Lamella Sedimentation Tank

Abstract

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h5.Aerial View of the Parallel Sedimentation Tanks
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The Lamella Sedimentation Tank design was design to test different variables, such as lamella spacing, lamella length and joint performance with sludge blankets, that can affect the overall sedimentation process. These variables are being test the limits of our design.

Introduction

Design and Methods of the Sedimentation Tank with Lamella

A preliminary design was done for the lamella tank. This design is still under review to determine if this algorithm was truly the best way to calculate the length of the lamella. It is intended that another design will be created that does not assume a set b (baffle spacing) value.

The same tank size was used for the lamella design as was used for the floc blanket design. The lamella were designed to be constructed in a grid because a material with a set baffle spacing of 11mm was thought a possible material. In this suggested grid set-up a series of channels of 11mm by 7mm in cross section would be set into the tank, this was done to ensure that a V _up of close to 100 m/day was met. If the lamella were adjusted to be parallel plates that extended the width of the channel (the current AguaClara design) then the baffles would have to placed much closer to each other to guarantee a V _up of 100 m/day.

The inlet design, the effluent launder design, tank drain design will all be the same as for the floc blanket sedimentation tank. The only difference is that the inlet will have to enter the tank below the lamella and thus two 22.5 degree angle bends are to be used to lower the inlet pipe level from where it exits the floc tank to where it needs to enter the sedimentation tank. This tank was designed to be versatile and the lamella placed close enough to the top of the tank to allow for the possibility that a floc blanket could be formed underneath the lamella if desired.

Grid Lamella Design with Set Spacing

Lamella Design
The lamella were designed out of corrugated plastic. Flow was designed to move upward through the grid of channels in parallel. The dimensions of the material proposed for use are listed below.

• b = 10.5 mm - the distance between corrugations
• w = 7 mm - the width of the channel (same as the width of the material)
• thickness = 0.40 mm - the thickness of the plastic material
• α = 60 deg

The first step in the lamella design was to determine the size of the lamella needed. A method very similar to the one used to determine the correct pipe size for the effluent launder was used to find length of the lamella. The equation for lamella length is implicit and thus the correct length was iteratively determined.
The implicit equation used can be seen below:

The following two equations were substituted into the above main equation.
Number of lamella sheets needs to fill the tank:

The number of cells in on sheet of lamella given the tank size:

The target V _up value was 100 m/day. This was left constant instead of being updated with each iteration. This was done because the program would not converge otherwise. After the program returned the converging value of Lamella Length, the active upward velocity and critical upward velocity needed to be calculated. The active upward velocity is different from the target upward velocity because the dead zone created from the lamella angle has to be taken into account. The active length of Lamella, active upward velocity and critical velocity were calculated using the following equations:

The initial program of used to determine the lamella length returned the total length of the material placed at an angle of 60 degrees. For construction uses and determination of the inlet position we needed to determine the vertical height of the lamella. For this calculation the follow was used:

Plate Lamella Design

It is important to meet two major parameters in the design of sedimentation tanks. The upward velocity at the bottom of the tank is important for potential sludge blanket formation and most likely should be between 70 and 120 m/day. The critical velocity up through the lamella should be around 10 m/day. A design algorithm with the lamella modeled as plates allowed for us to acurately meet both parameters by calculating the spacing between the lamella. This allowed for a more accurate design than the grid pattern of lamella allowed for. Another problem with the grid design of lamella was the larger inactive area. Because of the above two issues the grid design was discarded. The tank design for the plates can be found below.

This design was based off of the sedimentation tank design that completed for full size tanks built in Honduras. The inlet into the tank was the same design as the sludge blanket tank with the exception that the inlet included at 45 degree angle and a long radius 90 degree elbow in the connection from the floc tank to the sedimentation tank. The sludge manifold at the bottom of the tank was the same design but centered for more even sludge removal and the holes were enlarged to allow for faster drainage.