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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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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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