Restricted Flow of Hypochlorinators
Introduction
In Honduras, a Calcium Hypochlorite solution is used in the Agua Clara 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.
Experimental Model
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 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 the height of solution in the constant head tank was at any given time. By using the Easy Data to analyze the data, we would have a more accurate idea of when clogging occurred. In addition, a pH meter 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.
The values used to run the experiment were as follows. We chose to use 9.336 mg/L of calcium hypochlorite for this lab experiment because it is half way between 12201 mg/L and 6470 mg/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 of deionized water would be put into the 5 gallon stock tank. 158 mL 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 and the "cm of water" calibration file. The pH meter would be run using the program pH Meter.
Results
Using the flow rate of 68 mL/min, the model (assuming no restricted flow) would require approximately 4 hours and 9 minutes to be emptied (4.16 hours). Below are graphs of the height of water in the constant head tank plotted against the amount of time elapsed each experimental run. It is quite noticeable that in each consecutive run, the constant head tank is increasingly more filled by the 4.2 hour mark, which is marked by the red line. Furthermore, the "peak" point (which means that the stock tank has finally been drained) takes longer and longer to occur in each run, as is signified by the blue line. Finally, it has been noted that on October 18th, the graph never reaches a "peak" because it has finally clogged.
October 3, 2008
October 6, 2008
October 7,2008
October 17, 2008
October 17, 2008 extended
October 18, 2008
Conclusions
The flow of water through the model becomes increasingly restricted over each run. 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.
Goals
The next steps here are to figure out where the clog occurred. We can do this by isolating the sections at which we believe may be clogged. The next step will be to perform the experiment with a new method of preparing the solution (filtering it, settling for longer, etc) and recognizing whether or not it takes longer to restrict the flow rate. Another step would be to analyze the pH readings of the runs and to determine whether altering the alkalinity (adding acid) will stop the model from clogging.