Plate Settler Spacing
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Introduction
This research is focused on a deeper understanding of the sedimentation process so that plants can effectively remove the dirt particles from the system, allowing for successful chlorination of the outgoing water. Currently the plants all use lamella, which are corrugated parallel plates that decrease the distance that a floc must fall to be settled out. To make things simpler for a lab, we use tube settlers of various diameters, and experiment with different flow rates and turbidities, because these things fluctuate often in the plants in Honduras based on the season.
One of the main reasons for this focus is that sedimentation is the only process in which particulates are actually removed from the water. Flocculation and rapid mix may be working well, but if the particles are not being settled out, people will still be drinking dirty water. Low turbidity water is not only aesthetically pleasing, it's also safer because the disease causing pathogens are more likely to be exposed and killed when the water is chlorinated. AguaClara has had the goal of 1 NTU water since its beginning, and the Plate Settler Spacing team has been able to accomplish this in the lab, but now is trying to see how this can be transferred to our future plants in Honduras. Sedimentation is also the limiting factor for the height requirement of the plants, so if we can find a way to produce the same results with a smaller amount of space, the plant cost will decrease significantly.
Subteam Semester Goals
Weekly Subteam Progress.
Experimental Methods and Results
The following experiments are based on research completed in Summer 2008 and Fall 2008 .
Overview of Methods
As water flows through tube settlers, particles settle out of the flow based on their individual velocities relative to the tube length and diameter. A common parameter in describing sedimentation is the capture velocity. The capture velocity is the maximum velocity that will facilitate particle settling. Therefore, if a less dense floc is travelling through a settler at a velocity higher than the capture velocity, this floc will be unlikely to settle out of the flow. Below is the equation for capture velocity:
From the above equation, it is evident that capture velocity is dependent on the diameter, length, and angle of the tube settlers, as well as the flow rate through the settlers. The following experiments explore the interdependency of tube geometry, flow rate, capture velocity, and floc blanket involvement on the efficiency of sedimentation.
The same experimental setup is used for each experiment described below. The system consists of four major processes: rapid mix, flocculation, floc blanket formation, and sedimentation. The turbidity of the raw water influent is monitored by a turbidimeter. From the raw water bucket, the influent travels to a coil that rapidly mixes the raw water and alum dosage. Next, the flow travels to the tube flocculator__________ . From the flocculator, the water flows up through a 4.5 in. diameter column. A cone in the bottom of the column facilitates flow dispersion to create turbulent flow. It is here, in the column, that a floc blanket is formed. The height of the floc blanket is controlled by an effluent tube, located at the lowest desired floc blanket height, which is opened or closed by a solenoid valve. From the top of the column, water is pulled through the sedimentation tubes and a final turbidimeter by a peristaltic pump. This system is fully automated by a program called Process Controller. Figure 1 below offers a schematic to illustrate the system described above.
[Experiment 1: Variation of Flow Rates and Inner Tube Diameters]
Five flow rates are run through tube settlers of varying diameter. Trials are run with the inlets of the tubes submerged in the floc blanket and with the inlets of the tubes set above the floc blanket.
[Experiment 2: Variation of Tube Length and Inner Tube Diameters]
The capture velocity through the tube settlers is held constant by varying the length of the tubes based on their diameter. Trials are run with the inlets of the tubes submerged in the floc blanket and with the inlets of the tubes set above the floc blanket.
Experiment 3: Floc Blanket Formation
Ongoing research concerning the rate of formation and density of floc blankets.