Ammonia Came Back at 60 and Nitrate Below Detection. The Coliform Failure Had the Same Cause.

A laboratory certificate arrived on my desk with four parameters on it. Two had passed and two had failed, and the owner's covering message was about the two that failed. Fecal coliform was over the limit. Ammonia had come back near 60 mg/L. He wanted to know whether he needed a bigger chlorine system.

The two that passed were the more useful half of the sheet. BOD was inside its limit and so was TSS. That tells you the fast bacteria are alive and eating, and that whatever is holding solids back is holding them back well enough to keep them out of the effluent. A plant that passes BOD and TSS and fails ammonia is not a broken plant. It is a plant that is doing one job and not the other, and the question is which input the second job is missing.

I have written before about why ammonia is usually the parameter that fails and about a plant that could not hold its slow bacteria long enough to grow any. This one is the exact opposite case, and that is why it is worth writing down. This plant had all the retention time nitrification could ask for. It still could not nitrify, and the coliform failure on the same sheet turned out to have the same single cause.

Ammonia at 60 with nitrate below detection is not partial nitrification. It is none.

The number that decided the whole assessment was not the ammonia. It was the nitrate, and it was below the laboratory's detection limit.

Nitrification is a conversion. Ammonia goes to nitrite and nitrite goes to nitrate, and the nitrogen does not leave the water when that happens — it changes form. So if a plant is nitrifying badly, you see a reduced ammonia figure and a nitrate figure that has gone up to meet it. If a plant is nitrifying at half speed, the sheet shows you half of each. What you cannot have, if any nitrification is happening at all, is high ammonia and no nitrate.

Below detection means the conversion is not partially working. It is not happening. That is a different finding from a weak result, and it closes off a whole set of explanations immediately: this is not a plant that needs tuning, or a slightly longer retention time, or a little more alkalinity. Something is missing outright.

That is also why I ask for the full sheet and not the failures. An owner sends me the two red lines because those are the ones the regulator will ask about. The passing lines and the unremarkable lines are where the diagnosis actually lives, which is the same argument I make about influent results nobody has taken — the figure you did not think was interesting is usually the one that narrows the problem.

Sludge age was not the problem, and I checked that first

The first thing I do with an ammonia failure is divide the process volume by the daily flow and ask what mechanism keeps the slow bacteria in the tank, because on the last plant I wrote about that division was the entire answer. There, solids retention time equalled hydraulic retention time at about 1.3 days, below the point where nitrifiers wash out faster than they can reproduce, and no amount of air would have fixed it.

Here the same arithmetic came out at roughly nineteen days of sludge age against something like the four and a half to five days nitrification needs at this temperature. That is not marginal. It is four times the requirement, and it rules out washout as flatly as the nitrate figure rules out partial conversion. The bacteria had every opportunity to establish. They had not established.

When retention is ample and the conversion is zero, there is really only one input left worth checking, and it is oxygen. Nitrifiers need substantially more oxygen per kilogram of nitrogen than the heterotrophs need per kilogram of BOD, and they are last in the queue for it. In a tank that is short of air, the fast bacteria take what there is and the slow ones get nothing. The plant looks like it is working, because the parameter you can see is the one the fast bacteria are responsible for.

The air balance, and the timer nobody had revisited

So I costed the air out rather than guessing at it. Carbon removal alone — just the BOD, before you provide a single kilogram of oxygen for nitrification — needed on the order of 4.9 kW of blower power on the hardware installed at that site.

About 1.1 kW was ever running.

Not because machines had failed. Two blowers were installed and they were alternating on a one-hour timer, one on and one resting, turn and turn about. Half the installed air capacity was idle at any given moment by design, and the machine that was running was itself the smaller part of the requirement. Nobody had done anything wrong in the sense of neglect. Somebody had set a control philosophy years earlier, it had been handed down through however many changes of operator, and no one had ever gone back and asked what the plant's actual oxygen demand was.

An alternating timer is a design decision, and nobody revisits it

Alternating duty is a reasonable thing to want. It evens out running hours and it keeps a standby machine exercised rather than seized. The mistake is alternating two machines that are each sized as part of the duty rather than as the whole of it. Then the timer is not rotating a standby, it is halving the plant. I have argued the sizing rule elsewhere — one duty machine and one standby, not several small ones sharing the job — and this is what the other version looks like after a few years: an installation that reads as redundant on paper and is under-aerated every hour of every day.

It is worth saying that a low dissolved oxygen reading is not automatically a fault. I have left a plant running at 1 mg/L and been right to. The difference is what the plant is being asked to deliver. If you only need BOD removal, low oxygen that works is not a problem to solve. If the permit has an ammonia limit on it, the same reading is the reason you are failing.

The coliform failure had the same cause, and no chlorine system would have fixed it

Here is the part that made this worth a post rather than a paragraph in a report.

The owner's question was about the chlorine system, because fecal coliform had failed and chlorine is what kills coliform. It is a fair question and the answer is no — not a bigger dosing pump, not a stronger solution, not a longer contact tank. His disinfection could not work while the ammonia was at 60, and the reason is chemistry rather than equipment.

Chlorine dosed into water carrying ammonia does not stay as free chlorine. It combines with the ammonia to form chloramines, which are far weaker disinfectants and far slower. Free chlorine — the thing that actually produces a reliable coliform kill in the contact time a small plant has — only begins to exist once you have satisfied all the ammonia present and passed what is called the breakpoint. I have published the ratio before, in the course of explaining two ammonia-removal routes we will not sell: roughly 7.6 kg of chlorine per kilogram of ammonia nitrogen, and worse in practice once everything else in the water has taken its share.

