Agalteca Design

Agalteca has a population of 2,160 residents. A distribution tank is already in place, but another distribution tank is expected. Construction of an AguaClara water treatment plant started on September 30th, 2009.

Agalteca is unique in being the first plant to be designed largely using the Automated Design Tool.

Design Specifics

Entrance Tank

There are a number of constraints which must be met when considering the dimensions and location of the entrance tank.  These items are discussed below.

1. The elevation of the entrance tank is determined by the design of the rapid mix tube.  The current design of the rapid mix pipe has 10cm of headloss.  Therefore, the entrance tank will need to be located 10cm above the flocculator tank. 
2. Entrance tank height is determined by the headloss throughout the plant and physical size of the rapid-mix tube.  These values are added together to determine the minimum depth of entrance tank.
   1. 10 cm of free-board - this is a standard free-board dimension used in Agua Clara plants.
   2. 10 cm head loss rapid mix pipe - this is a calculated value based on the marco-mix and micro-mix orifices in the pipe
   3. 13.5 cm head loss flocculator - this is a calculated value based on the number of baffles and the baffle spacing
   4. 5 cm head loss launders - also a calculated value based on the number and size of the orifices in the outlet manifolds
   5. 5 cm head weir - designed value
   6. .61 m max height needed for macro scale rapid mix pipe - this is a physical dimension of the rapid mix pipe.  It is assumed that the coupling will be embedded in the concrete floor of the entrance tank.  The micro-mix plate will sit in the coupling with the rapid mix pipe and macro-mix plate placed inside this coupling. 
   7. Total height of entrance tank is rounded to 1.00 meters. 
3. Width and length of entrance tank is are 1 meter x 1 meter (inside dimensions.)  This is a critical dimension to accommodate the length of the lever-arm for the chemical doser.
4. Additional concern: Swing of the lever arm is represented by 33 cm of change in the water level.  The arm length is 40 cm on each side of the pivot.  10 cm of this is accounted for in the freeboard of the tank.  The lever arm will be mounted 30 cm above the entrance tank (preventing the arm from dipping into the water).  Additionally, the roof will need to be a minimum of 70 cm above the entrance tank wall.
5. Important points for entrance tank:
   1. Understanding that PVC dimensions are dependent upon the manufacturer, many of the above figures are guidelines as the PVC available in Honduras is undoubtedly of different dimensions than what we used in our design.
   2. The rapid mix pipe coupling is to be embedded in concrete in the floor of the entrance tank.  The micro-mix plate will fit inside the coupling with the macro rapid mix pipe sitting on top (the nipple connecting the elbow will serve to support the orifice plate).  Therefore, the rapid mix pipe needs to sit deep enough into the coupling that it is well supported.  A long coupling would be suitable here.
   3. The pipe above the rapid mix coupling will be a slip joint (not glued) for easy disassembling and cleaning.  Because the pipe above the coupling is to be removable, it may not be embedded in concrete.
   4. If the coupling is not embedded in the floor of the entrance tank, modifications must ensure that the top of the rapid mix pipe matches the elevation of the exit weir and that the water level above the rapid mix pipe is 33.5 cm is and 10 cm above the flocculator water level.   

      Flocculator

This is the first AguaClara design to use untapered baffles in the flocculator. The goal is to create a 6 mW/kg energy dissipation rate throughout the flocculator. There are 29 baffles in each of the first two channels to ensure that the water flows through the ports connecting channels, which are at the bottom, correctly. The last channel has 28 baffles because the inlet port is at the bottom and the inlet channel for the sedimentation tank is at the top. The baffles use polycarbonate sheets instead of cement which are easier to remove. This makes draining the system much easier.

Inlet Channel

The inlet channel was modified to have only one opening leading to the sedimentation tank instead of two. See the explanation for this under Sedimentation Tank. The concrete in the inlet channel will be rounded going into the pipe so that the transition of flow is smoother. The construction sequence for this transition will be done as follows:
before pouring the concrete for the base slab of the inlet channel, a 90 deg elbow will be coupled to the 6" pipe (substituting the chimeneys). The idea is thet the concrete follows the shape of the elbow in the bottom slab of the channel to reduce the expansion loss created by the change in direction of the flow. Please refer to the Construction sequence graphical explanation

Sedimentation Tank

The sedimentation tank design is significantly different from previous designs. The main problem that brought about the change is that too much sludge was settling out in the inlet manifolds at Cuatro Comunidades. There was originally a similar design for Agalteca, so it needed to be fixed. To see a list of ideas on how to fix this problem until the final design was chosen, click here. To see other potential changes that can be made if needed to correct this problem, click here.

The final design involves an inlet pipe down the center of the tank instead of two inlet manifolds under the slopes. The pipe will be a 6" diameter PVC pipe. Refer to the Inlet Manifold Equations for theoretical information. This pipe will subtitute the inlet manifolds bellow the slopes which will be filled in with concrete. The slopes will remain though so that flocs are directed toward the sludge drain if they settle.

Ideally, a floc blanket will be formed. In order to more likely achieve a floc blanket, the orifices in the inlet manifold pipe will point downward, to re-elevate settled sludge along the sludge drain cover. If the floc blanket fails, the pipe can be rotated 180 degrees.

One issue that has yet to be resolved is how to distribute the flow of water evenly throughout this pipe. This includes potentially tapering the pipe and determining the size and spacing of the orifices and their positioning in relation to the sludge drain orifices. With this design, we will have more flow through the last ports of the inlet manifold pipe as the presure recovery is higher than the major headloss through the manifold. Refer to Inlet Manifold Equations.

In order to purge the floc blanket to keep the desired concentration of flocs in it, we are designing a Floc Hopper whose objective is to trap flocs and concentrate them so that when we purge, not much water is wasted. The postitioning of the floc hopper to accomplish this is yet to be determined.

Wireframe Image

Conceptual Image

Inlet Manifold

An inlet manifold is used to transport the flocs from the inlet channel to the sedimentation tank taking into account two constraints. Floc breakup and Floc sedimentation.

Floc breakup occurs when the energy dissipation is very high. From initial tests done by the AguaClara team, the maximum energy dissipation rate to prevent floc break up was 6 mW/kg. Previous plants (i.e. Cuatro Comunidades), where designed with this constraint and showed heavy sedimentation. Velocity in the manifolds was very low and particles settled here. According to the Ten State Standards, the minimum velocity of water inside a pipe should be 0.15 m/s to prevent sedimentation.

The objective of the new design was to prevent particle settling inside the manifold without increasing the energy dissipation rate to prevent floc breakup. For the final design a pipe manifold was used.

Design parameters of the manifold, results of energy dissipation rate and velocity inside the manifold are shown in the following tables.

Design Parameters

Wireframe Image

Conceptual Image

Sludge Drain

The main innovation on the sludge drain is its cover. Initially the cover served also as a support for the slope, but now, as the slopes are built by a solid piece of concrete, this is not necesary any more.
One of the advantages of the new design is that it is much easier to clean, as the only removable parts will be the 6 inch PVC inlet manifold and the drain cover which was designed in sections to make it easy to remove. Also, the fact that the flocs will settle inside the sedimentation tank, which has a sludge drain specifically designed to purge all the sludge, will imply that after emptying the tanks the remaining sludge in the bottom of the sedimentation tank will be very little to none.
The idea of designing the sludge drain cover with small orifices is to create an even vacum along the entire length of the sedimentation tank to reduce dead spaces where sludge could be accumulated.

Plant Drawings

AutoCAD drawing Click to download