Pilot Plant- Sedimentation Tank

Abstract

A sedimentation tank is planned to be connected in series to the flocculator at the pilot plant. This sedimentation tank will allow us to isolate the process in the AguaClara design that needs to be optimized to allow us to achieve our overriding goal of consistently producing water under 1 NTU. Building this sedimentation tank allows for us to test not only the flocculation process but the sedimentation tank for design efficiency. Testing will include not only the traditional lamella design of sedimentation tanks but also the alternative of sludge blanket based sedimentation tank designs. These two processes will be tested side by side in parallel flow sedimentation tanks.

Introduction

The pilot plant sedimentation tank will be a vertical flow style tank and will only use a sludge blanket as its removal mechanism. This tank will be the first time that an AguaClara team has used a sludge blanket instead of plate settlers. 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 are 37'' of available water head at the end of the floculator which are available for our use. The piping connection between the flocculator tank and sedimentation tank also cannot have a shear value that excedes the max shear in the last baffle section ( GMax= 48.826 /s), or else the flocs made in the last section will be broken up.

The flow through the sedimentation tank has been limited to at most half of the flocculator tank flow so that it will be possible to construct a second tank sometime in the future to handle the other half of the flow. ideally this second tank would only use plate settlers so that we could make a comparison between a sludge blanket and plate settlers.  

Sedimentation Tank Design

The design of the tank was done during the Spring 2008 semester. The sedimentation tank was designed to be contained in a polyethylene tank of dimensions 24" x 24" x 36" (length × width × height) with a wall thickness of about 5/16". The design goal was to have enough area in the tank to create a sludge blanket. With the available water level height, which is taken directly from the water level in the floc tank, we did not have enough room to include plate settlers. Initially we wanted to split the plant flow rate (110 L/min) in half, allowing us the possibility of installing two parallel sed tanks for experimentation purposes, but we found that a flow rate of 55 L/min into the tank would require a 6 inch pipe to transport water from the floc tank to the sed tank without breaking up flocs. Due to cost restraints (a six in bulk head fitting would cost about $300) we had to limit this pipe to being 4 inches, and we did that by lowering the flow rate of the sedimentation tank down to 30 L/min. This constraint on the inlet pipe and flow rate required us to switch to a smaller tank than originally planned. This change was necessary to obtain an upward velocity of 100m/day. This low flow and smaller tank set-up will allow for parallel testing of tanks containing different sedimentation processes.

Given Variables:

• Length (L) = 23.125"
• Width (W) = 23.125"
• Height (H) = 36"
• Water Level (WL) = 30.5"
• Flow Rate (Q) = 30L/min

Below is an AutoCAD drawing of the proposed sedimentation tank design.

!Pilot Plant Construction and Research^CAD drawing of sed
tank.jpg!

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 Calculation:
To determine the upward velocity of the sedimentation tank the flow rate through the tank was divided be the cross sectional area of the tank. In order to make our model comparable to the sedimentation tanks that are built on a full scale the upward velocity needs to be the same. The upward velocity in Ojojona was found to be 100m/day. Thus this was the parameter we used for this model as well.

Insert Upward Velocity Equation.

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 Gav in the last section of the flocculator (Gav = 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 a pipe elbow is shown below. This equation is solved for D, the diameter of the pipe that would provide shear equal to Gav. 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. the original equation used was:

Insert Gbar = sqrt(epsilon/nu) equation here.

Where epsilon is the represented by the following equation.

Insert epsilon equation here.

Insert head loss equation here.

The head loss found in this equation is assumed to be just minor head loss. The minor head loss for this is assumed to be the dominationg factor because the pipe is designed to be relatively short in length and the bends and exit are our major concern for floc break up.

Velocity through the pipe is represented by Q/A for the pipe.

The residence time is found to be the volume over the flow rate through that volume. The volume used was assumed to be 2*D times the cross sectional area of the pipe, where D is the diameter of the pipe. (???? what this was we used????)

