Activated sludge treatment for high ammonium nitrogen wastewater: performance and microbial adaptation with synthetic media

Background High ammonium nitrogen concentrations are common in decentralized wastewater systems such as Motor Rest Areas (MRAs), where urine-derived wastewater with low organic carbon leads to an imbalanced C:N ratio, impairing microbial processes and thereby compromising biological nutrient removal. These conditions often render conventional systems ineffective, whereas supporting them with external carbon dosing or chemical treatment is impractical due to their high costs and operational complexity. Therefore, this study evaluates a modified activated sludge process with synthetic media to enhance nitrogen removal and microbial resilience under adverse conditions. The study was conducted in six lab-scale PMMA cylinders (4 L each), operated under controlled temperature and aeration in sequential aerobic–anaerobic phases, run as 36-hour treatment cycles. Two experimental groups (G1, G2) incorporated synthetic media, while the corresponding controls (C-1, C-2) relied solely on suspended activated sludge. Synthetic wastewater reflecting MRA streams was applied at two mean ammonium nitrogen concentrations (202.0 and 453.0 mg/L), and treatment performance was evaluated through physicochemical analyses combined with statistical tests and 16S rRNA-based microbial community structure analysis. Results Compared to the controls, the results showed that the implementation of synthetic media significantly improved ammonium nitrogen removal efficiency, with reductions of 72.6% and 36.6% in the modified systems. The C:N ratio analysis revealed a more pronounced decrease in the experimental groups, suggesting the sustained activity of the activated sludge despite carbon limitations. Under high ammonium nitrogen loading, synthetic media enabled the growth of nitrifiers such as Nitrospira and Nitrosomonas , and denitrifiers such as Thauera . Notably, elevated abundances of Planctomycetota (12.1–22.1%), the presence of marine-associated taxa and other uncultured genera indicate that the applied conditions may have supported potential nitrogen transformation pathways that remain unexplored. Conclusions The unique bacterial consortium identified provides a basis for further research on activated sludge biomass and its adaptive potential. The findings confirm that synthetic media improved the activated sludge process performance, highlighting its high applicability as a low-cost modification for off-site facilities facing similar biological treatment challenges.