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 baffle spacing of the flocculator. This program requires inputs from the user and from our bases 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 function follows the basis design procedure outlined in CEE 454: Sustainable Small Scale Water Supply. Our design calls for three sedimentation tanks. This assumption is based on the flow rates we think our plants should handle and an attempt to keep the footprint of the plant a reasonably sized rectangle or square. Since all the tanks are identical, the sedimentation function provides the dimensions for a single one (with one third of the total flow rate).

The width of the tank is a user input determined by the width of the material used for the plate settlers.

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.