Microplastic degradation and interaction with organic micro-pollutants within wastewater treatment systems

Abstract

Microplastic (MP) pollution is released to the aquatic environment within wastewater effluent due to its ineffective removal by traditional wastewater treatment plants (WWTPs). Once dispersed in riverine and marine ecosystems, MP pollution presents a health risk to animals and humans. Developing effective and sustainable treatment systems capable of removing and destroying MPs before they exit WWTPs is essential in preventing this flow of pollution to the environment. Advanced oxidation processes (AOPs) have gained recent attention as a potential sustainable treatment solution, capable of complete mineralisation of MPs in water. As an emerging area of study, information on the effectiveness, mechanism of action and real-world applicability of these treatment methods is greatly needed. This research fills gaps in the knowledge of the effectiveness of several AOPs for removal of MPs from wastewater.

Microplastic fibres originating from laundry washing are often found in high numbers in WWTP effluent due to their tendency to pass through filter membranes. In Chapter 4, the effect of UV irradiation and hydrogen peroxide AOP treatment (UVC/H2O2) on the removal of polyester MP fibres was studied. Physical and chemical degradation of the fibres was followed with a range on analytical characterisation techniques. A dose of 500 mg L-1 H2O2 under 4.0 mW cm-2 UVC irradiation caused mass loss of 52.7% after 48 h. The degradation of polyester resulted in the formation of shallow holes, pits and cracks across the fibre surface as well as changes to the abundance of oxygen containing functional groups. In real hospital laundry water, the rate of degradation was impeded by the chemical oxygen demand (COD) of the wastewater.

Incomplete degradation of MPs can result in the formation of smaller fragments with altered surface chemistry. In Chapter 5, the impact of sulfate based oxidation on this transformation of polyester fibre MPs was investigated. Persulfate activation by light, heat and ultrasound were compared for their effect on the chemical and physical characteristics of the MPs. Ultrasound significantly increased the fragmentation with smaller microplastic fibrils forming during treatment. The most aggressive treatment in terms of mass loss, UVC/PDS treatment (31.8 mW cm-2, 500 mg L-1), reduced the mass by 18.5% in 9 h with clear surface pitting and cracking. Based on the rate at which MPs are degraded, incomplete mineralisation is likely to occur in wastewater treatment systems. The physical and chemical transformation observed at lab scale carries implications for the interaction of partially degraded MPs with organic contaminants in wastewater and aid in our understanding of MP fate.

MPs in the aquatic environment interact with organic micropollutants including antibiotics and can substantially increase their environmental mobility. Chapter 6 takes the antibiotic ceftazidime as an example to study this type of adsorption interaction. The effect of MP weathering degree, polymer type, size and shape of MPs are investigated using lab-scale batch adsorption experiments. As already shown in aforementioned chapters, the weathering of MPs drastically alters the chemical and physical characteristics of MPs and therefore impacts their environmental behaviour. Ten different MPs were weathered under simulated environmental conditions in order to investigate this effect further. Among the MPs studied, weathered PET fibers exhibited the highest adsorption reaching 1.432 mg g-1 after 48 h of contact time. The formation of holes, cracks and fragmentation of the MPs increased the surface area and therefore the number of adsorption sites.

Alongside these lab-scale investigations, Chapter 3 considers how proposed solutions to MP pollution exist within a complex landscape of policy and environmental regulation. The environmental and waste legislation in Scotland and the UK is complex and is currently in the process of change. As a case study to exemplify this, the MP and chemical pollution of sewage sludge and its use as a fertiliser material for land application is reviewed. This chapter shows that a sustainable solution to microplastic pollution from WWTPs must be considered as part of a much larger waste management system.