Comparison of conventional activated sludge and aerobic granular sludge reactors for microplastics removal during municipal wastewater treatment

Abstract

The objective of this study was to investigate the performance of the conventional activated sludge (CAS) process and the relatively new biofilm process, aerobic granular sludge (AGS) for microplastics removal. In addition, the likely influence of aggregate extracellular polymeric substances on microplastics removal was assessed. Two lab-scale sequential batch reactors were operated for both processes for two experimental phases: an organic loading rate (OLR) of 0.9 and 0.6 kg COD m-3 day-1 with sodium acetate as the sole carbon source. The microplastics removal efficiency obtained in both phases was then compared. Microplastics removal was quantified using epi-fluorescent microscopy while adsorption to EPS was observed through EPS staining and confocal microscopy. The results showed that larger sized microplastics (100 – 150 µm) generally had higher removal efficiencies than smaller sized microplastics (40 – 90 µm). Overall, microplastics removal efficiencies obtained in the 0.6 kg COD m-3 day-1 phase (96% and 94% in CAS and AGS respectively) were higher than those measured in the 0.9 kg COD m-3 day-1 phase (69% and 77% in CAS and AGS respectively). This difference was likely due to the presence of more loosely bound EPS fractions and smaller-sized aggregates in the 0.6 kg COD m-3 day-1 phase. Furthermore, confocal microscopy investigations demonstrated that microplastics were mostly located either close to the surface of granules or at the edge of flocs suggesting that microplastics were adsorbed to the outer parts of aggregates. Therefore, both CAS and AGS technologies can effectively remove microplastics with small aggregate size and a disperse morphology.