Coupling fundamental mechanisms and operational controls in mainstream partial denitrification for partial denitrification anammox applications: A review

Nutrient management and stringent regulatory limits have challenged wastewater treatment plants to opt for innovative processes. Partial Denitrification Anammox (PDNA) for mainstream treatment applications have transitioned from pilot scale to full-scale within only 5 years, showing a realistic and sustainable solution with savings lying on carbon, energy, and footprint. This review aims to present the recent advances in the partial denitrification (PD) process and its coupling to the anammox process, analyze and critically review fundamental mechanisms and operational strategies responsible for stable PDNA process. A major consideration in the PD process is achieving a stable control of nitrate to nitrite conversion for sustainable nitrite accumulation, which can be achieved by controlling the intermediates involved in the different mechanisms of nitrate reduction. Although the magnitude of nitrite accumulation is reliant on various operational factors; bacterial ecology shift, C/N ratio, SRT decoupling and method of addition and gradient of carbon source (feast-famine conditions) also play an important role, but the significance of such factors has been often neglected. Overall, this study intends to provide insights on the critical parameter including the carbon gradients and feast-famine conditions as the influential operational factors for PD full-scale application. Further research is required to overcome the burdens remaining in the implementation of full-scale mainstream PDNA process.