Fate, characteristics, and potential threat of microplastics in sludge under various dewatering treatments

Microplastics (MPs), an emerging environmental pollutant, have been found in wastewater sludge with increasing frequency. Their occurrence, surface properties, and adsorption characteristics may be altered during various sludge dewatering processes. This study explored and compared the performance of four types of sludge dewatering processes (FeCl₃ + CaO, Fe²⁺ + H₂O₂ + CaO, Fe²⁺ + peroxymonosulfate (PMS) + CaO, and Fe²⁺ + CaO₂ + CaO) in improving sludge dewaterability, the fate and characteristics of MPs during treatments, and their effect on the adsorption of heavy metals by aged MPs. Results showed that iron-based advanced oxidation processes (Fe-AOPs) indicated superior performance in improving sludge dewaterability compared to conventional FeCl₃ + CaO treatment, as evidenced by the water content of sludge cakes being reduced to below 54.0 % (w/t) in Fe-AOPs. Fe²⁺ + PMS + CaO and FeCl₃ + CaO effectively reduced MPs concentrations in both dewatered sludge and filtrate, thereby mitigating potential environmental risks. The potential risk associated with heavy metal adsorption onto treated MPs was greater for Fe²⁺ + PMS + CaO than for FeCl₃ + CaO. In summary, Fe²⁺ + PMS + CaO offered a feasible method for sludge dewatering and MPs removal, particularly suited to sludge with low heavy metal concentrations. FeCl₃ + CaO treatment effectively mitigated co-toxicity between heavy metals and MPs, proving more suitable for sludge with high heavy metal content. This study offers new insights into the selection of appropriate sludge treatments regarding MPs.