BOD, COD and TSS explained

Three numbers on the permit

Almost every lagoon discharge permit in the United States is written around three measurements: BOD, COD and TSS. They are not interchangeable — each captures something different about the water leaving the lagoon — and understanding what each one means is the difference between reacting to a permit exceedance and seeing it coming. All three are determined by standardised laboratory procedures set out in Standard Methods for the Examination of Water and Wastewater, the reference that regulators and laboratories work from, so a number measured in one lab is comparable with the same number measured in another.

What each measure captures

BOD, biochemical oxygen demand, is the most direct proxy for readily biodegradable organic load. It is measured biologically: a sample is held for a set period, commonly five days — hence BOD5 — and the oxygen that micro-organisms consume as they break down the organic matter is recorded. The more readily biodegradable material there is, the more oxygen is demanded.

COD, chemical oxygen demand, asks a related but broader question. Instead of waiting for biology, a strong chemical oxidant is used, and it oxidises both the biodegradable and the non-biodegradable organics in the sample. Because it captures more, COD on a given sample is usually higher than BOD. The gap between them is informative: when COD rises much faster than BOD, the extra material is chemically oxidisable but not readily biodegradable, and it will not respond to biological treatment at any dose.

TSS, total suspended solids, is the simplest of the three to picture. It is the dry mass of particles suspended in the water, measured by passing a known volume through a filter, then drying and weighing what the filter retains. It says nothing about whether those solids are organic or mineral — only how much particulate matter the water is carrying.

None of the three is a direct read-out of the others. A lagoon can hold an acceptable BOD while its TSS climbs on resuspended solids, or hold TSS steady while a change in the incoming load pushes COD up ahead of BOD. Reading them together is what turns three isolated numbers into a picture of what the lagoon is actually doing.

How the sludge blanket feeds back into the numbers

The link between the blanket at the bottom of a lagoon and the numbers at the outlet is detention time. As the blanket grows it takes up volume and shortens the time water spends in the cell, so less of the incoming load is treated before the water reaches the outlet. The visible result is that effluent BOD and TSS drift upward, often gradually enough that the trend is missed until a sample breaches the permit.

There are more direct paths too. A poorly settling blanket, or one resuspended by wind or by seasonal turnover, puts solids back into the water column where they can leave with the effluent. Short-circuiting carries flow — and whatever it holds — towards the outlet without mixing through the cell. Each of these tends to push TSS up, and because suspended organic solids carry their own oxygen demand, BOD usually follows.

That is also why a treatment quantity is never quoted from permit numbers alone. What a lagoon needs is sized by applying the manufacturer's written dose table to the specific lagoon and its treatable fraction, using the figures you already hold — which is what a dosing plan does; see /dosing-plan.