What is biological nutrient removal (BNR)?

Nitrogen arrives as ammonia and organic nitrogen. In aerated zones, slow-growing bacteria oxidise ammonia to nitrite and then nitrate (nitrification), which takes a long sludge age, a lot of oxygen, about 4.6 pounds per pound of ammonia nitrogen, and alkalinity. In anoxic zones, where there is nitrate but no dissolved oxygen, other bacteria use the nitrate to oxidise organic carbon and release nitrogen gas (denitrification), which recovers part of the oxygen and alkalinity. The common Modified Ludzack-Ettinger (MLE) layout puts the anoxic zone first and recycles nitrate-rich mixed liquor back to it.

An anaerobic zone at the head of the process selects for phosphorus-accumulating organisms. They release phosphorus there while taking up volatile fatty acids, and then take up far more than they released once they reach the aerobic zone. Wasting sludge removes that phosphorus from the system. Many plants back up biological removal with alum or ferric salts to meet low limits reliably.

A2O adds an anaerobic zone to MLE so that both nutrients are removed; the four- and five-stage Bardenpho processes add a second anoxic zone for lower nitrogen; UCT and its variants protect the anaerobic zone from nitrate. Sequencing batch reactors and oxidation ditches achieve the same sequence in time or along the loop. Conventional BNR reaches about 8 mg/L of total nitrogen and 1 to 2 mg/L of total phosphorus. Enhanced nutrient removal, as required around Chesapeake Bay, aims for about 3 mg/L of nitrogen and 0.3 mg/L of phosphorus, which usually takes tertiary filters, an added carbon source such as methanol, and chemical polishing.

Nitrogen and phosphorus feed algal blooms and oxygen-depleted dead zones, so states write nutrient limits into NPDES permits as they renew them, driven by numeric nutrient criteria and watershed clean-up plans (TMDLs) for waters such as Chesapeake Bay, Long Island Sound, Puget Sound, the Great Lakes and the Mississippi basin. A renewal with a new nutrient limit normally comes with a compliance schedule, and with a project in the capital plan.