Denitrification kinetics and microbial evolution of anaerobic ammonia oxidation for low-acidity ammonia nitrogen wastewater leached from rare earth tailings

A significant amount of acidic ammonia nitrogen wastewater is produced after extracting rare earth elements from weathered crust elution-deposited rare earth ore. Anaerobic ammonia oxidation (Anammox) offers advantages such as low carbon emissions and environmental sustainability. However, this process is highly sensitive to environmental factors, which constrains its practical application. This study comprehensively investigated the nitrogen removal performance of anammox under low-acidity conditions and developed kinetic models to describe the recovery of denitrification, revealing the secretion dynamics of EPS and the evolution of microbial community structure. The results indicate that a decrease in pH led to an increased concentration of toxic FNA in anammox system, causing a decline in the functional bacterium <i>Planctomycetota</i>. This deterioration ultimately compromised or even eliminated the denitrification performance of the anammox system, reducing the average TNRE of the system to 53.66% or less. The kinetic study indicated that low acidity had significantly negative effects on the response time (λ), TNRE, and maximum total nitrogen removal rate (R_max) of anammox bacteria, which was following the modified logistic model (R₂ > 0.95). The EPS secretion was greatly reduced with decreased stability of the anammox system, but PN/PS values were not significantly altered, suggesting that immobilisation reduced the shock from pH changes. Meanwhile, acidity severely inhibited anammox functional bacterial activity, resulting in a decrease in the abundance of <i>Planctomycetota</i> by 29.91% and <i>Unclassified_Brocadiacea</i> by 29.57%. While acidic condition stimulated the expression of genes related to carbon metabolism, ribosome function, and amino acid biosynthesis, excessive acidity severely impaired overall metabolic levels.