What is Biological Nutrient Removal (BNR)?

Introduction

Biological Nutrient Removal (BNR) is the process of removing nitrogen and phosphorus from wastewater using bacteria under controlled environmental conditions — rather than chemicals. As nutrient discharge limits have tightened across the country in response to eutrophication in receiving waters, BNR has become one of the most important treatment technologies in municipal wastewater.

The nitrogen cycle and phosphorus cycle are both mediated by specific groups of bacteria. BNR exploits these metabolic pathways by deliberately manipulating the aerobic, anoxic, and anaerobic conditions in the treatment process.

Why Nutrient Removal Matters

Excess nitrogen and phosphorus in waterways causes eutrophication — explosive algae growth that consumes dissolved oxygen, kills fish, creates harmful algal blooms (HABs), and degrades recreational water quality. The Chesapeake Bay, Gulf of Mexico dead zone, Lake Erie, and thousands of impaired water bodies across the US are impacted by nutrient pollution.

Sources of nutrients in wastewater:

• Human waste (urine is ~80% of total nitrogen load in sewage)
• Detergents (historically the largest source of phosphorus; now largely reformulated)
• Food waste disposal, industrial discharges

Typical raw municipal wastewater:

• Total Nitrogen (TN): 25-50 mg/L
• Total Phosphorus (TP): 4-8 mg/L

Typical discharge permit limits requiring BNR:

• TN: 3-10 mg/L
• TP: 0.1-1.0 mg/L

Biological Nitrogen Removal: The Two-Step Process

Biological nitrogen removal requires two sequential steps, each performed by different groups of bacteria:

Step 1: Nitrification (Aerobic)

Ammonia (NH4-N) is oxidized to nitrate (NO3-N) by Nitrosomonas and Nitrobacter (and other nitrifying autotrophs).

NH4+ → NO2- → NO3-

Required conditions:

• Dissolved oxygen (DO) > 1.5 mg/L (typically 2+ mg/L)
• Long SRT (>10 days at 20°C; >20 days at 10°C)
• Alkalinity (nitrification consumes ~7.14 mg alkalinity per mg NH4-N oxidized)
• pH 7.5-8.5

Step 2: Denitrification (Anoxic)

Nitrate (NO3-N) is converted to nitrogen gas (N2) by heterotrophic bacteria under anoxic conditions.

NO3- → NO2- → N2O → N2 (off-gas)

Required conditions:

• No dissolved oxygen (DO < 0.3 mg/L)
• Carbon source (BOD or external carbon like methanol/acetate)
• Anoxic zone in the process flow

The key constraint in denitrification is carbon. Bacteria need a carbon:nitrogen ratio of approximately 4-6 g BOD per g NO3-N to denitrify. In lightly loaded plants or at sites with low influent BOD:TN ratios, external carbon (methanol, sodium acetate, glycerol) must be added.

Biological Phosphorus Removal: EBPR

Enhanced Biological Phosphorus Removal (EBPR) is accomplished by Polyphosphate Accumulating Organisms (PAOs) — particularly Candidatus Accumulibacter phosphatis.

The mechanism is counterintuitive: PAOs accumulate far more phosphorus under aerobic conditions than they release under anaerobic conditions, resulting in net phosphorus removal from the liquid stream into the biomass (sludge).

The PAO cycle:

1. Anaerobic zone: PAOs take up readily biodegradable COD (rbCOD) and store it intracellularly as PHA, drawing energy from stored polyphosphate — releasing orthophosphate (HPO4²⁻) into the liquid
2. Aerobic zone: PAOs take up far more phosphorus than they released, storing it as polyphosphate
3. Sludge wasting: PAOs are removed with the waste sludge, carrying the accumulated phosphorus out of the system

Critical requirements for EBPR:

• True anaerobic zone upstream of the aerobic zone (no DO, no nitrate)
• Adequate VFA/rbCOD in the influent to the anaerobic zone
• Nitrate must not recycle to the anaerobic zone (degrades EBPR performance)
• Stable operation — EBPR is sensitive to upsets

BNR Process Configurations

Numerous BNR process configurations have been developed to optimize nitrogen and phosphorus removal while minimizing aeration energy:

A2O (Anaerobic-Anoxic-Oxic)

Three-zone process: anaerobic → anoxic → aerobic. Provides both EBPR (anaerobic zone) and denitrification (anoxic zone). Basic BNR configuration for combined N and P removal.

Modified Bardenpho (5-Stage)

Anaerobic → Anoxic → Aerobic → Anoxic → Aerobic (re-aeration). Two anoxic stages allow more complete denitrification. Common where total nitrogen limits are tight (<5 mg/L).

UCT (University of Cape Town)

Modified A2O that recycles the RAS to the anoxic zone rather than the anaerobic zone. Prevents nitrate from entering the anaerobic zone, protecting EBPR performance.

JHB (Johannesburg)

RAS goes through a small anoxic pre-zone to strip residual nitrate before entering the anaerobic zone.

Sequencing Batch Reactor (SBR)

Fill, react, settle, decant in a single basin. Can be programmed for BNR by controlling aeration on/off cycles within the react phase.

Oxidation Ditch with BNR Zones

Channel-in-channel configurations create aerobic/anoxic/anaerobic zones within the ditch volume.

Chemical vs. Biological Phosphorus Removal

Chemical phosphorus removal adds metal salts (alum, ferric chloride, ferrous sulfate) to precipitate phosphorus as insoluble metal phosphate compounds, which are removed with the sludge.

Many plants use biological P removal for the bulk of removal with a chemical polishing step (small alum or ferric dose) to reliably meet tight limits.

BNR System Manufacturers

• Aero-Mod — SEQUOX/SEQUOX+ sequencing BNR (total N to ~3 mg/L, P <1 mg/L) with ClarAtor continuous clarification
• Fluidyne — ISAM (Integrated Surge Anoxic Mix) BNR
• Veolia Water Technologies — Biostyr, BioDenitro processes; acquired Krüger
• Kruger (Veolia) — BioDenitro/BioDenipho; Double/Triple Ditch
• Evoqua Water Technologies — Orbal BNR systems
• Newterra — Tri-Oval oxidation ditch with BNR
• Aqua-Aerobic Systems — AquaSBR and Aqua-Aerobic MBR