Sedimentation Tank Design & Construction (Spring 2008)
Design Objective
The design objective of the sedimentation tank is to create an open-to-atmosphere tank that achieves similar settling performance as the existing closed tank, without the need for the level control weir, that is easy to clean and maintain.
Theoretical Design
Since the sedimentation tank is open to the atmosphere, its height depends on the water level in the last channel of the flocculator. The lamellas are at an angle of 60° from horizontal to allow flocs to slide down along the lamella surfaces. The critical settling velocity of the sedimentation tank is set at 0.6 cm/s, which is within the literature range of 10 m/day to 20 m/day.
The lamellas are to be made from corrugated plastic and therefore have fixed dimensions. The number of lamellas is determined by the critical settling velocity, by the following formulas:
All the calculations were done in a MathCAD program.
Theoretical Design Results
It was found that the lamellas need to be 29 cm long, and that 13 lamellas are required to achieve the desired upflow velocity.
Physical Design
Open Top Design
This design will be very similar to the Summer 2007 design, with the exception that the top of the sedimentation tank will be open to the atmosphere. Due to this adjustment, the weir will no longer be necessary for plant water level control. Also, the sedimentation tank will now need to be almost as tall as the flocculator, to prevent overflowing. The water levels in the flocculator and sedimentation tank will be approximately the same.
Perpendicular Open Top Design
This sedimentation tank design will address the problem of uneven flow lengths, rates, and shears through each lamella of the sedimentation tank in the Summer 2007 Demo Plant. This problem is further described in the Summer 2007 Sedimentation Tank Design and Construction page.
The proposed sedimentation tank will be oriented perpendicular to the axis of the flocculator. In order to eliminate differences in lengths of flow paths through the sedimentation tank, water exiting the flocculator will be split into several tubes of equal diameters and lengths, which will enter the bottom of the sedimentation tank. The tubes entering the sedimentation tank will be evenly spaced.
The sedimentation tank itself will be redesigned so that lamella still form the optimal 60 degree angle. However, the lamella will now guide the water in a path parallel to the axis of the flocculator, so that the whole tank will have an incline, and will need to be either very bottom-heavy, or have a support for the upper ends of the lamella. This design, along with the new entry method, will normalize the head loss through individual channels in the sedimentation tank.
The proposed sedimentation tank will also incorporate an open-top design, similar to the flocculator of the Fall 2007 Demo Plant.
This design also allows us to consider the incorporation of a drain for built up sediments in the sedimentation tank, adding the functionality to clean the sedimentation tank while the plant is still running.
Figure 1. AutoCAD rendering of Perpendicular open top sedimentation tank
Open Top Parallel Lamella Design
This design is very similar to the first Open Top Design, but it will take into account the varied flow rates observed throughout the lamellas of the sedimentation tank. Instead of one row of lamellas, we will have two parallel sets of lamellas. Each set will have 7 lamellas, resulting in a total of 14 lamellas on the entire sedimentation tank.
Construction
This semester, we constructed two sedimentation tanks based on the Open Top Design and the Open Top Parallel Lamella Design. Most of the construction work was performed by Paul Charles of the Civil and Environmental Engineering Machine Shop.
Materials
In building the sedimentation tanks, we used materials similar to those used in previous plants. For the section of the tank containing lamellas, we used readily available corrugated plastic with 0.7 cm x 1 cm channels. The water entry section on the bottom of the sedimentation tank was made out of a 2" diameter clear acrylic rod. The Open Top Design also required a water collection section on the top, which was also made out of transparent acrylic. The Parallel Lamella Design required a double tap for the top of the tank, which was made out of acrylic as well. The plug at the end of the tank furthest from the flocculator was made of gray PVC.
The stands for the sedimentation tanks were made from thin acrylic slabs and a thumb screw.
Description of Construction Process
Open Top Design
For construction of the Open Top Design sedimentation tank, we reused materials from a previous duplicate construction of the Closed Top Sedimentation Tank from Summer 2007. To do this, we took apart the older sedimentation tank and reused the acrylic pieces on the top and bottom. The upper piece was further machined to make an oval-shaped hole in the top, resulting in a reservoir that was open to the atmosphere. The lamella section in the middle was made out of the same corrugated plastic material, but had to be constructed from scratch because we needed longer lamellas to match the height of the flocculator. To do this, we simply cut a diagonal section of the corrugated plastic sheet to the dimensions needed. The lamella sections were cut from a sheet of the same corrugated plastic that was used for the flocculator.
Parallel Lamella Design
For construction of this sedimentation tank, we could not reuse materials from previous models, since this design required pieces with different dimensions.
The acrylic section on the bottom was made from an acrylic rod. A hole was bored into the center of the rod by the Machine shop, resulting in a acrylic pipe. Then, two long oval holes were drilled out of one side to fit the lamella. The lamella sections were cut from a sheet of the same corrugated plastic that was used for the flocculator. The upper 3 cm (vertical height) of each channel baffle was removed, and holes were tapped into the sides 2.5 cm from the top of the tank. A square acrylic piece was machined with two holes on one side, joining into one hole into which a pipe fitting was inserted. This piece was joined to the two holes in the side of the sedimentation tank to serve as the water output pipe.
Sedimentation Tank Stands
The stands for the sedimentation tanks were made by drilling 1 inch holes in thin acrylic slabs. A hole was then tapped from the top of each stand to allow a thumb screw to be inserted to keep the tank from rotating around the cylindrical acrylic bottom.
Pictures of Finished Product
Open Top Design
Figure 2. Open top sedimentation tank
Parallel Lamella Design
Figure 3. Parallel lamella open top sedimentation tank
Post-Construction Modifications
Open Top Design
In the Open Top Design sedimentation tank, we found that flow was unequal through each of the lamella. Water tended to flow through the first 3 lamella at a very fast rate, through the next 3 lamella at a slow rate, and through the last ones at a very slow rate, or not at all. This was determined by adding red dye to the last column of the flocculator, and observing the movement of the dye through the sedimentation tank.
To rectify this, we designed the Parallel Lamella Design sedimentation tank, and had it constructed by Paul Charles of the Civil and Environmental Engineering Machine Shop.
Parallel Lamella Design
In the Parallel Lamella Design, we found that flow through the sedimentation tank was highly dependent on the angle at which the lamellas were oriented. Due to imperfections in the construction process, the flow through each set of lamella was uneven when the tank was exactly perpendicular to the table surface (parallel to plane of flocculator).
To rectify this problem, we varied the angle of the sedimentation tank while observing its effects on the flow rate in each set of lamellas. We found that a slight angle (with respect to the plane of the flocculator) was necessary to achieve equal flow in both sets of parallel lamellas. We hypothesized that this was due to very slight differences in the height of the orifice through which water exited each set of lamellas, which had a significant affect on the amount of head loss experienced by water flowing through each side of the sedimentation tank. By changing the angle of the whole tank, we solved this problem.