Sedimentation Tank Dimensions 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 flocculatorlamella spacing, among other things. 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.

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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.

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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.

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sedimentation program calculates the dimensions of one sedimentation tank considering the dimensions of inlet slopes, sed plate frame, lamella, sed launder, as well as dimensions of the inlet channel.

Firstly, the design of sedimentation tanks for a given flowrate Q, involves a selection of the number of sedimentation tanks from a user input. Based on the user input, the flowrate in one sedimentation tank and length of the sed tank are calculated:

and

where the width of the sedimentation tank was set to 42.5 inches, which represents the width of available lamella material, and the upflow velocity was set to 70m/day to allow for possible sludge blanket formation.

The wall height of the sedimentation tank was set to be equal to the height of the water level plus the height of the 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 level height is calculated as the sum of elevation of the sed slopes, 2*outerdiameter of the pipe used for the sed plate frame, height of the lamella, height of the water above lamella, and the distance between the top of the slopes and the bottom of the lamella. Two other factors also taken into consideration are the thickness of the concrete ledge used to hold up the launder on the inlet channel side and the extra space between the top of the lamella and the launder to offset any error during construction. Because of these two variables there is significant space between the top of the lamella and the launders, which results in an increase in the height of the water in the sedimentation tank, leading to a taller sedimentation tank.

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$$
HW_{Sed} = Z_{SedSlopes} + H_{SedBetween} + 2*outerdiameter(ND_{SedPlateFrame} ) + H_{SedPlate} + H_{SedAbove} + T_{ConcreteMin} + S_{LamellaLaunder}
$$

where, the elevation of the sed slopes is defined as:

\large
$$
Z_{SedSlopes} = Z_{SedSludge} + H_{SedTopSlope} + H_{SlopeThickness}
$$

The calculations of the lamella height can be found here.

The launders leaving the sedimentation tanks were designed in a similar manner as the sedimentation sludge drain manifold. The height of water above plate settlers has been defined as:

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$$
H_{SedAboveW} = outerdiameter(ND_{SedLaunder} ) + HL_{SedLaunder}
$$

The calculations of dimensions of the inlet channel can be found here.

Finally, therefore:

\large
$$
H_{Sed} = HW_{Sed} + H_{PlantFreeboard}
$$

and

\large
$$
Z_{MP} = HW_{Sed} - HW_{InletChannel}
$$

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