Marcala Plant Design

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This figure depicts a general top view of the layout of the Marcala plant. Only the baffle spacing for flocculation tanks C and D were calculated due to anticipated changes to the current flocculator design program. The spacing in tanks A and B are areas with more variation in potential baffle spacing's and since the baffles are not removable, the calculations will be delayed for these two sections. Due to the many constraints in the flocculation tank including the sloping floor and the cement beams under which it was preferred not to build, the spacing of the baffles was a unique challenge.

The spacing between the baffles in flocculation tanks C and D was calculated using Monroe Weber-Shirk's flocculator design program. Important care must ensure that the baffles are the correct spacing to keep the flocs from breaking apart. The results of the MathCAD program were slightly modified through the two tanks to accommodate the unique tanks. Additionally, the thickness of the ferrous cement baffles themselves had to be considered in the design. Each ferrous cement baffle will be approximately 3.8 cm in thickness.

The flocculator design program calculates the optimal open space between the baffles to be 54.6cm wide. However, due to the design constraint of not being able to put the baffles under an 8cm wide ceiling beam that runs horizontally approximately over the middle of tanks C and D, this spacing was not used for all of the baffles. The flow path between baffles in the beginning of tank C is 92 cm across and the open space between baffles is 51.75 cm wide, just over 3cm closer together than what was recommended. The velocity of the water is determined by the open space between the baffles multiplied by the flow rate, so this smaller distance causes the water to move faster than is recommended through this section of the flocculator. Greater velocities can cause flocs to break up, but since there will be a lot more flocculation occurring after this section with the closer baffles, any flocs that break up should clump together again as they move further along the flocculator. The spacing in this first section of 55.25cm when measured from the center of one baffle to the center of the next baffle (on center), and thus taking the baffle thickness into account, keeps any of the baffles from being under the ceiling beam. All the baffles further down the flocculator must be spaced with at least 54.6cm of open space between them so that the velocity does not increase thus breaking up more flocs. The baffles on the other side of the ceiling beam in tank C have the program recommended open spacing of 54.6 cm. The open space measurements were used for the calculation of the velocity because only the area where the water can go matters.

The baffles in tank D all have an open spacing between the baffles of 66.2 cm wide, with an on center spacing of 70cm. Since the open space is greater than that recommended by the flocculator design program, only distances which include the baffle thickness matter for the placement of baffles in tank D. A spacing of 70cm on center keeps all the baffles out from under the ceiling beam that was discussed previously. The turn from tank C to tank D is treated as a spacing between baffles, thus since the last baffle in tank C has the water flowing under it, the first baffle in tank D has an overflow path. The entrance into tank C from tank B has a water depth of 45 cm in the existing structure. This pathway will have to be changed so that the water depth is 77.2 cm above the divider. The table lists the spacing between each baffle in tanks C and D in distances measured from the center of each baffle because that is the measurement needed when placing the baffles in the actual tank.

The opening to the transition channel running from the flocculator to the sedimentation channels will be placed on the side of tank D at the very end of the flocculation system. This opening will be 90 cm wide (38 cm + 52 cm) and will cut into the area with the ceiling beam by approximately 38 cm. The rest of the opening will be taken from the wall in tank D. The figures show both the side and top views of the transitional chamber.

The top of the "up" baffles will have at least a 5cm free board and come almost all the way up to the top of the tank. Some room should be left between the top of the "up" baffles and the top of the tank if possible to keep any instances of overflow from flooding out of the tank.

The change in the spacing of tank D's baffles to a distance greater than the value recommended by the MathCAD Flocculator Program created a problem of short circuiting. Utilizing the 1.5b rule for the height of the flow path does not allow the "up" and "down" baffles to overlap. The 1.5b rule is suggested by Schultz and Okun for the height desirable above the baffles to provide sufficient area for the flocs to remain intact. Using the factor if 1.5 for the calculation of the height of the flow path for tank D's baffles allows the water to go straight through the tank instead of under and over the baffles. This figure illustrates the error that results if a baffle spacing of 66.2cm and a factor of 1.5 is used in tank D.

