Energy Dissipation Flocculation Design Program
This flocculator program determines the size of the flocculator channels and number and spacing of baffles based on a energy dissipation rate optimized by the Computational Fluid Dynamics team. This program also outputs arrays of the location of each baffle in the tank; these arrays are used by the AutoCAD scripts to draw the baffles. The programs used are Flocculator 3 (the design program) and floctank (the autoCAD script). In the scheme of the whole plant, the floc tank is drawn after the sed tank.
Top View
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Front View
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Flocculator Program Inputs and Outputs--under construction
Flocculation Tank Program Inputs
Flocculation Tank Program Outputs
Flocculation Tank AutoCAD Drawing Program
Flocculation Design Algorithm
Computational Fluid Dynamics (CFD) Results and Functions
The major design assumption that forms the basis of the flocculator program: that the headloss of the water traveling from the end of the flocculator to the sedimentation tank is negligible, i.e. that the height of the water at the end of the flocculator is the same as the water height in the sedimentation tank. Later on, this will come into play in connecting the sed to the floc tanks.
Linear interpolation is used (based on the CFD results) to get relationships between the ratio of water depth to baffle spacing, collision potential, and energy dissapation rate.
Flocculator Functions
The critical balance in the flocculator is between ensuring that the alum and entering water are meeting the energy dissapation (ED) and collision potential (CP) goals, and not breaking up flocs that have formed.
Calculating the amount of baffle spaces that give the target ED rate is done with an iterative code that relies on the CFD functions described above. Then, the collision potential per baffle and max ED-to-CP ratio determines the baffle distribution, e.g. ED.Target(Psi).
The number of spaces per channel is determined by an iterative code that finds the correct ED rate & cumulative CP at each channel. The last channel (the one immediately preceding the sed tank) is then forced to have an even number of baffles. This is because the water must flow under the last baffle and up to the sed tank.
Calculation of Flocculator Geometry
The width of the floc channel is calculated based on the CFD functions, but if this value is less than 45 cm, the width will default to 45 cm. This minimum value was set to ensure that a person can fit into the channel for construction purposes.
The number and spacing of floc spaces and floc baffles is calculated, as well as the CP, for the specific tank being drawn.
The center-to-center distance between baffles includes the spacing between baffles and the thickness of the baffles, for each channel. The total, continuous length of the flocculator is the product of the length of one floc channel and the number of floc channels.
The residence time in the flocculator is determined as follows:
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\large
$$
Ti.Floc = HW.FlocEnd \cdot L.FlocTank \cdot W.FlocChannel} \over {Q.Plant
$$
The height of water at the beginning of the flocculator is based on the height of water at the end of the flocculator (which is the same as in the sedimentation tank), plus the headloss through the flocculator. The head loss is determined per baffle (and per channel, and in the whole flocculator) using the HL function in the fluids function program. An additional 10 cm of freeboard space was added to the water level (HW) found at the beginning of the flocculator to determine the height of the flocculator walls.
The head loss per baffle in each channel:
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$$
HL.FlocBaffle = {Kp \cdot ({{HW.FlocEnd} \over {S.FlocBaffle) \cdot Q.Plant^2 } \over
Unknown macro: {2 cdot g cdot (S.FlocBaffle cdot W.FlocChannel)^2 }
}}
$$
Water flows between channels in the flocculator through ports cut in the concrete. The area of these ports is determined to ensure flocs will not be broken up. The width of this port is set to be the same as the baffle spacing in the final section minus a the thickness of the concrete lip needed to hold the baffle in place (S.FlocBaffle). It should be noted that this lip needs to be taken into account when determining baffle spacing for smaller plant when this length loss becomes significant. The height of the port is determined by the dividing the total port area, by the set width.
The length of the floc baffles is determined based on the height of water at the end of the floc tank and the height water needed for the turn.
Length of Lower Baffle:
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$$
L.FlocBaffleLower = HW.FlocEnd - S.FlocBaffle \cdot Pi.FlocBaffle
$$
The upper baffles are set to line up with the top of the tank rather than the waterlevel.
Length of Upper Baffles:
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$$
L.FlocBaffleUpper = H.Floc - S.FlocBaffle \cdot Pi.FlocBaffle
$$
The placement of the baffles in the flocculator is determined by an algorithm that creates a matrix of baffle displacements from the end of the flocculator. Water flows back and forth with flow direction alternating in each successive channel.