Two processes. One goal: remove nitrogen from wastewater before it reaches the environment. Here's how both work — and exactly what the exam expects you to know.
Nitrogen removal is one of the most important — and most tested — concepts on the wastewater operator certification exam. If you've studied activated sludge, you already know the basics of secondary treatment. Nitrification and denitrification are what take that process a step further, removing nitrogen compounds that standard BOD removal doesn't touch.
Understanding these two processes doesn't just help you pass the exam — it helps you understand what's actually happening inside your aeration basin every day. This guide covers both processes from first principles, explains how they connect to each other, and highlights the specific concepts that show up on Class I and Class II certification exams.
Wastewater contains nitrogen in several forms — primarily as ammonia (NH₃) from human waste and industrial sources. Discharging high concentrations of ammonia or nitrate into receiving waters causes serious problems:
Standard BOD removal in secondary treatment doesn't significantly reduce nitrogen. To meet permit limits, operators need to understand and control two separate but linked biological processes: nitrification and denitrification.
Nitrification is the biological conversion of ammonia to nitrate through a two-step process carried out by specialized bacteria. It happens in aerobic (oxygen-present) conditions and is a critical step in nitrogen removal.
Bacteria called Nitrosomonas oxidize ammonia (NH₃) to nitrite (NO₂⁻). This is the first and rate-limiting step — meaning it controls how fast the overall process goes.
Bacteria called Nitrobacter oxidize nitrite (NO₂⁻) to nitrate (NO₃⁻). This step typically happens quickly once nitrite is available, so nitrite rarely accumulates under normal operating conditions.
Know the bacteria by name: Nitrosomonas converts ammonia to nitrite. Nitrobacter converts nitrite to nitrate. The exam will ask you which organism does which step. A helpful memory trick: Nitrosomonas → "somonas" sounds like "so far" — it only gets you part of the way (to nitrite). Nitrobacter finishes the job.
Nitrifying bacteria are autotrophic — they get their energy from oxidizing ammonia and nitrite rather than from organic carbon. This makes them fundamentally different from the heterotrophic bacteria doing BOD removal, and it has important implications for how they respond to operating conditions.
When the exam describes a plant that "used to nitrify but stopped," think through the most common causes in order: low DO, cold temperature, pH drop, toxic slug, or insufficient SRT. Each has a different corrective action.
Denitrification is the biological conversion of nitrate to nitrogen gas (N₂), which escapes harmlessly to the atmosphere. It's the second half of the biological nitrogen removal process — and the step that actually gets nitrogen out of the water.
While nitrification converts ammonia into nitrate, it doesn't remove nitrogen from the system — it just changes its form. Denitrification completes the job by converting that nitrate into a gas that leaves the water entirely.
Denitrifying bacteria are heterotrophic — the same general class of organisms doing BOD removal. Under normal aerobic conditions, these bacteria use dissolved oxygen as their electron acceptor. But when oxygen is absent or very low (anoxic conditions), they can switch to using nitrate as their electron acceptor instead.
In doing so, they reduce nitrate (NO₃⁻) through nitrite (NO₂⁻) and ultimately to nitrogen gas (N₂). A carbon source is required as a food (electron donor) for this process — typically the BOD already present in the wastewater, or an external source like methanol if needed.
Anoxic ≠ Anaerobic. This is a common exam trap. Anoxic means no dissolved oxygen, but nitrate is present and being used as an oxygen substitute. Anaerobic means no oxygen and no nitrate — a completely different condition. Denitrification is an anoxic process, not anaerobic.
| Characteristic | Nitrification | Denitrification |
|---|---|---|
| What it converts | Ammonia (NH₃) → Nitrate (NO₃⁻) | Nitrate (NO₃⁻) → Nitrogen gas (N₂) |
| Oxygen requirement | Aerobic — requires DO | Anoxic — no DO, but nitrate present |
| Bacteria type | Autotrophic (Nitrosomonas, Nitrobacter) | Heterotrophic (same as BOD removal) |
| Carbon source | Not required (uses inorganic energy) | Required (BOD or external carbon) |
| Effect on alkalinity | Consumes alkalinity (~7.14 g/g NH₃) | Recovers alkalinity (~3.57 g/g NO₃⁻) |
| Effect on pH | Lowers pH | Raises pH slightly |
| Growth rate | Slow — requires long SRT | Faster — same organisms as BOD removal |
| Nitrogen removed? | No — changes form only | Yes — nitrogen leaves as gas |
This is one of the most important connections on the exam. Nitrifying bacteria grow slowly — their doubling time is measured in days, not hours like heterotrophs. If you waste sludge faster than nitrifiers can reproduce, you'll wash them out of the system entirely.
To maintain a stable nitrifying population, the sludge age (SRT) must be long enough for nitrifiers to reproduce before being wasted. The minimum SRT required for nitrification is typically:
In practice, most plants operating for nitrification run SRTs of 10–20 days or more to maintain a safety margin — especially through winter.
A plant has been successfully nitrifying all summer. As temperatures drop in the fall, ammonia levels in the effluent start climbing. The operator has not changed wasting rates. What's happening? The nitrifiers need a longer SRT in cold water — the existing SRT may have been sufficient in warm weather but is now too short. The corrective action is to reduce wasting to extend the SRT.
Denitrification doesn't only happen where operators intend it to. One of the most common activated sludge problems — rising sludge — is actually caused by unintended denitrification occurring in the secondary clarifier.
Here's what happens: if sludge sits too long in the clarifier without sufficient oxygen, the denitrifying bacteria begin converting nitrate to nitrogen gas. The gas bubbles attach to the sludge, causing it to float to the surface in large clumps — rather than settling as intended. The result is sludge escaping over the clarifier weirs into the effluent.
Rising sludge is often confused with bulking sludge, but the causes and corrective actions are very different:
On the exam, rising sludge clues include: sludge floating to the clarifier surface in clumps, the plant has been nitrifying well, and the problem often gets worse later in the day when DO in the clarifier drops. These details distinguish it from bulking.
At plants designed for biological nitrogen removal, operators control these processes through system design and daily operational decisions:
The WastewaterAce Complete Exam Guide covers nitrification, denitrification, activated sludge, disinfection, and all 12 major exam topic areas — with detailed explanations for every answer. Built for Class I and Class II operator exam prep.
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200 questions. 12 topics. Zero math. The Complete Exam Guide is built for operators who want to understand the process — not just memorize answers.
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