An equation was added to the end of the flocculator draining programming to calculate factor (f) by which the baffle spacing (b) should be multiplied by to calculate the distance between the bottom of the tank and the bottom of the "up" baffles, which is the same distance between the surface of the water and the top of the "down" baffles. This equation calculates a new f based on the height of the water (water), b, and an "overlap" variable. The "overlap" variable is the amount by which the user wishes the "up" and "down" baffles to vertically overlap so that the bottom of the "up" baffles are below the top of the "down" baffles by this distance. This makes the water go over and under the baffles. The amount of overlap is arbitrary and can be set by the user. This value is set at 2cm for the calculation of a new f for tank D.

The f factor for the baffles in tank D was calculated with

EQUATION 1

The f for tank D is 1.37, which provides a 2 cm overlap between the "up" and "down" baffles. The height of the flow path is thus 1.37*66.2 cm = 91 cm. Since a factor of 1.5 is not known to be strictly necessary in order to not break up flocs as they go around the baffles, a factor 1.37 is considered acceptable at this point. The final baffle spacings for tanks C and D are shown in the following figures.

The water treatment plant being built in Marcala, Honduras will have a different flocculation system from others AguaClara has designed. The baffles in the flocculation tanks will be made of 1.5 inch thick ferro cement slabs and these baffles will not be removable. As mentioned, an additional challenge with this design is devising a scheme for draining the flocculation tanks that would not cause short circuiting in the system or take an exorbitant amount of time to drain. A program called Final Floc Draining Time with Forces Calculated was created to model this process.

A simple method of emptying a flocculator tank without removing the baffles is to construct a small hole into the bottom of each down baffle. The size of the hole depends on the desired draining time. The larger the hole, the faster the tank will drain. However, putting a hole in the bottom of each down baffle would allow the flocs to short circuit and cause a new flow path. The holes are modeled as orifices and must have a minimal area, chosen to be less than 1% of the flow path area to minimize the likelihood that they would compromise the flocculation system. The orifices are also staggered to further reduce the risk of short circuiting; the holes will alternate between the left and the right side of the baffles. Another factor in selecting orifice size is the amount of time it would take for the tanks to drain. A compromise must be made between draining time and the increased short circuiting.

A MathCad program was written to model the use of different orifice sizes to drain a series of connected tanks. The space between each down baffle is treated as an individual tank and the height of the water in each assumed to be equal. At time zero, with the heights of the water even throughout the set of tanks, the only head is the distance between the water height and the outlet valve. As the first tank begins to drain it creates head between it and the next tank causing the second tank to begin to drain which in turn creates head for each consecutive tank. The program calculates the flow rate through each orifice based on the current head then calculates the new height of the water in each tank according to the equation for orifice flow.

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Since all tanks are not only draining but are also receiving water from the adjacent tank (with the exception of the beginning tank) the flow rate used to calculate the water height is the difference between the rate of flow into the tank and the flow rate out of the tank. The water heights during each iteration are calculated using the following equation.

EQUATION 3

The program continues to cycle through a while loop until it meets the stopping criteria of which water height in the last tank is sufficiently small. This value is chosen as one thousandth the initial height in the flocculation tank.

Inputs for tank draining program:
*h~initial~: 1.8m
*n~tanks~: 4
*base: 66.2cm
*width: 92cm
*percent_allowable_area: 1
*height of space: 1.4*base

The "base" variable refers to the distance between consecutive baffles and the "height_of_space" variable is the distance between the bottom of the up baffles and the floor of the tank in the deepest area of the tank. A "tank" in this program refers to the space between two down baffles. The number of tanks is the number of down baffles plus one for the end wall. The "percent allowable area" is the area of the orifice compared to the area on top of the up baffles (height_of_space*width).

An initial vector of orifice diameters is used for the program to loop through. The program outputs several plots describing the relationship of water heights in different tanks. There is also an accompanying animation of the process. Only the orifice sizes that short circuit less than 1% the flow are considered. As illustrated in Table 2 all orifice diameters considered meet this specification. The forces on each baffle as a result of tank draining are also calculated. This information is useful knowledge for construction as strength requirements for the baffles.

