Study on surface modification of HDPE waste plastics to make bio-carriers for nutrient-rich domestic wastewater treatment

For several decades, the valorization of plastic waste has been a top concern for the scientific community. Informed by prior research, this work explores the viability of reusing bottle caps as bio-carriers after their surface modification. The High-Density Polyethylene (HDPE) bottle caps were modified using KMnO 4 , H 2 SO 4 , HCl and NaH 2 PO 4 solutions, then employed as bio-carriers in the fixed bed bioreactor for treating nutrient rich domestic wastewater (NH 4 + in = 50 mg/L − 162 mg/L; COD in = 490 mg/L − 570 mg/L). The initial results revealed that the removal efficiencies for organic matter and ammoniacal nitrogen of these modified carriers were 81 ± 1.9% and 86 ± 9.5%, respectively. The 4.5% increase in specific surface area contributed to slightly higher densities of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) on the modified carriers. The addition of modified plastic caps as carriers in both anoxic and oxic tanks helped improved the total nitrogen reduction efficiency from 77.9 ± 4.0% to 82.3 ± 7.3%. Full-length 16 S rRNA metagenomic barcoding analysis revealed that the microbial communities within the modified carriers from oxic tank were highly diverse, abundant, and evenly distributed with 399 bacterial genera, 626 bacterial species detected. In addition to Caldilinea and Nitrospira , other prevalent species included Romboutsia (3.02%) and Thermomonas (2.96%), the latter known for its nitrate-reducing capabilities. The metagenomic analysis and treatment efficiency confirmed the possibility of using this kind of carriers for nitrogen removal from nutrient-rich wastewater.