Achieving advanced nitrogen removal from oxytetracycline wastewater by partial nitrification-endogenous...

Partial nitrification-endogenous denitrification (PNED) can achieve advanced nutrient removal from wastewater. Herein, an anaerobic/oxic/anoxic/oxic-sequencing batch reactor (AOAO-SBR) was used to treat real oxytetracycline (OTC) pharmaceutical wastewater via PNED. Partial nitrification with an average nitrite accumulation ratio of 91.5 % was achieved. When the influent total nitrogen was 105 ± 10 mg/L, the effluent of it was only 5.9 mg/L, and the removal efficiency was 94.8 %. In the typical cycle, multiple nitrogen removal pathways including endogenous denitrification, simultaneous nitrification-denitrification, and denitrification contributed to 68.0 %, 20.3 %, and 7.3 %. The effluent concentration of NH₄⁺-N was 5 mg/L, and NO₂^--N and NO₃^--N were not detected. The biodegradation pathways of OTC were proposed, 47.1 % of OTC was degraded and eight possible degradation byproducts were detected with low toxicity in the extracellular and intracellular. Moreover, Extracellular polymeric substances increased from 35.3 (mg/gVSS) to 74.4 (mg/gVSS) during 120 days, which acts as a critical role in OTC degradation. High-throughput sequencing results showed that the relative abundance of ammonia-oxidizing bacteria was 2.4 %, and nitrite-oxidizing bacteria were washed out, which was conducive to partial nitrification. Candidatus_Competibacter (13.9 %) enhanced nitrogen removal by endogenous denitrification. Thauera (13.5 %), Ottowia (9.2 %), and OLB13 (1.2 %) are the main OTC-degrading bacteria. This study provides a valuable reference to treat OTC pharmaceutical wastewater effectively.