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By far, the biggest hurdle in the development of AguaClara technology has been devising a method for accurately and precisely administering process chemicals while adhering to the fundamental AguaClara design constraint of creating solutions which do not rely on electricity. Modern water treatment plants have computerized control and precise metering pumps at their disposal and while an AguaClara engineer has neither of these, this does not diminish the need for accurate metering.
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The AguaClara engineers from academic year 2009/10 not only designed, but built and installed a Non-Linear Dose Controller (NDC) in a new AguaClara plant in Algateca, Honduras. The current NDC is installed on top of the entrance tank of the plant and consists of a float connected to one end of a lever arm and the dosing orifice connected to the other end.
Figure 1. NDC As Installed in Aglateca
The entrance tank is connected to the hydraulic flocculator by way of a rapid mix orifice. A change in flow rate through the plant is indicated by a change in the level of the entrance tank. This is transmitted to the lever arm by way of the float. For example, if the flow rate of the plant were to decrease from 100% to 80%, the entrance tank would drop by a proportional amount. Since the pivot is in the center of the lever arm, each centimeter of change in the entrance tank will translate to a corresponding change in the angle of the lever arm.
Figure 2. Entrance Tank Connection to Flocculator via Rapid Mix Tube
Alum is piped from a 120 g/L concentration stock tank to a constant head tank and from there to the metering orifice. The metering orifice is connected to the lever arm by way of a movable slide. The lever arm has an incremental scale on the top surface which corresponds to specific alum concentrations. The operator is able to set the chemical dose concentration according to the values on the scale and the lever arm will automatically adjust the flow rate of alum to maintain the correct concentration of alum as plant flow rates change.
Figure 3. Stock Tanks and NDC
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Recommended Work for Fall 2010
Orifice Validation and Scale Development (2 - 4 Persons)
The CDC team is unique in that research, design and development are all key components of this team's challenges. To help clarify the work this team needs to accomplish, 3 sub-teams should be created. This is a suggestion, not a requirement. The three sub-teams are described at length below along with a recommended number of participants in each sub-group.
Orifice Validation and Scale Development (2 - 4 Persons)
One of the priorities of AguaClara engineers is to ensure their designs are locally sustainable. It is easy One of the priorities of AguaClara engineers is to ensure their designs are locally sustainable. It is easy to create the walls and foundations from locally manufactured materials but becomes increasingly difficult when very specific components are needed to perform very specific tasks. This proved true with the float valve used in the constant-head tank and may prove true with a metering orifice. The team that originally developed the NDC was not able to fabricate precise (e.g. repeatable) orifices for their lab tests. This team also noted that surface tension effected accuracy of a metering orifice when less than 4 cm of head was available. The Summer 2010 team concentrated on documenting the extent of the precision problem and researched various solutions. It is the belief of the Summer 2010 team that we can not properly quantify the inaccuracies associated with surface tension until the material and manufacturing techniques are finalized since material finish, or smoothness, has a strong influence on surface tension.
The parameters for a metering orifice that adhere to the guidelines set forth in the AguaClara Mission Statement are:
Parameters and guidelines for selecting and fabricating a metering orifice are:
- The orifice must be made from a material that is impervious to all process chemicals that are used or will be used in the future in an AguaClara plant. This includes alum, chlorine and saturated lime.
- The material selected must also have certain machinable characteristics. Initial attempts at fabricating the orifice from plastic "quick connect" caps proved fruitless. These plastics do not machine well and produce unreliable metering between orifices of the same size.
- It it preferred that the orifice be manufactured from "commonly available" fittings of a "commonly available" material, particularly one that would be available in the communities/countries we service. Correctly defining "commonly available" is part of this challenge.
- Drilling methods are important and must be repeatable. Effects of drill-bit break-through will be at the exit region if drilled in the direction of flow. However, when drilling in the direction of flow the angle created by the centering bit must be consistent from one orifice to the next or a different K.vc is created. Drilling opposite the direction of flow is possible with hard materials where drill-bit break-through is less of a concern
- The orifice must be made from a material that is impervious to all process chemicals that are used or will be used in the future in an AguaClara plant. This includes alum, chlorine and saturated lime.
- The material selected must also have certain machinable characteristics. Initial attempts at fabricating the orifice from plastic "quick connect" caps proved fruitless. These plastics do not machine well and produce unreliable metering between orifices of the same size.
- It it preferred that the orifice be manufactured from "commonly available" fittings of a "commonly available" material, particularly one that would be available in the communities/countries we service. Correctly defining "commonly available" is part of this challenge.
The Summer 2010 Team attempted to determine if it was reasonable to expect that a precise metering orifice could be fabricated in-house. The extent of this research is documented on the team's wiki pages, however, a few key lessons-learned are summarized below.
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As the CDC design evolves and final decisions are made, it is important that appropriate materials are selected along the way. The CDC administers chlorine, alum and in the future saturated lime, making material selection a critical part of this design. Additionally, there are a plethora of fitting styles available on the market. Cost, longevity, ease of use and availability are all important considerations. It is important to have a working knowledge about materials, pipes, tubing and fittings before any decisions can be made. This information should be posted on the wiki as a reference for future teams.
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Once the materials and fittings are decided upon, this information needs to be incorporated into the Automated Design Tool. The goal is for the design tool to eventually include the CDC and to have a material list and AutoCAD drawing of the installation included as part of a standard design package.
Concluding, this sub-team
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will improve on the Constant Head Tank design and valve selection so that this device will "scale-up" as AguaClara plants increase in size. Additionally, this sub-team
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will make final decisions on the materials and fitting styles that will be used in the CDC.
Design Upgrade of the NDC (3-6 Persons)
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- The float could be attached by cable to a pulley mounted on the ceiling. The cable would be connected to the dose controller side of the lever arm since the cable (after returning from the ceiling) and will move in the opposite direction as the water level changes. The range of lever motion would be adjusted by moving the attachment position. The slight angle of the cable from the pulley on the ceiling to the lever would be inconsequential.
- This float could be in the entrance tank rather than in a separate connected tank! Two pulleys mounted on the plant ceiling could transfer this motion to the CDC allowing the CDC to be placed wherever is most convenient. Each pulley adds a potential friction point as corrosion and dirt accumulate.
- Have a float attached to a cable that wraps around a very large pulley that is connected to a lever arm. This introduces error into the lever displacement. The pulley changes the equation of motion. Instead of applying a change in height to the point of connection with the lever, the float applies a change in angular rotation. These would be similar for small angle displacements, but there will be significant error for large displacements.
- The float could be a large diameter, shallow float with a small submerged weight that gives it stability. The float must have a large submerged AREA so that the depth of submergence is not changed by more than perhaps 1 cm when the dosing slides are moved. The buoyant weight of the slides (including the vertical drop tubes) will be larger if they are not submerged in water. Thus the float area will need to be larger if the CDC isn't mounted on top of the entrance tank (where the drop tubes were partially submerged).
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