We Were Not Failing to Grow the Bacteria. We Were Losing Them.

There is a small plant we worked on for months that kept failing ammonia, and for a long time I could not tell you why.

It was a 5 m³/day system for a sixteen-room boarding house with a few shops on the ground floor — a pharmacy, that sort of thing, no restaurant. We had converted an existing three-chamber septic tank rather than build new: the first chamber became anaerobic and doubled as equalisation, the second and third became aeration, and an IBC tank sitting above ground became the clarifier, returning water to the front.

On paper it was fine. In practice the ammonia result came back failed, then failed again, then failed again.

Everything we could measure looked right

The dissolved oxygen was there. The conditions were right. We could grow the bacteria — we would see the plant come good, and then a few weeks later the ammonia would be back up and we would be starting again.

That pattern is the clue, and it took us too long to read it. A plant that cannot nitrify at all has a design problem. A plant that nitrifies and then stops, repeatedly, has something removing the bacteria.

And the bacteria that convert ammonia grow slowly. That is the whole difficulty with them. Lose a population and you are not waiting until tomorrow for it to come back, you are waiting two or three weeks. If something is flushing them out every few weeks, you will never see a stable result, and every reading you take in between will look perfectly reasonable.

It was the pump

The submersible pump moving water from the third chamber up to the clarifier kept burning out. The tanks were mostly underground and the clarifier was above ground, so that one pump carried the whole return.

Whenever the water level ran low, the pump burned. Whenever the pump was dead, the third chamber simply overflowed — and out went the biomass, and the alkalinity with it. Then somebody would replace the pump, the plant would recover over a few weeks, and the cycle would run again.

The client was also dosing chlorine into the system occasionally, which killed off whatever had managed to establish.

So we were not fighting a treatment problem at all. We were fighting an electrical one, and a habit.

What actually fixed it, in order

First, an overload relay on the pump. A component that costs almost nothing. It stopped the pump burning out on low water, which stopped the overflow, which stopped the plant losing its biomass every few weeks. That was the fix. Everything else was secondary.

Second, fixed-film media in the aeration chambers. Two or three bundles of polypropylene rope, made up rather like a jellyfish, hung with enough space between them for oxygen to move through. The point of media is that it gives the slow-growing bacteria something to hold onto instead of depending on a clarifier and a pump to send them back. If you cannot rely on the return, do not build a process that depends on the return.

We had tried moving media on that plant first, and it did not survive — it kept getting stuck, or escaped past strainers that were not up to the job. In our experience the fixed rope holds nitrifiers better. I cannot give you a clean scientific explanation for that, and I would rather say so than invent one. It is what we have seen on our own plants.

Since those two changes it has passed ammonia regularly.

What I take from it

When ammonia keeps failing on a plant where the readings look fine, look for the mechanical fault before you look at the biology. A transfer pump that trips. An overflow nobody logs. A clarifier quietly carrying solids over the weir. A cleaning routine that puts chlorine somewhere it should not be.

None of those appear on a laboratory certificate. All of them will empty your plant of the one population you cannot quickly replace. It is also the reason we treat the relay, the level switch and the spare pump as part of how a plant is operated and maintained rather than as somebody else's electrical scope.

And it is worth saying plainly that this was a small plant, which is exactly why it was hard. A 5 m³ system has nowhere to hide — one failed pump is the whole plant, where on a large works it would be one of several.

Every site is different, and I would not tell you your ammonia problem is a pump because ours was. I would tell you to rule it out before you buy anything. If it turns out not to be mechanical, then it is a treatment question, and there is more on how we go about removing ammonia and on which ammonia limit your discharge is actually held to.

If your plant keeps recovering and then failing again, send us the last few lab results in order rather than just the latest one — the pattern is usually more useful than the number. Book a free 15-minute consultation.