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We decided that a device like this would be costly to design and build for each community, but the idea of removing the CaCO3 before the FC was not completely discredited.
Initially we separated the restriction of flow problem into two categories. Communities that are experiencing the problem that currently have Aguaclara plants, and communities that could experience the problem that are going to use the Aguaclara FC in there existing chlorine feed system. The reason we separated the problem into two categories was that we assumed that the logistics of adding a settling basin of some type to an already existing Aguaclara plant would be different than adding it to a chlorine system from a chlorine stock tank in a community that is strictly chlorinating there water.
We then moved on to the restriction of flow in the FC 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, when the Ca(ClO)2 solution comes into contact with the atmosphere after initial settling, mixing or being forced through a small orifice (such as the one located in the float valve) will result in the precipitation of CaCO3 again. 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). This happens at pH's above around 8. The presence of Ca(OH)2 that also forms when Ca(ClO)2 is added to water (Equation 2) results in a pH increase to around 11 for a solution of Ca(ClO)2 of 30 grams/liter. This is high enough for the precipitation of CaCO3.
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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.
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It was decided that to deal with this occurrence, we would raise the level of the solution in the constant head bottle to a level above the orifice. This would result in the solution not coming into contact with the atmosphere at the orifice. The orifice would be submerged and calcium would not come into contact with CO2 until getting close to the surface of the solution in the constant head bottle.
In our laboratory experiments this has kept the system functioning with relatively high concentrations of Ca(ClO)2 for periods of time lasting more than a week as can be observed in "Restriction of Flow of Hypochlorinators" experiment. Presumably the experiment would have continued to function without obstruction, but it was terminated so a new test could be organized.
Another issue involved with the use of Ca(ClO)2, is safety. Calcium hypochlorite is considered a strong oxidizer. It is corrosive and causes burns to any area of the body it comes into contact with. It harmful if swallowed or inhaled and it will react with water. The MSDS recommends goggles, a ventilated hood, and proper gloves when in contact with this chemical. Specifically when inhaled it is extremely destructive to tissues of the mucous membranes and upper respiratory tract. Symptoms may include a burning sensation, coughing, wheezing, laryngitis, shortness of breath, headache, nausea and vomiting. Inhalation may be fatal as a result of spasm inflammation and edema of the larynx and bronchi, chemical pneumonites and pulmonary edema (J.T. Baker, MSDS 2008). There were instances this semester where individuals that were working with this chemical in the laboratory here at Cornell felt sick and or nauseous after accidental inhalation. These occurrences were the result of improper fume hood operation by the students. They were operating the fume hood with the window (or front door) open to high.
Another issue that was considered is if the Ca(ClO)2 solution is going to be pre-settled before being applied to the chlorine solution stock tanks, a sufficient volume of water must be added to the chemical in the container that the solution is being settled out of to completely dissolve the Ca(ClO)2 in solution. The solubility of Ca(ClO)2 is 210 g/L. In certain communities the concentration of Ca(ClO)2 required for chlorination of the water supply necessitates a large enough weight of Ca(ClO)2 that the amount will not dissolve in a 20 liter bucket of water, which is the procedure that some operators have been implementing.