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A linear dosing meter is being utilized in current AguaClara plants in Honduras to control the flow of alum, based on varying plant flow rates. In this system, the alum flows from the stock tank to the constant head tank through a small 0.23 cm orifice. This orifice is becoming clogged with chemical precipitate that is probably a combination of calcium, aluminum hydroxide, carbonates, or some other unknown substance, preventing the flow of alum into the entrance tank and consequently reducing the effectiveness of flocculation. The constant head tank orifice has to be cleaned multiple times per day by the operator.
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The experimental setup accurately mimics the conditions in the non-linear dose controller without incorporating the additional hassles of a large stock tank, constant head tank and entrance tank. As seen in Figure 1, the water circulates through a peristaltic pump to a small one liter reservoir intended to steady the pulsing input from the pump. The reservoir is connected to a tee, which connects a pressure sensor to the system just before the orifice. Water then drips through the smallest orifice, 0.1 cm diameter, into another one liter reservoir. This reservoir is connected to the pump, creating a closed system. The pump is set to discharge water at the same rate as the water flows through the orifice to maintain steady-state operation. The experiment was first be run with deionized water as a control. Then it was run with the maximum concentration of alum to be used in a plant, 125 g/L. As the experiment runs, Process Controller records the pressure directly upstream of the orifice. A safety sensor is connected to the second reservoir to provide further validation of the presence of a clog. As the pressure at sensor 1 increases due to a clog, the pressure at sensor 2 decreases due to decreased inflow from the orifice.
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