What is a Life Cycle Assessment?
Life cycle assessment (LCA) is a systematic set of procedures used to assess the environmental impacts associated with a product’s life, from raw material extraction to disposal scenarios by examining the inputs and outputs of materials and energy associated. In our life cycle assessment, we did not include the raw material extraction, but rather just our disposal methods.
The scope of the LCA
disposal of it. Impact categories in this study were climate change and primary energy demand. These impact categories were selected because they are widely accepted and understood indicators of a sustainable process. The primary energy demand contains energy derived from fossil fuel and non-fossil fuel resources. Climate change was estimated with global warming potentials (GWP 100).
Fig. 1: Model design
Detailed flowchart of the model developed for this study for the degradation of 1 kg of Expanded Polystyrene. The boundaries of the system encompass the materials, pre-treatment, pyrolysis, fermentation, and the disposal scenario.
The system boundaries of the study (Fig. 10) include the following processes: 1) Blending of Expanded Polystyrene, limonene and ethanol, 2) Filtration, 3) Extraction, 4) Pyrolysis, 5) Distillation, 6) Styrene vaporisation, 7) Batch or media preparation, 8) Fed-batch or fermentation; 9) Disposal scenario of the biomass.
Each step of the Polystyrene degradation process generates more than one product. For that reason, the values of inputs and outputs were partitioned to imply the physical relationship between each step. This principle is known as mass-based allocation, and it helps to reflects how the quantitative changes in the products are affecting the inputs and outputs of the system (Curran, 2012). The products of the operations no longer required for the following step were considered as waste in the corresponding assembly. The mass-based allocation was used for this study.
The assumptions and simplifications made for the modelling of each step of the pre-treatment, fermentation, and disposable scenarios are:
- Only the operational phases of the process were considered, the equipment installation, maintenance of the equipment used are excluded from the model.
- Formerly, the product manufacturing and use phases were supposed to have the same environmental impact for any plastic and does not have repercussions in the life cycle of the degradation process. To summarise, the manufacturing and use stages are not considered as part of the present LCA.
- The use of limonene and ethanol were already probed and analysed by an LCA and shown lower environmental impacts than the use of other solvents (Noguchi et al., 1998).
- The limonene and ethanol used are fully recovered after using it and re-incorporated into the system.
- For the disposal scenarios, the model considered three options and the allocation was made equally, which means that 33% of the biomass after de styrene degradation goes to landfill, 33% to composting and 33% to incineration.
Modelled Life Cycle
The first step for creating an LCA is to define the assemblies and subassemblies and to enter the data under the corresponding category. The model was designed to have three levels (Figure 2). The first level is the LCA 1 kg EPS degradation, which comprises two big assemblies: the Degradation of 1 kg EPS assembly and Disposable scenarios assembly. At the same time, the Degradation of 1 kg EPS comprises two sub-assemblies: pre-treatment and fermentation. Finally, the bottom level includes the materials and energy of each operation.
The environmental impact categories of interest for this LCA are Climate change, abiotic depletion, ecotoxicity, eutrophication and human toxicity. For that reason, the Life Cycle Impact Assessment was done with CML-IA baseline impact method, as this method comprises of these categories.
Fig. 2. Life Cycle Assessment using the CML-IA baseline impact method for the model of 1 kg EPS degradation with 0.35% of visualisation.
Results and Discussion
Life Cycle Assessment
The full Life Cycle network consist of 6560 nodes (see Figure 2). The thickness of the red line shows the negative impact of each assembly to the LCA. In contrast, green lines show environmental benefit impacts derived from the recycling scenario of the solvents of the disposable scenarios impacts. Two different analyses were used for the evaluation of this model: CML-IA baseline impact method Cumulative Energy demand method.
Using the CML-IA methods, in Figure 2 with 0.35% of visualisation, it is clear that the subassembly of pre-treatment is the high contributor to the overall process of EPS degradation. The categories evaluated using this method were Human Toxicity, Ecotoxicity, Eutrophication and Global Warming Potential.
Figure 3. Normalised impact categories for the full life cycle of EPS degradation.
The health risks in the human environment derived from the use of toxic substances in a system are incorporated into the human toxicity category (PreConsultants, 2014). This category is one of least reported for LCA studies and without a doubt one is in need of improvement. In terms of synthetic biology, one of the major concerns is the use of genetically modified organism (GMO) and the risks that this entails. The risk that is not included in the human toxicity or any other toxicity category. Because of this there are several detractors stating that LCA studies are not suitable to evaluate processes using synthetic biology in any of its stages (Seager, et al., 2017). Even if the toxicity impact were actually higher due to the use of GMOs, only in the fermentation assembly would be reflected. For our general purposes, the use of toxic chemicals continues to be a matter of importance in the pre-treatment assembly stage as can be seen in Fig. 4, rather than the use of GMOs.
