Nitrification in an aeration tank ends with higher nitrate and nitrite and lower alkalinity and pH.

What happens in the aeration tank after nitrification?

If you’ve spent any time around a wastewater plant, you know the machinery isn’t just metal and moving parts. It’s a living, breathing system, fueled by tiny microbes doing big jobs. One of the most important stages in nitrogen removal is nitrification. That’s the process that turns toxic ammonia into forms that are far less harmful to the environment. So what exactly should we expect in the aeration tank when nitrification is finished? The short answer is: nitrate and nitrite go up, while alkalinity and pH tend to go down. Let me unpack that a bit so it makes sense.

Two steps, two teams, one goal

Nitrification isn’t a one-step miracle. It’s a two-step dance carried out by two different groups of bacteria in the presence of oxygen. First, aerobic ammonia-oxidizing bacteria (AOB) take ammonia (NH3) and convert it to nitrite (NO2-). Then nitrite-oxidizing bacteria (NOB) take that nitrite and convert it to nitrate (NO3-). In other words, ammonia gets progressively oxidized, and nitrate usually ends up as the dominant end product, with nitrite often appearing as a brief intermediate.

What you see in the tank

Because nitrification is an aerobic process, oxygen is constantly banging away in the background. With oxygen in play and ammonia on the table, the chemistry moves in a predictable direction:

• Nitrate and nitrite rise: As ammonia gets oxidized, nitrite is formed first, and then nitrate. If you sample the water toward the end of nitrification, you’ll typically see higher concentrations of nitrate, and you might still measure some nitrite depending on how fast the two steps are happening relative to each other.

• Alkalinity falls: Alkalinity acts like a buffering reserve for pH. Each mole of ammonia converted to nitrite and then to nitrate consumes alkalinity (in chemistry terms, it consumes bicarbonate/carbonate buffering). That buffering reserve gets used up, so the system’s ability to resist pH changes softens, and the pH tends to drop.

• pH drops: As H+ ions are produced during the oxidation steps, the water becomes a little more acidic. You’ll often see a gradual dip in pH as nitrification runs its course, unless you’re actively offsetting it with alkalinity control or other pH-management strategies.

Putting those pieces together, the end-of-nitrification snapshot is pretty clear: more nitrate and nitrite, less alkalinity, and a lower pH. That’s why operators watch alkalinity and pH closely as nitrification progresses, and why some plants “feed” alkalinity back into the system with lime or other buffers to keep the process stable.

Why this matters for plant operation

Nitrification is a powerful engine, but it’s not free of consequences. The same processes that clean the water can shift the chemical balance inside the tank. Here are a few implications that surface in real-world operation:

• Oxygen demand stays high: Nitrification is an oxygen-hungry process. In meth terms, the rate of AOB and NOB activity is tied to how much oxygen is available. If DO pockets fall or diffusers get clogged, you’ll see the nitrification pace slow, and that can change the ratio of nitrate to nitrite you measure.

• pH-sensitive day-to-day shifts: A drop in pH doesn’t just affect the nitrifiers; it can influence metal solubility and other chemical equilibria in the tank. If the pH dips too far, you may need to intervene to protect the microbial community and the rest of the process train.

• Alkalinity insurance: Some plants keep a running buffer of alkalinity to absorb the acid generated by nitrification. Without that cushion, pH can crash, nitrification can stall, and you end up chasing a moving target rather than keeping the system steady.

A quick mental model you can carry around

Think of nitrification as a factory line. Ammonia enters the line, goes through two stations (AOB then NOB), and leaves as nitrate. Each station uses oxygen and, in the process, shifts the chemical balance of the water. The line doesn’t just spit out a finished product; it also changes the surrounding environment by consuming buffering capacity and releasing hydrogen ions. If you picture it that way, the signs you monitor—nitrate/nitrite levels, alkalinity, and pH—start to click into place.

A nod to denitrification: what comes next

If nitrification ends with a higher nitrate load, you might wonder what happens to that nitrogen. That’s where denitrification comes in. In zones with little or no oxygen and with a carbon source available, bacteria can convert nitrate into nitrogen gas, which escapes to the atmosphere. It’s the complementary half of the nitrogen cycle in many treatment trains: nitrification builds nitrate, and denitrification removes it in a separate, anoxic stage. So, the end-of-nitrification conditions aren’t the finish line—they’re a signal to steer the next stage appropriately.

A few practical notes for operators and students

While it’s tempting to memorize the “correct answer” to a quiz question and move on, the real value lies in understanding the mechanism behind the numbers you observe. Here are a few takeaways that tend to stick:

• Monitor the right levers: DO concentration, nitrification rate, and the alkalinity balance are your best friends when nitrification is underway. If you see nitrate rising faster than nitrite, that’s typically a sign the second step is catching up—but don’t ignore the pH and alkalinity trend.

• Manage alkalinity like a budget: Alkalinity isn’t just a number; it’s energy you can spend to keep pH stable. If you’re seeing a steady pH decline, consider whether you have enough buffering capacity and if lime or another buffer should be added.

• Temperature matters: Nitrification rates climb with temperature up to a point. Warmer water speeds up microbial metabolism, which can sharpen the transitions between ammonia, nitrite, and nitrate. Cold conditions can slow things down and skew the usual patterns.

• The role of the aeration system: Strong, evenly distributed aeration helps keep the conditions aerobic and supports stable nitrification. Diffuser performance, bubble size, and aeration strategy all play into how smoothly AOB and NOB do their jobs.

A gentle stroll through the nitrogen pathway

Let me connect a few dots, just to keep the mental map in good shape. The nitrogen we’re talking about isn’t just a lab curiosity—it’s a real-world balancing act. Ammonia is toxic to aquatic life in sufficient concentrations, so turning it into nitrate and nitrite isn’t just about cleanliness; it’s about safety and compliance. Yet nitrate in large amounts isn’t a perfect outcome either, especially in sensitive receiving waters or if the plant’s downstream processes rely on low nitrogen. That tension is what makes the nitrification step so carefully watched in every plant, from a small municipal facility to a large regional plant.

Analogies that help, but don’t oversimplify

If you’ve ever watched a well-oiled kitchen team, nitrification can resemble a two-chef system. One cook (AOB) starts chopping onions, turning ammonia into nitrite, while the second cook (NOB) finishes the job by turning that into nitrate. Both cooks need clean utensils, steady heat (oxygen), and a calm workspace (stable pH and alkalinity). Mess with any one piece—oxygen levels, buffering capacity, or temperature—and the whole choreography can stumble. It’s a reminder that wastewater treatment is less about heroic single moments and more about consistent, cooperative processes.

Putting it all together

So, if you’re looking at the end of nitrification in an aeration tank and a question pops up, the most accurate picture is this: nitrate and nitrite rise as ammonia is oxidized, while alkalinity and pH tend to fall because of the buffering and hydrogen ion production in the reactions. It’s a clean, compact summary of a pretty dynamic and nuanced biology-and-chemistry dance going on behind the scenes.

A closing thought

Wastewater treatment isn’t just about moving water from one place to another; it’s about steering a tiny, invisible workforce toward a safer, cleaner outcome. Nitrification is a cornerstone of that effort, and understanding its fingerprints—nitrate and nitrite increases paired with alkalinity and pH declines—helps operators anticipate needs, keep systems stable, and plan the next steps with confidence. The next time you hear about oxygen, bacteria, and the nitrogen cycle working in concert, you’ll know what to look for in the aeration tank—and you’ll see more than a tank of water—you’ll see a living, balancing system at work.