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¹ Marmara University, Department of Environmental Engineering, Maltepe, 34854, Istanbul, Türkiye DOI : 10.38042/biotechstudies.2008923 - The objective of this study was to investigate the long-term effects of titanium dioxide (TiO₂) nanoparticles (NPs) on the anammox process under low-temperature stress. In this context, two identical anammox bioreactors were subjected to varying temperatures starting from 35 °C and gradually decreased to 15 °C. The enrichment and test periods were completed in 210 days. The addition of TiO₂ NPs enhanced nitrogen removal efficiency at the optimal temperature of 35 °C, increasing ammonium and nitrite removal efficiencies from 95.8 ± 1.2% to 97.2 ± 1.0% and from 94.2 ± 1.6% to 98.7 ± 0.5%, respectively. Moreover, TiO₂ NPs addition significantly improved reactor performance under sub-optimal temperature conditions and provided stable operation at 25 °C with removal efficiencies of 86.5 ± 1.8% for ammonium and 92.3 ± 2.2% for nitrite. Since anammox systems are typically operated at temperatures around 30–35 °C to maintain high microbial activity, considerable heating energy is required in full-scale wastewater treatment plants. Therefore, the ability to sustain high treatment performance at lower temperatures provides a major operational advantage. As a result, the TiO₂-modified anammox system reduced heating energy demand by 43.5% and demonstrated a cost-effective and sustainable strategy for improving nitrogen removal performance in wastewater treatment applications. Keywords : Cold stress, Nitrite removal, Ammonium removal, Sequencing batch reactor, Inhibition