Restricted Flow of Hypochlorinators
Abstract
Honduran communities are using the AguaClara flow controller for chlorination. Some of these hypochlorinators are experiencing diminished flow rates after only 2 days of operation which makes dosing difficult. It is believed that the clogging is due to the precipitate of chlorine solution, calcium hypochlorite. The set up of the experiment consists of a stock tank, constant head tank, and a peristaltic pump which determined the flow rate. The procedure included allowing the solution to settle, filtering it through a cloth rag and changing the angle of the float valve to minimize exposure to atmospheric CO2 gas. From the research, it appears that steps such as the proposed procedure may be taken to reduce the detrimental effects of calcium carbonate precipitate on the hypochlorinators.
Introduction and Objectives
The hypochlorinators were experiencing diminished flow rates due to clogging. It is believed that the clogging is due to the precipitate of chlorine solution. This precipitate is calcium carbonate, and its formation is displayed in equation 1.
Eq 1. 2Ca(ClO)2 + 2CO2 → 2CaCO3 + 2Cl2 + O2
We wanted to determine where in the line the precipitate form, why it formed and under which conditions it formed most readily and least readily. Furthermore, we wished to develop a procedure that would eliminate clogging in the hypochlorinator.
The hypochlorinator may become clogged through two mechanisms. Clogging may be due to previously precipitated granular material precipitate from the original preparation of solution that is transported to clog site. Clogging may also be due to precipitation in situ via equation 1, where the CO2 is introduced in sufficient amounts to induce precipitation through atmospheric exposure to the solution.
Procedure
Initial set up:
When initially approaching the set up of the experiment, it was thought that it would be most revealing if the experiment was modeled as identically to the conditions in Honduras as possible. The set up consisted of a stock tank and a flow controller (figure 1). Using a concentration of 9.336 g/L, we created a 17 L solution of calcium hypochlorite and mixed it well. We allowed it to settle for 10 minutes and then attached it to the flow controller . The head difference of the outlet was set so that the flow rate was 68 mL/min. Despite this being very similar to the practical application of the hypochlorinator in Honduras, this method had some drawbacks. The major drawback was that the solution ran through the set up too quickly (within three hours) for any conclusions to be drawn from the experiment. This was because we were unable to obtain enough space for a drum large enough to hold the amount of solution that would have the set up running for a long enough time to draw any conclusions. Despite these failures, the initial set up allowed the observation of continual calcium carbonate formation during experimental runs. One observation that was seen was that large particles of calcium carbonate remained in the containers after the solution had run through completely. If the tank was not cleaned properly, these particles could clog the orifices in the system. It was hypothesized that the restriction was due to the continuous formation of calcium carbonate within the tubing.
Figure 1. Initial Experimental Set Up
Second set up:
The second set up of the experiment addressed the pitfalls of the first. The set up is displayed in figure 2. It consists of a stock tank, constant head tank, and a peristaltic pump which determined the flow rate. The peristaltic pump returned the effluent to the stock tank. This set up allowed for continuous flow, which would allow the set up to run long enough for concrete conclusions to be made. Also, because the looped set up required much less chlorine solution to be run through the system, it allowed the experimenters to minimize chlorine gas contact.
Figure 2. Second Experimental Set Up
In the new set up, it was important to test certain procedures that were aimed to determine the mode of clogging.. One procedure involved the production of the calcium hypochlorite solution. After the combination of the calcium hypochlorite and water, the solution was allowed to settle for a minimum of a day. After the settling period, the supernatant of the solution was filtered through a cotton rag. This filtered solution was then attached to the constant head tank and allowed to run through the system. This procedure was designed to limit the amount of granule related clogging, allowing observations to be made on alternative clogging mechanisms.
A second procedure that was tested involved an alteration to the float valve in the constant head tank. One theory was that the float valve was clogging (and the calcium carbonate was forming) due to the contact of the solution and set up parts with atmospheric carbon dioxide. In the original set up (figure 3), the massive amounts of atmospheric carbon dioxide was in contact with the float valve and in contact with the solution around the float valve orifice. This would lead to the formation of calcium carbonate coating on and around the float valve, which may induce clogging. A proposed solution to this phenomenon was to change the angle of the arm of the float valve (Figure 4). This would keep the orifice of the float valve submerged in solution so that the atmospheric carbon dioxide would not have the same effect on precipitation.
A 5 L solution of 30 g/L calcium hypochlorite was produced and mixed well. It was allowed to settle for a minimum of 24 hours. The supernatant of the solution was filtered through a cotton rag, and then allowed to run through the experimental set up. The peristaltic pump was set to 35 mL/min.
Figure 3. Original Float Valve angle
Figure 4. Modified Float Valve angle
Results and Discussion
The second experimental set up included a pressure sensor to determine changes in height in the stock tank. This would have theoretically reported when the system clogged and failed. Unfortunately, due to the set up of the lab, the sensor was subject to much unwanted noise and thus that data is not presentable. However, the beginning dates and end dates (and subsequent observations) are available for each experiment, and the information follows below. All information refers to the second experimental set up. For information on the first set up, see Restricting of Hypochlorinators midterm report At this point, it is important to differentiate between the attributes of precipitation due to atmospheric CO2 and the precipitate that forms with the initial preparation for the solution. Precipitation due to exposure to atmospheric CO2 will produce a uniform coating. Precipitation due to initial preparation will be granular.
On November 11th, 2008, the experimental set up was run with the parameters stated in the procedure. At this point, the float valve was in its original position (Figure 3). By November 13th, 2008, the float valve had clogged. After this experiment, the float valve was disassembled. The inside of the valve was entirely coated in precipitate. It is postulated that this is due to atmospheric CO2 because it was widely coated in precipitate.
On November 16th, 2008, the experimental set up was run with the parameters stated in the procedure. At this point, the float valve was in its new position (Figure 4). The set up used the solution from the first experimental run. An observation taken on November 17th noted that there were no signs of precipitation forming. By November 26th, the set up had not yet clogged, and the experiment was ended.
On December 1st, 2008, the experimental set up was run with the parameters stated in the procedure. The float valve was still in its new position (Figure 4). By December 4th, 2008, the set up had clogged. The set up was observed and the restriction of the flow was found to be at the orifice leaving the stock tank. The restriction appeared to be a large granule, which was from the original preparation and had not formed in the line. The restriction was removed, and the model ran well as was expected. The experiment had not clogged by December 7th, 2008, and it was ended.
On December 9th, 2008, the experimental set up was run with the parameters stated in the procedure, except this run was different because the solution had not been filtered through a cotton rag in order to see what effect this had on the clogging. The float valve was still in its new position (Figure 4). At time of this writing, the run had not yet ended.
Conclusion
From the research, it appears that steps may be taken to reduce the detrimental effects of calcium carbonate precipitate (eq. 1) on the hypochlorinators. The simple act of changing the angle of the float valve allowed for a vast improvement over the previous configuration. Furthermore, the designation of a proper settling and simple filtration technique can also improve the effectiveness of the hypochlorinators. A recommendation to change the angle of the float valve has already been made and the results are pending. More research may be performed. One idea that has yet to be implemented is the addition of a settling basin to the stock tank. Alternative filtration techniques to remove calcium hypochlorite may be tested. Similarly, methods to restrict the formation of calcium carbonate may also be tested.