Summer 2009 Research Plan

THEORY AND INTRODUCTION

Tube settlers are cylindrical devices used to filter and lower the turbidity of dirty water, and as can be seen from the final stages of the experimental schematic they are integral to the overall effectiveness and performance of the system we are analyzing, as they are involved in the final particulate settling mechanism and thus the final effluent turbidity. A controlled clay stock 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 and simultaneously filter it.

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

Dependent

energy dissipation rate (turbidity, velocity, turbulence)
residence time (in plate settler and flocculation tube)

EXPERIMENTAL DIRECTION

This summer, the plate settler team will first focus on the geometry of the tube settlers in order to gain insight into how parameters like the tubes' diameters and lengths affect the effluent turbidity, our main output and target of optimization. After the optimal geometry of the settlers has been determined under ideal conditions at two floc blanket levels--high and low--we need to test how robust this geometry actually is. To do so, we will create non-ideal circumstances by varying the influent turbidity and possibly the alum dosing in order to analyze how the system responds. Further, we hope to be able to characterize the system in terms of the fluid's residence time in the tube settlers in order to make correlations to the effluent turbidity.

In past experimentation things like energy dissipation and velocity gradients within the tube settlers have been of much importance. Consequently, we are also interested in developing a physical model of our system that will allow us to explain such phenomena as floc roll-up, wherein velocity gradient thresholds are exceeded and floc enters the effluent water.