LINK ANIMATION

This graph plots the draining time in minutes that corresponds to each of the possible orifice diameters in inches. As depicted, the time it takes to drain a tank decreases as the size of the hole increases. The hole should be chosen so that the tank drains in an acceptable amount of time. A drainage time of around 30 minutes is adequate for the Marcala plant. Due to the still evolving design for the baffle spacing in the flocculator tank, only tanks C and D were modeled. These tanks are far enough down the flocculation process that as long as the baffles are spaced either the distance the current flocculator model recommends or further apart, there should not be an issue with flocs breaking up. Since these flocculator tanks have different baffle spacing and different floor heights each tank was analyzed separately.

Tank C's draining time was modeled using the 54.6cm spacing and 4 "tanks" (areas between down baffles). These values allow tank C to drain in a little less than 17.5 minutes with a 2 inch hole. Tank D takes longer to drain. Though the program calculates the drainage time with a 2in orifice as almost 24min, the actual drainage time will be a little longer since all of the water in the channel up to the sedimentation tanks will be draining as well. These drainage times are acceptable, so a hole with a 2in diameter should be made in the corner of each down baffle, alternating the side of the baffle that the hole is on. Note this is only 0.22% of the total flow path area between the bottom of the up baffles and the floor in tank D. This area percentage should keep most of the water from short circuiting by going through this orifice. Since the height of the water changes throughout both tanks due to the sloped floor, the greatest water depth in each tank was used for calculations since this gives a more conservative estimate of drainage time.

This graph shows how the water level in each "tank" responds to draining. The plot of indicates that the "tank" closest to the drain loses water at the fastest rate while the last tank plotted, has a shallower curve indicating a slower draining rate. Note: The 'i' variable indicates the entire time over which the tank is draining.

The force on the baffle is due to the pressure exerted by the water in each tank. The pressure in each 'tank' is depicted here.

This force, calculated as pressure x area is constantly changing as the water heights decrease when draining the flocculator. Equation 3 calculates the heights of the water in the flocculator. These values are stored in a table using MathCAD. Using the equations below, the force on each baffle can be calculated as a function of the difference in water heights.

EQUATION 4

EQUATION 5

[#This graphillustrates the force on the outside of each baffle in Newtons as a function of time in seconds. The force rapidly reaches a peak then slowly goes down to zero. The further the baffle is from the drain, the less the pressure gradient between two consecutive baffles. When the operator 'unplugs' the flocculator, the strongest force exists on the baffle closest to the wall of the flocculator since the difference in head is greatest in the tank that is emptying to waste.

Force analysis provides valuable information on the structural parameters of the baffles. Using the above figure, it can be seen that the maximum force exerted on the baffles when draining is 1,169 lbf (5,200 N). Hence the baffles must be designed to withstand this force.

Diagrams of the floors in the flocculator were compiled through drawings sent by Fred Stottlemyer and a phone conversation with him. The sloping floors are meant to make draining the flocculators easier and to thoroughly clean out any floc build up on the bottom on the flocculation tanks.

Tanks A, B, C and D will have a floor with a downward slope of roughly 10 cm over the length of each section. Tank A will begin at 58 cm above the current floor, tank B will begin at 48 cm above the current floor, and tank C will begin 35 cm above the current floor. Tank D starts at the height that tank C ends at and slopes down to 15cm above the original floor. The diagrams are shown at the end of the drainage section.

Included in the tank drawings are the drain placements in the flocculator system. There are three main drains in the series of tanks. One is on the side of tank B just before tank C. Another one is at the end of tank C and the last drain is in the floor of tank D at the end of the tank. This last drain connects to a 4 inch pipe running under the floor of tank D with a 90 degree elbow. The water then leaves the system through the wall at the beginning of tank D. The pipe has a downward slope with the beginning being at a height of 15 cm and the end at 5 cm above the original floor level where it then drains out of the side of the tank. These drains have already been placed in the Marcala plant in anticipation of a sloped floor.