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 33.25'' 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 exceeds 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.  

Sludge Blanket 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 sedimentation 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 sedimentation tank to avoid 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 24.5 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) = 33.25"
• 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 by 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. We allowed our design velocity to exceed 100 m/day with the idea that we will probably have to modify the flow after construction to allow a stable sludge blanket to form.

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 that can be seen above.

Where epsilon is the represented by the following equation.

Insert epsilon equation here.

Insert head loss equation here.

The head loss substituted into this equation is assumed to be just minor head loss. 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 through the pipe is represented 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 the cross sectional area of the pipe times 2 diameters of the pipe.

Initially when this equation was solved, we used 55 L/min as the plant flow rate, and MathCAD returned that the pipe diameter would be 5.11 inches to achieve the desired Gav value. 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 24.5 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 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 4in off of center of the tank. The launder is placed 2 in beneath the water level. This value was assumed to be constant to allow for uniform flow through all of the launder orifaces. The diameter of the effluent launderer was calculated by iterating through pipe diameters to find the diameter pipe that matched up with the flow rate. The diameter was selected based on the following equation:

The best pipe diameter is 1.5 inches. The number of orifice holes was chosen so that the hole diameter would match up with a drill bit size while still giving the proper about of head. 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 assumed to be 15.

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 0.63, 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 19/64". This value was rounded to the next smallest drill bit size.

The head loss due to the orifice ends up being an important parameter because it effects how evenly the water will flow through the orifices. This quality is quantified in the parameter Q ratio, which is the ratio of flow through the first orifice divided by the ratio of flow through the last orifice. We have set the value of Q ratio to be 0.90.

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

D is the diameter of the orifice (21.6 cm), 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 4.55 cm. 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 24.5 L/min, but the plant flow rate is designed to be 100 L/min. This means that excess flow needs to bypass the sedimentation tank and go directly to the outlet. A large outlet weir is our proposed design for the alternative exit. Because the head loss out of the sedimentation tank is so high (44.979cm) the height of the water in the tank versus the weir height is assumed negligible(on the order of mm).

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 Cw is a constant (equal to 0.611+0.075*(H/Pw) 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 sed flow rate, and the available perimeter was set to be 120 cm. 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 the cut out, down into the trough and then out the existing 3" outlet. When this was calculated we got a water height above the weir of 2.12 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
Given the amount of sludge that was calculated the tank would 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. 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 have decided to create 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.

Plant Leveling Tank
The Plant leveling tank will handle the outflow from both the sludge blanket tank and the plate settler tank. Using one leveling tank and identical inlet and outlet piping for the two tank ensures identical flow (assuming that the head loss through the tanks themselves are negligible). We decided to make the head loss into the leveling tank the dominant head loss through the system, which then sets the flow rate through the sed tanks. Our design for the outflow system was an elevated 3'' pipe with an orifice set into the pipe below the water level. Using the orifice equation, with the flow set at 24.5 L/min, and a head loss found by multiplying the floc tank head variability by a predetermined safety factor of 12, we could find the required orifice size.

Results

The first sedimentation tank containing the sedimentation tank has been constructed. Flocs are entering the tank and not breaking up upon entrance so it has been determined that our entrance pipe calculations are reliable. Over the next week we are observing the tank and trying to see how the sludge blanket forms.

Conclusions