Anammox Bacteria Are Potentially Involved in Anaerobic Ammonium Oxidation Coupled to Iron(III) Reduction in the Wastewater Treatment System

Ferric and sulfate coupled ammonium oxidation enhanced nitrogen removal in two-stage partial nitrification - Anammox/denitrification process for food waste liquid digestate treatment

Liquid digestate of food waste is an ammonium-, ferric- and sulfate-laden leachate produced during digestate dewatering, where the carbon source is insufficient for nitrogen removal. A two-stage partial nitrification-anammox/denitrification process was established for nitrogen removal of liquid digestate without pre-treatment (>300 d), through which nitrogen (95 %), biodegradable organics (100 %), sulfate (78 %) and iron (100 %) were efficiently removed. Additional ammonium conversion (20 %N) might be coupled with ferric and sulfate reduction, while produced nitrite could be further converted to di-nitrogen gas through anammox (75 %) and denitrification (25 %). Notably, since increasingly contribution of hydroxylamine producing nitrous oxide, and up-regulated expression of electron transfer and cytochrome c protein, the enhanced ammonium oxidation was probably conducted through extracellular polymeric substances-mediated electron transfer between sulfate/ferric-reducers and aerobic ammonium oxidizers. Thus, the established partial nitrification-anammox/denitrification process might be a cost-efficient nitrogen removal technology for liquid digestate, benefitting to domestic waste recycling and carbon neutralization.

Evidence for the occurrence of Feammox coupled with nitrate-dependent Fe(II) oxidation in natural enrichment cultures

Feammox is a newly discovered process of anaerobic ammonium oxidation driven by Fe(III) reduction. Nitrate-dependent Fe(II) oxidation (NDFO) is the coupling of Fe(II) oxidation and nitrate reduction to produce N₂ under anaerobic conditions. It has not been reported whether the coupling of the two reactions exists in natural enrichment. In this study, enrichment culture experiments were carrired out to prove the occurrence of Feammox with NDFO. The results indicated that the nitrogen and iron cycle were formed during natural enrichment cultures, including Fe(III) reduction and NH₄⁺-N was oxidation to NO₃^--N, NO₂^--N and N₂, Fe(III) and Fe(II) were cyclically formed, and Fe(II) was oxidized with NO₃^--N reduced to N₂. The removal efficiencies of ammonium nitrogen and total nitrogen in the incubation were about 92.9% and 20% respectively. Organic carbon experiments indicate that sodium acetate can promote the initial NO₃^--N removal and a low concentration of organic carbon limited the NDFO process because iron-oxidizing bacteria are mixotrophic microorganisms. The added 9,10-anthraquinone-2,6-disulfonate (AQDS) in the later stage can promote NDFO to remove nitrate, thereby increasing the TN removal efficiency to 50%. ¹⁵N-isotope tracer incubations provided direct evidence for the occurrence of Feammox coupled to NDFO, with rates producing ³⁰N₂ of Feammox (0.024-0.0288 mg N·L^-1·d^-1) and NDFO (0.0465-0.0833 mg N·L^-1·d^-1) in three groups (Wetland/Wheat soil/Sediment). 16S rRNA sequencing further demonstrated that Pseudomonas, Rhodanobacter, Acinetobacter and Thermomonas were the dominant generas among the enrichment cultures, and these bacteria belonged to FeOB and FeRB, which may further promote Feammox coupled to NDFO in the cultivation system.