In a biological nitrogen removal system, efficient performance of the secondary clarifier is essential since the system depends on a high concentration of active biological solids being available in the biological reactor. Estimations of the current capacity of the clarifier can be made from observing maximum feasible solids flux to the clarifier which does not cause escape of sludge. The aim of this work is to demonstrate a method to establish maximum feasible solids flux, the limiting solids flux, from on-line operational data. In this method the solids flux into the clarifier is calculated from on-line measurements of effluent flow and of suspended solids in the biological reactor, and a known value of return sludge flow. From relations between solids flux and sludge blanket height, the value of limiting flux is estimated. This method is based on the assumption that the movement of sludge blanket height is directly related to changes in the solids flux. When the limiting solids flux is estimated from on-line measurements a measurement of the capacity of the clarifier is retrieved. The suggested method also gives an early warning to prevent sludge escape from the clarifier.

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