Unveiling interactions between anammox and denitrification from long-term steady-state scale and transient kinetic scale in a granular sludge system

Interactions in hybrid anammox and denitrification sludge (HADS) systems were unveiled through the effects of carbon to nitrogen (C/N) and nitrite to nitrate (Nit/Nat) ratios in long-term steady-state scale and transient kinetic scale. In nitrite-only feed (SAD1), increasing C/N ratio in long-term operation reduced effluent nitrate to 2.9 mgN/L, achieving a total nitrogen removal efficiency (TN_re) of 88.6%. Competitive interaction with full denitrification dominated in SAD1 as revealed by the transient experiment, causing that C/N ratio higher than 0.75 was unsustainable for HADS maintenance. In mixed feed (SAD2), reducing Nit/Nat ratio from 4 to 0 resulted in a TN_re of 93.8%, with anammox contributing 98.3%. Excellent synergistic interaction with partial denitrification in SAD2 was derived from the electron competition effect of nitrate on nitrite reduction. The optimal HADS operation occurred with C/N between 2-3 and Nit/Nat below 0.5. These results deepen the understanding of HADS system and facilitate its operation.