Initially when we solved this, we used 55 L/min as the plant flow rate, and MathCAD returned that the pipe diameter would be 5.11 inches to achieve shear equal to Gav. Due to the cost of 6 inch 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 4 inches was achieved. We found that the maximum flow for these conditions is 30 L/min, so we changed the flow rate of the plant. The option of having two 4" inlets was considered but this was discarded because then 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 launderer will span the length of the tank at the center of the width. The launderer is placed 20cm beneath the top of the water level, which is taken from a constant in the launderer MathCAD equations. The diameter of the effluent launderer was calculated by iterating through pipe diameters to find the diameter pipe that matched with the flow rate. The diameter was selected based on the following equation:

The best pipe diameter is 2 inches. The number of orifice holes in the pipe is equal the the length of the tank divided by the lamina spacing. We used a lamina spacing of 2.5cm, which is what would have been used for our plant if we had enough height to use them. Therefore, the total number of orifices is 45.

Orifice Diameter
The diameter of the orifices was calculated by finding the necessary area of the orifice based on the head at the launder, the minor loss of the orifice, and the flow rate through the orifice (plant flow rate divided by number of orifices). The equation to find the area is found below. With the area, the diameter of the orifice can easily be calculated by solving the area of a circle for the diameter. The minor loss coefficient of an orifice is 0.62, and the head, h, is equal to the height of the sed tank minus head loss through the launder minus the velocity head through the orifice. The diameter of each orifice was figured out to be .285 cm.

The head loss due to the orifice ends up being an important parameter because it affects how much water goes into the sedimentation tank, and how much bypasses it. We decided that we want a head loss of an order of magnitude or greater than the headloss in the weir at the end of the floc tank because this will minimize the effects if the flow through the floc tank changes. This will be explained further below.

Orifice Head Loss
The head loss through the orifice was calculated by using the following equation:

D is the diameter of the orifice (.285cm), 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 21.5 cm, a very high head loss. The head loss through an orifice is parallel to other orifices, so you do not add the head loss in each orifice. This parameter is the level that the water in the plant leveling tank needs to be below the water level in the sed tank in order for our plant to be operated at the designed water levels.

Secondary Outlet from the Flocculator
The flow rate of the sedimentation tank is designed to be 30 L/min, but the plant flow rate is designed to be 110 L/min. This means that 80 L/min needs to bypass the sedimentation tank and go directly to the outlet water. A large outlet weir is our proposed design for the alternative exit. Because the head loss out of the sedimentation tank is so high (21.5cm) the height of the water in the tank versus the weir height is assumed negligible.

It was important to check whether the height above the weir would be substantial enough that it would effect the flow rate of the sedimentation to tank. To do that we found an equation online that describes the water height above the weir.

Where C is a constant (taken to be 3 feet^1.5/s), Qweir is the plant flow rate minus the sed flow rate, and the diameter of the weir was set to 6 inches (we wanted a big diameter, so there was a larger circumference for the water to interact with). When this was calculated we got a height of 0.68 cm, 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 sed tank.

Hopper Design for Sludge Blanket
The volume of the sludge hopper should be big enough that it takes 48 hours to fill up. We created a parameter, PhiFloc, which is the ratio of how much the alum flow rate relates to the the volume of flocs created in a certain amount of time. The value (0.004) was taken from the mathCAD file used to create the ideal Gmax versus Gtheta curve. The majority of a sludge is assumed to be made of alum, and the flow rate of alum is 20 mL/min. Therefore, the rate of sludge build-up is the flow rate of alum times the ratio of the sed flow rate to the plant flow rate times PhiFloc. When calculated, the sludge rate was determined to be 3.429 x 10^-3 L/min. Therefore, the minimum hopper volume should be equal to the sludge rate times 48 hours, or 99 cm^2. We are still considering what will be the best way to build this volume in the tank, but possibilities are having a conical shaped bucket for sludge to fall into, or putting a lamina at an angle from one of the bottom sides.

Results

Currently our design is under review, with the hope of construction in the upcoming weeks.

Conclusions