Fe(III)/Fe(II) forwarding a new anammox-like process to remove high-concentration ammonium using nitrate as terminal electron acceptor

The study demonstrated a novel anammox-like process to remove high-concentration ammonium using nitrate as terminal electron acceptor under Fe(III)/Fe(II) cycle. Compared with NO2− in common anammox, NO3− used here is more available in practice, suitable for in-situ removal of high-concentration NH4+ in a single anaerobic system. The NOx− and Fe(II) produced from Feammox [Fe(III) reduction coupled to anaerobic ammonium oxidation] subsequently react together via NOx−-dependent Fe(II) oxidation to regenerate Fe(III) that potentially stimulates next round of Feammox. However, these processes couldn't be lasting due to inadequate Fe(III) regeneration because NOx− is non-dominant product during Feammox. In this study NO3− was added to supplement the insufficient NOx− to enhance Fe(III) regeneration and remove nitrogen successively. Results showed that periodically adding nitrate caused oscillations between Fe(III) and Fe(II) in the sludge, implying Fe(III) regeneration and consumption. Consequently, nitrogen removal of the digester with an initial total nitrogen of 1036.7 mg/L reached 90.1% after 98-day operation, much higher than that of control (41.6%) without NO3− addition. Adding NO3− in the digester to trigger Fe(III)/Fe(II) cycle for removing ammonium is just equivalent to an anammox-like process using NO3− as terminal electron acceptor to oxidize NH4+.