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Sep 23, 2026

Nitrification, temperature, sampling - how to avoid the common error sources in BOD determination

Determining biochemical oxygen demand (BOD) is one of the central parameters in the wastewater laboratory. Sampling, sample preparation and the choice of measurement method decide day after day on the laboratory effort, the significance and the legal certainty of the results. In practice, a whole range of interference factors repeatedly leads to distorted or implausible measured values - and anyone who does not know them often searches for the cause for a long time.

Where everyday laboratory work runs into difficulties

If the BOD value suddenly comes out too high although the sample appeared unremarkable, the cause is frequently nitrifiers. These bacteria oxidize ammonium to nitrate via nitrite and consume oxygen that has nothing to do with the actual BOD load. The result is systematically too high. The targeted use of nitrification inhibitors such as ATH stops this reliably.

Temperature is another underestimated factor. If the measurement sample is warmer than 21 °C, an overestimation threatens - and the respirometric measurement cannot distinguish between negative pressure resulting from BOD processes and negative pressure resulting from a drop in temperature. Below 15 °C the permissible start-up phase is often no longer sufficient. Even a one-degree Celsius temperature change during the adaptation phase influences the result.

Sample preparation also determines the quality of the measurement: inadequate homogenization, incorrectly selected dilution series or detergent residues on bottles and stirrers distort the results just as much as an incorrect oxygen supply.

Solutions that can be implemented directly in the laboratory

The new BOD handbook from WTW® brings together this practical knowledge on more than 60 pages and provides concrete assistance for everyday laboratory work. It conveys the fundamentals of BOD determination, compares the common methods and shows which microbiological boundary conditions, which oxygen supply and which sample treatment are decisive for reliable BOD values.

Topics covered include:

Fundamentals of BOD and respirometric measurement technology
Comparison of different BOD determination methods, including the OxiTop® system
Sampling, sample preparation and homogenization
Handling inhibitory and toxic substances
Influence of nitrification and suitable inhibitors
Dilution methods according to current standards
Evaluation and plausibility checking of measurement results
Typical error sources and practical tips for avoiding errors
Cleaning, maintenance and quality assurance

Particularly valuable for users are the numerous practical examples, quick-reference guides and tips for avoiding errors that enable direct implementation in the laboratory. The handbook is suitable both as an introduction to the topic and as a reference work for experienced users.

→ Download BOD handbook now