Anaerobic ammonium oxidation coupled to iron reduction in constructed wetland mesocosms

Acidimicrobiaceae sp. A6 (referred to as A6) was recently identified as playing a key role in the Feammox process (ammonium oxidation coupled to iron reduction). Two constructed wetlands (CW) were built and bioaugmented with A6 to determine if, under the right conditions, Feammox can be enhanced in CWs by having strata with higher iron content. Hence, the solid stratum in the CWs was sand, and one CW was augmented with ferrihydrite. Vertical ammonium (NH₄ ⁺) concentration profiles in the CW mesocosms were monitored regularly. After four months of operation, when reducing conditions were established in the CWs, they were inoculated with an enrichment culture containing A6 and monitored for an additional four months, after which they were dismantled and analyzed. During the four-month period after the A6 enrichment culture injection, 25.0 ± 7.3% of NH₄ ⁺ was removed from the CW with the high iron substrate whereas 11.0 ± 9.7% was removed from the CW with the low iron substrate on average. Since the CW with high NH₄ ⁺ removal had the same plant density, same bacterial biomass, same fraction of ammonium oxidizing bacteria (AOB), a higher biomass of A6, and a higher pH (NH₄ ⁺ oxidation by Feammox raises pH, whereas NH₄ ⁺ oxidation by aerobic AOB decreases pH), this difference in NH₄ ⁺ removal is attributed to the Feammox process, indicating that wetlands can be constructed to take advantage of the Feammox process for increased NH₄ ⁺ removal.