Anoxic Denitrification
Facultative bacteria convert nitrate to nitrogen gas in low-oxygen zones.
Denitrification uses facultative heterotrophic bacteria that, in the absence of free oxygen, use nitrate as an electron acceptor instead, converting it stepwise to nitrogen gas, which off-gasses harmlessly.
This process requires an anoxic zone (no free DO, but nitrate present) and a carbon source, either influent BOD or a supplemental source like methanol or acetic acid.
Common configurations include Modified Ludzack-Ettinger (MLE) with an anoxic zone ahead of the aerobic zone, using internal mixed liquor recycle (IMLR) to supply nitrate.
Insufficient carbon is the most common cause of incomplete denitrification; track your COD:N ratio if total nitrogen limits are tight.
A general rule of thumb is roughly 4-6 mg of readily biodegradable COD needed per mg of nitrate-nitrogen denitrified; methanol is the cheapest common supplemental carbon source but requires acclimated biomass since it isn't readily degraded by all heterotrophs.
Common mistake: sizing the anoxic zone purely on hydraulic retention time without confirming actual nitrate removal via before/after sampling — a zone that's the 'right size' on paper can still underperform if mixing or carbon delivery is uneven.
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