Simultaneous nitrification–denitrification in biofilm systems for wastewater treatment: Key factors, potential routes, and engineered applications

• Simultaneous nitrification–denitrification (SND) is emerging for water treatment. • Shortcut SND can cut organic and energy consumption as well as CO 2 emission by >20 %. • Nitrous oxide emission can be reduced by adjusting operational and physical factors. • Moving bed and aerobic granular biofilms are mature technologies for efficient SND. • SND biofilms can enable the combined removal of nitrogen and phosphorus. Simultaneous nitrification–denitrification (SND) is an advantageous bioprocess that allows the complete removal of ammonia nitrogen through sequential redox reactions leading to nitrogen gas production. SND can govern nitrogen removal in single-stage biofilm systems, such as the moving bed biofilm reactor and aerobic granular sludge system, as oxygen gradients allow the development of multilayered biofilms including nitrifying and denitrifying bacteria. Environmental and operational conditions can strongly influence SND performance, biofilm development and biochemical pathways. Recent advances have outlined the possibility to reduce the carbon and energy consumption of the process via the “shortcut pathway”, and simultaneously remove both N and phosphorus under specific operational conditions, opening new possibilities for wastewater treatment. This work critically reviews the factors influencing SND and its application in biofilm systems from laboratory to full scale. Operational strategies to enhance SND efficiency and hints to reduce nitrous oxide emission and operational costs are provided.