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:
The upward velocity parameter determines what size of floc particles are removed from the sedimentation tank. Thus in order for our design to be comparable to existing sedimentation tank design V _up needs to be the same. V _up was set to be 100 m/day. From this we calculated the flow rate through the tank to achieve this V _up.
Given that there are no lamella the calculations for this are fairly straight forward.

From this equation the flow rate was determined to be 24 L/min.

Inlet Pipe Calculations:
To determine the diameter of the inlet pipe we used the constraint that the maximum shear due to minor losses had to be less than G _cell in the last section of the flocculator (G _cell = 24/s). The predominant minor losses from the inlet pipe will occur at the exit point and the elbow. The minor loss coefficient for the elbow is 0.9 and the exit is 1.0 so K was set to be 1.0 for this design. The equation to calculate shear due to minor losses in a pipe is shown below. This equation is solved for D, the diameter of the pipe that would provide shear equal to G _cell. The equation used is very similar to the equation used to find the baffle spacing for the flocculator.

Variables:
K = 1.0 (minor loss for an exit)

• This equation was derived from a series of substitutions that can be seen above.
• The head loss term that is found in the above derivation is assumed to include only minor losses. The minor head loss for this pipe is assumed to be the dominating factor because the pipe is designed to be relatively short in length and the bends and exit are our major sources of shear, the primary locations where floc break up would occur.
• The velocity equals Q/A for the pipe.
• The residence time term is found to be the volume over the flow rate through that volume.
• The volume used was assumed to be the cross sectional area of the pipe times 2 diameters of the pipe.

Note: Initially during preliminary design with a large tank, we were using 55 L/min as the plant flow rate, and the required pipe diameter was 5.11 to achieve the desired G _cell value. Due to the cost of 15.24cm (6") bulkhead fittings (nearly $300), we had to find an alternative design. We decided to lower the flow until a pipe with inlet diameter equal to 10.16cm (4") was achieved. We found that the maximum flow for these conditions is 24 L/min, so we changed the flow rate of the sedimentation tank. The option of multiple inlets was considered but this was discarded because then 2 - 4 bulk head fittings would be needed. Thus, the overall flow of the tank was lowered and a smaller tank was chosen.

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. This value was chosen so that enough water head would be available to allow for relativley equal flow through the lauder orifices and so that a majority of the tank height would be available for the floc blanket. The diameter of the effluent launder was calculated by iterating through pipe diameters to find the existing diameter of pipe that gave the desired flow rate and orifice holes. The diameter was selected based on the following equation:

• Qratio is assumed to be 0.92, this indicates that flow through the first orifice will be 92% of the flow through the last orifice.
  The best pipe diameter is 3.81cm (1.5") inches. The number of orifice holes was tweaked until the orifice hole diameter would match up with a drill bit size and still gave the proper amount of head. (equations for this can be seen in the following section) This head loss was defined to be about 5 cm, or the head available above the launder. If the head loss through the launder orifices exceeded the available head, we would have encountered issues because the water in the outlet system would have had less energy than the surrounding system, which would have effected the flow rate and caused air to collect in the outlet pipe. The total number of orifices was determined to be 15.

Orifice Diameter
The orifice equation was used to determine the necessary area of the orifice that gives the correct amount of head and matches with an existing drill bit size. The flow rate used in this equation was the total plant flow rate divided by the number of orifices. The minor loss coefficient for an orifice is assumed to be 0.63. The head loss term in the equation is total head available less the head loss in the manifold pipe less the velocity component of head loss through the pipe.
This area was then converted into a diameter and rounded to the next smallest drill bit size (0.7541cm or 19/64").

Orifice Head Loss
The head loss due to the orifice is an important parameter because it effects how evenly the water will flow through the orifices. Thus we back calculated the actual head loss achieved given the diameter adjustment that was made to achieve an available drill bit size.
The head loss through the orifice was calculated by using the following equation:

D is the diameter of the orifice (0.7541cm or 19/64"), the minor loss coefficient of the orifice is 0.63, and the flow is the flow through one of the orifices. The minor loss for an orifice was calculated to be 4.54 cm. The head loss through an orifice is parallel to other orifices, so you do not add the head loss in each orifice. So the water level in the plant leveling tank will be approximately 4.54 cm since the launder orifice head loss will be significantly larger then the losses in the piping system between the two tanks.

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.