Run that against 60 mg/L of ammonia and the breakpoint sits somewhere around 450 mg/L of chlorine. Nobody doses a sewage plant at 450 mg/L. What the plant was doing instead was making a weak chloramine residual, measuring a residual on a test kit, recording that residual in a logbook, and calling it disinfection. The residual was real. It simply was not the species that kills anything quickly.

Which means the coliform line on that certificate was not a disinfection finding at all. It was the ammonia finding, appearing a second time at the other end of the plant. Fix the aeration and the coliform result follows without anyone touching the chlorine system. That is an unusually clean outcome and I do not expect it often, but it is the kind of thing you only see if you read the whole sheet as one plant rather than as a list of separate problems. It is the same shape as the phosphate that passes the clarifier and fails at the chlorine tank: the parameter fails where you measure it, not where it is caused.

Two passing parameters can be the most useful thing on a failing sheet

I keep coming back to BOD and TSS passing, because that pair is what made the rest of it quick.

If BOD passes, the heterotrophs have enough air for their own work and the carbon load is not overwhelming the plant. If TSS passes, solids are being held where they should be, which means the clarifier and the return are doing something, which in turn means a washout explanation has to be argued rather than assumed. Those two passes did more to narrow the problem than either failure did. The ammonia told me which job was not being done; the nitrate told me it was not being done at all; the passes told me where not to look.

This is why I am wary of a diagnosis built on one parameter. Ammonia and phosphate are the two that fail on most Philippine plants, and if you treat each failure as its own project you end up buying a chemical for one and a machine for the other when a single input was missing. A plant is one system. Four numbers taken on the same day are four views of it.

What we proposed, and in what order

The order matters more than the list, because an owner reading a report will act on the first item.

First, establish the real oxygen demand from the loading rather than from what is installed, and get air into the tank that matches it — which on this site meant revisiting the control philosophy before revisiting the hardware, since a timer setting costs nothing to change and the machines were not the immediate constraint. Second, hold the plant on that footing long enough for the nitrifiers to establish, which is weeks rather than days, and watch the sludge with a cone reading rather than waiting on a monthly certificate. Third, re-test ammonia and nitrate together, because the pair tells you whether the conversion has started in a way that neither figure tells you alone. Fourth — and only fourth — look at the chlorine system, by which point there may be nothing to look at.

There is a complication I am deliberately not opening here. Converting 60 mg/L of ammonia into nitrate does not make the nitrogen disappear, and a plant with a nitrate limit on its permit has to be asked what happens to all that nitrate once nitrification finally starts working. Solving one parameter in a way that creates the next one is a real trap and it deserves its own note, which I will write separately. For now it is enough to say that the downstream limit gets checked before the upstream fix is quoted.

I should also be honest about what this assessment was not. It was a reading of a laboratory sheet, a set of equipment details and an air balance. Equipment counts and control settings reported second-hand are reports until somebody stands in the plant room, which is why reading somebody else's documents is the beginning of a job and not the end of one. The arithmetic above — volumes, loading, sludge age, the breakpoint ratio — is the part nobody can dispute, and it is deliberately what the finding rests on.

What transfers from this, and what does not

Three things transfer, and they cost nothing.

Ask for the whole certificate, not the failures. Read ammonia and nitrate as a pair before you read either one alone, because the pair tells you whether a conversion is weak or absent and those are different problems with different prices. And before you buy a bigger disinfection system because coliform failed, check what the ammonia is doing, because chlorine cannot be free chlorine while there is ammonia left to combine with.

The numbers do not transfer. The air requirement I quoted is specific to that loading, that tank arrangement and that class of machine; a different plant with the same flow could need a different figure entirely. The sludge age figure for nitrification moves with temperature, and ours is a warm-water number. The breakpoint ratio is chemistry and holds, but the conclusion drawn from it only holds at a high ammonia concentration — at 5 or 10 mg/L the chloramine problem is a nuisance rather than the explanation for a failed coliform result.

And the diagnosis itself does not transfer. I would not tell you your ammonia failure is an oxygen problem because this one was, any more than I would tell you it is a retention problem because the previous plant I wrote about was. Those two plants had the same failed parameter and opposite causes. What I would tell you is to do the division and the air balance first, on paper, because between them they distinguish the two cases in an afternoon and they change what you spend money on. If the arithmetic is beyond your operator, that is not a criticism of him — a good operator can carry a plant but he cannot carry a design decision taken before he arrived.

If you want to work through your own numbers, our design generator will take a flow and a strength and give you a process train, the price estimator will put a budget figure against it, and the parameters by industry page is where to start if you are not sure what your wastewater should look like in the first place. For a plant that is already failing, the compliance page sets out what the limits actually require and the nutrient field guide covers the ammonia, nitrate and phosphate failures in one place. There are landing pages on ammonia removal and nitrate removal if you want the shorter version, and our operation and maintenance page covers what running one of these properly involves. You can see what we have built, and how we work in Cebu, or read the longer background on wastewater treatment and on choosing a provider here. If you would rather have a plant built or assessed than read about it, getting an STP and our consultations both start in the same place.

Other notes that bear on this one: what nitrification does to your alkalinity once it starts working, the carbon arithmetic for removing nitrate, what losing nitrifiers looks like, what happens when the air stops entirely, a new plant failing its first test, a BOD result arriving at the worst moment, what to submit after an inspection, what a plant smells like before the results come back, why we design at several strengths, fixed film against moving media, retrofit against new build, and why I want you to check our design.

If you have a certificate in front of you and you are not sure what it is telling you, send it over and we will read it with you. Book a free 15-minute consultation.