Removal of nitrogen from wastewater: Unsolved problems and possible solutions with partial denitrification/anammox systems

Nitrate (NO3-) is more stable than ammonia (NH4+) and nitrite (NO2-) among the majority of emerging pollutants in wastewater that can harm the ecosystem by depleting oxygen and water quality, thereby posing environmental and human health risks. Despite anaerobic ammonium oxidation (anammox) being a common method for nitrogen (N) removal from wastewater, it leaves behind residual NO3- of around 11 %. Maintaining the stability of N removal in the mainstream of wastewater (50-60 mg/L) is challenging due to the lack of intrinsic factors that suppress nitrite-oxidizing bacteria (NOB) and the accumulation of NO3-. Recently, partial denitrification/anammox (PD/A) has been recognized as a useful approach to eliminate NO3- and NH4+ from the sidestream (>= 500 mg/L of N) and mainstream wastewater simultaneously. This review explores the complexities of sustaining stability in PD/A systems, including managing fluctuating NH4 (+) and NO3- levels while keeping a COD/N ratio of <= 3 to facilitate optimal NO2- production for the anammox. This review discusses the significance of temperature, pH, free ammonia, and free nitrous acid levels for optimal total N removal efficiency and PD/A stability in the long term. Strategies used for initiating and restoring the PD/A systems include optimized inoculum selection, granular sludge seeding, stepwise acclimation, bioaugmentation, unique substance incorporation, and innovative system design. Additionally, various strategies aim to enhance the stability by modifying the substance composition, introducing inorganic carbon and NO3-, incorporating heavy metals, embedding the biomass in gel carriers, adding microalgae, and adjusting hydraulic and solids retention time. These solutions aim to address the challenges to improve stability in the PD/A process. Future research roadmaps and supporting portfolios should address advancements and emerging technologies tackling the ongoing challenges in PD/A systems. These developments emphasize the significance of wastewater treatment for sustainable water availability to support the United Nations Sustainable Development Goal 6 (UN SDG 6) established in 2015.