Discussion

In common with other LCA’s of plastic recycling processes, the boundaries of this LCA excluded the end-of-life impacts of the four products. These exclusions include disposal, landfilling, further recycling and reuse, which could provide a longer term view of the impacts. It is well known that in comparison to PP, steel has a lower impact in the disposal stage because of its easy recyclability (Guo et al. 2014). Thus it is reasonable to assume that if end-of-life options were considered, the conclusions of this study could be different. However in Australian context, we do not believe that including end-of-life impacts would alter the final conclusions. Firstly, based on a study of the long term durability of plastic fibres in alkaline environments (EPC 2012; Roque et al. 2009), all four scenarios would require the same level of maintenance and have equal life spans. Furthermore, despite being easily recycled, end-of-life concrete footpaths in Australia are almost always landfilled, as sorting and recycling for the small volumes is considered uneconomical. In essence, the processes that would lead to different end-of-life impacts are, in practice, unlikely to occur. We would also argue that the extent of these end-of-life impacts would not be sufficient to make up for the substantial differences that arise during production. Furthermore, because the use of fibres in footpaths is a relatively new application, end-of-life impacts are not well defined. Including these estimates would introduce more uncertainty, and may not be useful for decision makers. However, we would recommend that the potential for water based pollution (i.e. toxicity) associated with PP leaching during the use phase, be more carefully considered.

From a pragmatic point of view, the main objective of this research is to provide decision makers with accurate information on environmental impacts from plastic waste collection stage to the production of recycled PP fibre. This LCA study only considers the environmental impacts of the four products, and it is worth noting that other factors such as costs, markets for recovered products and national and international policy and regulations are not taken into account. Uncertainties regarding the quantities of required raw materials were considered in this study. However, uncertainty accuracy of data and data inventory has not been considered here, thus a more thorough uncertainty analysis using primary data is recommended for future studies.

Since this study is based on the Australian context and Australian inventory data, it is interesting to compare this work with the LCA by Shen et al. (2010), who assessed the impacts of recycled and virgin plastic fibres (PET) in a Western European context. Both studies used the same system boundaries, namely, starting from the point the plastic products become a waste and ending at the point that they become recycled plastic fibres. Although it was not clear if the PET fibres in Shen et al. (2010) were able to be used directly in concrete reinforcing, according to our judgement, the PET fibres would not require additional processing to be fit for purpose. Thus the comparative impacts remain valid and relevant to our study. Collection, sorting, washing, reprocessing, transportation and fibre production were included, but end-of-life impacts were not considered. The results in both studies are very similar, which gives us confidence in our methodology and sampled data. In comparison, the production of 40 kg domestic recycled plastic fibre in Australia has more global warming impact (109 kg CO2 eq) and eutrophication impact (0.069 kg PO4 eq) than those in Western Europe (81 kg CO2 eq and 0.044 kg PO4 eq). For the production of 40 kg virgin fibre, the two regions have a similar global warming impact (around 161 kg CO2 eq), while Australian eutrophication impact (0.095 kg PO4 eq) is much higher than in Western Europe (0.048 kg PO4 eq), likely the result of more sophisticated water treatment and recycling initiatives in Western Europe. However, across all impact categories the comparisons between virgin and recycled fibres follow similar trends, and differences in absolute values arise because of local variations. These include differences in collection methods and frequency, transportation infrastructure and distances, as well as differences in the maturity of sorting, reprocessing and fibre production methods. Moreover, the different plastic types considered in these studies also contribute to differences in the calculated impacts.

 
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