Achieving partial nitrification, enhanced biological phosphorus removal and in-situ fermentation (PNPRF) in continuous-flow system and mechanism analysis at transcriptional level

Simultaneous partial nitrification, biological phosphorous removal and sludge fermentation in continuous–flow system faced many challenges in wastewater treatment plants (WWTPs). In this study, a novel process was developed to achieve partial nitrification, enhanced biological phosphorus removal and in–situ fermentation (PNPRF) in continuous–flow system dominated by Tetrasphaera treating low carbon/nitrogen ratio (COD/N) real domestic wastewater. After 80 days of operation, no external carbon was added into the influent. Nitrite accumulation ratio (NAR) reached 99.4% and removal efficiency of PO43−–P was 100%. The effluent concentration of total inorganic nitrogen (TIN) was lower than 2 mg/L. Compared with traditional biological nitrogen and phosphorus removal processes, the daily sludge discharge in continuous–flow PNPRF decreased by 61.9%. DNA concentration of the sludge supernatant after anaerobic zone was 2.26-fold of that before anaerobic zone, and percentages of intact cells decreased by 6.9% and necrotic cells increased by 8.3%, indicating that cell lysis caused the reduction of sludge discharge. The negative effect of prolonged anaerobic phase on transcriptional response of nxrB gene was more significant than that of amoA and hao gene, which resulted in stable partial nitrification. The activities of ammonium–oxidizing bacteria (AOB) were higher than that of nitrite–oxidizing bacteria (NOB), and reverse transcriptional PCR (RT–PCR) showed that RNA expression of AOB was higher than that of NOB. Achieving partial nitrification, enhanced biological phosphorus removal and in–situ fermentation in continuous-flow system dominated by Tetrasphaera solved the problems of carbon source deficiency and large amount of sludge discharge in treating real domestic wastewater process.