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 an AutoCAD drawing of the proposed sedimentation tank design.

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.

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. A large outlet weir is our proposed design for this alternative exit. Because the head loss out of the sedimentation tank is relatively high (5 cm through the manifold and 13.7 cm out of the plant leveling tank) a small height variation (on the order of mm) of the water in the floc tank will have a negligible effect on the sedimentation tank flow rate.

Since the flow rate into the floc tank has been measured to be anywhere between 70 L/min to 110 L/min, It was important to check whether the height of water in the floc tank could change enough to effect the flow rate to the sedimentation tank. If the accessory weir is required to take in a smaller flow rate, the water height in the floc tank will lower. This is the equation that describes the water height above the weir as a function of the flow rate.

Where

where H is the height of water above the weir and Pw is the weir height), Q weir is the plant flow rate minus the sedimentation flow rate, and the available perimeter was set to be 120 cm. This perimeter values was chosen arbitrarily to attain a factor of safety of about 10 between the head loss coming out of the sedimentation tank and a head loss over the alternative weir in the flocculation tank. A factor of ten was not feasible because this would have required the head loss over the alternative weir to be 0.5cm which was not feasible. At 120 cm the head loss is 0.75cm. In order to minimize changes in the floc tank, the perimeter will be established by cutting a pipe laterally and letting water flow over the cut out, down into the trough and then out the existing 7.62cm (3") outlet. When this was calculated we got a water height above the weir of 7.6 mm, which is small enough when compared to the head loss in the launder to assume that it will not affect the flow rate into the sedimentation tank.

Hopper Design for Floc Blanket
Given the calculated amount of sludge the tank will create, a continuous sludge drainage system was created. This was done to decrease the size of the hopper in the tank. It was desired to keep the hopper small so that there would be minimal disturbance in the tank. φ _floc is the specific volume of the floc, it was found to be 0.016. Most of the sludge produced is assumed to be made of alum. The alum concentration is approximately 10 mg/L. This alum concentration is used in the φ _floc determination. The rate of sludge build-up is the sedimentation tank flow rate times φ _floc.

When calculated, the sludge volume accumulated over two days was determined to be 1.105 x 10^3 L. We created this hopper with a conical funnel with flex-hose connected at the bottom. This small funnel will continuously drain sludge out of the tank and into the outlet panel. The hose will be 2m long and 1.27cm (1/2") in diameter. The diameter was determined using the manifold equation used in the design of the effluent launder design. If no sludge consolidation occurs, then the flow rate for the sludge drain must equal φ _{floc~Q ~tank}, or approximately 380 mL/min.

Plant Leveling Tank

The plant leveling tank will handle the outflow from both the floc blanket tank and the plate settler tank. Using one leveling tank and identical inlet and outlet piping for the two tanks ensures identical flow (assuming that differences in the loss coefficients through the tanks themselves are negligible). We decided to make the head loss through the leveling tank the dominant head loss through the system, which then sets the flow rate through the sedimentation tanks. Our design for the outflow system was a surface piercing 7.62cm (3") pipe with an orifice cut into the pipe below the water level. It was desired that the flow rate through the sedimentation tank would remain constant at 24 L/min no matter the flow rate coming into the flocculator (as long as it was at least 24 L/min). 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. the depth of the orifice hole beneath the water surface was determined to be 13.673cm (18 times the head loss over the flocculator outlet weir). Then using the orifice equation, with the flow set at 24 L/min, this head was used to determine the orifice size (2.223cm or 7/8"). The orifice equation used was the same as used for the flocculator outlet but converted to a circular perimeter from a rectangle.

Tank Drainage Manifold

A tank drain system was designed to drain in 30 minutes (Q ~drain~ = 20 L/min. The design is similar to the effluent launder except this manifold is located at the bottom of the tank.

By the same method as was used for the tank effluent manifold the manifold diameter was determined to be 1.905cm (0.75") and the orifice diameter is 0.3572cm (9/64").
* The available head for the sludge ports is the total water depth (84.455cm or 33.25") minus the head loss in the manifold and minus the velocity head at the end of the manifold.
* Kminor was assumed to be 0 in the manifold and 1.0 for the pipe entrances and exits.
* The constant for the determining orifice diameter was found to be 0.63
*Qratio = 0.90
* 15 orifices were assumed to provide a similar layout to the effluent manifold.
The hole size in the drain manifold was increased to 1/4" to allow for faster tank drainage.

Lamella
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