Determination of the required surface area for activated sludge final clarifiers based on a unified database

The paper presents a methodology for the calculation of the surface area of activated sludge secondary sedimentation tanks, based on a simplification of the solids flux theory. The proposed methodology has the advantage of being based on the integration of settling data from tens of full-scale plants presented by several authors in different countries. The equations derived are separated by settleability ranges (very good, good, fair, poor, very poor), which allow the easy decision by the designer or operator with respect to the equation to be adopted. The values of the hydraulic and solids loading rates compare well with those presented in frequently used design manuals, but have the advantage of allowing continuous solutions for any MLSS concentration and of taking into account the influence of the sludge return ratio, usually not incorporated in the usual methods. The paper develops two general equations of simple use for the calculation of the maximum allowable value of Q/A, therefore allowing simultaneous compliance with the clarification and thickening requirements of the clarifier. The work includes formulae, charts and tables for the determination of the hydraulic and solids loading rates according to the proposed methodology.