This paper describes a method for the continuous estimation of the influent short-term biochemical oxygen demand (BODst) of a wastewater treatment plant. The method uses the dissolved oxygen (DO) concentration in the inlet and outlet of a closed, completely mixed respiration chamber during the transients between two modes of respiration measurement which are alternately executed. In one mode, sludge having the endogenous respiration rate is directed through the respiration chamber. In the other mode, sludge from the aeration tank directly flows through the respiration chamber and, in addition, a sample of influent is continuously introduced in the chamber. The influent BODst is estimated for each cycle of both modes by integrating the mass balances of BODst and DO over the respiration chamber. Knowledge of the degradation kinetic of readily biodegradable matter is not required. The proposed procedure is verified using both simulated and experimental data from a pilot plant. It is concluded that the procedure enables the estimation of the influent BODst with a relative error ranging from 2 - 6%.

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