Long-term effect of zero-valent iron on one-stage partial nitritation and anammox

Partial nitritation and anammox (PN/A) is an energy- and carbon-saving process for nitrogen removal from organic deficient wastewater. However, the PN/A effluent often requires polishing to eliminate the nitrate produced by anammox. Iron as the essential element for living organisms has been tested for nitrate reduction and phosphorus recovery in wastewater treatment processes. Here, we proposed a novel process coupling PN/A with iron-based denitrification for efficiently removing nitrogen and phosphorus in a single reactor. Two laboratory-scale bioreactors were operated under oxygen-limited conditions in parallel. Fe⁰ with a dosage of ∼ 27 mg Fe/L-wastewater was continuously dosed in the experimental reactor, with the other serving as a control. Throughout 300 days of operation, the results showed that experimental system always had significantly high removal efficiencies of organic carbon, nitrogen and phosphorus compared with control system. The total nitrogen removal achieved 90.1 ± 1.2 % at the rate of 0.96 ± 0.03 kg N/(m³•d) due to the improved activity of ammonia oxidizers, anammox bacteria and denitrifiers and enriched nitrogen conversion pathway (i.e., iron-based autotrophic denitrification) with Fe⁰ supplement. The total phosphorus removal was synchronously improved to 80.2 ± 9.1 % mainly driven by the phosphate precipitation with iron. This study provides new insights into achieving high-level nitrogen and phosphorus removal in a single reactor, and deepens our understanding of the improved nutrients removal mechanism mediated by iron.