Waste recycling is one of the essential tools for the European Union's transition towards a circular economy. One of the possibilities for recycling wood and plastic waste is to utilise it to produce composite product. This study analyses the environmental impacts of producing composite pallets made of wood and plastic waste from construction and demolition activities in Finland. It also compares these impacts with conventional wooden and plastic pallets made of virgin materials.
Two different life cycle assessment methods were used: attributional life cycle assessment and consequential life cycle assessment. In both of the life cycle assessment studies, 1000 trips were considered as the functional unit. Furthermore, end-of-life allocation formula such as 0:100 with a credit system had been used in this study. This study also used sensitivity analysis and normalisation calculation to determine the best performing pallet.
Result and discussion
In the attributional cradle-to-grave life cycle assessment, wood-polymer composite pallets had the lowest environmental impact in abiotic depletion potential (fossil), acidification potential, eutrophication potential, global warming potential (including biogenic carbon), global warming potential (including biogenic carbon) with indirect land-use change, and ozone depletion potential. In contrast, wooden pallets showed the lowest impact on global warming potential (excluding biogenic carbon). In the consequential life cycle assessment, wood-polymer composite pallets showed the best environmental impact in all impact categories. In both attributional and consequential life cycle assessments, plastic pallet had the maximum impact. The sensitivity analysis and normalisation calculation showed that wood-polymer composite pallets can be a better choice over plastic and wooden pallet.
The overall results of the pallets depends on the methodological approach of the LCA. However, it can be concluded that the wood-polymer composite pallet can be a better choice over the plastic pallet and, in most cases, over the wooden pallet. This study will be of use to the pallet industry and relevant stakeholders.
Pallets are used for storing, protecting, and transporting freight. They are the most common base for handling and moving the unit load, carried by materials handling units, such as forklifts. The pallet market is growing due to the rising standard of goods transportation, the adoption of modern material handling units in different industries, and market demand for palletised goods (McCrea 2016). It was estimated that the global pallet market reached 6.87 billion units in 2018 (Nichols 2020). More than 600 million European Pallets Association (EPAL) approved pallets are available to the global logistics industry. In 2019, 123 million wooden EPAL pallets and other carriers were produced, which is 1.2 million more compared to 2018 (EPAL 2020).
The global pallet market can be classified based on materials, sizes, and management strategies (Deviatkin et al. 2019). Among various segments of pallets, wooden pallets dominate the market share, followed by plastic pallets (Leblanc 2020). Wooden pallets are inexpensive and can easily be manufactured and repaired compared to rackable plastic pallets. One of the most significant downsides of wooden pallets is the cost to forests (Retallack 2019). Furthermore, wooden pallets are heavier than plastic pallets, imposing an environmental burden on freight shipment. Even though plastic pallets are lighter than wooden pallets, plastic pallets' production is an energy-intensive process. In addition, repairing plastic pallets is impossible because the materials have to be melted down and remoulded in the plastic pallet repairing process.
Waste recycling is one of the pathways taken by the European Union to move towards a circular economy, as highlighted in the circular economy action plan (European Commission 2020). The central idea of a circular economy is to minimise the consumption of virgin materials, which means that an item that can be recycled should not be landfilled or incinerated. The EU is planning to recycle 50% plastic and 25% wood waste by 2025, which will increase to 55% for plastic and 30% for wood by 2030 (European Commission, 2018). By following the EU's target, Finland's objective is to fortify its role as a pioneer in the circular economy by implementing the strategic programme for circular economy (Ministry of Employment and the Economy 2021). The transition to a circular economy is essential for Finland to strengthen its export-driven economy with minimum environmental impact.
The environmental benefits of recycled-based plastic products are well known and quantifiable (WRAP 2019). Also, materials made from wood waste can deliver low carbon-based products with less pressure on forests (WWF 2016). One of the possibilities for reducing the environmental burden of plastic and wood waste is to utilise these wastes for wood-polymer composite (WPC) products, such as WPC pallets. However, analysing the environmental performance of WPC pallets requires a complete life cycle analysis. Furthermore, it is important to consider that different materials have different life expectancies, reuse capabilities, and recyclability.
