Microalgal-Bacterial Granular Sludge under Emerging Stressors: Effects of Zinc Oxide Nanoparticles and Degradable Microplastics Derived from Polylactic Acid and Polycaprolactone

The increasing discharge of emerging contaminants (ECs) such as zinc oxide nanoparticles (ZnO NPs) and degradable microplastics (DMPs) into aquatic environments poses a growing challenge for next-generation wastewater treatment technologies. This study investigates the individual and combined effects of ZnO NPs, polylactic acid (PLA), and polycaprolactone (PCL) microplastics on microalgal-bacterial granular sludge (MBGS), a promising low-energy biotechnological system. Using photosequencing batch reactors, changes in extracellular polymeric substances (EPS), chlorophyll content, and microbial activity were monitored across multiple exposure scenarios. The results revealed significant, concentration-dependent alterations. While low-dose exposures (5 mg/L) to individual contaminants marginally stimulated ammonia oxidation or algal growth, combined exposures, especially ZnO+PCL and ZnO+PLA+PCL, induced synergistic inhibitory effects. EPS protein fractions decreased by up to 82%; chlorophyll content decreased by 52%, and specific ammonia and phosphorus uptake rates were reduced by more than 60% at a concentration of 10 mg/L. Principal coordinate analysis (PCoA) confirmed the pronounced system-level perturbations under mixed contaminant stress. These findings underscore the ecological vulnerability of MBGS to co-occurring ECs and highlight the need for pollutant-specific resilience strategies in the design and operation of sustainable wastewater technologies.