Characterisation and environmental assessment of recyclable waste from recycling centres

Recycling of waste has high priority in the European Union. The aim is to keep the functionality of resources within the anthroposphere, thereby reducing the pressure on the environment and increasing the security of supply. Despite an historical focus on energy recovery, Denmark is transitioning its waste management system towards increasing recycling of resources.

However, while acknowledging the large achievements in the last decades, recycling of household waste still holds large room for improvements. Contamination is considered one of the main issues as it may act as technical, safety, and market barrier to recycling, ultimately affecting the quality of the recycled products, i.e. the ability to maintain the material properties comparable with virgin resources.

Some material fractions exhibit larger recycling difficulties than others. Wood waste, plastic waste and small combustible waste are waste fractions for which characterisation studies are needed to estimate contamination levels and for which alternative management solutions may enhance current recycling.

Indeed, with increasing amounts of waste being recycled, there is a general interest to maximise the efficiency and effectiveness of recycling processes while ensuring clean and safe recycling loops. Similarly, it is necessary to ensure that recyclable resources do not enter waste streams not destined for recycling.

The goal of this PhD thesis was to assess the resource quality of wood waste, plastic waste and small combustible waste collected at recycling centres and link this to the recyclability of the selected waste fractions as well as the potential contribution to environmental savings. Recycling centres are manned collection points where the waste is sorted typically into 30-40 material fractions.

In Denmark, recycling centres represent the only collection method for wood waste and small combustible waste, and receives around 40% of source-separated plastic waste.Wood waste was sampled from three recycling centres and characterised according to product application, quality, and presence of material and chemical impurities.

Overall, wood waste mainly comprised Construction & Demolition and Furniture applications, which showed the highest contamination by material impurities and chemical impurities, respectively. Packaging wood waste was the cleanest product application, although it contributed with a minor share to the overall wood waste composition.

In particular, Low quality grade wood waste (i.e. wood waste treated for e.g. outdoor use and fibreboards) exhibited dramatically higher content of chemical elements such as As, Cr, Pb and PAHs. Improving the management of wood waste may entail routing the Low quality grade to energy recovery through separate collection.

Such alternative management system was tested by dynamic life cycle assessment (LCA). The results illustrated that global warming potential (GWP) savings could be increased by 5-58 times when recycling activities target only the upper qualities of wood waste. In this case, wood waste should preferably be recycled to floorboards or insulation boards, which can ensure substantial GWP savings due to substitution of long-lived or energy-intensive product.Hard plastic waste, plastic film waste and PVC waste were sampled from three recycling centres and characterised in terms of product applications, quality, polymer, presence of material impurities and colour.

The material composition appeared widely diversified across the three waste fractions: while plastic film waste mostly comprised Non-food packaging made of LDPE, PVC waste included only Construction applications. Conversely, hard plastic waste consisted of a wide number of applications and polymers, making it a very heterogeneous fraction to recycle.

The Low quality applications (Non-food packaging, Automotive, Construction and Other) were characterised by a larger presence of non-plastic parts, multi-polymer products and coloured products than High quality applications (Food packaging). Consequently, the material losses in case of recycling Low quality plastic in a typical European mechanical recycling plant were 117% larger than recycling High quality plastic waste, as demonstrated by material flow analysis.

As the effective removal of impurities influences the efficiency of recycling processes and the quality of the recyclates, three recycling scenarios were evaluated in terms of environmental and financial impacts. The results indicate that a mechanical recycling technology efficiently removing impurities can lead to large environmental savings and cost-effectiveness, as it produces recycled plastics characterised by high quality.

Small combustible waste was sampled from eight recycling centres and characterised with respect to material fraction, product application and, in the case of recyclable materials, other properties relevant when addressing the recyclability of the waste. Actual combustible materials constituted on average 38% of the waste, while 54% was identified as Recyclable materials, mainly in the form of plastics, textiles, paper and wood waste.

If these Recyclable materials, currently incinerated as small combustible waste, were redirected to recycling processes, the national household recycling rate for glass, paper, cardboard, metals, plastics, wood, WEEE, textiles and garden waste (calculated after sorting losses) could be increased by 16%.

Furthermore, the recycling of the Recyclable materials would enable save 27 kg CO2-eq/capita/year, representing an increase in avoided emissions of 30% compared to the current national savings from recycling the same recyclable fractions. The role of quality was demonstrated to be crucial throughout the recycling chain of wood waste, plastic waste and small combustible waste, indicating that from a circular economy perspective “better recycling” may be preferable over “more recycling”.