Moving Bed Biofilm Reactor Using Plastic Waste as Biofilm Carriers for Ciprofloxacin-Containing Wastewater Treatment

Pharmaceuticals and other organic matter can remain in municipal wastewater after conventional treatment, while MBBR carriers are typically made from virgin plastics. This study investigated mechanically modified post-consumer polypropylene (PP) drinking straws and perforated high-density polyethylene (HDPE) bottle caps as alternative microbial biofilm carriers for organic pollutant removal in a laboratory-scale MBBR. The reactors treated real secondary wastewater mixed with recycled sludge (1:1) and spiked with 0.1 mg/L ciprofloxacin (CIP). Three experimental series evaluated the effects of carrier filling ratio (10% and 30% PP straws) and carrier type (30% PP straws and 30% HDPE bottle caps) on pH, conductivity, UV254 absorbance, extracellular polymeric substances (EPSs), and UV280 absorption associated with wastewater containing CIP. A fourth series compared the more effective carrier configuration (30% HDPE bottle caps) with and without the addition of CIP. Increasing the filling ratio from 10% to 30% improved process stability and UV254 behavior, while the 30% HDPE bottle caps showed the most favorable overall performance. At equal filling ratio, HDPE bottle caps exhibited more stable behavior under the tested conditions than PP straws, achieving lower UV254 absorbance and more stable UV280 absorbance. The EPS dynamics reflected the growth and life cycle of the biofilm, while the no-CIP control confirmed that the UV280 fluctuations were related to the CIP-containing wastewater and not only to the soluble organic matter originating from the biofilm. Overall, post-consumer HDPE bottle caps showed promising potential as low-cost alternative MBBR carriers for treating ciprofloxacin-containing wastewater, while supporting the utilization of plastic waste and circular economy approaches in wastewater treatment.