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The sedimentation process, which is completed occurs directly after flow through the floc blanket, is the final separation process of technique in our water purification system. By the time the water enters the tube settlers, the turbidity has been uniform flow has been established, and the floc blanket has already dramatically reduced by the water turbidity. The design of the sedimentation section of the apparatus requires serious consideration of flow rates, tube geometry, and other parameters.
The design of our column 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 independence of these flow rates is maintained by the additional waste outlet just located above the floc blanket (see figure below). The water flows up through six tube settlers and manifold, through a turbidity meter, and is then wasted. This flow is controlled by the The same pump circulating alum to the system controls this flow. In order to maintain a steady flow, a buffer may be utilized.
The flow rate (Q) through the tube settlers is calculated based on a desired upflow velocity (Vup) of 100 m/day through the tube settlers (this number is based on previous pilot plant research). Note that the tubes are . We note here that the tube settlers are angled at the standard sixty-degree angle from the horizontal. degrees from the horizon in order to reduce plate length. We also note that flow through the tubes is assumed to be laminar, resulting in a parabolic velocity profile with the maximum velocity (Va) occurring at the center of the tubes. The desired upflow velocity is dependent primarily on the critical velocity (Vc) necessary to allow flocs to settle out of the water: 
Below is the calculation of the flow rate, regulated by a pump, through the tube settlers.
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First of all, the ratio of the vertical height of the tube to the tube diameter must be at least twenty-four in order to allow enough distance for the flocs to settle out of the flow (this value is based on the critical velocity and tank dimensions). In addition, a decrease in diameter allows for a decrease in plate length and, consequently, a decrease in sedimentation tank depth. However, if the diameter is too small, flocs may be impacted by shearing at the tube surfacethe resultant shear force may rupture the flocs. Also, previous research (see Summer 2008 above) shows indicates that various problems that are difficult to identify and predict quantitativelyquantify, such as clogging, exhibit at small diameters.
Since flow through the tube settlers is laminar, the shear stress is greatest at the tube surface where va is zero. 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 1 Pa.
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into the maximum shear stress equation yields:
Since substituting our parameters into this calculation equation yields an unreasonably small minimum diameter of 20 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 2 mm), and the length of the tubes was set to be twelve inches. Table 2 .304 m. The table below lists the parameters of the tube settlers settler apparatus for this experiment.
Results and Discussion
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