The BOD (biological or biochemical oxygen demand) value indicates the amount of oxygen which is needed for the biological degradation of organic substances in water in mg O2/ l.
Generally, the biological oxygen demand serves as a sum parameter for the assessment of pollution of sewage.
Unlike the special factor BOD5, which according to the standardized lab method the measurement of the biochemical oxygen demand excluding the nitrification part follows after 5 days, the total BOD includes this bacterial oxidation of ammonia to nitrate which allows a continuous online monitoring.
The nitrification is an important part of the sewage treatment processes and should be taken into consideration for optimal control of waste water treatment plants. Hence, determination of BOD5 is hardly suitable for optimum control of WWTPs.
Common BOD analysers allow a determination within 10 to 30 minutes, whereby short-term measurements of the oxygen consumption through O2 electrodes dominate. However, as oxygen is hardly soluble in water, very often low measurement results are found, which then need to be extrapolated. Essentially, this method really only is suitable in determining very easily degradable substances.
The biomass or sludge of a waste water treatment plant not only needs oxygen to decompose substances, but also for the endogenic respiration, the so-called activated sludge respiration or sludge activity. This oxygen turnover of the bacteria is measured in mg/l*min (volume/time).
This amount of the total oxygen consumption is measured separately online in the BioMonitor - by use of the reference cascade, through which only the sludge and air is led. The decrease of oxygen concentration of the inlet air corresponds to the ASR.
On comparing the results of the sludge activity to the results at the cascade's drain gives the conclusion as to the anaerobic decomposability of the biomass and even toxications may be highlighted.
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