14.2 Nutrient Removal: Nitrification, Denitrification & Anaerobic/Anoxic/Aerobic Zones

14.2 Nutrient Removal: Nitrification, Denitrification & Anaerobic/Anoxic/Aerobic Zones

Exam Focus: "Nutrient removal processes (e.g., anaerobic/anoxic/aerobic)" is a named Treatment Process line item, and "Nutrient removal/enhancement" appears again under chemical dosing. Ammonia limits are now common even at small plants, so nitrogen chemistry has become genuine Class I material.

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1. Nitrification

Nitrification is the aerobic, two-step biological oxidation of ammonia:

Step 1:  Ammonia (NH4-N)  -->  Nitrite (NO2-N)     by ammonia-oxidizing bacteria (e.g., Nitrosomonas)
Step 2:  Nitrite (NO2-N)  -->  Nitrate (NO3-N)     by nitrite-oxidizing bacteria (e.g., Nitrobacter, Nitrospira)

The organisms responsible are autotrophs — they build cell material from inorganic carbon rather than from organic matter — and they are slow growers. Everything difficult about nitrification follows from that slow growth rate.

What Nitrifiers Require

The Two Numbers Worth Memorizing

• Oxygen demand: approximately 4.6 pounds of oxygen per pound of ammonia nitrogen oxidized — over and above the oxygen required for carbonaceous BOD. Turning on nitrification substantially increases aeration demand.
• Alkalinity destruction: approximately 7.14 pounds of alkalinity as CaCO3 per pound of ammonia nitrogen oxidized.

The classic nitrification failure. A plant in a soft-water region begins nitrifying, alkalinity is consumed faster than the influent supplies it, mixed liquor pH falls below 6.5, and nitrification stops — which then lets ammonia rise, alkalinity partially recover, pH rise, nitrification restart, and the cycle repeat. The fix is to monitor effluent alkalinity as a routine process control test and supplement with caustic soda, lime, or soda ash to hold a residual.

2. Denitrification

Denitrification is the anoxic conversion of nitrate to nitrogen gas:

Nitrate (NO3-N)  -->  Nitrite  -->  Nitric oxide  -->  Nitrous oxide  -->  Nitrogen gas (N2)

The organisms are ordinary heterotrophic bacteria — the same ones doing BOD removal — operating in a zone that has nitrate but no free dissolved oxygen. Given a choice they will always use oxygen, which is why an anoxic zone must genuinely exclude it.

Requirements

1. Nitrate present — you cannot denitrify what you have not first nitrified.
2. Essentially zero dissolved oxygen. This is why anoxic zones are mixed mechanically, never with air (Section 10.2).
3. A carbon source. Raw influent BOD is the cheapest, which is why anoxic zones are usually placed first, receiving raw influent and a nitrate-rich internal recycle. Where nitrate must be removed after the aerobic zone, a supplemental carbon source such as methanol is fed.

What Denitrification Gives Back

• Alkalinity recovery: approximately 3.57 pounds as CaCO3 per pound of nitrate nitrogen reduced — roughly half of what nitrification destroyed. A plant that both nitrifies and denitrifies has a far easier alkalinity balance than one that only nitrifies.
• Oxygen credit: roughly 2.86 pounds of oxygen equivalent recovered per pound of nitrate nitrogen reduced, because the nitrate itself supplies the oxidizing power that would otherwise come from blower air.

Denitrification in the wrong place. Nitrate that reaches a secondary clarifier blanket and sits there will denitrify, and the nitrogen gas bubbles float sludge to the surface — the rising sludge covered in Section 3.4. The distinction matters: denitrification in an anoxic zone is the process working; denitrification in the clarifier is a problem, corrected by increasing RAS rate to shorten blanket detention time.

3. Common Zone Arrangements

4. Enhanced Biological Phosphorus Removal

Phosphorus-accumulating organisms (PAOs) can store phosphorus far in excess of their metabolic needs, but only if they are cycled through the right sequence:

1. Anaerobic zone — no dissolved oxygen AND no nitrate. PAOs take up volatile fatty acids from the influent and store them as internal polymers, and to power that uptake they release phosphorus into solution. Soluble phosphorus therefore rises in the anaerobic zone; this is normal and necessary.
2. Aerobic zone. PAOs metabolize their stored polymers and take phosphorus back up in excess of what they released — the phenomenon called luxury uptake.
3. Wasting. The phosphorus is now inside the cells, so it leaves the plant only in the wasted sludge.

Operating implications that get tested:

• The anaerobic zone must exclude nitrate as well as oxygen. Nitrate arriving with RAS lets denitrifiers consume the volatile fatty acids that PAOs need, and biological phosphorus removal collapses. This is why plants with combined nitrogen and phosphorus removal manage where nitrate-bearing RAS is returned.
• Waste consistently. Skipping wasting keeps phosphorus in the system.
• Secondary release occurs whenever phosphorus-laden sludge sits under anaerobic conditions without volatile fatty acids present — in a deep clarifier blanket, a gravity thickener, or a sludge holding tank. The phosphorus goes back into solution and returns to the head of the plant in the side stream. Keep blankets thin and side streams managed.

5. Chemical Phosphorus Removal

Where biological removal is absent or insufficient, phosphorus is precipitated chemically with a metal salt — alum, ferric chloride, or ferrous salts — or with lime.

• Dosing points include ahead of the primary clarifier, into the aeration basin or its effluent, and ahead of a tertiary filter for final polishing.
• Metal salt addition consumes alkalinity and depresses pH, which interacts directly with the nitrification alkalinity budget above.
• Chemical addition increases sludge production, and that additional chemical sludge loads thickening, digestion, and dewatering.
• Multiple dosing points at lower total dose usually outperform a single large dose at one location.