Microplastic and natural sediment in bed load saltation: Material does not dictate the fate.

Microplastic (MP) pollution is a well document threat to our aquatic and terrestrial ecosystems, however, the mechanisms by which MPs are transported in river flows are still unknown. The transport of MPs and natural sediment in aquatic flows could be somewhat comparable, as particles are similar in size. However, it is unknown how the lower density of MPs, their shape and their different material properties impact transport dynamics. To answer this, novel laboratory experiments on bed load saltation dynamics in an open-channel flow, using high-speed camera imaging and the detection of 11,035 individual saltation events were used to identify the similarities and differences between spherical MPs and spherical natural sediments transport. The tested MPs and sediment varied in terms of size and material properties (density and elasticity). Our analysis shows that the Rouse number accurately describes saltation length, height, transport velocity and collision angles equally well for both MPs and natural sediments. Through statistical inference, the distribution functions of saltation trajectory characteristics for MPs were analogous to natural sediment with only one sediment experiment (1.4% of cases) differing from all other plastic experiments. Similarly, only nine experiments (9.3% of cases) showed that collision angles for MPs differed from those of natural sediment experiments. Differences observed in terms of restitution become negligible in overall transport dynamics as turbulence overcomes the kinetic energy lost at particle-bed impact, which keeps particle motion independent from impact. Overall, spherical MP particles behave similarly to spherical natural sediments in aquatic environments under the examined experimental conditions. This is significant because there is an established body of knowledge for sediment transport that can serve as a foundation for the study of MP transport.

Microplastics in the riverine environment: Meta-analysis and quality criteria for developing robust field sampling procedures.

Current sampling approaches for quantifying microplastics (MP) in the riverine water column and riverbed are unstandardised and fail to document key river properties that impact on the hydrodynamic and transport processes of MP particles, hindering our understanding of MP behaviour in riverine systems. Using ten criteria based on the reportage of the catchment area, river characteristics of sampling sites and approach, we reviewed the sampling procedures employed in 36 field-based river studies that quantify MP presence in the water column and benthic sediment. Our results showed that a limited number of studies conducted reliable sampling procedures in accordance with the proposed quality criteria, with 35 of the 36 studies receiving a score of zero for at least one criterion, indicating the omission of critical information relating to the study's sample size and the physical and hydraulic characteristics of the sampled river. On the other hand, a good number of studies adequately documented the spatial information of the sampling sites, the vertical location of sample collection, and sampling equipment used. An idealised MP sampling approach is presented to ensure that future studies are harmonised and variables underpinning MP transport in rivers are reported. In addition, a meta-analysis on MP particle characteristics from these studies found that concentrations in the riverine water column and benthic sediment are highly variable, varying by five and seven orders of magnitude respectively, and are heavily dependent on the sampling equipment used. Polypropylene (PP), polyethene, (PE), polystyrene (PS), polyethylene terephthalate (PET) and polyvinyl chloride (PVC) were the most frequently reported MP polymers, while irregular-shaped particles, fibres, spheres, and films were the most commonly reported shapes in the river studies. These results highlight the urgent need to standardise sampling procedures and include key contextual information to improve our understanding of MP behaviour and transport in the freshwater environment.

Microplastics removal from a primary settler tank in a wastewater treatment plant and estimations of contamination onto European agricultural land via sewage sludge recycling.

Wastewater treatment plants (WwTPs) remove microplastics (MPs) from municipal sewage flow, with the resulting bulk of MPs being concentrated within generated sewage sludge which is frequently recycled back onto agricultural land as accepted practice in many European countries as a sustainable fertiliser resource. This circular process means that MPs successfully removed from WwTPs are deposited into the soil and able to return into the natural watercourse by means of run-off or infiltration to groundwater. This study quantifies the removal efficiency of MPs with size ranging between 1000 and 5000 µm in a primary settlement tank (PST) at a WwTP serving a population equivalent of 300,000 and provides MP concentrations in the generated sewage sludge. Our study revealed that the proportion of MPs partitioning in a PST to settled sludge, floating scum and effluent was 96%, 4% and 0% respectively, implying 100% removal of MPs of 1000-5000 µm in size. The generated sewage sludge was estimated to contain concentrations of approximately 0.01 g of MPs or 24.7 MP particles per g of dry sewage sludge solid, equivalent to ∼1% of the sewage sludge weight. Using these figures and data from the European Commission and Eurostat, the potential yearly MP contamination onto soils throughout European nations is estimated to be equivalent to a mass of MPs ranging between 31,000 and 42,000 tonnes (considering MPs 1000-5000 µm in size) or 8.6×1013-7.1×1014 MP particles (considering MPs 25-5000 µm in size). An estimated maximum application rate of 4.8 g of MP/m2/yr or 11,489 MP particles/m2/yr, suggests that the practice of spreading sludge on agricultural land could potentially make them one of the largest global reservoirs of MP pollution. Hence, recycling raw sewage sludge onto agricultural soils should be reviewed to avoid introducing extreme MP pollution into the environment.