Most wastewater treatment plants have several secondary clarifiers or even more sets of clarifiers including several secondary clarifiers, and in practice it is a well known problem that equal distribution of the load to the single clarifier (or set of clarifiers) is very difficult—not to say impossible—to obtain. If the problem is neglected, quite a big percentage of the total clarifier capacity—measured as the max. allowed hydraulic load—can be lost. Further, return sludge rates are seldom controlled by any other means than as a (typically too high) percentage of the inlet to the wastewater treatment plant—giving a varying and too low suspended solids concentration in the return sludge, which again can lead to an unnecessary use of polymer in the pre-dewatering of the surplus sludge taken from the return sludge. A control of the return sludge rate divided into two parts - control of the total return sludge flow and control of how the total flow shall be distributed between the secondary clarifiers - is able to solve the mentioned problems. Finally, as shall be demonstrated on full scale wastewater treatment plants, a considerable increase of the hydraulic capacity of the treatment plants can be obtained.

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