The key wastewater-related measurements, which detect the concentration of the discharged waste stream in terms of its oxygen demand, are discussed in Section 8.7. The sensors discussed in this section measure the biological population and the biological oxidative activity by detecting the amount of adenosine triphosphate (ATP) and the changes in its concentration.In a detailed study of the control parameters for the activated sludge process, measurements of great interest are biochemical oxygen demand (BOD) and chemical oxygen demand (COD). BOD and COD reduction, biological population density, and biological oxidative activity are important indicators of this process. It has been found that the amount of ATP is proportional to the viable biomass in a sample. It has also been found that changes in ATP concentration measure the oxidative capabilitycapability of the biomass. 1 Thus, it is of great interest to measure the ATP content of samples in the activated sludge process as well as in rivers, lakes, and other receiving waters. Sensitive methods for ATP analysis have been developed based on the observation that the luminescent reaction in fireflies is absolutely dependent on the presence of ATP. The in vitro light-yielding reactions are given in EquationsATP assay procedures have been developed based on the reactions just described. Briefly, the procedure involves rapid killing of the live bacterial cells and immediate extraction of ATP into aqueous solution. The latter is then treated with firefly lantern extract, and the light emission of the resultant solution is measured with a photometer. The firefly lantern extract and the ATP required for calibration are commercially available.