Simultaneous removal of humic acid and nitrate by combining micro-electrolysis and autotrophic denitrification

Abstract

Due to increasingly stringent discharge standards for total nitrogen (TN) in wastewater treatment plant effluents and presence of residual organic pollutants (e.g., humic acid, HA, a typical refractory organic) in secondary effluents, new challenges have emerged for water reclamation. These residual organics contribute to membrane fouling in subsequent microfiltration units and serve as precursors to disinfection by-products, making the simultaneous and efficient removal of nitrate and refractory organic contaminants critical for improving reclaimed water quality. In this study, we employed iron-carbon micro-electrolysis (IC-ME) to achieve synchronous removal of nitrate and HA in a continuous-flow reactor system. The reactor was operated for 233 d, including 32 d of validation with real wastewater. With synthetic wastewater containing 20 mg/L NO₃⁻−N and 15 mg/L HA, the system achieved 96.3% ± 3.6% TN and 97.1% ± 4.6% HA removal. During real wastewater treatment (influent: 24.1 ± 0.9 mg/L NO₃⁻−N and 5.9 ± 0.6 mg/L HA), HA and TN removal efficiencies reached 94.6% ± 7.3% and 92.8% ± 2.9%, respectively, with effluent TN consistently below 2 mg/L. Three-dimensional fluorescence analysis confirmed that HA was effectively degraded from the reactor bottom to top. HA addition facilitated the transformation of Fe₃O₄ precipitates into FeO(OH) and Fe(OH)₃, suggesting reduced passivation film formation. Moreover, NapA gene abundance increased and f_Rhodocyclaceae and f_Methanobacteriaceae were the dominant microbes. Thus, IC-ME is a robust technique for treating complex real wastewater, thus providing an environmentally benign solution for concurrent nitrogen and refractory organics removal in advanced wastewater treatment.

Graphical abstract

Keywords

Nitrate removal / Iron-carbon / Humic acid / Iron passivation / Microbial community

Highlight

	● With HA, TN removal efficiency reached 96.3% ± 3.6%.• HA degraded with increase in reactor height.
	● Dense Fe3O4 decreased with increase in metastable FeOOH and Fe(OH)3.
	● Methanogenic archaea and autotrophic denitrifying bacteria abundance increased.
	● unclassified_f_Rhodocyclaceae was the dominant autotrophic denitrifying bacteria.

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