On-Line Control of Polymer Addition to Prevent Massive Sludge Washout

An experimental method that quantifies the effect of polymer dosing on sludge settling characteristics is proposed. This method consists of recording batch settling curves at a grid of sludge and polymer concentrations. The effect of the polymer was found to depend on the mixing time between the dosing of the polymer and the start of the batch sedimentation. The recorded effects could be successfully implemented in a 1D dynamic settler model. From the literature it was concluded that keeping the sludge blanket below a certain critical height is an effective way of controlling the effluent suspended solids. From a model-based analysis this strategy appeared to be sound. Different control strategies using, respectively, the sludge blanket height feedback (FB)], the hydraulic loading feedforward (FF)], the solids loading (FF), or the sludge volume loading (FF) were tested for their ability to keep the sludge blanket below the critical height. The control strategy based on the hydraulic loading was the least efficient with respect to minimizing polymer dosage. The others appeared equally effective provided that they were properly tuned. Using the excess of the critical sludge blanket height as a measure of effectiveness, strategies based on more than one measured variable appeared to be less sensitive to suboptimal tuning.