A compromise situation at which both nitrification and denitrification can occur simultaneously requires the bioreactor to operate at the minimum DO concentration necessary to support nitrification ( 0.5 mg/L) (Munch, et al 1996).
The major challenge is creating the optimal conditions necessary for an efficient SND process: appropriate hydraulic/solids retention times, optimal aeration rates, and suitable feed introduction strategy. These conditions must be met while minimizing odor generation and the accumulation of free ammonia gas. Another potential difficulty is the selection of filamentous bacteria at low DO that interfere with settling and can cause sludge bulking in a reactor. However, these same bacterial flocs (sludge bulking) may provide sites of anoxic zones where denitrification can take place.