Plate Settler Spacing
Introduction
This research is focused on a deeper understanding of the sedimentation process so that plants can remove flocs from clean water as efficiently as possible, allowing for successful chlorination of the effluent water. Currently, AguaClara plants use lamella in their clarification systems, which are a network of sloped, stacked plates that create narrow channels through which clarified water can flow.
To emulate lamellar sedimentation, we use tube settlers of various diameters, which simulate the effects of the lamellar plates.
A majority of flocs that enter these inclined channels settle out due to the force of gravity, significantly reducing effluent turbidity to increase the effectiveness of chlorination. There is no control of influent water parameters going to the plant, so there is a keen interest in developing a settling system that is robust -after floc blanket clarification that will produce less than 1 NTU water.
Ultimately, we hope to optimize the lamella design in order to achieve effluent water with a turbidity of 1 NTU or less -- even under water chemistry fluctuations and alum dose variation. At present the plate spacing, capture velocity, and velocity gradient formation are three of our key design limiting constraints.
Velocity Gradient Research
Past research has brought to light the importance of velocity gradients within the tube settlers. Flocs at the bottom wall of the tube that experience an upward velocity pull greater than the force of gravity pulling them down will roll up the wall and exit with the effluent water. Flocs are fractal particles whose effective diameters are sensitive to shear stress, organic material in the water, and influent turbidity, so determining their response to the velocity gradient may require more than just a simple force balance. Furthermore, the maximum velocity gradient achieved at steady state will fluctuate with a plant's conditions and choices for flow rate through their lamella.
Subteam Semester Goals and Future Challenges
Weekly Subteam Progress.
Research Plan
Experimental Methods and Results
Fall 2009
Experiments with Saturated Water Influent
This section contains the experiment run in collaboration with the Floating Floc team to test the effect of saturated water in the influent on the plate settler performance
Experiments with the Velocity Gradient
This section contains the model derived to estimate floc roll up in relation to particle size and velocity. Also, experiments were run to collect data to support the model
Experiments with Natural Organic Matter
This section contains experiments testing the effect of natural organic matter (humic acid) on the plate settler performance
Summer 2009
Experiments Varying Alum Concentration
This section contains the experiments in which the alum dose was varied in order to test how well the selected geometry (d = 15.1mm) performed when subjected to non-ideal conditions
Experiments with the Velocity Gradient
The team has conducted physical modeling of the system to investigate the phenomenon of floc roll-up.
Filter Foam
In current plant designs, head loss through the entrance ports in the sedimentation tank is much greater than the head loss from flow through the plate settlers. Because head loss through the ports is much greater than head loss through the plate settlers, flow through each of the plate settlers in plants in Honduras is not equal. A geotextile foam was placed on top of the plate settler to create head loss to equalize the flow.
Spring 2009
Experiments with Flow Rates and Inner Tube Diameters
This section contains two related experiments. The first experiment varies flow rates and the inner diameter of the tube settler. From these results, the ideal L/d ratio is identified and held constant for a certain flow rate and inner diameter by varying the tube length.
Floc Blanket Formation
Experiments exploring the rate of formation and density of floc blankets.
Fall 2008 Preliminary Testing
Fall 2008
This section contains preliminary observations and data that preceded the experiments run during spring 2009, above.
Summer 2008 Preliminary Testing
Experiment Using Straws for Tube Settlers
These early experiments explored the impact of the diameter of tube settlers on effluent turbidity. Tube settlers were modeled using drinking straws.
Additional Information
Annotated Bibliography of Relevant Literature