A strategy for controlling secondary clarifiers during dry weather and rain is described. The strategy is now implemented and is being tested on a full-scale wastewater treatment plant. The plant - Aalborg West Wastewater Treatment Plant - is operated by using the patented Biodenipho process.

The flux theory as formulated by A. Vesilind, in combination with measurements of sludge level, suspended solids and flow at critical points, form the bases for a dynamic on-line control of the clarifiers. The settling velocity used in the flux theory is measured and calculated on the basis of the installed sensors. In the test period it is expected that the control strategy will show capability of several improvements compared to traditional control strategy. The dry weather strategy should result in a reduction of return sludge pumping and thereby save energy. Moreover, improvement of the sludge characteristics is expected because of less pumping of sludge.

The rain strategy should show possibility of significant reduction in storm overflows in the plant and in the sewer system upstream to the treatment plant. Thereby the capacity of the plant will be improved and the total effluent load to the recipient reduced.

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