Recycling of plastic waste have been implemented for tens of years but very limited reuse in environmental engineering, in particular in wastewater treatment. This study evaluated the potential of reusing waste polyethylene terephthalate (PET) sponges as bio-carriers for the remediation of domestic wastewater. The test was conducted with domestic wastewater from the University administration building, using a lab-scale fixed bed bioreactor and run for nearly 03 months. Initial results revealed that the COD removal efficiencies were observed within the 70–80% range, yielding a mean effluent concentration of less than 50 mg/L, which adheres to the national standards of QCVN 14:2025/BTNMT, Column A. Ammonia-nitrogen (NH4-N) reduction efficiencies reached substantial levels of 94–96%, indicative of the robust proliferation of nitrifying microbial assemblages sequestered upon the substrate surface. Metagenon analysis rendered the presence of anaerobic bacterial genera, such as Romboutsia and Clostridium, together with specialized denitrifying groups, including Thauera and Pseudoxanthomonas, provides biological evidence of possible simultaneous nitrification and denitrification (SND) mechanism, thereby optimizing total nitrogen removal in a single treatment stage. These findings suggest that recycled PET can be served as a viable alternative to conventional, high-cost biocarriers within both aerobic and anaerobic biological treatment configurations.