According to International Organization for Standardization (ISO), life cycle assessment (LCA) is one of the environmental management techniques that "addresses the environmental aspects and potential environmental impacts throughout a product's life cycle from raw material acquisition through production, use, end-of-life treatment, recycling, and final disposal" (EN ISO 14040:2006; EN ISO 14044:2006). Several LCA studies have been conducted on pallets focusing on pallet manufacturing, management strategies and supply chains, repair intensity, and pallets manufactured from various materials, such as wood, virgin plastic, cardboard, and waste plastic. Gasol et al. (2008) conducted an LCA study to compare the environmental performance of wooden pallets with high reuse intensity and low reuse intensity in the European context, and with the findings showing that due to transportation, high reuse intensity pallets have more adverse impacts on climate change than low reuse intensity pallets. Bengtsson and Logie (2015) performed an LCA comparing one-way wooden pallets, disposable compressed cardboard pallets, pooled softwood pallets, and plastic stackable pallets in Australia and China. The study results pointed out that pooled softwood pallets have the minimum environmental impact among all types of studied pallets. Tornese et al. (2018) examined pallets' economic and climate change impacts, demonstrating that manufacturing a pallet causes more damage to the environment than repairing a pallet. The study also identified that the cross-docking system has equivalent emissions as the take-back system due to higher transportation distance. Almeida and Bengtsson (2017) compared the LCA of waste plastic-based pallets with wooden pallets and virgin plastic-based pallets and demonstrated that plastic waste-derived pallets outperform all other alternatives. Franklin Associates (2007) compared the environmental impacts of pooled pallets versus non-pooled pallets. The study indicated that pooled pallets have less of an environmental burden than non-pooled pallets. Ko?í (2019) studied the environmental impact of wooden pallets, primary plastic pallets, and secondary plastic pallets. The study found that wooden pallets have a better environmental impact than primary and secondary plastic pallets if energy recovery occurs. Furthermore, the study also showed that the weight of the pallet plays a significant role on its total environmental impact.
The authors of previously conducted LCA studies analysed various pallets, making their cross-comparison a difficult task. Previous literature, including the above mentioned studies, have conducted LCA from an attributional point of view and excluded consequential LCA, which is thought to be an important method for identifying the changes in the system as a consequence of using a particular pallet. It is important to investigate the differences in the results, conclusions, and suitability of attributional and consequential LCA for cases where waste recycling is included. Furthermore, all the former studies assumed that various pallets perform equally well during their life cycle. None of the studies considered that pallets made with different materials have different life expectancies, repairing times, and recycling rates. In addition, end-of-life (EoL) is an integral part of the cradle to grave LCA. The methodological difference of the EoL allocation might have a significant impact on the overall result of LCA. It is found that the allocation of the environmental burdens of the EoL of the pallets was absent in the studies as mentioned earlier.
The goal of this LCA study was to calculate and assess the environmental impacts of manufacturing, utilising, and disposal of pallets made of different materials. Both attributional LCA (ALCA) and consequential LCA (CLCA) methods were used in the study. An ALCA investigates the environmental impact of the physical flows to and from a product's life cycle and its subsystems (Ekvall et al. 2016). In contrast, consequential LCA investigates the environmental impacts of the product system and the systems linked to it that are expected to change for production, consumption, and disposal of the product (Ekvall et al. 2016). Despite the ISO 14040/44 standards not explicitly distinguishing between the two types of LCAs, there is a clear difference in the definition of the scope for those assessments, as described below. The study results are intended to guide the selection of materials for the production of pallets.
Scope of the ALCA study
The attributional LCA follows the cradle-to-grave approach, meaning that the product system includes the processes starting with the provision of raw materials from the environment in the form of elementary flows, i.e. the flows created by nature, through the use of the pallets and ending with their disposal and with the release of emissions into air and water, and to the generation of waste.
The system boundary of the ALCA comparing the impacts of the pallet's production, use, and EoL is shown in Fig. 1. The modelling started with producing the raw materials and the energy generation for the pallets, such as wood harvest, timber production, and plastic production. It should be noted that the system boundary for WPC starteds from the collection of waste. Once the materials are produced and delivered to the production facilities, the pallets are manufactured. Nails are used to secure the parts of the wooden pallets, whereas plastic and WPC pallets are compressed into the required shape and do not require any fixing elements. The pallets are then delivered to a pallet pooling company, which operates by delivering the produced pallets to customers who can use them for their own purposes. After which, the pooling company collects the pallets and repairs them in the case of wooden pallets, if needed. After being used, the pallets are crushed for incineration. In the case of wooden pallets, ferrous metals are separated before incineration. By incinerating wooden, plastic and WPC pallets' waste, energy is substituted. Nevertheless, materials are also substituted by separated ferrous metals from wooden pallets.
