Two biological processes. One removes ammonia. The other removes the nitrate left behind. Together they're nitrogen removal — and together they're heavily tested on every Class II exam and above.
Nitrogen removal is one of the most important and most tested topics on the wastewater operator certification exam — particularly at Class II and above. If your facility has a permit limit for ammonia, total nitrogen, or nitrate, understanding nitrification and denitrification isn't optional. It's the core of how you meet those limits.
This article covers both processes in full: what they are, which bacteria do the work, what conditions they need, what disrupts them, and exactly what the exam tests on each one.
Nitrogen enters wastewater treatment plants primarily as ammonia (NH3), released when organic nitrogen compounds break down. Ammonia is a problem in receiving waters for two reasons: it exerts an oxygen demand on the receiving stream (nitrogenous oxygen demand, or NOD), and at high enough concentrations it's toxic to aquatic life.
Many facilities also have total nitrogen or nitrate limits to prevent eutrophication — the explosive algae growth that occurs when nitrate-rich water reaches lakes, bays, and estuaries. That's what drives the need for both nitrification and denitrification in sequence.
Bacteria convert ammonia (NH3) first to nitrite (NO2), then to nitrate (NO3). Requires oxygen. Removes ammonia from the water — but doesn't eliminate nitrogen from the system.
Different bacteria convert nitrate (NO3) to nitrogen gas (N2), which escapes to the atmosphere. Requires an anoxic zone — no dissolved oxygen. This is how nitrogen actually leaves the system.
Nitrification is a two-step biological process carried out by autotrophic bacteria — bacteria that get their energy from inorganic compounds rather than organic carbon. They're slow-growing, sensitive to environmental conditions, and easily disrupted. That's why nitrification is often the most difficult process to maintain consistently.
Nitrosomonas bacteria handle the first step — converting ammonia to nitrite. Nitrobacter bacteria handle the second step — converting nitrite to nitrate. Both are autotrophs, both are slow-growing, and both require dissolved oxygen to function. Under normal conditions, nitrite does not accumulate in the process — Nitrobacter converts it to nitrate almost as fast as Nitrosomonas produces it.
Nitrite accumulation (elevated NO2 in process or effluent) is a sign that nitrification is incomplete or disrupted. It almost always means something has inhibited Nitrobacter more than Nitrosomonas — often a toxic compound, low DO, or a sudden temperature drop. This is a common exam troubleshooting scenario.
Nitrifiers grow very slowly — minimum doubling time of 8–12 hours. Your SRT must be long enough that nitrifiers aren't wasted out faster than they can reproduce. Typically 10–15+ days for reliable nitrification. This is the single most important operational requirement.
Nitrification is aerobic — it requires oxygen. Minimum DO of 1.5–2.0 mg/L in the aeration basin. Below this, nitrification slows significantly. Many facilities targeting nitrification maintain 2.0+ mg/L to provide a buffer.
Nitrifiers are sensitive to pH. Optimal range is 7.5–8.5. Below pH 6.5, nitrification essentially stops. Also note: nitrification itself consumes alkalinity — which can drive pH down over time if not managed.
Nitrification slows significantly in cold water. Below 10°C (50°F), nitrifiers become barely active. This is why facilities often struggle to nitrify in winter — and why some permits have seasonal ammonia limits instead of year-round limits.
This is one of the most exam-tested details about nitrification: nitrification consumes alkalinity. Specifically, it consumes approximately 7.14 mg of alkalinity (as CaCO3) for every mg of ammonia-nitrogen oxidized. If your influent alkalinity is low, nitrification can drive the pH down to a point where it inhibits itself. Many facilities add sodium bicarbonate or lime to maintain adequate alkalinity when nitrifying.
Denitrification completes the nitrogen removal cycle. Where nitrification converts ammonia to nitrate (keeping nitrogen in the water in a different form), denitrification converts nitrate all the way to nitrogen gas — which leaves the system to the atmosphere. This is how nitrogen is actually removed.
Denitrification is carried out by heterotrophic bacteria — the same general class of bacteria responsible for BOD removal. Under anoxic conditions (no dissolved oxygen present), these bacteria use nitrate as their electron acceptor instead of oxygen. They're facultative anaerobes — they can work with either oxygen or nitrate, and they'll use whichever is available.
No dissolved oxygen present. With DO available, bacteria will use oxygen (more efficient) instead of nitrate. True denitrification requires an anoxic zone — DO below 0.2 mg/L. This is the most critical design requirement.
