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Overview
A linear dosing meter is currently being utilized in current AguaClara plants designed forin 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 becomes becoming clogged with a chemical precipitate that is probably a combination of calcium and , aluminum hydroxide, carbonates, or carbonatesome 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.
A more ideal system would clog at most once per day. The objective of this experiment is to determine the cause of the clogging in Honduras (precipitant or foreign materials) (I'm not sure what else it could be? Hair?) and estimate the frequency of clogging in the new non-linear dosing system. If the new orifice (0.1 cm diameter) clogs frequently with lab grade alum, the system will need to be redesigned to incorporate a larger orifice. If the source of the clogging is determined to be foreign material (little or no clogging occurs with lab grade alum) a strainer will be included in the overall system design to remove the foreign material.
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Next, alum was used in order to determine the time it would take for a clog to form in our 0.1 cm orifice. The experiment was run for four days but the most conclusive data was seen within the first hours. The pressure readings, from pressure sensor 1, are shown in Graph 2 below.
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The elevated pressure, shown in Graph 2, seems indicative of a clog, but the spike was not repeated at all for the remainder of the experiment. If the clog was "blown out" due to the increase in pressure then it should simply reform after another 4 hour time span, forming a clogging cycle. (Precipitation and blow out could possibly be more intermittent than you would periodically predict.) Yet this cycle of perpetual clogging and clearing of the orifice was not observed. This was unexpected and might indicate erroneous pressure readings for the spikeThe precipitation and blow out could possibly be more intermittent than one might be able to periodically predict.
Conclusions/Future Work
The 0.1 cm orifice appeared to clog after 4 hours, indicating that a larger dosing orifice might need to be used. Further testing will need to be performed in order to verify these results, and determine the clog time for any larger orifice we select. An ideal system would only require cleaning once a day. (Already stated this earlier) For the Agalteca plant, based off these experimental results, we are recommending increasing the diameter of the smallest dosing orifice to 0.152 cm (0.06 inch) to minimize the occurrence of clogs. In addition, we are recommending using 51 grams/liter stock tank solution, which again should minimize the clogging frequency. The downside to using a lower concentration alum is that the stock tank will have to be changed more frequently, yet this should help with
Since lab grade alum was utilized in this experiment and a clog was still present, the presence of foreign material, might not be the main cause of the clogs present in the Honduran plants. An inline strainer would still be recommended for might be beneficial to include in future dosing systems (Is the source water utilized to mix the alum really that dirty that we would need to strain it?), yet but it would be more effective to verify the Honduran operators are cleaning the system regularly and ensuring no foreign matter is making it into the dosing system. Yet it should be noted that this would not eliminate the occurrence of clogs since they may also appear to be caused by the precipitation of aluminum and calcium species in the water.