Hydrothermal processing of non-recyclable plastic waste for solid deconstruction

Abstract

Plastic is a useful material in terms of its ease of manufacturing as well as it’s everyday applications. However, the world’s population has rapidly grown, and so has the amount of plastic waste generated due to lack of infrastructure to handle it at present. This plastic waste eventually ends up choking the ecosystem as well as the food chain, with humans being affected the most for being at the top of this food chain. Many developed countries, until recently, exported all their non-recyclable plastic waste to developing countries in South-East Asia as well as China, keeping developed countries clean even though they are the primary cause of pollution. New Zealand faces similar problem at the moment with different rules applied for disposal of plastic waste by different city councils and have struggled to effectively treat and dispose of waste since the ban on import of plastic waste by previously generous developing countries. In this thesis, three of the primary sources of plastic waste, i.e., low-density polyethylene (LDPE), polypropylene (PP) and acrylonitrile butadiene styrene (ABS) were chosen due to the challenges it provides for conventional sources of recycling. In order to reduce the solid content of the plastic waste and recover valuable materials, laboratory-scale batch oxidative and non-oxidative hydrothermal processing was carried out between temperatures ranging from 250°C to 400°C. A considerable amount of plastic waste solid reduction of over 96% was observed through oxidative hydrothermal processing at all temperatures. However, non-oxidative hydrothermal processing introduced us with the challenges related to plastic waste with this technique. A mixture of this plastic waste was also processed at similar conditions to show the low level of selectivity with hydrothermal processing compared to mechanical and chemical recycling. Soluble chemical oxygen demand showed different trends for different materials due to the competing effects of solubilization and oxidation. However, with the introduction of green energy in future, this processing can help towards long term management of plastic waste providing the environmental benefit and at the same time improving financial and commercial benefit through the recovery of volatile fatty acids.