The describes the development of a prototype device in which parallel computer control techniques are used for the rapid on-line measurement of the biological oxygen demand (BOD). Wastewater samples diluted with fresh water are fed into a whirl bed reactor in which microorganisms are grown. A computer-based feedback controller keeps the oxygen consumed in the reactor to a constant value by dynamically adjusting the dilution ratio of fresh water to waste water. The BOD is assessed by monitoring the controller output changes in the dilution ratio. The application of a linear quadratic control algorithm implemented by a specially designed two-processor computer architecture has resulted in a measurement delay of 2 min. >