Flocculator Tank Construction History

Spring 2007

Design

The construction of the tank was started during the Spring 2007 semester. The floc tank was designed to be contained in a polyethylene tank of dimensions 182.9 cm × 91.4 cm × 121.9 cm (length × width × height) with a wall thickness of about 0.8 cm.#tankThe design goal was to divide the tank into 3 separate sections, basically condensing a long, narrow flocculation tank into a more compact space by snaking the flow back and forth. The initial design divided the total minimum mixing value (20,000) evenly among the three sections, with each section having an even velocity gradient (G) of 45 s-1.

In order to divide the tank into three sections, a divider system had to be built in the tank. Originally it was planned to purchase two plastic sheets to function as the dividers and weld them vertically into the tank. This option was soon rejected due to two major concerns: difficulties of welding inside the cramped spaces of the tank, and lack of strength in the welds. After much contemplation, the maintenance shop proposed a plan to build a structure that would provide support and flexibility. The sketch shows that the #dividerswere welded onto a base slab and the completed module was placed into the tank. A port hole (hole through which water flows between sections) was cut in each divider prior to welding. The dimensions of the dividers are approximately 182.9 cm x 121.9 cm with a 0.6 cm thickness. The choice of material for the dividers as well as the base slab is high-density polyethylene and was specifically chosen for its non-reactive property in water treatment process, and most importantly its ability to be welded.
Below is a list of fixed parameters (or "givens") and the values of G to be used in the initial setup.

Givens:

• Tank dimensions: 182.9 cm × 91.4 cm × 121.9 cm
• Tank wall thickness: 0.8 cm
• Tank divided into 3 sections (serpentine flow path)
• Total minimum mixing value (Gθ) = 20,000
• Initially 1st, 2nd, and 3rd sections of tank to have velocity gradient (G) of 45 s-1

Installation of Tank

Constant communication with the Cornell Water Treatment Plant was needed to attempt to integrate the pilot plant into their facility. Some modifications was necessary after transportation and set up in the tank at the plant. A frame was fabricated to hold the rapid mix in place on the side of the tank. The inlet pipe had to be redirected to fit into the rapid mix pipe. Directing the inlet pipe directly straight down into the top of the rapid mix pipe would give us the maximum flow from that pipe. The outlet pipe was reconstructed to prevent leakage. Water from the treatment plant is directed to us before treatment. After the water is treated in our system the effluent exits the tank and reenters the treatment plant at the same point through a nearby drain.

Spring 2008

The first half of the Spring 2008 semester was spent making significant modifications to the flocculation tank. When the tapered baffle configuration was installed in the flocculator the baffles around the port holes were set farther apart than in the original configuration and thus the port holes had to be enlarged to ensure flocs were not being broken up. The holes were enlarged so that the port hole area was equal to the cross-sectional area of water below a top baffle. Sludge that had settled to the bottom of the tank was removed and tank was cleaned before it was started up for the first time that semester.

Additional sand was added to the bottom of the tank before the baffles were placed in. The purpose of the sand was to block or greatly reduce flow in the space between the divider module and tank wall. It was also desired that the sand reduce bypass of water underneath the baffles. When the tank was turned on it was determined that a significant amount of flow was still by-passing the system and flowing under the divider module, despite the several inches of sand that were added to the third section. It was noticed when the tank was filled that that the first and third sections were filling at almost the same rate. Before caulking was done additional sand was added to the bottom of the tank in the third section to try and see if significant enough head loss could be created in the tank to cause most of inflow to follow the correct flow path. Upon refilling the tank the upflow was still noticed. A clear PVC tube was inserted into the gap and the difference in water height in the tube and in the tank was measured. It was found to be about ¾". This was deemed significant, however caulking was still deemed necessary. The tank was drained, cleaned and cleared of sand again so that caulk could be added to seal the gap.

Summer 2008

Several changes were implemented while the tank was being caulked. A digital flow meter was added to the inlet pipe. This meter was put in place for several purposes. First, it was added to make it easier to alter the plant flow rate without draining and refilling the tank to determine the flow rate. Secondly it was added to provide instant feedback to the flow measuring bucket that was added to the inlet of the tank.

The structural changes needed to be implemented the flow measuring bucket were also implemented while the tank was being caulked. These changes included building a brace and stand for the bucket to sit over the tank, above the first baffle. In order to accommodate the bucket the rapid mix tube had to be cut and extended so that instead of flowing directly into the tank it flowed into the bucket and then the bucket's weir deposited water into the tank. Alum inlet also had to be extended upward because after the rapid mix extension the alum was entering lower than the raw water inlet pipe and this could cause back flow down the alum tube not to mention leaking in this part of the rapid mix set-up that was not glued down. The raw water inlet after structural modifications were made to the rapid mix tube and bucket was in place.

The final modification that was made in the tank was to add a submersible light to the third section. This florescent light was added to allow us to be able to see flocs in the water. The light was attached to a pole and then suspended a few feet below the surface of the water. This light was held in place by two ring stand clamps attached to the light pole and to two separate bracing poles that rested across the section dividers.