Internal‐Wave Action in Gravity Clarifier

Because the flow in a gravity clarifier is stratified, internal waves can develop in regions where a large concentration gradient exists. In this study, we determine the locale of internal waves inside the clarifier tank and show that they are present only in the upper third of the tank. We also find that the increase in clarification efficiency resulting from the use of a flow contraction baffle is not combined with a decrease in internal‐wave amplitude within the clarifier. Furthermore, the ability of internal waves to reduce the rate of sedimentation through remixing of the settling solids is questioned. We also propose a new method of assessing the performance of clarifiers by measuring the power spectrum of the concentration fluctuations in the effluent. This technique suggests how effluent is being withdrawn from the clarifier and should be suitable for all types of clarifier designs. For a baffled clarifier, a measured increase of concentration fluctuations in the effluent is correlated with an improvement in solids removal using the baffle. Based on these experimental results, we present a physical model for the mechanism of baffle action that supposes the baffle prevents the formation of an eddy near the clarifier end wall.