Hazard characterisation of microplastics in wastewater
Interactions between microplastics and chemical contaminants in wastewater can alter the contaminants’ impact.
Most wastewater contains chemical contaminants and microplastics, which enter via households, industry and the environment. An experiment was conducted to study how microplastics affect chemical contaminants in wastewater and how their presence alters the risk these chemicals pose.
Microplastics were added to wastewater and then exposed to marine conditions like those they would encounter after leaving the treatment plant. The chemical contaminants adhered to the microplastics in wastewater and remained bound when the particles were exposed to seawater. The sample was then exposed to conditions reflective of a marine organism's gut, which released the bound chemical contaminants. Marine organisms that consume microplastics are likely to be exposed to greater concentrations of contaminants than would be present in the surrounding environment.
The NSW EPA used these findings to support a precautionary approach to chemical regulation and as evidence to support actions to minimise microplastics entering wastewater treatment plants.
Most of the waste that goes down a drain is likely to end up passing through a wastewater treatment plant. During this process, screens remove larger plastic items and particles, but microplastics pass through. Most of these microplastics end up in biosolids, but some get discharged to waterways, where they may be consumed by marine animals.
Plastics are known to attract and retain various chemicals on their surfaces through a process called sorption. Once a chemical is sorbed onto a plastic, it may either be retained on the plastics surface or released back into the environment in a process known as desorption.
To better understand the impacts of microplastics on contaminants in wastewater treatment, sorption and desorption of chemical contaminants were measured in samples of wastewater, seawater, and within a simulated marine organism gut.
To determine which contaminants to examine, wastewater samples were collected from 2 wastewater treatment plants and tested for phenol, polycyclic aromatic hydrocarbons, and phthalates. From this, 3 contaminants of concern were identified for sorption and desorption analysis:
- bisphenol A (BPA)
- benzalkonium chloride
- triclosan.
Four of the most abundant microplastic types were examined:
- polyethylene terephthalate (PET) fibres
- polyethylene (PE) fragments
- PE synthetic fibres
- PE microbeads.
Contaminant sorption to microplastics was tested in wastewater, with desorption subsequently assessed in collected samples of seawater and simulated marine organism gut conditions over a 72-hour period.
All 3 contaminants were found to sorb to all types of microplastics examined, with triclosan consistently exhibiting the highest sorption rate, followed by benzalkonium chloride, and then bisphenol A (BPA).
The sorption rates of contaminants are largely determined by their different levels of solubility in fats. All forms of PE had similar sorption rates, which were higher than those of PET. PET likely shows lower sorption as it has a more ordered, tightly packed chemical structure with fewer potential sorption sites.
The high salinity and pH of seawater did not release contaminants back into the water column. Conversely, exposure to more acidic simulated gut conditions resulted in a significant release of benzalkonium chloride and triclosan from all microplastics. This suggests that marine animals consuming microplastics are likely exposed to higher concentrations of contaminants than they would encounter in the surrounding water alone.
The results of this study demonstrate that chemical contaminants can sorb to microplastics and may be delivering contaminants in higher concentrations to marine organisms.
The interaction between microplastics and chemical contaminants needs to be considered when managing wastewater. This means that relatively low levels of chemical contaminants may have a larger impact in the presence of microplastics.
The NSW EPA takes a holistic approach to managing contaminants, considering likely immediate, secondary, and long-term impacts. The NSW EPA has reduced microplastic concentrations entering wastewater treatment plants by banning plastic microbeads in rinse-off personal care items. Additional measures are also proposed to ban microbeads in drain-bound cleaning products and design standards to reduce microfibre release from washing machines.
Read the full report: Hazard characterisation of microplastics in wastewater (PDF 1MB).