Horizontal Flow 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 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 maximize the opportunity for flocs to form and to produce flocs that are small enough that they can be transported to the sedimentation tank without breaking.
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). The design program from horizontal flow is integrated into the same program used to design vertical flow flocculators and an algorithm is used to select between the two designs.

To create a consistent relation between vertical and horizontal flow, generic notation is used. J represents the flow travel distance between flow direction changes. The flow area, which is the cross sectional area that is perpendicular to the flow of the water, is P*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 sometimes). The optimal J/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, J 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. See Calculation of Flocculator Geometry for this equation.

Calculation of Flocculator Geometry

The depth of water at the end of the flocculation tank is set by the user. The different options are described in the algorithm that chooses between horizontal and vertical. If the floc tank is shallower than the sed tank, the floc tank is elevated so that the water levels match.

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 function which forces the number of spaces to be an odd integer:

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 total number of channels is found by dividing the total target collision potential by the collision potential per space and rounding that value up according to how many spaces are in a 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 plus the headloss through the flocculator. The head loss is determined per baffle (and per channel, and in the whole flocculator) based on the minor loss coefficient for flow around a baffle. An additional 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:

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.

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.

Drain Design

Port Sizing

The time it takes to drain the flocculator can be approximated by:

where D is the nominal valve diameter, h_o is the initial water height in the flocculator, A_res is the plan area the entire tank, and K_minor is the minor loss coefficient associated with the valve and subsequent expansion. Since there is a valve in every other channel, the maximum number of channels being drained by any given valve is two. Applying this equation to two channels gives the calculated value for the idealized size of the valve, which is then rounded up to the next available nominal diameter. For a flocculator that has only two channels, each of the channels will have a valve in it. In this case, the above equation will be applied to a single channel only.

Male Adapters

The calculated nominal diameter is that of the slip side of the adapter and is used to calculate its corresponding inner and outer diameters. The outer diameter of the slip side is also the inner diameter of the threaded side. The outer diameter of the threaded side is used as the inner diameter of the valve which fits it.

Drain Slopes

Since the center of the valve is aligned with the floor of the flocculator, slopes are required in the floor of the tank. The slopes have a width equal to the diameter of the valve and a depth equal to half the diameter (placing the center of the valve at-grade) with a slope of 30 degrees. If the distance the slope extends into the channel is longer than the spacing between baffles, the slope would extend through a baffle.  To correct this problem, the distance the slope extends into the channel will be set to a distance of 5 cm from the nearest baffle.

Gate Valves

Gate valves are placed at the base of every other channel in the flocculator at-grade to allow for draining. Additionally, the design requires a drain in the first and last channel of the flocculator, so if there is an even number of channels, the first two channels (the ones closest to the entrance tank) will each have a valve.