2.2.1. Description

Composting is a naturally occurring process of aerobic breakdown of natural matter by microorganisms. Reyes-Torres et al. [22] carried out a systematic review of green waste composting. The raw input material for composting is biowaste, as defined earlier. The process itself is relatively simple. However, the composting plant has to be controlled and operated well. In another case, there is an increased risk of excess odour, and greenhouse emissions production, low quality of the output products, and the process itself can be considerably slower [22]. Details of the composting process and technology used can be found in [23] and are not described in this paper in detail.

Fermentation, when compared with composting, occurs without access to oxygen—it is an anaerobic process. It is also often described as the anaerobic digestion process. The input material for this treatment process is any biologically degradable compound called the substrate. The output products are two—digestate, which is usually present in liquid form; and biogas with its two main components, methane and carbon dioxide. The process of anaerobic digestion is a complex of chemical reactions and its description and other details on fermentation processes are given in, for instance, in [12]. Fan et al. [24] analysed the carbon emission footprint in pre-and post-treating MSW in fermentation processes.

An LCA analysis is used to determine GHG production from these two processes. As the reference unit, the disposal of one ton of separately collected biowaste (SEP-BIO) is set.

The following processes are to be considered within the system boundaries (Figure 2): the specific waste treatment process (fermentation, composting) itself, including the further treatment of intermediates from the disposal process and all linked material and energy, flows related to the need for materials and supplies. In addition, the emissions from the collection and transport of the inputs (SEP-BIO) are considered. Additional benefits such as energy and secondary materials (e.g., fertilisers, organic matter) result from the disposal processes. Corresponding amounts of energy or products/articles do not need to be produced conventionally from primary processes. The environmental impacts that would be associated with the conventional manufacturing/production of each of the substituted primary raw material are thus "saved" or "avoided". The provision and maintenance of infrastructure (construction, service and repair of buildings, machine, industrial facilities) are not considered, as they are not expected to have a decisive influence.

**Figure 2.** System boundaries for separately collected biowaste (SEP-BIO) treatment routes: (**a**) composting; (**b**) fermentation.
