For many years, composting was regarded as the most ecologically sensible way to utilize green waste. In the context of alternative energies, in recent years, green waste has increasingly been used as an energy source in biogas plants. In addition, the efficiency of energy use in the KVA’s has risen sharply in recent years and more and more raises the question of which is the optimal use of green waste from an environmental point of view.
This question was pursued in a research project of the BfE, in which the various types of utilization were examined with the method of life cycle assessment, with the aim of providing decision-making bases for the optimal material or thermal utilization of green waste. On the one hand eco-inventories in ecoinvent quality for different procedures (composting, fermentation and thermal utilization in a KVA) for different kinds of green goods were created. On the other hand, the basis of a tool was developed that can model the system of green waste utilization of a particular municipality or region and take into account the various interactions and regional conditions.
To achieve this goal, the following research questions had to be clarified:
- Emissions during composting and fermentation
Measured values from various research projects were discussed and evaluated together with an international panel of experts so that reliable emission factors are now available. These are much lower for both fermentation and composting than the values used to date in eco-inventories.
- Evaluation of humus formation
Various ways of assessing the humus in life cycle assessments were evaluated, further developed and discussed with international experts. Thus, a method could be found with which the organic substance in the compost or digestate can be assessed in life cycle assessments. The calculations have shown that the consideration of the organic substance has a relevant influence on the result.
- Heavy metals in compost and digestate
In the evaluation of heavy metals, it has been shown that great uncertainties occur. Depending on the evaluation method, the heavy metal emissions dominate the entire balance sheet. A revision of the methods or the corresponding inventories is necessary from our point of view. In this regard, some ideas were presented.
These research questions could be answered to a large extent and, if necessary, open questions could be identified. The created data sets for different types of greenery and utilization are made available to users of life cycle assessments via ecoinvent. This provides significantly better data for assessing the utilization of green waste. As system modeling has shown, this leads in part to confirmation and partly to other results compared to older studies. So it has, for example It has been shown that not one recovery process is always ecologically superior to the other, but that the choice depends on the type of greenery and the given circumstances.