Organic micropollutants (OMPs) are commonly detected in municipal wastewater. Conventional activated sludge processes partially remove these compounds, allowing them to enter receiving waters and pose ecological risks. Biotransformation, governed by microbial community composition and activity, is the main pathway for OMP removal. Aerobic granular sludge (AGS), with its distinct structure and microbial communities compared to conventional activated sludge, has emerged as a promising alternative. Full-scale AGS reactors contain predominately large granules (>1 mm), alongside medium (0.2–1 mm), and small (<0.2 mm) fractions, which differ in morphology and microbial composition and may influence OMP biotransformation. To date, the potential of different AGS size fractions for OMP biotransformation at environmentally relevant concentrations (1 µg L−1) remains poorly understood. This study evaluated the biotransformation of 23 OMPs (pharmaceuticals and industrial compounds) under nitrifying, aerobic heterotrophic, and denitrifying conditions, using batch microcosm with six AGS size fractions collected from a full-scale AGS plant. Eight OMPs (sulfamethoxazole, atenolol, furosemide, benzotriazole, trimethoprim, diclofenac, metoprolol, and gabapentin) showed biotransformation efficiencies above 10 % under at least one condition. Under aerobic conditions, smaller fractions showed higher biotransformation rate (Kbio), reflecting increased nitrifier and aerobic heterotroph activity. Under denitrifying conditions, three OMPs were biotransformed > 10 %, but Kbio did not correlate clearly with denitrifying activity, likely due to heterogeneous denitrifier distribution across size fractions. At the system level, AGS showed slightly lower Kbio than activated sludge, as smaller, more active AGS fractions comprised less than 40 % of total biomass in full-scale reactors. This study is the first to assess OMP biotransformation across AGS size fractions, highlighting the combined effects of granule size and bioconversion conditions. The findings provide insights for optimizing AGS systems, including potential granule size adjustments, to enhance OMP biotransformation and reduce environmental impacts. ... Read more

Aerobic granular sludge (AGS) process is an effective wastewater treatment technology for nutrient and organic matter removal and is being widely applied worldwide. To date, its performance in removing organic micropollutants (OMPs), particularly under wet weather conditions when operation differs, remains poorly understood. This study evaluated the occurrence and removal of OMPs, including 19 pharmaceuticals and 2 industrial compounds, in a full-scale AGS plant during one year under both dry and wet weather conditions. Under dry weather conditions, influent concentrations of 5 pharmaceuticals and 1 industrial compound exceeded 1 μg L−1. Rainfall resulted in diluted OMP influent concentrations, but also caused a significant increase in the influent load of 6 OMPs with positively charged functional groups, likely due to mobilization of sewage sediments that had adsorbed these OMPs. Under dry weather conditions, average removal efficiencies of 14 compounds were greater than 20 %, with 6 of these compounds detected in the sludge phase, and thus likely removed through sorption. Under wet weather conditions, OMP removal efficiencies decreased by 8 % to 38 %. Shortened aeration reaction time significantly reduced (p-value<0.05; R2>0.5) the removal of 8 potentially biodegradable compounds, while the impact on sorption-driven removal was limited for 6 compounds. Effluent OMP load increased under wet weather conditions, mainly due to reduced removal efficiency, rather than the discharge of OMPs adsorbed onto suspended solids. Under dry weather conditions, the AGS plant exhibited comparable or slightly higher OMP removal efficiencyies than activated sludge plants; however, differences in performance under wet weather conditions remain unclear due to limited data on activated sludge systems. Overall, this study is the first to assess OMP removal in a full-scale AGS plant under wet weather, showing the impact of increased flow on the sorption and biotransformation of OMPs. ... Read more

Protozoa contribute to water purification through predation in wastewater treatment systems. Full-scale aerobic granular sludge (AGS) reactors treating municipal wastewater contain AGS of varying sizes, with those larger than 2 mm dominating. These size fractions exhibit different sludge morphologies and microbial communities. To date, little is known about protozoan communities and their role in the removal of human-associated bacteria (like pathogens) in AGS plants, particularly across different size fractions. This study conducted uptake experiments with fluorescent Escherichia coli, as a model for human-associated bacteria, followed by microscopic observation to investigate protozoan communities and their predatory behavior in six AGS size fractions and activated sludge collected from full-scale municipal wastewater treatment plants. Sessile ciliates, particularly Epistylis and Vorticella, dominated protozoan populations across six AGS size fractions, with Epistylis being more abundant in larger AGS fractions (>1 mm) and Vorticella in smaller fractions (<1 mm). Additionally, microcosm experiments under aerobic (including predation) and anoxic conditions (excluding predation) revealed that predation was likely to be the main E. coli removal pathway, contributing an additional 0.5 to 2.5 log10CFU mL^–1reduction over a combination of non-predatory biological and abiotic processes. Larger AGS fractions showed greater predation capacity, linked to higher Epistylis abundance, while activated sludge, dominated by Vorticella, resembled smaller AGS fractions with lower predation capacity. These findings advance the understanding of the distribution of protozoan communities and their contribution to E. coli removal by predation in AGS wastewater treatment. ... Read more

Aerobic granular sludge (AGS) is an effective wastewater treatment widely applied worldwide for the removal of nutrients and organic matter. However, limited information is available on the effectiveness of full-scale AGS plants for removing organic micropollutants (OMP), particularly during wet weather. This study investigated the occurrence and removal of OMPs, including 19 pharmaceuticals and 2 industrial compounds, in a full-scale AGS plant during both dry and wet weather over one year. We selected a full-scale AGS plant located in Utrecht, the Netherlands as the targeted AGS plant, and collected monthly 24-hour composite water samples and grab sludge samples f rom May 2023 to April 2024. 21 OMPs were extracted from water and sludge samples and measured by LC/MS. Concentrations of ammonia and organic matter (both particulate and soluble) were measured with HACH Lange GMBH kits.

