COD vs BOD: two tests, two different questions
Chemical oxygen demand is the oxygen equivalent of the matter in a sample that a strong chemical oxidant can oxidise under acid conditions. The conventional test digests the sample with dichromate in sulphuric acid at elevated temperature, usually for two hours, and converts the dichromate consumed into an oxygen equivalent. It is fast, repeatable and close to complete, and that completeness is the point: it counts what bacteria cannot touch as well as what they can.
Biochemical oxygen demand is the oxygen micro-organisms consume while oxidising the sample over a fixed incubation. The conventional test runs five days at 20 degrees Celsius, in the dark, with a seed. It measures what the organisms present can actually use inside that window, which is both its value and its limitation.
So the two numbers answer different questions. COD asks how much oxidisable matter is in the water. BOD asks how much of it a biological population can consume in five days. Where the population is well adapted and the water is not inhibitory, the second follows the first closely, and the ratio between them is high. Where it does not, the size of the gap is the answer to a question that neither figure answers alone.
What each test does and does not see
COD sees most organic compounds, including the ones that resist biological attack, and it also sees certain inorganic species that dichromate oxidises: sulphide, sulphite, ferrous iron, nitrite, and chloride where it is not masked. A COD figure can therefore be raised by matter that no biological process will remove, and a saline wastewater will read high on COD for reasons that have nothing to do with its organic load.
BOD sees the biodegradable fraction, and only the part of it that is consumed in five days. Carbonaceous demand is what the figure is usually quoted for, although nitrifying organisms carried in with the seed also consume oxygen inside the window and inflate the result. The other side of the same coin is inhibition: anything that stops the organisms, whether a toxicant, an extreme pH or a disinfection residual, suppresses the test. A toxic wastewater can therefore return a low BOD not because the matter is refractory but because the organisms were stopped before they could work.
The BOD to COD ratio in wastewater as a first read
Dividing BOD by COD on the same sample gives the fraction of the oxidisable matter that a biological population can take up in five days, and the BOD to COD ratio in wastewater is read as a first estimate of biodegradability. The bands commonly quoted come from municipal and mixed industrial practice: above about 0.5 the water is usually described as readily biodegradable and a biological stage is a comfortable fit; between about 0.3 and 0.5 a biological route is still realistic but expects an acclimatised culture and a longer retention of biomass; below about 0.3 the biological route will struggle on its own. Read those bands as a starting method rather than as a specification, because they are conventions from experience and not thresholds that a particular wastewater has to obey.
And a ratio is a coarse instrument by construction. It is one number summarising a mixture, and two wastewaters with the same ratio can be entirely different problems. Reading the treatability of industrial wastewater from it is a starting point rather than a conclusion.
Where the ratio misleads
- A low ratio does not mean refractory matter. COD includes settleable and particulate solids, and a large suspended fraction depresses the ratio while being removable by settling or screening rather than by biology. Measuring both tests on a filtered sample separates the two questions.
- A low ratio can be an inhibition result rather than a biodegradability result. A toxicant, a pH excursion or a residual oxidant stops the organisms in the bottle and the BOD falls for a reason the plant will not reproduce once the discharge is diluted or neutralised.
- A low ratio can be a seed result. An unacclimatised seed cannot degrade an industrial compound in five days, while the culture in a plant that has seen the discharge for months can. The bench test understates the plant.
- Nitrification in the bottle inflates BOD and therefore the ratio, so a sample with a nitrifying seed and a high ammonia load reads better than its carbonaceous demand deserves.
- Chloride and other inorganic reductants inflate COD, so a high-salinity wastewater reads a ratio that is too low until the interference is masked.
- Two different COD methods, or a change of laboratory, give two different ratios for the same water. Comparing ratios across methods is comparing methods.
- The five-day window is a convention. A compound that degrades slowly may not appear in it at all, so a low ratio can mean slow rather than impossible.
The soluble COD in wastewater answers more
Measuring both tests on a filtered sample, conventionally through a 0.45 micrometre filter, gives the soluble pair, and the soluble COD in wastewater is what a biological stage is really being asked about. The settleable part is a physical duty that belongs upstream, to screening, grit removal and primary settling; the soluble part is the job that reaches the biology. Where the total ratio is low and the soluble ratio is high, the plant needs better solids removal rather than a different biology.
Two figures and a ratio are still not a treatability study. pH, temperature, salinity, the presence of a specific inhibitor and the availability of nitrogen and phosphorus all decide whether the biology can work, and none of them appears in the ratio. They belong in the same sample report.
From the ratio to a process decision
How to determine it, where the ratio is ambiguous: run the BOD series over five, ten and twenty days, or use a respirometer on the actual seed, and read the shape of the oxygen uptake curve rather than a single point on it. A curve that continues to climb says the matter is biodegradable and slow; a curve that flattens early says it is not going to be consumed by that population in a useful time.
Where the ratio is high, the decision is straightforward: a biological stage, sized on the organic and nitrogen load. Where it is moderate, the decision is about time and adaptation, which usually means a longer retention of biomass and an equalisation volume rather than a different process. Where the soluble ratio is genuinely low on a water that has been tested with an adapted seed, the choice is between breaking the refractory fraction down before the biology, with a chemical oxidation step such as Fenton or micro-electrolysis, and removing the residual by physicochemical means at the end of the train. Both routes are covered on the chemical and advanced treatment page, and which one applies is decided by the compounds in the water rather than by the ratio alone.
Common mistakes
- Treating the ratio as a pass mark for biological treatment and dropping the question once it is above a threshold.
- Comparing a BOD and a COD measured on different samples, or on the same sample at different dilutions, where one of the two was outside its working range.
- Reading a suppressed BOD as evidence of refractory matter without checking for toxicity, pH and residual oxidant.
- Comparing COD figures from two laboratories using different digestion methods or different chloride masking.
- Quoting the ratio of the raw wastewater when the question being asked is about the soluble fraction that reaches the biology.
- Forgetting that the design figure is the load and not the concentration: the same ratio at twice the flow is twice the problem.
Self-check before the ratio decides the process
- Were the COD vs BOD tests run on the same sample, at the same time, by the same laboratory?
- Is the sample representative of the load the plant will see, including the peak production day rather than a quiet one?
- Has the ratio been computed for both the total and the filtered sample?
- Was the sample checked for inhibitors, and was the BOD run with an acclimatised seed where an industrial discharge is involved?
- Is the COD figure quoted with its method, so that it can be compared with earlier results?
- Has a longer-duration or respirometric test been run where the ratio sits in the ambiguous band?
- Has the discharge standard been stated, so that the required removal is read from the permit rather than from the ratio?
- Is the load, in kilograms per day, recorded alongside the concentration?
From a ratio to a treatment train
The ratio is the first page of a treatability question and not the last. It says whether the biology has a chance on its own, and the rest of the answer comes from the specific compounds, the discharge standard, the flow, and a test run on the real water with a real seed. The design influent quality chart and the effluent standards to process train chart set the two ends of that calculation, and where the refractory fraction has to be broken down before the biology, the chemical and advanced treatment page describes the dosing and oxidation equipment that does it. What a single packaged unit will remove from your water is confirmed with the factory for your influent analysis and your discharge standard rather than assumed from a table.