Module 3 · Denitrification

What Anoxic Means

An anoxic zone has little or no free dissolved oxygen. Yet nitrate or nitrite is present. This is different from an anaerobic zone. An anaerobic zone is where free oxygen, nitrate, and nitrite are all absent. The difference matters because the microbial reactions are different.

Key: = nitrate= nitrite

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In an anoxic zone, denitrifying microbes use oxidized nitrogen in place of dissolved oxygen while they consume biodegradable carbon. The nitrogen is changed through intermediate forms and ends mainly as nitrogen gas that leaves the liquid.

Anoxic zones need mixing to keep solids in contact with wastewater. Mixing should not add the oxygen used in an aerobic zone. Staff watch DO, nitrate, mixing, carbon availability, and recycle flows together.

Oxidation-reduction potential, or ORP, is a sensor signal used in some plants. It can support local trending. It does not replace direct DO and nitrogen measurements required by the control plan.