Biological Nutrient Removal (BNR)

Biological Nutrient Removal (BNR) refers to biological treatment processes specifically designed to remove nitrogen and phosphorus from wastewater, beyond the standard BOD/COD reduction provided by conventional secondary treatment. BNR is increasingly required as discharge standards tighten to control eutrophication — the excessive algal growth and oxygen depletion caused by nutrient-rich discharges — in lakes, coastal waters, estuaries, and other sensitive receiving water bodies.

Nitrogen removal proceeds through two linked biological stages. In the aerobic nitrification stage, ammonia-oxidising bacteria and nitrite-oxidising bacteria sequentially convert ammonium to nitrite and then to nitrate. In the anoxic denitrification stage, facultative bacteria use available organic carbon as an electron donor to reduce nitrate to nitrogen gas, which escapes harmlessly to the atmosphere, in a mixed-but-unaerated anoxic zone. Common process configurations used to achieve this include the MLE (Modified Ludzack-Ettinger) arrangement, where an anoxic zone precedes the aerobic zone with an internal recycle stream carrying nitrate back for denitrification; the four-stage Bardenpho process, which adds further anoxic and aerobic stages for deeper nitrogen removal; and oxidation ditch geometries, which achieve simultaneous nitrification-denitrification within a single looped reactor through controlled dissolved oxygen zoning.

Phosphorus removal is achieved either biologically or chemically, and frequently through a combination of both. Enhanced Biological Phosphorus Removal (EBPR) uses an anaerobic zone in which phosphorus-accumulating organisms release stored phosphorus while taking up volatile fatty acids, followed by an aerobic zone in which these same organisms take up phosphorus beyond their normal metabolic requirement. This luxury uptake allows net phosphorus removal from the system via routine sludge wasting. Chemical phosphorus removal, by contrast, uses alum or ferric salt dosing to precipitate phosphate as an insoluble metal phosphate, offering a more predictable and controllable removal pathway, particularly useful for meeting stringent limits or handling load variability.

CPCB and state pollution control board total nitrogen and total phosphorus limits are increasingly specified for discharge to eutrophication-sensitive water bodies, and for STPs discharging near lakes, estuaries, or coastal zones. This is pushing many existing plants to add BNR capability. Retrofitting BNR onto an existing conventional activated sludge or MBBR plant typically requires adding anoxic — and sometimes anaerobic — zones with internal mixed-liquor recycle, which can be a genuine space and civil-works challenge in plants that were not originally designed with nutrient removal in mind.