Mainstream Nitrogen Removal in Wastewater Treatment: Nitritation, Anammox, and Partial Denitrification

The revised EU Urban Wastewater Treatment Directive (UWWTD) EU 2024/3019 imposes stricter nitrogen discharge limits (8-10 mg N/L) and mandates energy neutrality by 2045 for WWTPs >10,000 population equivalents, challenging conventional biological nutrient removal processes. Mainstream partial nitritation and anammox (PN/A) processes for autotrophic nitrogen removal are a good alternative for achieving energy-neutral wastewater treatment. However, maintaining stability under fluctuating mainstream conditions, along with low water temperatures (<20°C) and low nitrogen concentrations (<100 mg N/L), remains difficult. This chapter evaluates the current state of mainstream PN/A processes, focusing on pilot-scale developments and bottlenecks associated with suppressing nitrite-oxidizing bacteria (NOB) activity. By analyzing the ecological niches of microbial guilds, this analysis explores why NOB, such as Nitrospira, persist under mainstream conditions and how metabolic stressors can be leveraged to select for them. Moreover, the possible replacement of NOB species is discussed, highlighting the difficulty of their absolute suppression. Given the challenges of supplying nitrite to anammox via nitritation, alternative metabolic pathways, including partial heterotrophic denitrification and sulfur-driven processes, are discussed. While stoichiometry dictates that, in PN/A processes, approximately 11% of the converted nitrogen remains as nitrate, coupling anammox with complementary processes provides a resilient, resource-efficient approach. Ultimately, the future of nitrogen removal depends on flexible, interconnected treatment schemes that enable regulatory compliance.