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BOD5 calculator
Choose whether the sample was diluted. Without dilution, the oxygen at the start and after 5 days of incubation in the dark is enough. With dilution, add the sample and bottle volumes, and optionally - a blank (seed) correction.
Enter oxygen at the start and after 5 days of incubation.
How we calculate
Without dilution: BOD5 = D1 − D2, where D1 and D2 are the dissolved oxygen at the start and after 5 days.
With dilution: BOD5 = (D1 − D2) / P, where P = sample volume / bottle volume. With a blank (seed) correction: BOD5 = ((D1 − D2) − (B1 − B2) × f) / P, where B1/B2 are the blank oxygen before and after incubation, and f is the ratio of seed in the sample to the blank (with the same amount of seed in both bottles, f = 1).
The factor f depends on how the sample was seeded. If the seed was added to the dilution water and the blank is that same seeded water (as in EN ISO 5815-1), then f = 1 − P, i.e. the share of dilution water in the bottle. If you add the seed separately to each bottle (APHA 5210B), f = seed volume in the sample bottle ÷ seed volume in the seed control; with the same volume in both, f = 1.
The validity criteria come from APHA 5210B. If you work to EN ISO 5815-1, apply the criteria of that standard.
The calculator does not assess water quality: what a BOD5 value means depends on the type of sample (surface water, raw wastewater, treated effluent) and the purpose of the test. Compare the result with the requirements that apply to that matrix, e.g. a water classification or a water permit.
Frequently asked questions
When is sample dilution needed?
When the sample is polluted enough that microorganisms would use up all the oxygen in the bottle before the 5 days are up (and the result would be underestimated). It is usually diluted with aerated water, sometimes seeded with microorganisms - then a blank (dilution water/seed only, without sample) is also needed for correction.
What is "P" (the sample volume fraction)?
It is the share of the actual sample in the volume of the whole incubation bottle, P = sample volume / bottle volume. E.g. 15 ml of sample in a 300 ml bottle gives P = 0.05. The stronger the dilution, the smaller the P and the more the result is "amplified" when converted to 1 litre.
Do I have to include a blank (seed)?
Not always. If the sample contains its own sufficient microflora (typical wastewater, surface water) and is not additionally seeded, skip this section - then BOD5 = (D1 − D2) / P. The blank correction is used when a microbial seed has been added to the dilution (e.g. for toxic samples or those poor in microorganisms).
Does the result include nitrification?
The calculator works only from the oxygen concentrations you enter and cannot tell what the oxygen was used for. Without a nitrification inhibitor the depletion also includes the oxidation of ammonia - in the effluent of a plant with nitrification this can be a large part of the result. In EN ISO 5815-1 the addition of allylthiourea (ATU) is part of the method; in APHA 5210B a result obtained with an inhibitor (TCMP) is reported as CBOD5. Record next to the result whether an inhibitor was used.
BOD5 together with the other indicators, in one session
A BOD5 result says more alongside dissolved oxygen and other indicators from the same session. In LimnoLog you add it to the right sample - under the sampling date, not the day you read the bottle after incubation - and see it in context, also as a time trend.
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