SND (Simultaneous Nitrification-Denitrification)
Nitrification and denitrification occurring in the same basin via DO gradients within the floc itself.
SND happens when floc particles have an oxygen-rich outer layer supporting nitrification and an oxygen-depleted core supporting denitrification, without a physically separate anoxic zone.
Running aeration basins at a lower, carefully controlled DO (often 0.5–1.5 mg/L) can promote SND and reduce total nitrogen without dedicated anoxic zone construction.
SND is harder to control precisely than a designed anoxic/aerobic zone system, and pushing DO too low risks losing nitrification entirely rather than gaining denitrification.
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