The study presents an evaluation of the oxygen requirement and sludge production, modeled in terms of fundamental processes reflecting different biochemical transformations in activated sludge systems. Modern modeling concepts define substrate utilization and endogenous decay as major processes requiring final electron acceptors. Substrate and sludge components may be defined in terms of different parameters. Chemical oxygen demand (COD) and biochemical oxygen demand (BOD5) are the traditional substrate parameters. COD also serves to define biomass along with volatile suspended solids (VSS) parameter. Estimation of oxygen requirement and sludge production for aerobic activated sludge systems covering basic biochemical processes are defined for different substrate and sludge parameters, considering that each calculation is associated with a different stoichiometry on the basis of a selected parameter set. The German regulations are examined and the biochemical bases of the coefficients in the regulatory expressions are set in terms of fundamental model constants.