Microbial interactions elucidate the mechanisms of hydraulic retention time altering denitrification pathway in a sole pyrite-based electrochemical bioreactor (PEBR)

In the current context of low-carbon wastewater treatment, pyrite-based autotrophic denitrification (PAD) has gained attention as an energy-efficient and environmentally sustainable method for nitrogen elimination. However, the limited dissolution of pyrite and the associated slow autotrophic denitrification rate restrict its practical application. To tackle this, a pyrite-based electrochemical bioreactor (PEBR) was constructed and the microbial effect of hydraulic retention time (HRT) on denitrification efficiency and sulfide or iron oxidation in the PEBR system was investigated. It was found that upon the conclusion of phase V (HRT = 12 h), the nitrate removal efficiency (NRE) reached 92.53% ± 0.96%, and the concentration of NH₄⁺-N in the effluent reached 2.63 ± 0.57 mg/L with a minimal accumulation of NO₂⁻-N (0.03 ± 0.05 mg/L) when the optimal treatment performance was obtained. As the HRT increased, the proportion of heterotrophic denitrification decreased substantially to 1%. Desulfobacterota, a sulfate-reducing bacteria (SRB), became dominant, with a relative abundance ranging from 0.04% to 19.44%. The PAD-related genera, such as Thiobacillus and Ferritrophicum, exhibited a positive correlation with HRT, indicating that PAD was enhanced with the extension of HRT. The functional genes related to Fe²⁺ intracellular oxidation (e.g., korA/B) positively correlated with HRT. The positive correlation of dsrA/B with HRT highlighted the role of dissimilatory sulfate reduction (DSR) as a primary contributor to reduced sulfate production. Furthermore, the variations in the relative abundance of aprA/B for sulfate reduction with the extension of HRT reflected that HRT affected sulfate reduction probably via the APS→SO₃²⁻ process. This study might shed light on the optimization of HRT in PEBR for the treatment of nitrogenous wastewater.