Sedimentation Tank Design

Introduction

The pilot plant sedimentation tank is a vertical flow style tank and uses only a floc blanket as its removal mechanism. This tank was the first close to full scale sludge blanket sedimentation tank that the AguaClara team has constructed. As with other AguaClara designs, the tank will be run by the elevation head in the flocculator tank and will not require electricity. There are several design restraints due to the current set up at the pilot plant. There was 33.25" of available water head at the end of the flocculator to power the flocculator. The piping connection between the flocculator tank and sedimentation tank also cannot have a shear value that exceeds the shear in the last baffle section ( G _cell = 24 /s), or else the flocs made in the last section will be broken up upon entrance to the sedimentation tank. The design goal was to have enough area in the tank to create a floc blanket and an upward velocity of 100m/day. 100m/day is the upward velocity of full scale plants in Honduras, keeping this parameter the same made the two designs comparable. This low flow and smaller tank set-up allowed for parallel testing of tanks containing different sedimentation processes.

Below is a table of the calculated pipe dimensions and other calculated design parameters. The method behind each number can be found in the following design.

Given Variables:

• Length (L) = 58.7 cm (23.125")
• Width (W) = 58.7 cm (23.125")
• Height (H) = 91.4 cm (36")
• Water Level (WL) = 84.5 cm (33.25")
• Flow Rate (Q) = 24 L/min

Upward Velocity:
From the reasoning shown here, the flow rate was determined to be 24 L/min.

Inlet Pipe Calculations:
Through the calculations found here, the inlet pipe was determined to be 4" in diameter.

Launder Calculations:

Pipe Diameter
The effluent launder will span the length of the tank about 10.16cm (4") off of the center of the tank. The launder is placed 5.08cm (2") beneath the water level. The best pipe diameter is 3.81cm (1.5") inches. The total number of orifices was determined to be 15. Calculations can be found here.

Orifice Diameter
The Orfice diameter was chosen to be (0.7541cm or 19/64"). Calculations shown here.

Orifice Head Loss
The minor loss for an orifice was calculated to be 4.54 cm. Calulations are shown here.

Secondary Outlet from the Flocculator
The flow rate of the sedimentation tank is designed to be 24 L/min, but the flocculator flow rate is designed to be as high as 110 L/min. This means that excess flow needs to bypass the sedimentation tank and go directly to the existing outlet in the flocculation tank. The calculations for the solution can be found here.

Hopper Design for Floc Blanket
Given the calculated amount of sludge the tank will create, a continuous sludge drainage system was created. More details are found here.

Plant Leveling Tank
In order to ensure this the head loss out of the plant leveling tank (which controls flow in the sedimentation tank) had to be significantly greater than the head loss over the alternative exit in the flocculator. We chose a factor of safety of 18, thus the head loss out of the plant leveling tank would be 18 times the head loss out of the flocculator.More details are included here.

Tank Drainage Manifold
A tank drain system was designed to drain in 30 minutes (Q ~drain~ = 20 L/min). The manifold diameter was determined to be 1.905cm (0.75") and the orifice diameter is 0.3572cm (9/64"). The hole size in the drain manifold was increased to 1/4" to allow for faster tank drainage. Here are more details.

Lamella
Lamella Design