Abstract The parameters required to describe the metabolism of activated-sludge according to the equations of Michaelis and Menten or Monod can be determined by simple batch experiments. For the sake of clarity one should distinguish between “long-run” batch experiments with a period of unlimited growth, like the BOD determination, and “short-run” experiments with a very small relative cell growth. In this paper the possibilities of a simple short-term oxygen demand test are examined. This method is expected to be applied in practice for process control, viz. continuous monitoring of influent BOD, measuring the concentration of viable sludge bacteria and for measuring toxicity.