Denitrifiers need organic carbon as their energy source. In most facilities, incoming wastewater BOD serves as the carbon source. Facilities with low influent BOD may need to add an external carbon source — methanol is the most common, though acetate and glycerol are also used.
Obviously — denitrification requires nitrate to convert. This is why nitrification must come first, or nitrate must be recycled from a downstream aerobic zone back to an upstream anoxic zone.
Denitrifiers are less pH-sensitive than nitrifiers but still prefer near-neutral conditions. Also note: denitrification recovers approximately half the alkalinity consumed by nitrification — about 3.57 mg alkalinity per mg nitrate-N removed.
In a biological nitrogen removal (BNR) system, nitrification and denitrification are operated in sequence — but they need opposite conditions. Nitrification needs oxygen; denitrification needs the absence of it. This is solved through zone design.
The most common configuration for municipal plants. Wastewater flows first through an anoxic zone, then into the aerobic zone. Nitrate produced in the aerobic zone is recycled (via internal recycle pumps) back to the anoxic zone where incoming BOD serves as the carbon source for denitrification. This is the most efficient use of influent BOD because carbon is available at the front of the process.
Aerobic zone comes first (nitrification), followed by an anoxic zone (denitrification). The problem: most of the readily biodegradable BOD has already been consumed in the aerobic zone, so denitrification in the post-anoxic zone is carbon-limited. An external carbon source is often required. Used when total nitrogen removal needs to be very high.
The internal recycle in a pre-anoxic BNR system recycles nitrate-rich mixed liquor from the aerobic zone back to the anoxic zone. This is distinct from RAS (return activated sludge), which recycles settled sludge from the clarifier. Both recycle streams exist — they serve different purposes. Mixing them up is a common exam mistake.
One of the most recognizable symptoms of denitrification problems is rising sludge in the secondary clarifier. Here's what happens: if nitrate accumulates in the sludge blanket at the bottom of the clarifier, denitrification can occur there. The nitrogen gas produced attaches to the sludge floc and causes it to float to the surface in clumps or mats.
Rising sludge is different from bulking sludge. Bulking is a settling problem — the sludge settles slowly because of filamentous bacteria. Rising sludge settles fine, then floats back up due to gas production. The fixes are also different:
| Topic | What the Exam Tests |
|---|---|
| Bacteria involved | Nitrosomonas (ammonia → nitrite), Nitrobacter (nitrite → nitrate), heterotrophs (denitrification) |
| Nitrification requirements | Long SRT (10–15+ days), DO ≥ 1.5–2.0 mg/L, pH 7.0–8.5, adequate alkalinity |
| Denitrification requirements | Anoxic conditions (DO < 0.2 mg/L), carbon source, nitrate present |
| Alkalinity relationships | Nitrification consumes ~7.14 mg alk/mg NH3-N; denitrification recovers ~3.57 mg alk/mg NO3-N |
| Nitrite accumulation | Indicates incomplete nitrification — Nitrobacter inhibited more than Nitrosomonas |
| Rising sludge | Denitrification in clarifier — caused by nitrate in sludge blanket; fix by increasing WAS/RAS |
| BNR configuration | Pre-anoxic vs. post-anoxic; internal recycle purpose; carbon source requirements |
| Winter nitrification | Temperature drops slow nitrifiers — may need to increase SRT or add supplemental alkalinity |
| Nitrification | Denitrification | |
|---|---|---|
| What it does | Converts ammonia → nitrate | Converts nitrate → nitrogen gas |
| Bacteria | Nitrosomonas, Nitrobacter (autotrophs) | Heterotrophs (facultative anaerobes) |
| Oxygen | Required (aerobic) — DO ≥ 2.0 mg/L | Must be absent (anoxic) — DO < 0.2 mg/L |
| Carbon source | Not needed (autotrophs) | Required — BOD or external carbon |
| SRT requirement | Long (10–15+ days) | Less critical |
| pH range | 7.0–8.5 (optimal 7.5–8.5) | 6.5–8.0 |
| Alkalinity effect | Consumes ~7.14 mg/mg NH3-N | Recovers ~3.57 mg/mg NO3-N |
| Temperature sensitivity | High — slow below 10°C | Moderate |
| Result | Ammonia removed — nitrate remains | Nitrogen leaves the system as N2 gas |
The WastewaterAce Complete Exam Guide covers nitrogen removal, activated sludge, process control, and all 12 core exam topics — 200 questions with full explanations for every answer.
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