The results show that Influent concentrations of 5 pharmaceuticals and 1 industrial compound exceeded 1 ?g L-1, which were diluted by rainfall. Influent loadings of 7 OMPs (absolute OMP amount in ?g day -1) were significantly increased during wet weather, likely from sewage sediment resuspen sion and urban runoff. Average removal efficiencies of 11 compounds achieved greater than 20%, with 5 of them exceeding 50%, in the AGS plants. Simple linear regression results between OMP removal efficiencies and flow rates showed that biodegradable OMPs were more strongly affected by increased influent volumes (as indicated by steeper slopes) than OMPs primarily removed through sorption. Additionally, correlation analysis results showed that the removal of soluble organic matter was significantly correlated (p-value < 0.05) with the removal of 14 OMPs, suggesting that organic matter removal may be an indicator for OMP removal in the AGS plant. After AGS treatment, 8 compounds in the effluent remained above their predicted no-effect concentration levels, indicating potential ecological and human health risks in the receiving water. During wet weather, effluent OMP loadings increased mainly driven by reduced removal efficiency. Additionally, compared to activated sludge plants, the AGS plant exhibited comparable or slightly higher OMP removal efficiency during dry weather. Overall, this study is the first to investigate OMP removal during wet weather in a full-scale AGS plant and propose the potential impact of increased flow rates on biodegradable OMPs or OMPs mainly removed through sorption. ... Read more

Aerobic Granular Sludge (AGS) is a relatively new approach to wastewater treatment. As opposed to activated sludge with mainly flocculant biomass of relatively even composition, the gra nules in AGS differ in size and composition. Biomass ranges from flocs similar to activated sludge to granules of >6 mm. Granules also have distinct biomass due to the stratification of redox conditions across the granules. Typically organic micropollutants (OMPs) are removed via sorption and biodegradation during wastewater treatment, however this has yet to be thoroughly invested for AGS systems. We hypothesized that different sized granules have potentially different sorption behaviour due to the differe nces in extracellular polymeric substances (EPS) and have different biodegradation patterns due to the differences in biomass composition among size fractions. Therefore, we investigated the capacity of AGS to remove OMPs via sorption and biodegradation by performing controlled batch experiments in the lab. We separated the sludge into different sizes in order to understand the role of different sludge composition on OMP removal.

We observed notable sorption (>40% removal) for 10 of the 24 OMPs tested in our study (Figure 1). The 10 OMPs include 3 fluoroquinolones (norfloxacin, ofloxacin, and ciprofloxacin), 3 macrolides (clarithromycin, azithromycin, and erythromycin), 2 beta-blockers (propranolol and atenolol), tetracycline, and citalopram. We noted that all of these 10 compounds are ionizable, with 6 positively charged compounds and 4 zwitterionic compounds at pH 7. Considering that sludge biomass typically has a negative charge, this seems to indicate electrostatic interactions between the 10 OMPs and th e sludge. Larger fractions contributed more to sorption than smaller granules and flocs, as expressed per unit of biomass. Specifically, the normalized Kd¬ for large fractions was up to 100%larger than for small fractions, suggesting that larger fractions contribute more to sorption in real AGS systems than smaller fractions and flocs. However, sorption kinetics were most likely retarded by diffusion limitations in large granules. Biodegradation was observed for a number of compounds. Overall, this study elucidated the roles of sorption and biodegradation in the removal of OMPs in AGS WWTPs, showing that these processes are size fraction dependent. ... Read more

Aerobic granular sludge (AGS) is gaining popularity as an alternative to activated sludge for wastewater treatment. However, little information is available on AGS regarding the removal of organic micropollutants (OMPs) through sorption. In this study, the sorption behavior of 24 OMPs at environmentally relevant concentrations (1 μg/L) was investigated in six sludge fractions of varying sizes (>4 mm, 2–4 mm, 1–2 mm, 0.6–1 mm, 0.2–0.6 mm, and <0.2 mm) from a full-scale AGS reactor using batch experiments. Sorption was significant (removal efficiency >40 %) for 10 OMPs, including 4 zwitterionic and 6 positively charged pharmaceuticals, indicating the importance of electrostatic interaction for OMP sorption in AGS systems. Larger granules exhibited a higher sorption coefficient and capacity than smaller AGS fractions, probably due to increased extracellular polymeric substance content for larger granules. Equilibrium OMP sorption was only reached after 72 h in granules larger than 2 mm, indicating an effect of longer diffusion distance for OMPs into larger granules. Additionally, compared to activated sludge, AGS demonstrates a similar or even slightly higher sorption capacity for 10 OMPs at 1 μg/L. Overall, this study is the first to investigate the sorption behavior of six AGS size fractions for OMPs at environmentally relevant concentrations (1 μg/L) and propose the possible roles of different-sized sludge in OMP sorption in the full-scale AGS reactor. ... Read more