Energy Dissipation Flocculation Design Program
This flocculator program determines the size, number, and spacing of the flocculator channels and baffles, based on the results of the Computational Fluid Dynamics (CFD) team. The tank is designed give an optimal energy dissipation rate to mix the alum (coagulating chemical) with the incoming water and to maximize the opportunity for flocs to form.
The 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 in place in the flocculator.
The programs used are Flocculator 3 (the design program) and floctank (the autoCAD script). In the scheme of the whole plant, the flocculation tank is drawn after the sedimentation tank (so many of our variables are constrained by an already-drawn sedimentation tank).
Top View
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Front View
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Flocculator Program Inputs and Outputs
Flocculation Tank Program Inputs
Flocculation Tank Program Outputs
Flocculation Tank AutoCAD Drawing Program
Flocculation Design Algorithm
Each section outlined below corresponds to its equivalent MathCAD code, identified by the same titles.
-The first part mainly informs and establishes the equations and design ideas used in the actual drawing of the tank.
-The second part determines the number, spacing, energy dissipation, and collision potential of the necessary baffles.
-The third part determines the width, height, size, and the other parameters needed to draw the flocculation tank with the baffles inside it. This section is heavily relied upon by the MathCAD code in floctank that draws the plant.
-The last part creates and outputs the matrix of baffle positions.
Computational Fluid Dynamics (CFD) Results and Functions
The research that went into this program was done by the CFD team. As such, many of the equations used have their basis in experimental findings; some of their work can be directly applied to the daily drawing of flocculation tanks, and some support the parameters used in this program.
For example, a table of minor loss coefficients indicates that the minor loss coefficient is very large for small h/b values, where h/b values are the ratio of water depth to baffle spacing. Linear interpolation is used to create functional relationships as a function of these h/b ratios. Then, an iterative code that determines the spacing of the floc baffles uses these relationships. Calculating the collision potential per baffle and the spacing that gives the target energy dissipation rate continue to use the parameters set by the research team.
Flocculator Functions
The critical balance in the flocculator is between ensuring that the alum and entering water are meeting the energy dissipation (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 height of water at the end of the flocculation tank is set to be the same as the height of water at the beginning of the sedimentation tank.
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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$$
Ti.Floc = HW.FlocEnd \cdot L.FlocTank \cdot W.FlocChannel} \over {Q.Plant
$$
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 these ports varies by channel, as the baffle spacing may be different from channel to channel.
The width of these ports is set to include 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 energy dissapation rate through the ports is set to be same as around the baffles; the dimensions of the port are calculated to be the same as that of the baffles.
Position Calculations for Each Baffle
The length of the lower floc baffles is determined based on the height of water at the end of the floc tank and the water as it goes around each 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 algorithms that create a matrix of baffle displacements from the end of the flocculator (see this drawing program for step-by-step details).
Water flows in from the entrance tank, and out to the sed tank. The flow direction alternates up-down through each channel as it flows around the baffles and through the ports.