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A sedimentation tank is planned to be connected in series to the flocculator at the pilot plant. This sedimentation tank will allow us to isolate the process in the AguaClara design that needs to be optimized to allow us to achieve our overriding goal of consistently producing water under 1 NTU. Building this sedimentation tank allows for us to test not only the flocculation process but the sedimentation tank for design efficiency. Testing will include not only the traditional lamella design of sedimentation tanks or but also the alternative of sludge blanket based sedimentation tank designs. These two processes will be tested side by side in parallel flow sedimentation tanks.
Introduction
The pilot plant sedimentation tank will be a verticle vertical flow style tank and will only use a sludge blanket as its removal mechanism. This tank will be the first time that an AguaClara team has used a sludge blanket instead of plate settlers. As with other AguaClara designs, the tank will be run by the elevation head in the flocculator tank and will not require electricity. There are several design restraints due to the current set up at the pilot plant. There are 37'' of available water head at the end of the floculator which are available for our use. The piping connection between the flocculator tank and sedimentation tank also cannot have a shear value that excedes the max shear in the last baffle section ( GMax= 48.826 /s), or else the flocs made in the last section will be broken up.
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The design of the tank was done during the Spring 2008 semester. The sed sedimentation tank was designed to be contained in a polyethylene tank of dimensions 111.25 cm × 92.5 cm × 87.5 cm 24" x 24" x 36" (length × width × height) with a wall thickness of about 0.8 cm. 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 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 tank without 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 sed 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.
Given Variables:
- Length (L) = 11123.25cm125"
- Width (W) = 9223.5cm125"
- Height (H) = 87.5cm36"
- Water Level (WL) = 7630.25cm5"
- Flow Rate (Q) = 30L/min
Below is an AutoCAD drawing of the proposed sed sedimentation tank design.
!Pilot Plant^CAD Plant Construction and Research^CAD drawing of sed
tank.jpg!
Upward Velocity Calculation:
To determine the upward velocity of the sedimentation tank the flow rate through the tank was divided be 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.
Insert Upward Velocity Equation.
Inlet Pipe Calculations:
To determine the diameter of the inlet pipe we used the constraint that the maximum shear due to minor losses had to be less than Gav in the last section of the flocculator (Gav = 24/s). The predominant minor losses from the inlet pipe will occur at the exit elbow, and the point and the elbow. The minor loss coefficient for the elbow is 0.9 and the exit is 1.0 so K was set to be 1.0 for this design. The equation to calculate shear due to a pipe elbow is shown below. This equation is solved for D, the diameter of the pipe that would provide shear equal to Gav. The equation used is very similar to the equation used to find the baffle spacing for the flocculator.
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Variables:
K = 1.9 0 (minor loss for an elbowexit)
This equation was derived from a series of substitutions. the original equation used was:
Insert Gbar = sqrt(epsilon/nu) equation here.
Where epsilon is the represented by the following equation.
Insert epsilon equation here.
Insert head loss equation here.
The head loss found in this equation is assumed to be just minor head loss. The minor head loss for this is assumed to be the dominationg factor because the pipe is designed to be relatively short in length and the bends and exit are our major concern for floc break up.
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 of the pipe. (???? what this was we used????)
Initially when we 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 Gav. 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.
Launder Launderer Calculations:
Pipe Diameter
The effluent launderer will span the length of the tank at the center of the width. The launderer is placed 20cm beneath the top of the water level, which is taken from a constant in the launderer MathCAD equations. The diameter of the effluent launderer was calculated by iterating through pipe diameters to find the diameter pipe that matched with the flow rate. The diameter was selected based on the following equation:
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