Horizontal Flow Flocculation Design Program

*THIS PAGE IS UNDER CONSTRUCTION WHILE THE HORIZONTAL FLOW PROGRAM IS BEING CREATED. PARTS OF THIS PAGE ARE LIKELY TO CHANGE**

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 and the specification of horizontal flow. Horizontal flow is used for very high flow rates to avoid building the plants even taller, instead making them wider. Horizontal flocculators are also easier to drain than vertical flocculators made of cement baffles because individual drains are not needed for every lower baffle as they are in the vertical flocculator. The tank is designed given 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. For now, the horizontal flocculator is designed to have untapered baffle spacing. It is much more complicated to make tapered baffle spacing and its benefits are being questioned.
The programs used are Flocculator 3 Horizontal w Standard Vars JP v2 (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). Eventually the design program from horizontal flow will be integrated into the same program used to design vertical flow flocculators. The program will decide whether to use vertical or horizontal flocculation based on the flow rate.

To create a consistent relation between vertical and horizontal flow, generic notation is used. Tau represents the distance to turn. The flow area, which is the cross sectional area that is perpendicular to the flow of the water, is Rho*S.

Top View

Flocculation Design Algorithm

Each section outlined below corresponds to its equivalent MathCAD code, identified by the same titles.

To view the first part that informs and establishes the equations and design ideas used in the actual drawing of the tank refer to the Vertical Flow Flocculation Design Program page as these equations and ideas are the same.

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.

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 number of baffle spaces that gives the target ED and CP is done with a non-iterative code (as opposed to the vertical code which is iterative). The optimal Tau/S value of 3 found from the CFD calculations can be used for horizontal flocculators since the flocculator can be made as wide as necessary (whereas in vertical flocculation, Tau was constrained by the height of the sedimentation tank), so iteration is not needed. The space between two baffles is determined using the following code:

The number of spaces per channel is determined by a non-iterative code also since the length of the channels are fixed to be the length of the sedimentation tank. Unlike in vertical flow flocculation, channels will always have an odd number of spaces to ensure that water flows into successive channels and eventually into the sedimentation 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, i.e. no head loss occurs between the two:

The "height" of the water in the floc channel is equal to the height in a vertical flow tank, therefore

The number and spacing of floc spaces and floc baffles is calculated, as well as the Collision Potential, for the specific tank being drawn. The number of floc spaces is determined using the following Round function which forces the number of spaces to be an odd integer:

The above function was previously a floor function, but rounding to the next nearest odd number instead of flooring to it gets the spacing closer to that which provides the optimal energy dissipation rate.

When calculating the spacing, L represents the length of the sed tank L.Sed and T represents the thickness of a baffle T.FlocBaffle. The minimum baffle spacing is 45 cm, which is the width that a human could walk through if needed for maintenance.

The center-to-center distance between baffles includes the spacing between baffles and the thickness of the baffles, for each channel. This is an array with an element for each channel. Since the horizontal design is untapered, each channel should have the same baffle spacing, but the code was kept as similar to the vertical code as possible. The exception is the last channel, which might have corrected spacing to make up for the wide entrance into the inlet channel.

The residence time in the flocculator is determined as follows:

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 functions 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:

Water flows between channels in the flocculator. There are no ports as there were for the vertical flocculator because they were only necessary to maintain the vertical flow pattern. The width is the same as the baffle spacing in the previous channel and the height is the height of the floc tank. The width of these ports may vary by channel, as the baffle spacing may be different from channel to channel.

Position Calculations for Each Baffle

The length of the lower floc baffles and upper floc baffles that was seen in vertical flow is now the length of the left and right baffles (both are on the same height level). These baffles are set to line up with the top of the tank rather than the water level. The baffles are oriented to switch from one side of the tank channel's wall to other ("east" to "west") so that the flow of water is smooth back and forth through the tank. The baffles must be staggered in opposite directions in each channel so that the baffles at the end will form the necessary channel connecting the larger floc channels.

Length of "Lower" Baffle = Length of "Upper" Baffle:

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 of how the lamina, baffles, and other tank details are drawn). All baffles are also placed at the bottom of the tank, which is represented in a Z-matrix for the baffles.