Experimental study of activated sludge batch settling velocity profile

The last step of activated sludge wastewater treatment consists in separating the residual particulate matter from water. This separation process takes place in a clarifier: a large tank in which sludge particles settle while clear water overflows. In order to improve numerical models of clarifiers, a better understanding of activated sludge settling mechanism is required. Laboratory batch settling experiments involving acoustic Doppler velocity measurement methods have thus been developed. The raw Doppler signal was recorded over durations of up to 22hours and then post treated. At the very beginning of the sedimentation process 3D turbulence produced by the initial stirring generates a high velocity standard deviation. 3D turbulence dissipates after a few minutes. Low amplitude fluctuations of the vertical velocity remain, whereas the average velocity decreases at any vertical location. Vertical velocity profiles exhibit two areas: an area of rising velocity in the lower part of the blanket and an area in which velocity fluctuates around a uniform settling velocity in the upper part. This last area decreases constantly and disappears completely after some minutes. A change in concavity of the rising velocity profiles after three hours rest indicates a change in the physical mechanism of consolidation.