BOD - Biochemical Oxygen Demand
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.
BOD5 and Total BOD
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.
Online measurement of BOD
Measurement using cascades and O2-Sensor
Similar to the working principle of a biological treatment plant, in this WWTP simulation, the sewage is mixed with activated sludge under aeration and continuously led through the drain cascade. On leaving the reactor cascade the remaining oxygen will be determined with the O2 sensor, whereby the volumes of waste water, activated sludge and air are specifically defined and monitored. With this method it is important to use the plants-own biomass to simulate the real biology of the particular waste water treatment plant. Hence, the measurement results have specific relevance relating to the WWTP and its unique properties.
The BioMonitor regulates not only the optimal control of the oxygen input to the activated sludge reactors of the WWTP, but monitors the incoming contaminations of the WWTP's influent. This enables the operator to even out possible peak pollutions, thereby maintaining aeration at a regulated level, as usually it is these peak pollutions which cause additional aeration, which in turn is energy consuming and expensive.
BOD determination by O2-Electrode
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.
Activated Sludge Respiration (ASR) and Sludge Activity
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 is 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.