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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.
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- Length (L) = 23.125"
- Width (W) = 23.125"
- Height (H) = 36"
- Water Level (WL) = 30 33.5"
- Flow Rate (Q) = 30L/min
Below is an AutoCAD drawing of the proposed sedimentation tank design.
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Parameter | Value | |
|---|---|---|
Inlet Pipe | 4 in | |
Launder Diameter | 2 in | |
Number of Holes | 15 | |
Hole Diameter | x 3/16in | |
Tank Drain Diameter | 1.5in | |
Number of Holes | 15 | |
Hole Diameter |
| |
Secondary Leveling Outlet Weir Pipe Diameter | 3in | |
Weir Height | 30.5 in | |
Hopper Removal Diameter | 0.5in | |
Hopper Removal Line Length | 2m |
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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 8 2 in beneath the water level. This value is presumed was assumed to be a constant for AguaClara designs and also allows a signifigant amount of head above the launder to facilitate flowto allow for uniform flow through all of the launder orifaces. 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 steady5 cm, or the head available above the launder. If the head loss through the launder orifices exceeded the available head, we would have encountered issues because the water in the outlet system would have had less energy than the surrounding system, which would have effected the flow rate. The total number of orifices was found assumed to be 15.
Orifice Diameter
The diameter of the orifices was calculated by finding the necessary area of the orifice based on the head at the launder, the minor loss of the orifice, and the flow rate through the orifice (plant flow rate divided by number of orifices). The equation to find the area is found below. With the area, the diameter of the orifice can easily be calculated by solving the area of a circle for the diameter. The minor loss coefficient of an orifice is 0.63, and the head, h, is equal to the height of the sed tank minus head loss through the launder minus the velocity head through the orifice. The diameter of each orifice was figured out to be 3 9/16". This value was rounded to the next smallest drill bit size.
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The head loss due to the orifice ends up being an important parameter because it affects effects how much water goes into the sedimentation tank, and how much bypasses it. We decided that we want a head loss about 20 times greater than the headloss in the weir at the end of the floc tank because this will minimize the effects if the flow through the floc tank changes. This will be explained further belowevenly the water will flow through the orifices. This quality is quantified in the parameter Q ratio, which is the ratio of flow through the first orifice divided by the ratio of flow through the last orifice. We have set the value of q ratio to be 0.90.
Orifice Head Loss
The head loss through the orifice was calculated by using the following equation:
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D is the diameter of the orifice (21.285cm)6 cm, the minor loss coefficient of the orifice is 0.63, and the flow is the flow through one of the orifices. The minor loss for an orifice was calculated to be 214.5 cm, a very high head loss55 cm. The head loss through an orifice is parallel to other orifices, so you do not add the head loss in each orifice. This parameter is the level that the water in the plant leveling tank needs to be below the water level in the sed tank in order for our plant to be operated at the designed water levels.
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 (44.979cm) the height of the water in the tank versus the weir height is assumed negligible.
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Hopper Design for Sludge Blanket
The volume of the sludge hopper should be big enough that it takes 48 hours to fill up. We created a parameter, PhiFloc, which is the ratio of how much the alum flow rate relates to the the volume of flocs created in a certain amount of time. The value (0.004) was taken from the mathCAD file used to create the ideal Gmax versus Gtheta curve. The majority of a sludge is assumed to be made of alum, and the flow rate of alum is 20 mL/min. Therefore, the rate of sludge build-up is the flow rate of alum times the ratio of the sed flow rate to the plant flow rate times PhiFloc. When calculated, the sludge rate was determined to be 3.429 x 10^-3 L/min. Therefore, the minimum hopper volume should be equal to the sludge rate times 48 hours, or 99 cm^2. We are still considering what will be the best way to build this volume in the tank, but possibilities are having a conical shaped bucket for sludge to fall into, or putting a lamina at an angle from one of the bottom sides.
Plant Leveling Tank The Plant leveling tank will handle the outflow from both the sludge blanket tank and the plate settler tank. Using one leveling tank and identical inlet and outlet piping for the two tank ensures identical flow (assuming that the head loss through the tanks themselves are negligible). We decided to make the head loss through the leveling tank the dominant head loss through the system, which then sets the flowrate through the sed tanks. Our design for the outflow system was an elevated 3'' pipe with an oriface sut into the pipe below the water level. Using the oriface equation, with the flow set at 30 L/m, and a head loss found by multiplying the floc tank head variability by a predetirmined safety factor, SF, we could find the required oriface size.
Results
Currently our design is under review, with the hope of construction in the upcoming weeks.
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