Anoxic fraction-driven nutrient removal improvement mechanism in high-rate anoxic/microaerobic systems for sewage treatment

This study elucidated the underlying mechanism on the anoxic fraction-driven nutrient removal improvement in a high-rate anoxic/microaerobic (H-A/M) system (>1.50 kg chemical oxygen demand (COD) m −3 d −1 ; 0.5–1.0 mg L −1 dissolved oxygen in the microaerobic zone) during sewage treatment. The H-A/M system achieved stable COD and NH 4 + -N removals averaging 98–99 % and 98–100 %, respectively, at anoxic fractions of 0.20, 0.35, 0.45 and 0.55, while it solely achieved 50 % NH 4 + -N removals at an anoxic fraction of 0.65. Allocating more abundant organic carbon (from 80 % to 99 % of the daily COD decrement in the A/M tank) to the anoxic zone through the use of a larger anoxic fraction (from 0.20 to 0.55) increased the amount of total nitrogen (TN) (from 1078 ± 57 to 1386 ± 52 mg d −1 ) and total phosphorus (TP) (from 45 ± 9 to 117 ± 20 mg d −1 ) removed in the anoxic zone. It finally achieved a higher TN (from 69 % to 85 %) and TP (from 27 % to 38 %) removal in the H-A/M system. The conventional anoxic denitrification pathway predominantly (60–79 %) contributed to TN removal in the H-A/M system in all cases, while the negligible contribution (5–10 %) of heterotrophic nitrifying bacteria to TN removal should be attributed to insufficient organic matter supply in the microaerobic zone. Anoxic fraction (0.45 vs. 0.20, 0.35 and 0.55) influenced TP removal in the H-A/M system (38 % vs. 27–31 %) by maximizing the contribution of the anoxic denitrifying phosphorus removal pathway to TN removal (34 % vs. 16–21 %). • It elucidated the anoxic fraction-driven mechanism to improve nutrient removal performance. • An anoxic fraction of ≥0.45 allocated ≥91 % COD to anoxic zone to generate ≥82 % TN removal. • Anoxic fraction influenced anoxic denitrifying phosphorus removal and further TP removal. • Aerobic chemolithotrophic pathway contributed ≥80 % ammonia oxidization.