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The design of the tank was done during the Spring 2008 semester. The sedimentation tank was designed to be contained in a polyethylene tank of dimensions 24" x 24" x 36" (length × width × height) with a wall thickness of about 5/16". The design goal was to have enough area in the tank to create a sludge blanket. With the available water level height, which is taken directly from the water level in the floc tank, we did not have enough room to include plate settlers. Initially we wanted to split the plant flow rate (110 L/min) in half, allowing us the possibility of installing two parallel sed sedimentation tanks for experimentation purposes, but we found that a flow rate of 55 L/min into the tank would require a 6 inch pipe to transport water from the floc tank to the sed sedimentation tank without to avoid breaking up flocs. Due to cost restraints (a six in bulk head fitting would cost about $300) we had to limit this pipe to being 4 inches, and we did that by lowering the flow rate of the sedimentation tank down to 30 L/min. This constraint on the inlet pipe and flow rate required us to switch to a smaller tank than originally planned. This change was necessary to obtain an upward velocity of 100m/day. This low flow and smaller tank set-up will allow for parallel testing of tanks containing different sedimentation processes.
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Upward Velocity Calculation:
To determine the upward velocity of the sedimentation tank the flow rate through the tank was divided be by the cross sectional area of the tank. In order to make our model comparable to the sedimentation tanks that are built on a full scale the upward velocity needs to be the same. The upward velocity in Ojojona was found to be 100m/day. Thus this was the parameter we used for this model as well. We allowed our design velocity to exceed 100 m/day with the idea that we will probably have probably have to modify the flow after construction to allow a stable sludge blanket to form.
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Variables:
K = 1.0 (minor loss for an exit)
This equation was derived from a series of substitutions . the original equation used was:Insert Gbar = sqrt(epsilon/nu) equation herethat can be seen above.
Where epsilon is the represented by the following equation.
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Insert head loss equation here.
The head loss found in substituted into this equation is assumed to be just minor head loss. The minor head loss for this pipe is assumed to be the dominationg dominating factor because the pipe is designed to be relatively short in length and the bends and exit are our major concern for sources of shear the primary locations where floc break up would occur. Velocity The velocity through the pipe is represented by Q/A for the pipe. The residence time is found to be the volume over the flow rate through that volume. The volume used was assumed to be 2*D times the cross sectional area of the pipe , where D is the diameter times 2 diameters of the pipe. (???? was this was we used????)
Initially when we this equation was solved this, we used 55 L/min as the plant flow rate, and MathCAD returned that the pipe diameter would be 5.11 inches to achieve shear equal to the desired Gav value. Due to the cost of 6 inch bulkhead fittings (nearly $300), we had to find an alternative design. We decided to lower the flow until a pipe with inlet diameter equal to 4 inches was achieved. We found that the maximum flow for these conditions is 30 L/min, so we changed the flow rate of the plant. The option of having two 4" inlets was considered but this was discarded because then 4 bulk head fittings would be needed. Thus the overall flow of the tank was lowered and a smaller tank was chosen.
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