BNR Biological Nutrient Removal


Biological Nutrient Removal (BNR) is a process used for nitrogen and phosphorus removal from wastewater before it is discharged into surface or ground water.

The rising concentration of harmful nutrient compounds – specifically nitrogen and phosphorus – in municipal wastewater treatment plant discharge causes cultural eutrophication (nutrient enrichment due to human activities) in surface waters. Summer algal blooms are a familiar example of this eutrophication, and can present problems for ecosystems and people alike: low dissolved oxygen, fish kills, murky water, and depletion of desirable flora and fauna.

Because conventional biological processes designed to meet secondary treatment effluent standards typically do not remove total nitrogen (TN) and total phosphorus (TP) to the extent needed to protect receiving waters, wastewater treatment facilities are increasingly being required to implement processes that reduce effluent nutrient concentrations to safe levels. This can be a challenge for wastewater treatment plants because it usually involves major process modifications to a plant, such as making a portion of the aeration basin anaerobic and/or anoxic, which reduces the aerobic volume and limits nitrification capacity.

Clarifier solids loading usually limits the concentration of biomass available for nitrification, so it’s common to increase bioreactor volume to increase treatment capacity. This can be very expensive and even impossible if space is limited.


One cost-effective solution for Biological Nutrient Removal is the ActiveCell® process developed by Headworks BIO. By placing specially designed ActiveCell media into activated sludge basins, the benefits of fixed-film, attached-growth biological systems are combined with those of the suspended growth activated sludge process. This hybrid process is referred to as Integrated Fixed-film Activated Sludge (IFAS) technology.

Typically installed as a retrofit, IFAS technology allows existing activated sludge plants to increase capacity and support Biological Nutrient Removal without additional clarifier or aeration basin volume. Each individual piece of ActiveCell media has a high surface area-to-volume ratio that supports biofilm growth for active biology above and beyond the limits of the suspended growth activated sludge system.

The ActiveCell IFAS process is typically divided into stages that include anaerobic, anoxic, and aerobic volumes, similar to some well-known Biological Nutrient Removal configurations. Within the ActiveCell IFAS process, media is filled in the aerobic stages and retained by stainless steel wedge-wire screens at the effluent end.

Reactor capacity can be increased for organic loading and can support more advanced wastewater treatment due to longer sludge age. Unlike with typical suspended growth activated sludge systems, the additional fixed-film biomass in the ActiveCell process does not need to be settled out – and therefore does not increase the solids loading to the secondary clarifier. The fixed-film biomass helps the process respond to organic or hydraulic shock loads and to recover from upsets.

Portions of existing aerobic zones can be partitioned into anaerobic or anoxic zones for advanced Biological Nutrient Removal treatment and the addition of ActiveCell media to the remaining aerobic zones increases the Solids Retention Time (SRT) for nitrification.

The team of process design experts at Headworks BIO employs advanced modeling and design tools to determine the size, location, and quantity of ActiveCell media for each stage of the process to achieve the desired level of nutrient removal.

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