Sedimentation Tank Design Program

The design of the sedimentation tank is a critical piece of the design of the entire plant. Its properties, such as depth and critical velocity, are important in determining the dimensions and lamella spacing. This program requires inputs from the user and from our basis of design in order to determine the design and dimensions necessary to generate the AutoCAD drawing and design report.

Sedimentation Tank Design Program Algorithm

Sedimentation Tank Inputs
Sedimentation Tank Outputs
Sedimentation Tank AutoCAD Drawing Program

Algorithm

The sedimentation program calculates the dimensions of one sedimentation tank and the breaks down of what portion of the tank is allotted for the inlet slopes versus the lamella. The details of the inlet slopes, drain pipe, launders and lamella to be in the tank are calculated in separate programs. All sedimentation tanks in the plant are designed to be identical.
The number of tanks is specified by the user. Based on the number of tanks given by the user the flow rate through one tank must be calculated first.

This flow rate will then determine the cross sectional area of the tank given the desired upflow velocity. The upflow velocity is set to allow for possible sludge blanket formation (70m/day). The width of the tank is a user input determined by the width of the material used for the plate settlers (42in = 1.07m). The specified width allows for the tank length to be calculated.

The height or depth of the tank is simply the water height in the tank plus the plant freeboard of 10cm. This plant freeboard is a design assumption used through out the design algorithms to give a buffer to allow for possible variation in water levels without resulting in tank overflow. The water height is set in the basis of design. It should be noted that this value will have to be higher if larger plants are designed.

Within the given waterdepth the tank is divided into sections. The user is asks to give a ratio (Pi.SedBottomHWSed)of how the sedimentation tank should be divided, this fraction represents the portion of the tank devoted to the inlet slopes below the lamella, this includes the drain pipe at the bottom of the tank. The rest of the tank is aportioned to lamella and effluent launders. The user given fraction is multiplied by the design water depth to give a vertical height of the tank occupied by the inlet slopes and the drain pipe.

For deeper plants The height of the platform is calculated based on the bottom of the platform being even with the bottom of the inlet channel.

The height of the tank is the sum of four separate calculations. The space beneath the plate settlers is assumed to be 1 m. This is enough space to allow for the formation of a sludge blanket. Below the plate settlers the walls will be sloped for drainage purposes. This slope should be between 45 and 60 degrees to ensure that the flocs will slide down to the sludge drain. The length of the sloped walls is determined by the width of the tank and the slope. This sloped section can be couple with a straight section to guarantee 1 m of space below the lamella. The space between the top of the plate settlers and the water surface is equal to .25*spacing between adjacent launders. Since each tank has 1 launderer, the space between adjacent ones is equal to the tank width. This ratio should ensure equal flow of effluent through the plate settlers. The depth of the middle section of the tank is determined by the length and slope of the plate settlers. The total depth of the tank is equal to the sum of these three depths, plus a freeboard depth. Freeboard space is empty space between the top of the tank and the water surface.

Finally, the length of the sedimentation tank is determined by the active area of the tank. This is the area that is actually used for sedimentation purposes and it is determined by the [#critical velocity] in the tank.

Critical velocity is the rate at which a particle must fall to ensure that it settles out in the plate settlers. If the critical velocity is too large, flocs will not settle out. However a small critical velocity comes at the expense of area (so it is not practical to have an unnecessarily small velocity). Observations of the Ojojona plant recommend a critical velocity of approximately 15 m/day need a reference or a calculation to support this number , although anywhere between 10 and 20 m/day is allowable Do we have data to support this?. Critical velocity is also dependant on the upward velocity in the tank. We are designing our tanks to have an upward velocity of 100 m/day. We've found that this is the velocity allows for the formation of a sludge blanket in the bottom of the tank. Since a portion of the tank's length is rendered unusable due to the sloping of the lamella, the actual length of the tank is greater than the active length. Explain the dual constraints of critical velocity and upflow velocity and detail how both constraints could be met simultaneously or how they could both be set as maximum values.

The [#number of lamella] in the tank is determined by the tank's active length and the lamella spacing. Our design uses a lamella spacing of 5 cm and a slope of 60 degrees. These design parameters have proved successful in Ojojona. A channel runs along the width of each sedimentation tank and this limits the active length of the tank. Additionally, the diameter of the effluent launder must be accounted for in this calculation. The number of lamella in each tank is calculated based on this shorter length.

The sedimentation function is also responsible for calculating the height of the platform for the chemical storage drums. This height is calculated by adding the water depth in the channel and the water depth in the sedimentation tank.

Sedimentation is a basic step of most traditional water treatment processes. In our plant it comes between flocculation and chlorination. It uses gravity to separate water from the particles - particles settle to the bottom of the tank while the clean water rises to the top. Any particle settling faster than the critical velocity of the tank should settle out. Critical velcoty is a function of he flowrate and setting area of the tank. A particle's settling velcotiy depends on its size and density. Larger particles settle faster and therefore are thereby easier to separate from the clean water. Our design employs plate settlers which lessen the distance a particle must fall in order to settle out. This increases the tanks efficiency.

Outlined in CEE 492, the sedimentation algorithm relies on design assumptions and simple geometric relationships to determine the dimensions and number of plate settlers. The width of the tank is determined by the width of the plate settler material and an upward velocity of 100 m/day is set to allow for the formation of a sludge blanket. Additionally, the lamella are installed 5cm apart at an angle of 60° from the horizontal. From these initial conditions, the algorithm determines the tank's length and depth as well as the number of plate settlers. The depth of the tank is divided into three sections - the space above the plate settlers, the space below the plate settlers and the space with the plate settlers. Below the plate settlers the walls are sloped toward the drain pipe to aide drainage. Due to the angle of the lamella, the tank's entire length is not used for sedimentation. Additionally, the channel between the flocculator and sedimentation tanks and the launder pipe diameter decrease the active length of the sedimentation tank and must be accounted for when determining the number of plate settlers. PICTURE. Since the sedimentation tank determines many of the inputs for other functions, it is run at the beginning of the design process. This means that the sedimentation function has no way of knowing the launder pipe diameter. The current design assumes a pipe diameter of 15cm, however in the future, this diameter should be found iteratively after the launder function runs. The effluent launders and sludge drain pipe are also designed in separate functions. Finally, the sedimentation function is also responsible for calculating the [#height of the platform] for the chemical storage containers. This is based on the water height in the channel and height of the tank.

Sed Figure 1: Height of chemical storage platform

The function works for a limited range of inputs. For instance, if the slope of the walls is too steep the depth of water below the plate settlers will be too great. Similarly, an upward velocity of 100 m/day should typically result in a critical velocity of 10-20 m/day. This ensures that even the smaller flocs which fall slowly will settle out. However, if the length of the lamella or the lamella spacing are changed dramatically, the critical velocity will not be in this range and the function will return an error. These error messages should not cause the program to crash, so they need to be incorporated in the LabView screen so the user is made aware.

There is currently some discussion within the team regarding which paramters should be assumed and which should be calculated by the function. Currently, the Master Program assumes values for upward velocity, the length of lamella, the lamella spacing, and the angle of the lamella. Critical velocity is a function of these assumptions, and so it does not vary for different flow rate or tank dimensions. One possible solution is to assume BOTH the critical and the upward velocities. The user would input the length of the lamella and the lamella spacing. The length of the tank would then be calcualted to satsify both requirements. These changes to the sedimentation algorithm should be explored in the future as the team refines its design process.