Denitrification and phosphorus uptake by DPAOs using nitrite as an electron acceptor by step-feed strategies

Denitrifying phosphorus accumulating organisms (DPAOs) using nitrite as an electron acceptor can reduce more energy. However, nitrite has been reported to have an inhibition on denitrifying phosphorus removal. In this study, the step-feed strategy was proposed to achieve low nitrite concentration, which can avoid or relieve nitrite inhibition. The results showed that denitrification rate, phosphorus uptake rate and the ratio of the phosphorus uptaken to nitrite denitrified (anoxic P/N ratio) increased when the nitrite concentration was 15 mg·L^?1 after step-feeding nitrite. The maximum denitrification rate and phosphorus uptake rate was 12.73 mg NO ₂ ^? -N·g MLSS^?1·h^?1 and 18.75 mg PO ₄ ^3? -P·g MLSS^?1·h^?1, respectively. These rates were higher than that using nitrate (15 mg·^L?1) as an electron acceptor. The maximum anoxic P/N ratio was 1.55 mg PO ₄ ^3? P·mg NO ₂ ^? -N^?1. When the nitrite concentration increased from 15 to 20 mg NO ₂ ^? -N·L^?1 after addition of nitrite, the anoxic phosphorus uptake was inhibited by 64.85%, and the denitrification by DPAOs was inhibited by 61.25%. Denitrification rate by DPAOs decreased gradually when nitrite (about 20 mg ·^L?1) was added in the step-feed SBR. These results indicated that the step-feed strategy can be used to achieve denitrifying phosphorus removal using nitrite as an electron acceptor, and nitrite concentration should be maintained at low level (<15 mg·^L?1 in this study).