<p>To overcome challenges of energy-intensive nitrification and nitrite-limited anammox, this study developed a biochar-assisted anaerobic NH₄⁺-N oxidation process using bicarbonate (HCO₃^⁻) as a potential electron acceptor. While the biochar-free system primarily accumulated NO₂^⁻-N, bamboo biochar enabled the complete nitrification to NO₃^⁻-N, even under low-temperature stress (12–15&#xa0;°C). The prolonged operation under lower temperatures (&lt; 12&#xa0;°C) diminished this enhancement and reduced nitrification efficiency, with persistent effects even after temperature recovery to 25 °C. Morphological and 16S rRNA sequencing results revealed distinct microbial communities in this bicarbonate-driven system compared to conventional anammox sludge. Biochar enhanced the resilience of the system against low-temperature stress by selectively enriching specific taxa, such as nitrifying bacterium <i>Nitrobacter</i> and the functionally associated <i>nxrB</i> gene, both of which were critical for complete nitrification. Machine learning with XGBoost modeling effectively predicted the nitrite accumulation ratio (NAR) and nitrogen removal efficiency (NRE), identifying operating temperature as the significant positive factor. The negative contribution of biochar dosage to NAR prediction further confirmed its role in prompting complete nitrification. Overall, this study presents a promising complete nitrification process to address low-temperature stress and electron acceptor limitations in NH₄⁺-N removal.</p> Graphical Abstract <p>Highlights</p> <p>• NH₄ ⁺ -N oxidation using HCO₃ ⁻ as an alternative electron acceptor is acclimated</p> <p>• Byproducts distribution and NAR depend on biochar and operation temperature</p> <p>• Biochar promotes the complete nitrification under lower-temperature stress</p> <p>• Biochar selectively enriches Nitrobacter and upregulates <i>nxrB</i> gene</p> <p>• Machine learning identifies temperature as the key factor in predicting NAR and</p> <p>NRE</p> <p></p>