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Flow in to the system from the bottom of the settling column creates a jet in the center of the column, which is far from uniform flow. A typical dissipation ratio is 1:10; therefore, it would take on the order of 10 m for jet to dissipate in a column with a diameter 10 cm, as in the current setup. A cone with a ratio of 4:6 is used a diffuse to maximize the lateral spread of the jet. In addition to the cone, there is a mesh directly on top of the cone made of a 1 cm thick plastic sheet with uniformly distributed holes of 0.5 cm diameter. This mesh will break up the single jet coming from the column inlet, into several smaller, weaker jets that will dissipate quicker. The preliminary stages of the experiment will test the effectiveness of the cone and mesh system in dissipating the jet and allowing for optimal floc blanket formation.
Sedimentation
The sedimentation process, which is completed directly after the floc blanket, is the final separation process of water purification. By the time the water enters the tube settlers, the turbidity has been dramatically reduced by the floc blanket. As seen in the figure below, the apparatus allows for independent control of the flow rates through the floc blanket and the tube settlers because of the additional waste outlet just above the floc blanket. The water flows up through the tube settlers and manifold, through a turbidity meter, and is wasted. This flow is controlled by the same pump circulating alum to the system. In order to maintain a steady flow, a buffer may be utilized.
The flow rate is based on a desired upflow velocity of 100m100 m/day through the tube settlers (this number is based on previous pilot plant research). Note that the tubes are at the standard sixty-degree angle from the horizontal. Below is the calculation of the flow rate, regulated by a pump, through the tube settlers.
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Since flow through the tube settlers is laminar, the shear stress is greatest at the tube surface. Below is a calculation of the minimum diameter based on the shear stress at the tube surface. We assume that floc integrity begins to suffer under shear stresses of 1Pa1 Pa.
Since this calculation yields an unreasonably small minimum diameter of 20µm20 µm, shear stress is not the limiting factor for tube settler diameter. Therefore, the diameters tested in this experiment are based on floc size, the length/diameter ratio of twenty-four mentioned above, and material availability. The minimum diameter was set to be at least twice the diameter of a floc (estimated at 2mm2 mm), and the length of the tubes was set to be twelve inches. Table 2 below lists the parameters of the tube settlers for this experiment.
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