Most Operational and Maintenance (O&M) costs occur during
the same year as the waste is collected/treated/disposed, but for
some technologies costs may occur over time, for example in the
case of leaching from landfilled waste leading to future costs for
leachate treatment. Such future costs should be converted into
net present value (NPV), Eq. (9). The same applies to revenues
obtained in the future, for example from utilising biogas generated
from landfilled waste.