Figure 4. Human toxicity potential for EPS degradation assemblies. The results are referred to 1,4-dichlorobenzene equivalents/kg emission
The ecotoxicity potential categories indicate the impacts of the release of toxic substances to water and soil (PreConsultants, 2014). The pre-treatment processing continues to be the life cycle process which generates most of the environmental impacts. Over the three categories Freshwater, Marine aquatic and Terrestrial (Fig. 5) this pattern is followed. In the normalised results for the complete life cycle, the impacts of the Marine aquatic ecotoxicity (Fig. 3) stand out above the rest. The ecotoxicity categories once again failed to include the potential impact of using GMOs in the process. For this reason, the data is presented as an initial point for future work. This limitation is mainly due to the use of pre-existed methods for LCIA. For a better suitability of LCA for SynBio processes is needed an impact category that includes the risks of using GMOs, for example, the release of them, the current policies for their use, the problems of reproducibility and mutations in the strains.
Figure 5. Ecotoxicity potential for the EPS degradation assemblies. The results are referred to 1,4-dichlorobenzene equivalents/kg emission
The category that includes the potential impacts of disposing of macro-nutrients into the environment is Eutrophication (PreConsultants, 2014). By consulting the inventory for this impact, can be said that the substance responsible is the ammonia in the system. Making the Fermentation assembly the main contributor (Fig. 6).
Figure 6. Eutrophication potential. The results are expressed as kg PO4 equivalents per kg emission.
Global Warming Potential and Energy Demand
The Climate change category is implied in the Global Warming Potential (GWP) as the climate change is related to the emissions of greenhouse gases to air (PreConsultants, 2014). The carbon footprint of a material is one of the biggest issues in discussions of sustainability.
It is evident that the pre-treatment processing is contributing a major charge of GWP of the whole life cycle. However, the correlation suggested of the GHG gases due to the use of fossil fuels for energy production becomes more evidently with the GWP emissions of the Pre-treatment processing related to the energy consumption of the same assembly. However, this pattern is not followed by the Disposable scenario performance, because even when the Disposable scenarios are using energy for the process, their contribution to the emissions is negligible with respect to the Pre-treatment emissions.
Figure 8. Cumulative Energy Demand for EPS degradation assembly.
Cumulative Energy Demand Method
The cumulative energy demand is a valuable parameter to assess the sustainability performance of a process, rather than only focussing on the impacts generated by the use of substances. In Figure 8 can be observed that the Pre-treatment processing entails a great number of operations, for that is not surprising that is contributing with highest impact to the overall life cycle.
In an optimal condition of EPS pyrolysis and energy optimisation of the pre-treatment processing are achieved then the impacts should show a decrease. Making the whole life cycle more feasible. Correspondingly, the second step to improve is the yield of styrene degradation as the current rate is approximately 600 mg/L per 48 hours of fermentation. One possible scenario is the use of only the enzymes for the styrene degradation or couple a bioprocess of production, in that way the charge of energy and materials of the fermentation can be cushioned by the production of a high-value compound.
Figure 9. Life Cycle Assessment using the Cumulative Energy Demand method for the model of 1 kg EPS degradation with 1.5% of visualisation.
Analysis using the CML-IA method.
The thickness of the red line shows the negative impact of each step in the Pre-treatment assembly. In contrast, green lines show environmental benefit impacts derived from the recycling scenario of the solvents. However, the use of the solvents and the process involved as extraction are the ones contributing in big measure to the environmental charge.
As was mentioned before, the Pre-treatment processing is contributing with the 71.4% of the total impacts.
Analysis using the Cumulative Energy Demand.
The Pre-treatment assembly is contributing with the 57.4% of the whole degradation process.
The percentages for both methods point the Pre-treatment process as the most expensive in terms of energy demand and environmental impacts, making it susceptible for more study and research for making it more affordable.
The fermentation process which is the main focus of the project shows that it can be a feasible scenario in terms of environmental impacts and energy demand. However, the process has to be considered as a whole entity and for that reason; improvement to the Pre-treatment processing should be implemented.
For the LCA model the three options of Landfill, incineration and composting were evaluated in equal conditions. One of the conclusions is that the option with least environmental impact is the incineration, which could be the chosen option for the final step of Polystyrene degradation process.
Noguchi, T., Tomita, H., Satake, K., & Watanabe, H. (1998). A new recycling system for expanded polystyrene using a natural solvent. Part 3. Life cycle assessment. Packaging Technology and Science: An International Journal, 11(1), 39-44.
Seager, T. P., Trump, B. D., Poinsatte-Jones, K., & Linkov, I. (2017). Why life cycle assessment does not work for synthetic biology PRéConsultants, B. V. (2014). SimaPro. SimaPro 4 User's Manual.
Curran, M. A. (Ed.). (2012). Life cycle assessment handbook: a guide for environmentally sustainable products. John Wiley & Sons.