A number of technologies that are being applied to mixed municipal waste streams in Australia and elsewhere have been highlighted in this report. They vary from composting processes (which are net consumers of energy) to anaerobic digestion processes, which are net exporters of energy.
Costs associated with municipal waste processing are closely related to the complexity of the processes used. Composting processes that are used simply for volume reduction and stabilisation of municipal waste prior to landfilling are the lowest cost, and carry the lowest risk, but have the lowest landfill diversion rates. These processes are widely used in places like Germany, where experience has shown that it is difficult to produce composts that meet agricultural standards.
When a mixed waste composting process is geared towards producing saleable compost, it becomes more complicated. Contaminants need to be removed from the incoming waste stream or from the raw compost to meet standards for agricultural uses. These types of processes carry the greatest risk as far as sale of the resulting compost. Marketing of mixed waste-derived compost, when green waste-derived compost is available in the same market, has proven difficult in parts of Australia.
The trend in some parts of Europe (such as Germany), where waste incineration is not favoured, is towards plants that remove recyclables, remove the high calorific fraction, then use anaerobic digestion of the organic components of mixed wastes, with composting then used to stabilise the residuals from these plants before landfill disposal. Very often there are additional processes to remove combustible (but not easily recyclable) items and materials from the residuals (such as plastics) and produce refuse derived fuels.
Anaerobic digestion processes are more technically complex than composting processes, and therefore have a higher capital cost. However they produce a commodity (green energy), which is in high demand, and less stabilised material for landfilling than composting processes. To assist in understanding the differences between the various technologies currently used in Australia and selected technologies currently used Europe and North America, their features are shown in Table 17 and Table 18 below. Other technologies used outside Australia are compared in Table 19 and Table 20.
Table 17 Comparison of technologies used for municipal waste streams in Australia – (1)