Fall 2008 Semester Summery and Conclusions
As time progressed this fall, we began to understand more fully the problem that was presented to us. It became apparent that adding another chemical to the current Aguaclara water treatment system was not a reasonable solution to the precipitation problem. While adding hydrochloric acid (HCl) to the solution of calcium carbonate (CaCO3) would be an effective way to reduce pH and prevent the CaCO3 from forming, it would not be very cost effective. Using HCl would not only add to the cost of water treatment in terms of money spent, but also in terms of complicating plant operation. As a result, a different solution was pursued.
Next, the thought of settling out the CaCO3 before it enters the hypochlorinator was investigated. If the precipitated CaCO3 could be removed or reduced from solution before it comes into contact with the flow control device (FCD), this would ideally solve the problem. The fact that this chlorination treatment process will be expanding to many more communities, made investigating these problems more crucial, especially in communities that do not operate with full Aguaclara water treatment plants. We began by examining the flow restrictions that may arise in the Honduran communities that expressed interest in adding an Aguaclara FCD to their chlorine treatment systems. The first solution proposed was to design a settlement basin that could be implemented into the system which would allow the CaCO3 to precipitate out before it entered the FCD like the one that can be observed in (Figure 1).
settling tank.bmp
Figure 1. Calcium Hypochlorite Feeder designed by Ralph Morrison and company (US Patent Number 6,045,706).
We decided however that a device like this would be costly to design and build for each community, but the idea of removing the CaCO3 before the FCD was not completely discredited.
We then moved on to the restricted flow of FCD problems faced by communities with full Aguaclara water treatment plants. As is discussed in the flow control precipitation experiment Calcium Carbonate Settling Observations laboratory, the higher the concentration of calcium hypochlorite(Ca(ClO)2), the larger the concentration of precipitate that initially settles out. If the solution of Ca(ClO)2 is created and left to settle, it was observed that the amount of precipitate stops forming after three days, as long as the solution is not flowing or being mixed with the atmosphere. As was observed in the Final Restriction of Flow of Hypochlorinators experiment, if the Ca(ClO)2 solution comes into contact with the atmosphere even after initial settling, mixing, or as the result of being forced through a small orifice (such as the one located in the float valve), the precipitate will continue to be formed. It is believed that this is the result of the solution coming into contact with CO2 in the atmosphere which results in the formation of CaCO3 (Equation 1).
Equation 1. 2Ca(ClO)2 + 2CO2 → 2CaCO3 + 2Cl2 + O2
As was observed in the Carbonate Settling Observations laboratory, more precipitate can form after a concentrated solution of Ca(ClO)2 is diluted with non-distilled water as the result of the calcium's interaction with CO2 (Equation 1). Precipitate could also form as a result of its interaction with other chemical species located in the dilution water.
We have now come to the conclusion that if the majority of the precipitant is removed from settling, the only other way that the flow will be impeded will be as a result of Ca(ClO)2 solution coming into contact with the atmosphere. Locations where this could happen are at the surface of the stock tank that holds the Ca(ClO)2 solution, the surface of the constant head bottle where the float valve is located, and the orifice located in the float valve where the solution is forced into the constant head bottle (Figure 2).
Insert pic
As was examined in the Final Restriction of Flow of Hypochlorinators experiment, if there is a solid material present like plastic or metal when the Ca(ClO)2 solution comes into contact with the atmosphere, the precipitate will form on the surface of the plastic or metal (Figure 3).
Insert pic
If precipitation forms in the stock tank or at the surface of the constant head bottle over time after the initial settlement, it would flow out of the tanks with the rest of solution or settle out at the bottom of the containers, as was observed in the Final Restriction of Flow of Hypochlorinators experiment. If it continued through the system with the flow of solution we are confident that this small amount of precipitate would not build up, or impede flow at any point in the system. This leaves us with the float valve orifice. Every instance where we were able to restrict flow from the orifice in the float valve there was a buildup of precipitate all over the surface of the float and the arm holding the float to the rest of the valve. It appears that precipitate is slowly forming in the orifice where the solution first comes into contact with the atmosphere after leaving the tube that flows from the stock tank, eventually restricting flow (Figure 4).
orifice.bmp