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In Honduras, a Calcium Hypochlorite solution is used in the Agua Clara AguaClara flow controllers to disinfect the drinking water. Unfortunately, due to the precipitation of calcium carbonate, the system clogs which leads to large decrease in the flow rate (and thus dosing) of the chlorine solution. These experiments have been developed in order to determine the point of restricted flow and to devise a method to diminish the clogging.
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A model very similar to the model used during the Summer 2008 Demo Plant team was used. A slightly more analytical approach was taken in developing this semesters semester's model. A pressure sensor was inserted by means of a t-joint at the bottom of the constant head tank. The sensor would collect information on what was used to measure the height of solution in the constant head tank was at any given as a function of time. By using the Easy Data EasyData to analyze acquire the data, we would have a more accurate idea of when clogging occurred. In addition, a pH meter probe was added to the 5 gallon stock tank. This was done in order to aide future studies on the effects of alkalinity on calcium bicarbonate precipitation. The final change was that the flow rate was determined by changing the angle of opening within the valve at the exit of the constant head tank. Shown below is the experimental model from the summer with the changes written on the image.
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The values used to run the experiment were as follows. We chose to use 9.336 mgg/L of calcium hypochlorite for this lab experiment because it is half way between 12201 mg12.201 g/L and 6470 mg6.470 g/L, which were the average concentrations used at Tamara and 4 Communidades (The towns with the most severe issues with flow rate). The flow rate would be run at 68 mL/min, because it is the average of the reported range of flow rates in Honduras.
Procedure
15 mL L of deionized water would be put into the 5 gallon 20 L stock tank. 158 mL g of Calcium Hypochlorite powder would be added to this stock tank. 2 L of deionized water would be added on top of this in the stock tank. The carboy being used would then be simply stirred and shaken for 5 minutes in order to mix the solution thoroughly. The tank would be allowed to settle for 10 minutes, and then the experiment would be run. The 7kPa sensor would be set up using Easy Data EasyData and the "cm of water" calibration file. The pH meter probe would be run monitored using the program pH Meter.
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October 18, 2008
Conclusions
The Unfortunately, the model empties out much quicker than the estimated 4 hours and 9 minutes. This was likely because of a mistake in calculating the flow rate initially. However, because the model was never significantly altered between runs of the experiment, we expect that the flow rate remain constant (despite the fact that it was not what was calculated). And because the model takes longer in each subsequent run to empty, it can be inferred that the flow of water through the model becomes increasingly restricted over each run. It was hypothesized that the sudden spike in each run that signifies the emptying of the stock tank due to erroneous noise that occurs with the sudden end of flow. It is noted that on October 17th, 2008 the constant head tank increases gradually as opposed to rapidly. It is possible that this is due to restricted flow higher in the line (the exit of the stock tank, for example). The model finally clogs on October 18, 2008. When clogged, the the stock tank and constant head tank are still full with solution, which indicates that the closure is in the line below the constant head tank. We believe that it is clogged at the t-joint, because of the parts within the t-joint, however the closure may also be at the valve. The closure may be at the valve because of the fact that it isn't fully opened.
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