The materials flow cradle in the model is “waste in bin” from different sources, such as households and industries. Thus, the environmental and economic impact from the waste sources (comprising activities such as cleaning, sorting and transport to recycling station) is excluded from the system studied. The waste flows are then followed through the waste management system, calculating e.g. changes in composition and emissions depending on the fate of the waste stream. The solid line in Fig. 1 encloses the waste management core system, i.e. where the waste is treated. The dashed line also includes the waste management downstream system, i.e. the use of waste derived products like biogas and sludge as well as the waste sources which are necessary for input of waste to treat even if they are a zero-emission upstream system.
The main function of a waste management system is to treat a certain amount of waste from the system area in a proper way. Today, many waste management systems also provide other functions (benefits) in addition to treatment of the waste, such as recovery of energy and recycling of materials and nutrients. As different waste management systems (or different designs of a planned waste management system in a municipality) can produce different amounts of these functions (e.g. electricity, district heating, vehicle transport, materials and nutrients), comparisons are hard to make. In order to be able to compare, the conventional production (i.e. not derived from the waste management system) of these functions has been added. This makes it possible to level out the output of functions from the waste management system in each scenario, which will give a constant output of the functions for all scenarios. One can say that the conventional system compensates for the waste management system. The above mentioned functions are henceforth called functional units in conformity with the ISO standard, and the system for conventional production of the functional units is hereafter called the compensatory system (see Fig. 2 for an illustration).