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The pilot plant sedimentation tank will be a 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.
The flow through the sedimentation tank has been limited to at most half of the flocculator tank flow so that it will be possible to construct a second tank sometime in the future to handle the other half of the flow. ideally this second tank would only use plate settlers so that we could make a comparison between a sludge blanket and plate settlers.
Sedimentation Tank Design
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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 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 to modify the flow after construction to allow a stable sludge blanket to form.
Insert Upward Velocity Equation.
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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 was 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.
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Pipe Diameter
The effluent launder will span the length of the tank about 4in off of center of the tank. The launder is placed 2in 8 in beneath the water level, this value was chosen to be close to the surface of the water and while allowing a few inches for water height variation. The . This value is presumed to be a constant for AguaClara designs and also allows a signifigant amount of head above the launder to facilitate flow. The diameter of the effluent launderer was calculated by iterating through pipe diameters to find the diameter pipe that matched up with the flow rate. The diameter was selected based on the following equation:
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The best pipe diameter is 2 inches. The number of orifice holes was chosen so that the hole diameter would match up with a drill bit size while still giving the proper about of head. This head loss was defined to be about 20 times the water height over the accessory outlet weir. The goal was to make this height change of water in the floc tank insignificant with respect to head out of the sedimentation tank. The head out of the sedimentation tank needed to be the dominating head value to ensure that flow through the sedimentation tank is always uniform and steady. The total number of orifices was found to be 15.
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Secondary Outlet from the Flocculator
The flow rate of the sedimentation tank is designed to be 30 L/min, but the plant flow rate is designed to be 110 L/min. This means that 80 L/min needs to bypass the sedimentation tank and go directly to the outlet water. A large outlet weir is our proposed design for the alternative exit. Because the head loss out of the sedimentation tank is so high (2144.5cm979cm) the height of the water in the tank versus the weir height is assumed negligible.
Since the flow rate into the floc tank has been measured to be anywhere between 70 L/min to 110 L/min, It was important to check whether the height above the weir would be substantial enough that it would effect the flow rate of the sedimentation to tank. To do that we found an equation online that of water in the floc tank could change enough to effect the flow rate to the sedimentation tank. If the accesory weir is required to tank in a smaller flowrate, the water height in the floc tank will lower. This is the equation that describes the water height above the weir as a function of the flowrate.
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Where C Cw is a constant (taken to be 3 feet^1.5/sequal to 0.611+0.075*(H/Pw) where H is the height of water above the weir and Pw is the weir height), Qweir is the plant flow rate minus the sed flow rate, and the diameter of the weir was set to 6 to 3 inches (we wanted a big diameter, so there was a larger circumference for the water to interact withto minimize tank changes and the floc tank currently has a 3'' outflow piping system). When this was calculated we got a water height of 0.68 above the weir of 2.12 cm, which is small enough when compared to the head loss in the launder to assume that it will not affect the flow rate into the sed tank.
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