The BOD Came Back High Three Weeks Before the Inspection

Someone called us on a Tuesday. Their annual sampling for the discharge permit was three weeks away, and the self-monitoring result that had just come back from the laboratory read BOD 82 mg/L against a Class C limit of 50. The plant had passed the year before. Nobody had touched the blowers. The operator was, in the caller's words, doing the same thing he had always done.

BOD failing on its own is not the common case for us. In the failed laboratory results that reach us, ammonia and phosphate are the two that go first, and BOD usually passes quietly in the background. So when BOD is the number in trouble, the first thing we do is stop looking at the BOD.

The second number on the sheet answered most of it

TSS on the same result read 148 mg/L against a limit of 100. Both had failed, and roughly in proportion to each other.

That matters because of what the BOD test actually measures. It measures the oxygen demand of everything in the bottle, and suspended solids are organic matter. Solids carry BOD with them into the sample. When TSS and BOD fail together and in proportion, we treat it as a solids problem until something proves otherwise, rather than as a sign that the biology has stopped working. The Class C limits are 50 mg/L BOD and 100 mg/L TSS; this result was not two independent failures, it was one failure counted twice.

What we found standing at the tanks

We started where we always start. We do not track MLSS, we read a cone, and the cone came back around 140 mL/L — comfortably inside the 50 to 200 mL/L range we work to. It settled inside thirty minutes and the water above it was clear, so this was not bulking sludge either. The biomass was healthy. Nothing about the aeration tank looked like a plant in trouble, and there was no foam on the surface telling us otherwise.

Two things were wrong, and neither was biological.

The sample was being drawn from the chlorine contact chamber. That chamber had never been drained, and there was settled sludge sitting in the bottom of it — the same chamber where phosphate goes back into solution after you thought you had removed it. A sample dipped out of a chamber with a sludge bed in it is not a sample of what the plant produces.

The second thing was the date. The sample had been taken the morning after the building's quarterly kitchen deep-clean, and the grease interceptor had been pumped out the day before. The plant had received, in one day, a slug of everything a grease interceptor is supposed to hold back over three months.

The one extra number we asked the laboratory for

Before changing anything, we asked for the resample to be run as filtered BOD alongside total BOD. It costs very little and it settles the argument.

Total came back at 74 mg/L. Filtered came back at 11.

Eleven milligrams per litre of soluble BOD means the dissolved treatment was finished. The bacteria had done their work. Sixty-three milligrams of the result was organic matter riding out of the plant as solids. That single pair of numbers told us we were not buying a bigger aeration tank.

What we did

We changed nothing chemical. We never let compliance depend on a chemical that may not be on the shelf the week you need it, and there is no chemical that fixes solids leaving a clarifier anyway.

We changed the wasting schedule first. They were wasting sludge on Saturdays, because Saturday was when the vacuum truck came. So the blanket in the clarifier climbed all week and was at its highest every Friday. We moved them to wasting a small amount several times a week against the cone reading, and left the truck schedule to sort itself out afterwards.

Then the weir and the scum baffle, which had a gap in it. Solids that reach a weir leave with the water; there is nothing downstream to catch them.

Then the sample point. The permit describes the final discharge point, and that is where the sample belongs — after the polishing stage, not out of a chlorine chamber with a sludge bed in it.

Then the peak. The building was putting its kitchen load through in a narrow window, and holding some of it back through equalisation costs nothing but tank volume that was already there. We have written before about how the flow you declared is the flow you are held to, and a plant that sees its whole day's load in four hours is being run at a number nobody wrote down.

Last, we told them when to resample: an ordinary Wednesday, mid-morning, with nothing unusual happening in the building. Not after a wash-down.

The resample read BOD 26 and TSS 38. No new equipment, no new tank, and the plant they had was the plant they kept.

The version of this where we would have said something different

If the filtered BOD had come back at 40 or 50 instead of 11, none of the above would have been the answer. A high soluble BOD means the dissolved organics are not being consumed, and that is either not enough air, not enough tank, or more load arriving than the plant was drawn for. That is a design conversation, not a three-week conversation, and we would have said so on the same visit rather than let somebody spend a month adjusting a wasting schedule that was never the problem.

It is worth knowing which of the two you are in before the inspection rather than after it. The price estimator will tell you roughly what capacity your load actually needs, and the parameters your industry is held to are worth checking against what your laboratory is actually testing for — we have seen establishments sampling for the wrong list entirely.

On inspections, since that was what started the phone call

The sample the regulator takes is a grab sample, on a day of their choosing, and there is no arranging your plant around it. That is not a reason for anxiety and it is not a reason to get clever about timing. It is an argument for margin.

A plant designed to produce exactly 50 mg/L passes only on the days when nothing goes wrong. Design it to produce 20 and a deep-clean, a pumped interceptor and a blocked baffle can all land on it at once and it still comes in under the line. That margin is bought once, at design and build, and it is among the cheapest things in the whole project — cheaper than a failed result, a re-test, and the letters that follow. It is also the difference between water you may discharge and water you can actually reuse, which is the same margin doing a second job.

The other half of it is that a good operator can hold a plant like this together for years and nobody notices. He cannot carry a bad design, and when he leaves, the housekeeping that was keeping the numbers down leaves with him. We have watched plants fail on turnover alone, with no equipment fault anywhere.

Where this stops being transferable

Everything above worked because the biology on that site was genuinely healthy and the failure was physical. That is one of several ways a BOD result goes high, and on another plant the same lab sheet would have meant something else entirely — a young plant that has not finished growing its biomass reads much the same on paper as a plant that is simply overloaded, and we have written about the first of those at some length.

Plants carrying FBBR-style fixed-film media behave differently again. They hold biomass on the media rather than only in suspension, so a rising clarifier blanket is less of the whole story and a solids carry-over failure tends to be smaller when it happens. That is our experience across our own installations, not a rule we would ask anyone to design around.

And the wasting schedule that fixed this plant would starve a smaller one. A 5 m³/day plant has almost no buffer — the small ones are the hard ones — and taking sludge out of it three times a week on the same reasoning would do damage. Every site is different, which is why we go and look at one before saying anything about it.

If you have a result in front of you and you are not sure which of these two situations you are in, send it to us. Book a free 15-minute consultation and we will read the lab sheet with you.