The quantity of organic pollutants present in wastewater is classically evaluated by measuring the quantity of dissolved oxygen during five days; it is quantified by the so called BOD5 parameter (Biological Oxygen Demand) [1]. This work constitutes the first step of an overall strategy targeting to improve the monitoring of BOD5. We focus on the development of a microsystem approach allowing monitoring the O2 consumption induced by the biodegradation process of organic matter. To evaluate the organic pollutants concentration, we use Escherichia coli as bacterial indicator, confined in a PDMS-glass chip. Their metabolic activity in presence of organic molecules is deduced from their oxygen consumption. These measurements are ensured by optical sensors present in each of the five instrumented chambers of the chip. The results show that the microsystem approach is suitable to measure simultaneously different concentrations of organic load, and that it is possible to reduce the analysis time. By examining the O 2 diffusion towards the walls of the device, we analyse the different part of the experimental results; it allows predicting, in the future, a precise evaluation of the BOD value within a few hours.