Simultaneous partial nitrification, Anammox and nitrate-dependent Fe(II) oxidation (NDFO) for total nitrogen removal...

Sustainable nitrogen removal from wastewater at reduced energy and/or chemical consumptions is challenging. This paper investigated, for the first time, the feasibility of coupled partial nitrification, Anammox and nitrate-dependent Fe(II) oxidation (NDFO) for sustainable autotrophic nitrogen removal. With NH₄⁺-N as the only nitrogen-containing compound in the influent, near-complete nitrogen removal (a total of 97.5 % with a maximal total nitrogen removal rate of 6.64 ± 2.68 mgN/L/d) was achieved in a sequencing batch reactor for a 203-d operation without organic carbon source addition and forced aeration. Anammox (predominated by Candidatus Brocadia) and NDFO bacteria (such as Denitratisoma) were successfully enriched, with total relative abundances up to 11.54 % and 10.19 %, respectively. Dissolved oxygen (DO) concentration was a key factor affecting the coupling of multi (ammonia oxidization, Anammox, NDFO, iron-reduction, etc.) bacterial communities, resulting in different total nitrogen removal efficiencies and rates. In batch tests, the optimal DO concentration was 0.50-0.68 mg/L with a maximal total nitrogen removal efficiency of 98.7 %. Fe(II) in the sludge not only competed with nitrite oxidizing bacteria for DO to prevent complete nitrification, but promoted the transcription of NarG and NirK genes (10.5 and 3.5 times higher than the group without Fe(II) addition) as indicated by the reverse transcription quantitative polymerase chain reaction (RT-qPCR), resulting in increased NDFO rate (by 2.7 times) and promoted NO₂^--N generated from NO₃^--N, which back fed the Anammox process, achieving near-complete nitrogen removal. The reduction of Fe(III) by iron-reducing bacteria (IRB) and hydrolytic and fermentative anaerobes enabled a sustainable Fe(II)/Fe(III) recycling, avoiding the need in continuous Fe(II) or Fe (III) dosage. The coupled system is expected to benefit the development of novel autotrophic nitrogen removal processes with neglectable energy and material consumptions for the treatment of wastewater with low organic carbon and NH₄⁺-N contents in underdeveloped regions, such as decentralized rural wastewaters.