Mainstream anammox in inverse fluidized bed bioreactors

This study reports the first integration of mainstream anammox into inverse fluidized bed bioreactors, herein termed Invammox, synergizing respective strengths to overcome longstanding limitations of each process. A rapid (<3 h) biofilm attachment protocol was developed to effectively define optimal combination of initial particle loading as 30 % of the reactor by volume and initial seed biomass as 1 g VSS/L for startup. Upon seeding slow-growing anammox bacteria, Invammox spontaneously exhibited hydrodynamic uniformity, avoided biofilm overgrowth, and sustained resilient stability over 6,000 hours. The system demonstrated exceptional resilience and biomass retention under deliberately induced disturbances. Concurrently, internal liquid recirculation promoted thinner biofilms, sustained specific anammox activity up to 0.61 kg NH₄⁺-N/kg VSS·d, threefold higher than the inoculated sidestream granular anammox biomass. A full-scale system treating 1.5 MGD municipal wastewater, designed based on the process kinetics, achieved a 62 % reduction in reactor volume (from 4,194 to 1,594 m³) with comparable operating costs. This work positions Invammox as a state-of-the-art platform for mainstream anammox, and partial nitrification/anammox, as well as establishes a complementary paradigm for scalable, energy-efficient nitrogen removal in biological wastewater treatment.