Biological Nutrient Removal: Complete Guide for Operators

Biological nutrient removal is how your plant gets nitrogen and phosphorus out of wastewater using the bugs instead of (or alongside) chemicals. If you're studying for a Grade 2, 3, or 4 exam, BNR questions are common on many state certification exams - and they're not gimmies. This guide breaks down the nutrient removal process, configurations, control parameters, and exam traps you need to know.

What Exactly Is Biological Nutrient Removal?

BNR uses microorganisms to convert or capture nitrogen and phosphorus so they don't end up in the receiving water. Nitrogen removal happens through nitrification (ammonia to nitrate) followed by denitrification (nitrate to nitrogen gas). Phosphorus removal relies on a special group of bugs called PAOs that hoard phosphorus in their cells, which you then waste out with the sludge.

The reason plants invest in BNR comes down to permits. Excess nitrogen and phosphorus cause algal blooms, oxygen depletion, and dead zones in receiving waters. Nutrient-sensitive watersheds like the Chesapeake Bay, Long Island Sound, the Great Lakes, and Florida's estuaries have driven increasingly tight limits. Florida's commonly cited Advanced Wastewater Treatment (AWT) standard per Florida Statute 403.086(4) is a good example: 5 mg/L BOD, 5 mg/L TSS, 3 mg/L TN, 1 mg/L TP.

There's no single national nutrient limit. Your permit depends on your state, your receiving water, and any applicable TMDL. But the trend is clear: limits are getting tighter, and BNR knowledge is becoming essential.

How Does Nitrogen Removal Work in BNR Wastewater Systems?

Nitrogen removal is a two-act play. First you convert ammonia to nitrate (nitrification), then you convert nitrate to nitrogen gas that bubbles off harmlessly (denitrification). Skip either act and you've still got a nitrogen problem.

Nitrification: Ammonia to Nitrate

Nitrification is an aerobic process run by slow-growing autotrophic bacteria. The two key players are Nitrosomonas, which oxidizes ammonia to nitrite (the rate-limiting step), and Nitrobacter, which oxidizes nitrite to nitrate. Nitrospira actually dominates the NOB role in many real plants, but exams still test Nitrobacter.

Nitrifiers are autotrophs. They don't need BOD. They use inorganic carbon (alkalinity/bicarbonate) as their carbon source and ammonia as their energy source. This is a favorite exam distinction - don't confuse what nitrifiers need with what denitrifiers need.

The numbers you need to know:

• Oxygen demand: 4.57 mg O2 per mg NH3-N oxidized
• Alkalinity consumed: 7.14 mg/L as CaCO3 per mg NH3-N oxidized
• DO target: ~2.0 mg/L in the nitrification zone. Below 1.0 mg/L, rates drop significantly. Above 3.0 mg/L, you're wasting energy and carrying DO into your anoxic zone.
• pH sweet spot: 7.5-8.0. Below 6.8 you see major rate loss. Below 6.0 it essentially stops.
• Temperature effect: The rate roughly halves with each 7-10 degree C drop. Below 7 C, conventional suspended growth nitrification becomes extremely difficult.
• Alkalinity reserve: Keep effluent alkalinity at 50 mg/L minimum, 100 mg/L preferred.

SRT is everything for nitrification. Those slow-growing nitrifiers need enough time in the system to reproduce before you waste them out. If you want a deeper dive, check out the mean cell residence time formula and how to calculate it. Typical conservative design targets (with a safety factor of 2x or more):

The operational play: raise your MLSS (and SRT) in autumn to stockpile nitrifiers before winter hits.

Denitrification: Nitrate to Nitrogen Gas

Denitrification is an anoxic process run by heterotrophic facultative bacteria like Pseudomonas. They prefer free oxygen but can switch to the oxygen bound up in nitrate when DO drops out. Even small amounts of DO in the anoxic zone reduce denitrification rates.

Critical numbers:

• DO threshold: Anoxic zone DO must stay below 0.2-0.3 mg/L
• Carbon requirement: 2.86 mg COD per mg NO3-N reduced (stoichiometric). Real-world demand is higher. Denitrifiers are heterotrophs - they NEED organic carbon (BOD).
• Alkalinity recovery: 3.57 mg/L as CaCO3 per mg NO3-N reduced - denitrification gives back some of the alkalinity that nitrification burned.
• Net alkalinity impact: Nitrification consumes 7.14, denitrification recovers 3.57, so you lose ~3.57 mg/L per mg of nitrogen cycled.

When influent BOD isn't enough, you might need external carbon. Methanol is the classic choice at roughly 3:1 (methanol to NO3-N by mass). Other options include MicroC, glycerol, and acetic acid.

The internal mixed liquor recycle (IMLR) brings nitrate-rich mixed liquor from the aerobic zone back to the anoxic zone. Typical ratios run 1Q to 3Q, with about 2Q being the centerline. Going above 4Q rarely helps because you drag too much DO into the anoxic zone.

BNR plants typically run at higher SRTs and lower F:M ratios than conventional activated sludge. If you're working through how to calculate the F:M ratio, keep that in mind.

How Does Phosphorus Removal Work?

You've got two paths for nitrogen phosphorus removal: biological (EBPR) and chemical. Many plants use both.