There are no strict or specific requirements for modelling the EoL in LCA, and several allocation methods exist, such as 0:100 approach, 100:0 approach, 100:100 approach, 50:50 approach, etc. (Allacker et al. 2017). 0:100 EoL method can be conducted in two different ways, such as 0:100 with no credit for avoiding virgin materials and 0:100 with credit for avoiding virgin materials (Allacker et al. 2017). The system boundary of the study ends at the recovery of energy and material from the EoL phase. Therefore, in this study, the 0:100 EoL method with credit system had been used.
In the CLCA, the correct way of modelling environmental impact is to use marginal production technology data for the substituted product. Marginal production technologies are those technologies that are changed by the small changes in demand (Weidema et al. 1999). It was found from this study that a significant amount of heat and electricity substitution was impacted when wood and plastic waste were not incinerated but used for WPC pallet production. In this case, marginal heat and electricity were used in the modelling of CLCA. Biomass will be the prime heat production source in Finland by 2030 (Ministry of Employment and the Economy 2017), and wind and solar power will provide the maximum share of electricity by 2030 (SKM Market Predictor 2019). Therefore, the biomass-based heat source was selected as the marginal heat source and wind, and solar-power-sourced electricity was selected as the marginal electricity source in CLCA modelling. The more detailed information on the selection of marginal heat and electricity is presented in the supplementary materials.
Selection of the pallets
A great variety of pallets exists, as dictated by the specific requirements of customers. However, this study exclusively focused on pooled pallets, with the dimension of 1200 mm?×?800 mm, made of either wood, plastic, or WPC. The pallets with the above-specified dimension are widely known as EUR pallets and are the most widely used type of pallets in Europe (EPAL 2019).
Table 1 specifies the key parameters of the studied pallets in their baseline scenario. Wooden pallets are made of virgin wood, which is a mixture of softwood and hardwood as specific to Finnish conditions. The studied wooden pallets were block-type pallets, which are commonly used in Europe. Based on the review of LCA studies of wooden and lightweight plastic pallet by Deviatkin et al. (2019), the expected lifetime of the wooden pallets is 20 cycles, yet the number ranged between 5 and 30 cycles in most of the publications reviewed. The repair need of 7 cycles was estimated based on the mass of produced EUR pallets in Finland (3.2?×?103 kg), alongside with repaired (25?×?103 kg) and reused (167?×?103 kg). The expert views from a Finnish pallet pooling company suggested that the expected lifetime of the wooden pallets is somewhat higher, whereas the repair need for the pallets occurs on average after every 12 cycles. The variations in the expected lifetime of the pallets were examined in the scenario analysis of this study. It was assumed that, at the EoL, 90% of wooden pallets are incinerated, whereas 10% are used as a bulking agent in composting facilities.
The plastic and WPC pallets are identical in structure and production method. Plastic pallets are manufactured using injection moulding, whereas WPC pallets are produced by extrusion followed by a compression moulding process. Both pallets are made to allow their nesting, thus saving the space occupied by the pallets. The exact height occupied by wooden stackable pallets can fit 1.7 times more plastic or WPC pallets. According to the literature on plastic pallets, plastic pallets are more durable than wooden pallets (Deviatkin et al. 2019). The expected lifetime of Double Sided Plastic Pallets could be 66 cycles, whereas the lifetime ranges from 50–100 in most of the studies reviewed (Deviatkin et al. 2019). In this study, the lifetime of plastic pallets was considered to be 66 cycles by following the review study conducted by Deviatkin et al. (2019). The WPC pallets were assumed to be of comparable properties as plastic pallets in these terms. Plastic and WPC pallets are suitable for demanding applications, such as those with expected exposure to water, or specific industrial demands, like those of the pharmaceutical industry. Such features of plastic and WPC pallets are, however, not considered in this study. Once damaged, neither plastic nor WPC pallets can be repaired.
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