Efficiency of lagoon-based municipal wastewater treatment in removing microplastics.

Municipal wastewater treatment plants act as a sink, but also are a source of microplastics in the environment. A conventional wastewater lagoon system and an activated sludge (AS)-lagoon system in Victoria (Australia) were investigated through a two-year sampling program to understand the fate and transport of MP in such treatment processes. The abundance (>25 µm) and characteristics (size, shape, and colour) of the microplastics present in the various wastewater streams were determined. The mean values of MP in the influent of the two plants were 55.3 ± 38.4 and 42.5 ± 20.1 MP/L, respectively. The dominant MP size of influent and final effluent was <500 µm, with 25-200 µm accounting for >65 % of the total MP; synthetic fibres were the dominant MP in all wastewater streams. Influent MP concentration was significantly higher in summer than in other seasons for both systems, which was related to the lower plant inflow due to less stormwater entering the sewer during summer. The promising MP removal capability of the lagoon system (97 %) was attributed to its lengthy wastewater detention time (total HRT >250 days, including the storage lagoons) that would allow effective separation of MP from the water column via various physical and biological pathways. For the AS-lagoon system, the high MP reduction efficiency (98.4 %) was attributed to the post-secondary treatment of the wastewater with the lagoon system, in which MP was further removed during the month-long detention in the lagoons. The results indicated the potential of such low-energy and low-cost wastewater treatment systems for MP control.

Investigating the fate and transport of microplastics in a lagoon wastewater treatment system using a multimedia model approach.

Effluents of municipal wastewater treatment plants (WWTPs) are a major source of microplastics (MP) in the terrestrial and aquatic environment; there is growing concern over the environmental and health impacts of MP pollution. In this study, the MP removal (MP cut-off size= 25 µm) in a lagoon-based wastewater treatment system was predicted by developing a model based on the multimedia modelling approach and utilising MP-specific properties for improving the understanding of the fate and transport of MP in such treatment processes. The high MP removal efficiency of the lagoon treatment system as predicted by the model (99.3%) and determined with the site wastewater samples (97%) could be attributed to its high HRT (>200 days, including that for the storage lagoons) that would allow effective MP removal with the system. Evaluation of the model predictions of MP concentration demonstrated reasonable alignment with measured concentrations in the facultative, maturation and winter storage lagoons of the system. Further evaluation of model predictions for various MP size classes (25-100, 100-200, 200-500 and >500 µm) obtained reasonable predictions for MP within the size range of 25-500 µm, indicating that the model is better used for predicting MP within that size range. The sensitivity analysis revealed the model predictions to be sensitive towards the operating/water quality parameters in the order of influent wastewater flowrate, MP concentration in influent wastewater, and MP settling rate in the water column of the lagoon. The study showed the potential of the developed model as a quantitative assessment tool for better management of MP in lagoon-based WWTPs.