreading and the subsequent runoff emissions are
highly context-sensitive and difficult to estimate. These direct emissions attributed to digestate spreading
are therefore assumed be approximately of the same level as the spreading and runoff emissions of the
artificial fertilizer that is used as a system expansion product.
2.2. System description
The study consists of five cases. The first, called Case 0 here, is the reference scenario and represents
current practice. Organics are not separated from household waste, which is combusted at the Heimdal
district heating plant to provide district heating to the municipality of Trondheim. Case 1 also examines
incineration of the waste, although in this case, the waste is transported to a combined heat and power
(CHP) plant in Sundsvall, Sweden to provide district heating and electricity to the Sundsvall community.
The remaining three cases investigate the anaerobic digestion of source separated organic household
waste to produce biogas. The biogas is upgraded and used as vehicle fuel in Trondheim city buses in all
three cases. The difference between the cases is the location of the biogas facility; in Case 2, a new biogas
plant is built in Trondheim. In Case 4, the organic waste is transported to an existing biogas facility in
Verdal 90 km away. Finally, in Case 5, the waste is transported to an existing biogas plant in Sundsvall,
450 km from Trondheim. For cases 4 and 5, the compressed, upgraded biogas fuel is transported back to
Trondheim for use in the buses there. An overview of the scenarios is presented in Feil! Fant ikke
referansekilden.. The direct emissions resulting from the spreading of digestate on fields such as nutrient
runoff and further methane release is not considered in this study, nor is the collection of the organic
waste.