Life Cycle Analysis of PET to BHB Process
This life cycle analysis (LCA) evaluates the environmental impact of recycling polyethylene terephthalate (PET) plastic into beta-hydroxy-butyrate (BHB), quantifying the environmental advantages while also identifying key emission hotspots. The analysis focuses on the CO₂ emissions in units kg of CO₂ from plastic recycling, transportation, processing, and purification. The functional unit is 1 kg of PET plastic.
The recycling process of breaking down PET plastic into usable pellets produces significantly lower carbon emissions compared to the production of virgin plastic. Studies indicate that using recycled PET cuts CO₂ emissions by approximately 75% (Keul et al., 2024). Data from three different PET recycling facilities in Taiwan, which primarily utilise physical processing techniques such as washing, label removal, and shredding, show an average production amount of 0.631 to 0.741 kg of CO₂ per kg of PET recycled (Hu, 2025; Figure 2). The processing stage, where sorted and separated material is converted into usable feedstock for new products, such as pellets and flakes, is the most significant contributor to carbon emissions (Figure 3).
According to data collected in 2021, approximately 85% of plastic waste ends up in landfills, while 10% is incinerated, with a very small percentage being recycled (Osborne, 2022). PET waste in landfills undergoes decomposition that releases 0.253 kg of CO₂ per kg of plastic, excluding methane, a greenhouse gas 23 times more potent than CO₂ (IEA Bioenergy, 2003); whereas, incineration emits 0.673 to 4.605 kg of CO₂ per kg of plastic. Combined, the CO₂ emissions released from non-recycled plastic produce up to 0.676 kg of CO₂ per kg of PET waste. Recycling prevents these emissions by diverting plastic from disposal and, additionally, reducing demand for virgin plastic, which generates 5 kg of CO₂ per kg of PET (including 2.3 kg from production and 2.7 kg of embedded carbon) (Bauer et al., 2022). While recycling produces 0.631 to 0.741 kg of CO₂ per kg of PET recycled, relying on the production of virgin plastic and environmentally destructive disposal methods can result in a high of 5.676 kg of CO₂ produced, which is approximately 7.66 to 9 times greater than the amount produced through recycling.
A key component of our process's carbon footprint is the analysis of transportation; this model will utilise diesel-powered truck transport, the most cost-efficient and commonly used mode of transport in China, where we are based (Li et al., 2014). Given that this aspect of the project is purely hypothetical, we are basing the transportation information on facilities that can perform the processes necessary to produce our high-value products (HVP: PCA, PHB, and BHB). These calculations are based on the assumption that one trip is made from the recycling facility to the biorefinery, which holds 36,287.39kg of plastic pellets (Issacs & Issacs, 2024). The distance from Shanghai INTCO Industries (recycling facility) to Shanghai Yusong Biotechnology (biorefinery) is 68.6km. Assuming the diesel consumption of 10L per 100km holds for the model truck, 6.86L of diesel will be used. Based on the calculation methods provided by Michelin, 1 L of diesel produces 2.54 kg of CO₂(How to Calculate Your Fleet's CO₂ Emissions | MICHELIN Connected Fleet, 2025). Therefore, in one trip of 36,287.39kg of PET plastic, 17.424 kg of CO₂ would be produced. However, since the functional unit of this assessment is 1kg of plastic, dividing our initial number of 17.424kg of CO₂ by 36,287.39 kg of PET plastic would produce 0.00048 kg of CO₂ from transportation produced for every 1kg of PET recycled. It is also worth noting that our HVPs would hypothetically be produced at Shanghai Yusong Biotechnology. Hence, the only remaining transportation would be from the biorefinery to businesses that would sell our HVPs.
