Biological nutrient removal (BNR) is a wastewater treatment process that uses microorganisms to strip nitrogen and phosphorus from sewage before it’s discharged into rivers, lakes, or coastal waters. These two nutrients are the primary drivers of algal blooms and oxygen-depleted “dead zones” in surface waters, and BNR is the most widely used strategy for keeping them in check. A well-designed BNR system typically removes 70 to 90% of both nitrogen and phosphorus, with optimized plants capable of exceeding 90%.
Why Nitrogen and Phosphorus Matter
When excess nitrogen and phosphorus reach a lake or estuary, they fuel explosive algae growth. As that algae dies and decomposes, bacteria consume dissolved oxygen faster than it can be replenished. Fish, shellfish, and aquatic plants suffocate. This process, called eutrophication, is responsible for recurring dead zones in places like the Gulf of Mexico and Chesapeake Bay. Wastewater from cities and towns is one of the largest controllable sources of these nutrients, which is why treatment plants are increasingly required to reduce them before discharge.
Modern permits in the United States commonly target effluent concentrations of 10 mg/L for total nitrogen and 1.0 mg/L for total phosphorus. Those numbers represent what BNR can reliably achieve at facilities treating primarily domestic wastewater. Some plants push even lower, reducing phosphorus from incoming levels as high as 15 mg/L down to 0.1 mg/L in the final discharge.
How Nitrogen Removal Works
Nitrogen arrives at a treatment plant mostly as ammonia, dissolved in sewage. Removing it biologically requires two steps that happen in sequence: nitrification and denitrification.
In the nitrification step, bacteria in an oxygen-rich (aerobic) zone convert ammonia into nitrate. This is a straightforward conversion, but the bacteria responsible grow slowly and are sensitive to cold temperatures, which is one reason BNR plants in northern climates need careful design. In the denitrification step, a different group of bacteria in an oxygen-free (anoxic) zone convert that nitrate into harmless nitrogen gas, which bubbles off into the atmosphere. These bacteria need a carbon source as fuel, and they get it from the organic matter already present in the sewage. Nitrate-rich water from the aerobic zone is recycled back to the anoxic zone so the denitrifying bacteria can do their work.
The key insight is that you need at least two separate environments: one with oxygen for nitrification and one without oxygen for denitrification. The engineering challenge is creating both within the same treatment system and moving the right water between them at the right rates.
How Phosphorus Removal Works
Phosphorus removal relies on a specialized group of microorganisms called polyphosphate-accumulating organisms, or PAOs. These bacteria have an unusual metabolism that can be exploited with alternating conditions.
In an anaerobic zone (no oxygen, no nitrate), PAOs release stored phosphorus into the surrounding water while absorbing simple carbon compounds from the sewage. Think of it as the bacteria trading phosphorus for food. Then, when those same bacteria move into an aerobic zone with plenty of dissolved oxygen, they take up far more phosphorus than they originally released, packing it into their cells as dense polyphosphate granules. The net effect is a large transfer of phosphorus from the liquid into the bacterial cells. When those cells are removed as sludge, the phosphorus goes with them.
The process is sometimes called enhanced biological phosphorus removal (EBPR). It depends heavily on having enough easily digestible organic matter in the anaerobic zone. If the carbon supply is too low, PAOs can’t compete effectively and phosphorus removal drops.
Common Process Configurations
Because nitrogen and phosphorus removal each require different oxygen conditions, BNR systems string together a series of zones or reactors in a specific order. The most common configurations are designed to handle both nutrients simultaneously.
A2/O Process
The A2/O (anaerobic, anoxic, aerobic) process is the simplest layout for combined nitrogen and phosphorus removal. Wastewater flows first through an anaerobic zone, where PAOs release phosphorus and take up carbon. It then enters an anoxic zone, where denitrifying bacteria convert recycled nitrate into nitrogen gas. Finally, it passes through an aerobic zone, where ammonia is converted to nitrate (nitrification) and PAOs take up phosphorus in large quantities. Nitrate-rich water from the aerobic zone is pumped back to the anoxic zone to keep denitrification going.
Research on this configuration shows that most of the organic matter in sewage is consumed in the anaerobic and anoxic zones, leaving the aerobic zone with a lighter organic load. That actually helps nitrification, because the slow-growing nitrifying bacteria face less competition from other microbes.
Modified Bardenpho Process
The modified Bardenpho process uses five stages: anaerobic, first anoxic, first aerobic, second anoxic, and second aerobic. The extra anoxic and aerobic stages at the end give the system a second pass at denitrification, which pushes total nitrogen levels significantly lower than a three-stage A2/O can achieve. The second aerobic stage is short, mainly serving to strip residual nitrogen gas from the water and polish the effluent before discharge.
This configuration is rated as excellent for nitrogen removal and good for phosphorus removal. Beyond nutrients, the five-stage process also reduces suspended solids, organic pollutants, and even some heavy metals and viruses simply because the wastewater spends more time in contact with active biology.
What Affects Performance
BNR systems are living processes, which means they’re sensitive to their operating environment in ways that chemical treatment is not.
Temperature is the biggest variable. Nitrifying bacteria slow down substantially in cold water, so plants in colder climates need to hold onto their bacterial populations longer (a parameter called solids retention time) to compensate. Research on cold-weather BNR has shown that maintaining a sufficiently long solids retention time allows effective nitrogen and phosphorus removal even at low temperatures, with the added benefit of producing roughly one-third less sludge compared to chemical phosphorus removal methods.
Carbon availability is the other critical factor. Both denitrification and biological phosphorus removal depend on having enough organic matter in the incoming sewage. If the wastewater is dilute, or if too much carbon gets consumed before it reaches the right zone, performance drops. Some plants supplement with external carbon sources like fermented food waste or fermented waste activated sludge. Studies using these supplemental sources have achieved nitrogen removal rates of 82 to 92% and phosphorus removal rates of 73 to 99%, depending on the specific carbon source and system design.
Competing chemistry can also interfere. If nitrate leaks into the anaerobic zone (through poor flow control or inadequate denitrification), it disrupts the conditions PAOs need to release phosphorus, undermining the entire EBPR process.
BNR Compared to Chemical Treatment
The alternative to biological nutrient removal is chemical precipitation, where metal salts like ferric chloride or aluminum sulfate are added to wastewater to bind phosphorus into solid particles that settle out. Chemical treatment is simpler to operate and less sensitive to temperature, but it comes with trade-offs.
Chemical precipitation generates more sludge, roughly 50% more by some estimates, and that sludge contains metal compounds that limit options for reuse or land application. It also only addresses phosphorus. Nitrogen removal still requires a separate biological process, so most plants pursuing both nutrients end up with BNR anyway. Operating costs for chemical addition are ongoing (you’re constantly buying and dosing chemicals), while BNR, once built, runs primarily on the energy needed to aerate and pump water. Many modern plants use a hybrid approach: BNR as the primary system with chemical polishing to meet very stringent phosphorus limits.
Nutrient Recovery From BNR Sludge
One advantage of concentrating phosphorus in bacterial cells rather than chemical precipitates is that it opens the door to nutrient recovery. The phosphorus-rich sludge from EBPR systems can be processed to release phosphorus in a concentrated form, which is then crystallized into a slow-release fertilizer called struvite. This effectively turns a waste product into a marketable resource, partially offsetting treatment costs while reducing the volume of sludge that needs disposal. Nitrogen recovered during sludge processing can similarly be captured as ammonium sulfate fertilizer. These recovery pathways are increasingly common at larger BNR facilities, particularly in regions facing both strict discharge limits and rising fertilizer prices.