RESILIENCE AND ADAPTATION OF THREE TYPES OF ANAMMOX INOCULUM SLUDGE DURING THE START-UP OF BENCH-SCALE REACTORS UNDER CHRONIC METAL STRESS
nitrogen removal; trace elements; anaerobic ammonium oxidation; start-up; microbial resilience.
The ANAMMOX (ANaerobic AMMonium OXidation) process represents an efficient alternative for autotrophic nitrogen removal in wastewater, but is recognizedly sensitive to environmental disturbances, including heavy metals and trace elements at elevated concentrations. In this study, the resilience and adaptation of the process were evaluated during the start-up of three sequential laboratory batch reactors (SBR) with different inoculum compositions: R1 (anammox biomass), R2 (activated sludge supplemented with anammox sludge), and R3 (UASB reactor sludge supplemented with anammox sludge). In the initial phase (days 0–39), an accidental thousand-fold overdose in the trace element solution concentration imposed chronic metal stress on all three reactors. Performance was monitored through total nitrogen (TN) removal efficiency, stoichiometric coefficients (ΔNO₂⁻/ΔNH₄⁺ and ΔNO₃⁻/ΔNH₄⁺), and the coefficient of variation (CV). None of the reactors exhibited complete process collapse. After a period of operation (days 100–164), the stoichiometric coefficients converged toward values close to the theoretical one (ΔNO₂⁻/ΔNH₄⁺ ≈ 1.32), indicating progressive process stabilization. No statistically significant differences were observed between reactors in either phase (ANOVA, p > 0.05), showing that inoculum composition was not a determining factor for system resilience. The results indicate that the anammox process developed partial functional resilience following severe metal perturbation.