Ammonia and Phosphate Are the Two That Fail
When a client sends us a failed laboratory result, we can usually guess the parameter before we open the file.
It is almost never COD. It is rarely BOD on its own. With decent filtration it is not TSS either. Across the results that reach us, the two that fail are ammonia and phosphate — and of those two, ammonia is the one that decides how expensive the fix will be.
Two workforces in one tank
The reason is that a sewage treatment plant contains two populations that look identical in the tank and behave nothing alike.
The organisms that consume BOD are fast, hungry and hard to kill. In a working plant they roughly double over a matter of hours, and they shrug off most of what a building throws at them.
The nitrifiers that convert ammonia reproduce on a completely different timescale — a day or two to double rather than a few hours, and slower still as the water cools. They need roughly 4.57 mg of oxygen per mg of nitrogen oxidised, consume about 7.14 mg of alkalinity as calcium carbonate doing it, and are set back for weeks by a single dose of chlorine or surfactant. Those two figures are standard stoichiometry and worth knowing, because they explain most ammonia failures without anyone needing to visit the site.
So a plant that is under-aerated, or wasting sludge too aggressively, or running short of alkalinity, still produces clear water and a passing BOD. It simply stops nitrifying. The result looks like a plant working well with one number wrong on it. The four things that quietly kill nitrifiers are worth reading before you spend anything.
Phosphate fails for a different reason
Biological uptake alone often does not reach the limit. Closing the gap usually pushes the design toward more deliberate phosphorus removal — dosing, mixing, and more sludge to handle afterwards. That is a change in operating cost as much as capital cost, and it is the sort of thing that should be decided at design stage rather than discovered later. Phosphate is its own problem with its own economics.
Nitrate is the one people worry about unnecessarily
Its limit is 14 mg/L for Classes B and C and it did not change in 2021, while ammonia sits at 3 or 4. In the results we see, ammonia goes first by a wide margin. Once it has failed, the nitrate figure rarely adds anything.
That is a pattern from our own casework rather than a rule of nature. Plants with unusual influent, industrial discharges or a denitrification stage in the wrong condition can certainly fail nitrate on its own. But if you are triaging a sheet and deciding what to chase first, ammonia is where the money is.
What to measure before you buy anything
Which parameters even apply to you depends on what your facility discharges — the significant parameters vary by industry, and a good number of owners discover theirs from a DENR letter rather than from whoever built the plant.
Before ordering cultures or dosing anything, measure four things: dissolved oxygen in the aeration tank at the busiest hour, pH and alkalinity in the same tank, how much sludge is actually leaving the system each week, and whether anything strong is reaching the drain line. In our experience the answer is usually sitting in one of those.
If ammonia is the failing parameter, that is a specific problem with specific causes, and it is worth identifying which one before spending. Send us the lab sheet and we will tell you what we read from it. Book a free 15-minute consultation.
Stoichiometric values for oxygen demand and alkalinity consumption in nitrification are standard design figures (Metcalf & Eddy, Wastewater Engineering). The oxygen figure is theoretical; observed demand is somewhat lower once cell synthesis is accounted for.