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

•The efficient nitrogen removal achieved by HRT regulation at a low temperature.•The lowest effluent DON and DTN concentration were in R12h and R24h respectively.•High HRT was beneficial for PHB accumulation but detrimental to ATP accumulation.•DON had negative correlations with ATP and amoA/16S abundance.•DTN had negative correlations with PHB and the transcription of nirS/16S and nxrA/16S.