Effect of hydraulic retention time on nitrogen removal and functional gene quantity/transcription in biochar packed reactors at 5 °C: A control-strategy study

This study investigated the effect of hydraulic retention time (HRT = 8, 12, 16, and 24 h) on effluent dissolved organic nitrogen (DON), dissolved total nitrogen (DTN), and functional gene quantity/transcription in biochar packed reactors over a 125-day operation at 5 °C. The lowest effluent DON concentration (0.21 ± 0.14 mg/L) and DTN concentration (10.74 ± 0.41 mg/L) were in R_12h and R_24h, respectively. Adequate HRT (>12 h) was a necessary parameter for poly-β-hydroxybutyrate (PHB) accumulation (PHB/MLSS: 0.1181-0.1522), but higher HRT was detrimental to adenosine triphosphate (ATP) accumulation from 62.77-66.31 µg/g SS (R_8h) to 48.21-48.39 µg/g SS (R_24h). Effluent DON had a negative correlation relation with ATP and the relative abundance of amoA (p = 0.02), and effluent DTN had a negative correlation relation with PHB (p < 0.01), the relative abundance of napA (p = 0.01), and the transcriptional quantity of nirS (p = 0.14) and nxrA (p = 0.03).