Enhanced Biological Phosphorus Removal (EBPR)

EBPR relies on PAOs (Polyphosphate Accumulating Organisms). Candidatus Accumulibacter is recognized as the primary PAO in most systems. Ordinary heterotrophs contain about 1.5-2% phosphorus by cell mass. PAOs can accumulate 5-15% or more. Get these P-hoarders thriving, waste them out, and the phosphorus goes with them.

EBPR works in two phases:

Anaerobic zone (P-Release phase): Fermentative bacteria break down soluble organics into VFAs. PAOs absorb VFAs and store them as PHB. To generate energy without oxygen or nitrate, they release orthophosphate. The counterintuitive part: dissolved P in the anaerobic zone should RISE - sometimes 3-4x influent concentration. That's actually a healthy sign.

Aerobic zone (Luxury Uptake phase): PAOs burn stored PHB and take up MORE phosphorus than they released. Soluble P can drop below 0.1 mg/L in optimized systems. The phosphorus is locked in sludge solids and removed when you waste.

The core requirement is a truly anaerobic zone - no DO AND effectively no nitrate. ORP is often observed in the -150 to -400 mV range, with about 1 hour detention time.

Carbon-to-phosphorus ratio: A commonly cited rule-of-thumb is a 20:1 BOD5:TP ratio for successful EBPR. The VFA-specific guideline is approximately 7-10 mg VFA (as COD) per 1 mg P removed.

What kills EBPR:

1. Nitrate contamination of the anaerobic zone - the #1 failure mode. Denitrifiers outcompete PAOs for VFAs.
2. GAO competition - Glycogen Accumulating Organisms consume VFAs but don't accumulate phosphorus. High temps (above 28-30 C) and low pH (below 6.9-7.0) favor GAOs.
3. VFA shortage - low BOD:P ratio, stormwater dilution, or septicity in the collection system.
4. Secondary P release - clarifier sludge blanket goes anaerobic; PAOs release P without subsequent aerobic uptake.

Chemical Phosphorus Removal

Metal salts (alum and ferric chloride) precipitate phosphorus. Typical dose is 1-3 moles metal per 1 mole phosphate. Alum and ferric work near neutral pH (roughly 5.5-7.5 for alum, 5.5-8 for ferric).

Lime operates at pH ~11. Do NOT confuse lime's high-pH mechanism with alum/ferric on the exam.

Chemical P removal produces more sludge than bio-P and consumes alkalinity. If you're running nitrification and chemical P removal simultaneously, watch for compounding alkalinity loss.

What Are the Main BNR Process Configurations?

This is where it all comes together. Different configurations combine anaerobic, anoxic, and aerobic zones depending on your nutrient removal targets.

Know that MLE is the configuration you're most likely to see for nitrogen removal, and A2/O is the standard combined N+P config. The 5-Stage Bardenpho is the answer when the question asks about achieving the lowest possible TN and TP.

What Control Parameters Matter Most?

Here's your cheat sheet for the parameters you'll adjust and monitor in a BNR plant:

What Are the Most Common BNR Troubleshooting Scenarios?

Cold weather nitrification loss: Effluent ammonia climbs as winter sets in. The fix is proactive - increase SRT in late autumn, bump DO to 2.0-3.0 mg/L, and keep alkalinity above 50 mg/L. Size SRT for worst-case winter conditions.

EBPR failure from nitrate contamination: Elevated effluent phosphorus despite having an anaerobic zone. Check nitrate in your RAS and anaerobic zone influent. If RAS NO3 is above 2-3 mg/L, you need better denitrification upstream. Turn on chemical P as a backstop.

Alkalinity crash: Effluent alkalinity dropping below 50 mg/L, pH below 6.8, nitrification declining. Supplement with soda ash, lime, caustic, or sodium bicarbonate. Optimize denitrification to recover that 3.57 mg/L per mg NO3-N.

Secondary P release: Effluent ortho-P higher than what you measure at the end of the aerobic zone. Your clarifier sludge blanket is probably going anaerobic. Increase RAS to minimize sludge residence time and keep blanket depth down. Monitoring your sludge volume index can help spot settling issues before they cause secondary release.

What BNR Exam Traps Should You Watch For?

These are the questions that trip people up on Grade 2-4 exams:

1. Anoxic vs. anaerobic. Not the same. Anoxic has nitrate. Anaerobic has neither DO nor nitrate.

2. Alkalinity direction. Nitrification CONSUMES alkalinity (7.14 mg/L per mg NH3-N). Denitrification RECOVERS alkalinity (3.57 mg/L per mg NO3-N).

3. Autotrophs vs. heterotrophs. Nitrifiers are autotrophs - no BOD needed. Denitrifiers are heterotrophs - they NEED BOD. If the question asks "which process requires organic carbon," denitrification and EBPR are the answers.

4. Lime vs. alum/ferric pH. Lime works at pH ~11. Alum and ferric work near neutral pH.

5. SRT sizing. Size for worst-case winter temperature, not annual average. Safety factor of 2x or more over theoretical minimum.

6. DO in the anoxic zone. Even small amounts above 0.2 mg/L inhibit denitrification. Questions about DO carryover from IMLR are common.

For the most current information on how BNR fits into nutrient permit requirements, check the EPA's nutrient policy and data resources. State-specific requirements vary, so always verify your permit limits with your state regulatory agency.