Fate and Control of Microplastics in Wastewater Treatment Plants

<p dir="ltr">Since the 1950s, global plastic production has increased dramatically, contributing significantly to plastic pollution. Microplastics (MPs), defined as plastic particles less than 5 mm in size, have become a major environmental concern due to their potential ecotoxicological impacts on ecosystems and marine organisms. They are mainly generated from the breakdown of larger plastic debris as a result of prolonged exposure to environmental conditions such as sunlight, wind, rainfall, and temperature fluctuations. Over time, these MPs may further fragment into nanoscale particles (<1 μm), commonly referred to as nanoplastics (NPs). MPs and NPs are emerging contaminants of global concern because of their persistence, ubiquity, and potential ecological risks in aquatic environments. Due to their small size and large surface-area-to-volume ratio, MPs/NPs are more readily ingested by aquatic organisms and have a higher potential to adsorb and release hazardous substances in water. NPs, in particular, pose more severe biological and ecological risks than MPs. Their colloidal nature allows them to remain suspended for long periods, increasing their potential for long-distance transport in water bodies and making them more difficult to remove using conventional treatment processes.</p><p dir="ltr">Wastewater treatment plants (WWTPs) play a crucial role in controlling pollution but also act as significant pathways for MP/NP release into the environment. MPs and NPs enter WWTPs via domestic and industrial wastewater from a variety of primary and secondary sources. Examples include microbeads and pellets used in cosmetics and cleaning products, as well as synthetic fibers released during laundry washing. Although conventional WWTPs remove over 90% of MPs, a notable fraction of smaller MPs (20-300 µm) and NPs (<1 µm) still pass through and are discharged into natural water bodies. Additionally, water shear forces generated during treatment processes such as mixing, pumping, and bubbling can further fragment MPs into NPs, potentially increasing NP concentrations beyond current estimates and elevating ecological and health risks. In WWTPs, larger MPs (≥300 μm) are generally removed during pre-treatment, and medium-sized particles (100-300 μm) during secondary and tertiary stages. However, smaller MPs (20-100 μm) and NPs (<1 µm) frequently bypass all treatment stages, highlighting the limitations of current wastewater treatment technologies for their effective removal.</p><p dir="ltr">Conventional treatment methods such as sedimentation, coagulation and dissolved air flotation (DAF) are designed primarily to remove organics, nutrients, suspended solids, and fats, oils, and grease (FOG) rather than MPs/NPs. Nevertheless, research has shown that some of these processes can also achieve partial removal of MPs from the liquid phase. Among them, DAF commonly used in primary treatment shows promise for MP removal due to the particles’ small size, hydrophobicity, and low density. However, DAF performance is highly dependent on parameters such as air pressure, bubble size, water flow rate, and pH. The addition of coagulants can enhance MP removal, with optimal dosage playing a key role. To address these challenges, this study aimed to better understand the fate and control of MPs in WWTPs. Therefore, the specific objectives of this study include, (i) detailed mapping (identifying, classifying, and quantifying) of MP in the WWTP, (ii) understanding the fragmentation mechanisms of MP into NP in the WWTP, (iii) removal of MP from the primary wastewater treatment stage using DAF process and (iv) optimization of DAF process for MP removal from wastewater.</p><p dir="ltr">To assess MP occurrence and removal in existing treatment systems, a year-long monitoring study was conducted at three water reclamation plants in Victoria, Australia. Wastewater samples were collected from different treatment stages. Various types of MPs, including polyester, polyethylene terephthalate, polypropylene, polyethylene, and polystyrene, were detected. Fibrous MPs were the most prevalent, accounting for 52-57% of all MPs. The treatment processes achieved high removal efficiencies (92-100%), with primary treatment alone removing over 60% of total MPs. However, a considerable number of MPs (3.63 × 10⁶ to 1.7 × 10⁸ MPs/day) were still released into the environment via effluent discharge. A reduction in MP size from influent (>500 μm) to effluent (100-300 μm) was observed, suggesting possible fragmentation into NPs during treatment. This finding highlights the importance of understanding and quantifying NP formation from MP breakdown within WWTPs.</p><p dir="ltr">In the next phase, the mechanisms of MP fragmentation into NPs during wastewater treatment were investigated. To simulate real WWTP conditions, mechanical mixing, sonication, and homogenization were applied to generate shear forces to break down weathered PE and PS particles sized 106–250 μm. Experiments showed that these processes produced energy densities ranging from 32 to 100 kJ/L, and the smallest NP (54.2 nm) was generated from the fragmentation of 100 μm PS particles during homogenization. Crack propagation on the MP surface under continuous stress from water shear forces was identified as the dominant fragmentation mechanism. The experimentally obtained NP size distributions were compared with modelled sizes estimated from crack propagation and failure models, showing strong agreement. This correlation supports the reliability of these models in predicting NP sizes likely to occur in WWTPs and informs strategies to minimize their environmental impact through effective removal. Overall, the theoretical models used in this study demonstrated strong reliability in predicting experimental results for MP fragmentation into NPs under controlled conditions using various devices and applied forces. The fracture energy model, in particular, proved effective in estimating fragmentation driven by water-induced shear forces. These findings highlight the importance of optimizing early-stage treatment processes to prevent MPs from progressing to later treatment stages, where they may fragment further into NPs. </p><p dir="ltr">Given the significance of enhancing early-stage removal, this study further explored the optimization of the DAF process for MP removal. DAF, widely used for solid-liquid separation in water treatment, offers a promising approach to capture MPs before they fragment or enter secondary treatment stages. A coagulation-assisted DAF system was evaluated, with operational parameters, including air pressure (3-5 bar), alum dosage (50-200 mg/L), polyacrylamide dosage (5-20 mg/L), water flow rate (400-700 mL/min), and pH (3-8), systematically optimized. Coagulation-assisted DAF achieved MP removal efficiencies exceeding 90%, likely due to enhanced hydrophobic interactions and charge neutralization. Removal efficiency increased to 95% in the presence of FOG, which enhanced MP hydrophobicity and supported the formation of larger, more buoyant flocs. Air pressure was found to be a critical factor in generating microbubbles (61-135 μm) at a fixed air saturation time of 3 minutes, significantly influencing bubble formation. NBs, however, showed potential advantages over MBs due to their higher surface area, greater stability, and ability to generate reactive oxygen species, which enhance pollutant aggregation and removal. Under 5 bar pressure, varying the saturation time produced a range of bubble sizes: MBs (5-203 µm) at 1-15 min and NBs (2 µm-395 nm) at 15-30 min. These were integrated into a micro-nanobubble (MNB) assisted DAF system. The optimized MNB-DAF process achieved up to 97% MP removal, even in the presence of dissolved organic matter and FOG, which further promoted MP aggregation and flotation. MBs provided the buoyant lift required for flotation, while NBs enhanced aggregation and stabilized bubble-MP interactions.</p><p dir="ltr">In summary, this research provides novel insights into the occurrence, fragmentation and removal of MPs in the WWTPs, highlighting the urgent need for improved primary treatment technologies. The proposed MNB-DAF approach offers a robust, scalable, and sustainable strategy to mitigate MP pollution at its source, with potential application in full-scale wastewater treatment systems.</p>