The Bacteria That Remove Ammonia Are Divas. The Rest of Your Plant Is Not.

Most of the sewage treatment plants I get called to in the Philippines are already passing BOD and TSS. The water in the final tank looks clear. The operator is proud of it. And then the lab result comes back and ammonia has failed again, and nobody in the room understands how water that clear can fail anything.

I have had this conversation so many times that I now say the same line at the start: your plant has two completely different workforces living in the same tank, and you have only been feeding one of them.

Two workforces, two personalities

The bacteria that eat BOD are the easy ones. They are fast, they are greedy, they are forgiving. Give them air and organic load and they double in a matter of hours. You can starve them for a weekend, shock them with a bad batch of wastewater, wash half of them out of the clarifier, and by the next day they are back at work. They do not complain.

The bacteria that convert ammonia into nitrite and then nitrate are a different species of employee entirely. They are slow. They double once every day or two, not every few hours. They do not eat your BOD at all — they get their carbon from the alkalinity in the water and their energy from the ammonia itself. They need more oxygen per kilo of work than anyone else in the tank. They stop working if the pH drifts. They quit if there is chlorine, or a solvent, or a big slug of surfactant. And when you finally kill them, they do not come back tomorrow. They come back in two or three weeks, if you are lucky and you fixed the reason.

That is why I call them the divas of the plant. Everything else in the process is tolerant. These ones are not, and they are the ones your discharge permit is actually testing you on. If ammonia is the parameter that keeps failing on your plant, this is the specific problem we deal with.

The four things that quietly kill them

When ammonia fails, I do not immediately assume the plant is undersized. Nine times out of ten it is one of four things, and none of them are visible in the water.

Sludge age. This is the biggest one, and it is the one nobody checks. Because nitrifiers grow so slowly, if you waste sludge faster than they can reproduce, you are literally throwing your ammonia removal into the sludge truck every week. In our climate the water is warm, which helps, but you still need to hold the biomass long enough for them to build a population. When an operator tells me he wastes sludge "whenever the tank looks thick," I already know what the ammonia result will be. How much sludge a plant should actually make is worth understanding before you set that schedule.

Alkalinity. Every milligram of ammonia nitrogen the bacteria oxidise consumes about 7.14 milligrams of alkalinity as calcium carbonate. That is not a small number. On a plant with strong ammonia and soft, low-alkalinity influent, the process eats its own buffer, the pH slides down through 6.5, and the nitrifiers slow down and then stop. The operator sees ammonia failing and assumes he needs more bacteria. What he actually needs is buffer. I have fixed ammonia on more than one plant with nothing but alkalinity dosing and a pH controller.

Oxygen. Oxidising ammonia takes roughly 4.57 milligrams of oxygen per milligram of nitrogen, on top of everything the BOD bugs are already using. If your dissolved oxygen sits at 1 mg/L, the fast bacteria will take what they need first and the nitrifiers will get the leftovers, which is nothing. I want to see 2 mg/L and above in the aeration tank, measured, not assumed from the sound of the blower. Half the "undersized plant" complaints I investigate are aeration problems — a blower running at the wrong duty point, diffusers half-blinded, or one blower quietly out of service for months.

Shock. Chlorine from the laundry line. A disinfectant flushed down a floor drain during general cleaning. A hospital or dressing plant sending a strong slug at 2am. The BOD population shrugs it off. The nitrifiers die, and you find out fourteen days later on a lab sheet. This is one of the few situations where dosing bacteria genuinely helps — and only once you have stopped whatever caused it.

The plant that taught me this

The project I still think about was a food processing plant — hams, hotdogs, beef jerky — and it was early in my career. I made several mistakes there, and one of them was about ammonia, although at the time I thought my problem was fat. The full account is here.

The obvious problem was the fats, oils and grease. I underestimated how much of it was emulsified rather than free-floating, and I specified a grease interceptor when what that wastewater needed was dissolved air flotation. That was mistake one, and it cost me.

Mistake two was quieter and, in the long run, more expensive. To simplify the layout, the domestic wastewater from the plant's toilets and the process wastewater from production were combined into a single treatment line. It made the piping neat. It also meant the whole combined stream now had to meet the stricter set of parameters, including ammonia, when the process side on its own would never have been tested that way. I had, with one drawing decision, given myself a nitrification problem I did not need to have — on a plant that was already fighting FOG, on the second floor of a building, in a neighbourhood where the neighbours could smell every bad day we had.

And they did complain. The odour complaints went to the Barangay, then to the City Health Office, and eventually to the Department of Health. That is not a place any engineer wants his design to end up.

What finally stabilised that plant was retrofitting FBBR-style fixed-film media into the aeration tank. I did not do it because I had read that it helps nitrification. I did it because I was desperate for more biomass in a tank I could not make bigger. But that is exactly why it worked. Fixed-film media lets the slow-growing bacteria hold on to something instead of depending on the clarifier to send them back. Once they had a home they could not be washed out of, they built up a population and stayed. Ammonia came down and stopped bouncing.

The other thing I want to say about that job is not technical. The client's previous contractor had collected ninety percent of the payment and then walked away from the site. So when I arrived with problems of my own, the trust was already gone. I made a decision then that I have never regretted: I did not send another bill until the plant passed. We worked that site at all hours, through holidays, until it ran the way I had promised it would. That is what won the client back. I only had to do it once, because I never repeated those design mistakes.

What I do differently now

I keep domestic and process streams separate unless there is a real reason to combine them, and if I do combine them, I say out loud in the design review which parameter just got harder because of it.

I size the biological stage for the sludge age nitrification actually needs, not the sludge age that makes the tank cheapest to quote. This is where a lot of Philippine STP bids are won and where a lot of Philippine STPs fail two years later. A tank that is undersized for nitrification will still pass BOD and TSS on day one, so the problem does not appear until the plant is somebody else's headache. If you want to see what a properly sized plant costs before anyone quotes you, the estimator will give you a figure.

I check alkalinity in the design stage, not after the plant fails. If the numbers say the process will eat its own buffer, dosing goes into the design and into the operating cost, honestly, from the start.

And I put fixed-film media in any plant where I know the operator will be part-time, the flow will swing, or the site cannot give me the tank volume I would prefer. It is insurance for the slowest, most valuable bacteria in the system.

How to check your own plant this week

If your ammonia is failing and your BOD is passing, do not order more bacteria yet. Measure four things first: dissolved oxygen in the aeration tank at the busiest hour of the day, pH and alkalinity in the aeration tank, how much sludge you are actually wasting per week, and whether anything strong — chlorine, disinfectant, a chemical clean — is reaching the drain line. In my experience the answer is sitting in one of those four numbers.

And if the plant has been ammonia-compliant before and suddenly is not, count backwards two weeks from the failed result. Whatever happened on the plant that week is your suspect. The divas do not react immediately. They react late.

If you want a second opinion on why your ammonia is failing, or you are reviewing an STP design and you want someone to check whether it can actually nitrify, book a free 15-minute consultation with us. Bring your last lab result and your aeration tank readings and we can usually tell you where to look.