Determination of biodegradable organic substances in wastewater has significant importance
on the environmental pollution control. Traditional analysis of biochemical oxygen demand
(BOD) involves 5 to 7 days incubation period and is thus ineffective for operation of activated
sludge processes in wastewater treatment plants or determination of the concentration of biodegradable organic pollutants in polluted waters. Because of the need for express method
of BOD determination, biochemical oxygen demand (BOD) biosensors have been developed.
A biosensor is defined as a self-contained integrated device capable of providing specific
quantitative analytical information using a biological recognition element [1]. BOD sensors
are easy to handle, portative and offer quick response. Last aspect is especially advantageous
as BOD sensors enable to evaluate the value of biochemical oxygen demand in 60 minutes.
Mathematical modelling of biosensors response provides several calibration methods and also
gives a link among the design, fabrication and measurement procedure of the device. Most
frequently the biosensor output signal is analysed according to the stabilised initial and final
values. It has been shown that the total oxygen uptake measurement needs 15-20 min and the
recovery time between measurements can be as long as 3-4 h [1]. The alternative approach to
the steady-state analysis of a biosensors response [1, 2, 3] is through the mathematical
modelling of the dynamic behaviour of the output values. Dynamic method is usually
preferred as it allows to carry out faster measurements. In this study, the calibration of the
BOD sensor involves both methods and also the calculation of time constants describing the
non-steady-state processes in a BOD sensor. For the dynamic change of the biosensor output
signal a model of transfer processes expressing the exponential decay and lag time of
response by a transfer function has been developed [1]. The function is characterised by an
inflection point at which the change of the output signal shows a maximum value and by a
time constant. The values of all the named variables depend on the substrate concentration
affording to calibrate the biosensor by several methods.