Summer 2009 Research Plan

THEORY AND INTRODUCTION

Tube settlers are designed to provide a low capture velocity to facilitate the removal of small flocs and hence lower the turbidity of flocculated suspensions. They are integral to the overall effectiveness and performance of the AguaClara water treatment plants, as they are involved in the final particulate settling mechanism and thus the final effluent turbidity. The experimental apparatus contains a controlled clay stock that is added to raw water and mixed to a turbidity of 100 NTU. This raw water is sent through rapid mixing with a fixed dosage of alum, which then enters a flocculator (where the floc is actually created). From this stage, the water enters a settling column where it forms a floc blanket. The tube settlers draw water from this column.

Tube settlers use the concept of capture velocity. Particles enter the settlers at a specific velocity, which for the purposes of our early experiments will be kept constant. The capture velocity is important because particles traveling faster than this quantity fall to the bottom of the tube settler as they enter and cascade back down to the floc blanket from which they were taken. This allows for clean, less turbid water to pass through the tube and out to be chlorinated. As the turbidity of the effluent water decreases, chlorination becomes more and more effective because pathogens in the water have more exposure to the cleaning chemical. Ultimately, this results in safer drinking water.

PARAMETERS

Independent

plate settler length
plate settler diameter
plate settler angle
capture velocity
upward velocity
turbidity of source water
coagulant dosage
natural organic matter

Dependent

velocity gradient
residence time (flocculator and floc blanket)

EXPERIMENTAL DIRECTION

This summer the plate settler team will work on generating a controlled ideal experiment as a basis for understanding the robustness of the plate settling system. We plan to test how the settlers function under a wide variance of conditions, since changes in alum dosage and influent turbidity inevitably affect the sedimentation process in many ways. 

Velocity gradients within the tube settlers have proven to be very important.  Particles that are supposed to settle out somehow make it into the effluent, and we would like to figure out why this is happening.  Since it is likely that the density of our floc particles strongly depends on variable external parameters (like alum dosage and influent turbidity), our direction is highly geared towards a fluid dynamics anlysis.