Evaluation of Ultrasonic Pretreatment Effects on Biodegradable and Non-biodegradable Microplastics in Sewage Sludge

Microplastics in sewage sludge represent a formidable obstacle to the use of biosolids in agricultural lands. Ultrasonic pretreatment is extensively utilized in sludge management to enhance biodegradability, reduce organic contaminants, and improve process efficiency through cavitation-induced physical and chemical effects. This research evaluates the effects of conventional ultrasonic sludge pretreatment—operating at a frequency of 20 kHz, a power level of 400 W, a temperature of 25°C, and 15 minutes—on sludge containing biodegradable polyethylene (PE) and non-biodegradable polyvinyl chloride (PVC) microplastics. Structural, morphological, and chemical analyses were conducted using Scanning Electron Microscopy (SEM) and Fourier Transform Infrared Spectroscopy (FTIR). The SEM results, obtained at magnifications of 60× (PE) and 100× (PVC) with scales of 500 µm and 300 µm, revealed no major physical modifications, such as cracks, fractures, or surface erosion, in either PE or PVC microplastics. Similarly, surface morphology remained largely unchanged across all examined resolutions, including 500 nm and 300 nm, suggesting their structural stability following treatment. Likewise, FTIR spectra demonstrated the chemical resilience of both polymers, as not major alterations were observed in their characteristic absorption peaks (e.g., 2920 and 2850 cm⁻¹ for PE, 1430 and 1250 cm⁻¹ for PVC). Overall, the findings indicate that conventional ultrasonic sludge pretreatment exerts negligible influence on the physical and chemical integrity of the studied microplastics, highlighting their resistance to ultrasonic cavitation and potential persistence in sewage sludge used for agricultural applications.