Performances and mechanisms of anaerobic nitrogen removal through Fe (III)/Fe (II) cycle with sponge iron biofilm without external iron addition

Abstract

Anaerobic ammonia oxidation coupled to Fe (III) reduction (Feammox) plays an important role in the natural nitrogen cycle. However, for its application in wastewater treatment process, the source of iron is a major limitation. In this study, Feammox process was successfully achieved without external iron addition using sponge iron biofilm. The products of Feammox process, nitrate and Fe (II), provided substrates for nitrate dependent ferrous oxidation (NDFO). Therefore, nitrogen removal and Fe (II)/Fe (III) cycle were achieved in reactor and nitrogen removal efficiency by Feammox and NDFO pathways was up to 58.4 %. The X-ray diffraction indicated the sludge contained magnetite and lepidocrocite, which provided Fe (III) for the occurrence of Feammox. Microorganisms related to Feammox process were dominated by Geothrix. It was found Feammox activity was similar at pH of 8 and 7, while inhibited at pH of 6, which was caused by the decrease of Feammox bacteria. And Feammox activity decreased with the decrease of temperature, in which the ammonia removal rate was 2 times faster at 30 °C than at 10 °C. Under micro‑oxygen aeration, the ammonia removal rate improved and Feammox and NDFO processes still played roles in system, nevertheless, under high dissolved oxygen condition, Feammox bacteria-Geothrix and NDFO bacteria Thiobacillus would be gradually eliminated from the system.