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. 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. The use of a tube flocculator allows for a more controlled flocculation environment as compared to flocculation with baffles. The flocculation design parameters are based on creating optimal conditions for floc blanket formation (please refer to Fall 2008 for tube flocculator parameters). From the flocculator, the water flows up through a 10 cm 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 (in our system 55 cm), 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.
Figure 1. Schematic of experimental apparatus.
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.