In our project, the process of converting PET waste to BHB involves three main steps: PET to TPA, TPA to PHB (where PCA can be produced as a byproduct), and then PHB to BHB. While further optimisation could occur, for the time being, this assessment accounts for the processes using three different bioreactors in the same facility. The energy required to operate a 500L bioreactor for 24 hours was found to be within the 8.4-37.2 kWh range, where the energy is allocated to aeration, agitation, temperature control, and pumping (Garcia-Ochoa & Gomez, 2009). The electricity used was assumed to have a carbon intensity of 545 g CO₂/kWh, in accordance with the average Chinese carbon intensity in 2022, resulting in a CO₂ emission of (8.4 to 37.2 kWh) × (0.545 kg of CO₂/kWh) = 4.578 to 20.274 kg of CO₂ per bioreactor. In total, running three 500L bioreactors for 24 hours would increase this range to 13.734 to 60.822 kg of CO₂ (Patel, 2025). Nonetheless, since the functional unit used for this LCA is 1 kg of PET, further calculations must be done using the yield value of converting PET to vanillin (VAN). Under the assumption that the conversion of PET to BHB is similar to the conversion of PET to VAN, as well as the assumption that the conversion of PET to TPA is efficient, there is a molar conversion rate of 71.1%, demonstrating a VAN production of 658.55 mg/L from 1992 mg/L of TPA (Li et al., 2024). With the given information, it is known that in a 500 L bioreactor, approximately 0.996 kg ((1992 mg/L × 500 L)/1000) of TPA will be used as feedstock. Thus, dividing the given range by 0.996 kg, 13.734 to 60.822 kg of CO₂, would result in the carbon emissions of running three bioreactors under the functional unit of 1 kg PET being 13.789 to 61.066 kg of CO₂. However, it is worth noting that the production of VAN from PET is highly inefficient, yielding only 259.2 mg/L from 20× diluted PET hydrolysates, which is significantly less than when using TPA as a feedstock; while viable, further research must be conducted to optimise the yield for commercial use (Li et al., 2024).
Following the production of BHB, filtration is needed to ensure a pure product. Filtration involves the separation of unwanted particles from biological materials to obtain contaminant-free products. It is a widely used process to ensure quality products that are safe for consumption by potential buyers. As there is insufficient information or research on BHB-specific purification processes, the following information regarding CO₂ emissions is based on the purification of cell-culture-based biopharmaceuticals, such as monoclonal antibodies, using the same method our product would employ: filtration. According to this study, the produced carbon emission for the purification process would be approximately 100 kg of CO₂ per 1000L batch from the bioreactor (Bernhard Spensberger et al., 2025); for our project, which would run three 500 L bioreactors, this results in a value of 150 kg of CO₂ per 1 kg PET used as feedstock for the bioreactor.
In total, the carbon emissions under the functional unit of 1 kg of PET range from 164.42 to 211.81 kg of CO₂.
While recycling lowers emissions compared to virgin plastic production, the production and purification process remains a significant contributor. Nonetheless, while recycling is limited by quality degradation, meaning most plastics inevitably end up in landfill or are unrecyclable, upcycling PET into high-quality products ensures that it is no longer plastic. This LCA confirms that PET recycling is environmentally favourable; however, optimising logistics for the subsequent steps (e.g., electric trucks, localised facilities, renewable energy sources) would further reduce the carbon footprint. Several studies have assessed the environmental impact of recycled plastics, and one such study by Zhang et al. (2020) identified organic chemical agents and the fibre-drawing from the recycling process as major contributors to environmental impact and costs. Their study suggested that adjusting the energy mix to reduce coal-fired electricity could mitigate pollution emissions from recycling PET plastics. Additionally, since this assessment was conducted on the assumption that the production process occurred in China, it must be noted that China's grid relies heavily on coal (~60% of electricity in 2023), which has high CO₂ emissions; if the facility later switches to renewables (e.g., solar/wind) to power the machines, the carbon intensity could drop significantly (IEA, 2023). Future assessments should explore alternative transport methods and energy-efficient recycling technologies.
Moreover, a large portion of the carbon footprint came from the bioreactors. In the future, additional research and optimisation of the process should enable the engineered E. coli to achieve a higher product yield of BHB from PET. Currently, the data indicate a low production value of directly converting PET into a high-value product, resulting in high carbon emissions for a minimal amount of product. Furthermore, a large portion of the electricity consumption of the bioreactors is due to the process of aeration, which is unnecessary in fermentation. Thus, the development of a PET to BHB pathway using fermentation could hypothetically reduce electricity consumption even further. With further biological advancements on the pathway to maximise yield, the production rate of BHB from PET would increase, thereby maximising yield and minimising the carbon footprint. It is essential to note that this life cycle assessment was conducted based on external research, with the assumption that electricity usage and production rates would be comparable to those in our project. This assumption introduces potential errors in the calculations; therefore, values must be approached with caution.