Organic carbon source excites extracellular polymeric substances to boost Fe-mediated autotrophic denitrification in mixotrophic system.

Fe-mediated autotrophic denitrification (ADN) can be suppressed by iron oxide coverage resulting from Fe corrosion. The mixotrophic denitrification (MDN) coupling Fe-mediated ADN with heterotrophic denitrification (HDN) can circumvent the weakening of Fe-mediated ADN over operation time. But the interaction between HDN and Fe-mediated ADN for nitrogen removal of secondary effluent with deficient bioavailable organics remains unclear. When the influent COD/NO-N ratio increased from 0.0 to 1.8-2.1, the TN removal efficiency was promoted significantly. The increased carbon source did not inhibit ADN, but promoted ADN and HDN synchronously. The formation of extracellular polymeric substances (EPS) was also facilitated concomitantly. Protein (PN) and humic acid (HA) in EPS increased significantly, which capable of accelerating electron transfer of denitrification. Due to that the electron transfer of HDN occurs intracellularly, the EPS with the capacity of accelerating electron transfer had a negligible influence on HDN. But for Fe-mediated ADN, the increased EPS as well as corresponding PN and HA facilitated TN and NO-N removal significantly, while accelerated the electron release originating from Fe corrosion. The bioorganic-Fe complexes were generated on Fe surface after used, meaning that the soluble EPS and soluble microbial products (SMP) participated in the electron transfer of Fe-mediated ADN. The coexistence of HDN and ADN denitrifiers demonstrated the synchronous enhancement of HDN and ADN by the external carbon source. From the perspective of EPS and related SMP, the insight of enhancing Fe-mediated ADN by external carbon source is beneficial to implement high-efficiency MDN for organics-deficient secondary wastewater.