The performance and mechanism of sodium carboxymethyl cellulose in promoting the anaerobic ammonium oxidation coupled with Fe (III) reduction.

Anaerobic ammonium (NH4+-N) oxidation coupled with Fe(III) reduction (Feammox) is a novel promising nitrogen removal pathway, but the unsatisfactory nitrogen removal induced by the low bioavailability of Fe(III) compounds limited its engineering potential. In this study, three easily available exogenous reinforcing substances (ERSs), humic acid (HA), sodium carboxymethyl cellulose (CMC-Na) and sodium sulfite (Na2SO3), were used to improve Fe(III) bioavailability and Feammox performance. All the three ERSs notably improved nitrogen removal and electrochemical conditions during the 36-h batch test. CMC-Na outperformed the other two, and removed 99.3% of NH4+-N under the optimum dosage of 8 mg/L. The 136-d long-term experiments further verified the long-term stability of CMC-Na, and the NH4+ and TN removal efficiency increased from 48.18% and 14.70% to 99.33% and 68.60%. Fe(II) and Fe(III) compounds were inter converted, with FeCl3 converted into iron oxide hydroxide, and then hematite. The addition of CMC-Na also increased the INT-ETS activity and the synthesis of ATP and NADH, thus promoting the intercellular energy metabolism. Consequently, the functional microorganisms and nitrogen and iron metabolism genes were enriched in the Feammox system, achieving nitrogen transformation through multiple nitrogen metabolism pathways. This study has verified a new exogenous substance, which can enhance the intracellular metabolism of microorganisms and increase the total amount of electron transfer.