A value-added insight of reusing microplastic waste: Carrier particle in fluidized bed bioreactor for simultaneous carbon and nitrogen removal from septic wastewater

Turning microplastic litter into reusable products is a resource-saving and eco-friendly strategy of disposing of plastic pollution. This work realized a value-added insight of reusing microplastic waste by processing the recycled microplastic items with clay waste into composite particle (CP). Furthermore, the systematic evaluation of CP particle was carried out in a Liquid-solid fluidized bed bioreactor (LSFBBR-CP) treating septic sewage with low COD to nitrogen ratio (C/N ≤3). The commercial inorganic particle, bioceramsite (BC), was tested in the LSFBBR-BC for comparison. During the 257-day stepwise operation, two LSFBBRs constantly achieved effluent of TCOD <45 mg L⁻¹ and low observed sludge yield (Y_obs ≤0.067 g VSS g^-1 COD). Comparatively, the lower fluidization velocity for the CP particles (u_l, 0.67-1.20 cm s^-1) not only contributed to high biomass attachment (17.2 mg VSS g^-1 CP particles) but also favored nitrifiers (Nitrosomonas, Nitrosospira, Nitrobacter and Nitrospira) enrichment. Additionally, the LSFBBR-CP outperformed the LSFBBR-BC in energy conservation by consuming 41.1–47.4% less energy for fluidization, due to the lower u_l. Contrary to BC particles’ attrition regarding to particle size decrease and surface chemicals loss, the CP particles exhibited superiority in physio-chemical stability. Those comparative advantages confirmed that the CP could be a promising alternative for BC as a carrier particle in LSFBBR, suggesting that turning microplastic litter into reusable carrier particle for FBBRs is feasible.