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.