Meeting/Event Information

ABCs of BNR - An Introduction and Evolution of Biological Nutrient Removal

October 05, 2020
2:00 PM - 3:00 PM
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Biological Nutrient Removal (BNR) is a way to lower the BOD, nitrogen and phosphorus, with naturally occurring microorganisms without the need of chemicals. The addition or limitation of oxygen to the mixed liquor determines which bacteria are metabolically converting micronutrients.
The mixed liquor enters, first, an anaerobic zone, second an anoxic zone, and third an aeration zone. The mixed liquor travels in series through two or three anaerobic stages. An internal recycle reintroduces nitrates into the system. Denitrification requires organic matter and minimal DO to occur. The third zone is aerobic. The microorganisms consume BOD and oxygen for energy. Nitrification bacteria grow slowly, to be efficient a sludge age of 10 days is typical.
During denitrification in the anoxic zone the nitrate converts to nitrite followed by nitric oxide, nitrous oxide and finally nitrogen gas (approximately 4 hours).
Biological phosphorus removal occurs in an anaerobic environment. When PAOs are exposed to an anaerobic environment they become stressed and begin to create polyhydroxybutyrate (PHB) from VFAs as a form of storage to be used later. When the PAOs are reintroduced to an aerobic environment they consume the released phosphorus and additional phosphorus from the mixed liquor.
To improve the nitrogen removal, the Modified Bardenpho or 5-stage system was created. The post-anoxic zone further denitrifies, and the fifth zone reaerates which prevents phosphorus release in the secondary clarifier sludge blanket. The TN is reduced significantly, to levels as low as 2-3 mg/L.

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