The U.S. Environmental Protection Agency’s (EPA) Water Infrastructure Resiliency and Finance Center, in collaboration with the...
Within the last twenty years, calcium hypochlorite has proven to be an excellent alternative to gaseous chlorine and liquid bleach. Calcium hypochlorite is stable over a wide range of ambient temperatures, making it possible to store tablets for extended periods of time with almost no loss of strength. It can be dissolved into a relatively low-strength solution that does not require secondary containment. It disinfects for all areas of water purification and is a Class III oxidizer, which is considered a manageable fire hazard. The chemical also poses no lethal threat to people or the environment.
Calcium hypochlorite tablets have one less than ideal characteristic—the dissolution process into water can be a challenge to control. Depending on the manufacturer, calcium hypochlorite tablets come in a variety of sizes and shapes. Regardless of size or shape, tablets are indiscriminately piled in a vertical column where, depending on the system, the lowest layer is exposed to water being passed over and around the tablets.
Higher flow rates contact more tablets, bringing more chemical into the solution. Some technologies utilize a single pass of high-volume, low-velocity water to erode the tablets, while others recirculate all or part of the water in order to control the solution strength. Another utilizes a water spray that dissolves the tablet through erosion to accomplish the same process.
Manufacturers are sensitive about the technology, arguing that its methods either erode, dissolve or accomplish a little of both in order to get the tablets into solution. However, it really does not matter because the end result is to dissolve the tablets in water at a consistent strength. All concepts wind up with more or less solution in a tank, where it is metered into the system as needed for disinfection with a balance of residual left to protect the clean water through distribution.
There are a couple of variables that can make this process somewhat complicated to control. First, there is the issue of tablet size and shape. The larger the tablet, the less dense the “stack,” creating more space between the individual tablets. As the tablets dissolve, their arrangement in the feeder will vary and so will the amount of area contacted by the moving water. Variable surface contact area results in variable strength. It seems like a little thing, but regardless of size, a tablet standing on its edge has much less area contacting the water than if it happens to be lying flat. As the column of tablets is consumed, this arrangement is constantly changing.
There is also the issue of humidity and length of time that tablets are stored in the damp environment of a feeder. Systems that have a low consumption rate may be exposed to this moist environment for several weeks. During that process, tablets absorb moisture and become softer, requiring less interaction with water in order to be dissolved. Therefore, unless the system is adjusted over time, the solution strength can vary as the tablet ages and becomes softer. Finally, the tablet-forming process can produce variations in hardness or consistency, which affects the solution make-down process.
Given these variables, the real issue is how to respond to changes quickly and accurately enough to maintain the target residual. Even with changes in water flow or velocity across the tablet bed, it can take several minutes before there is a noticeable change in the dissolution process. Because all systems feed a solution reservoir, there is the issue of as much as 20 gal of solution that must be consumed before there is a measurable difference in solution strength entering the process stream.
Without question, there are several manufacturers that are successful in their approach to these issues; however, as stated earlier, one has to question if there is not an easier way. What is needed is a fast, easily controlled means by which the dissolution process can be changed—one that will easily interface with digital controls or, in some cases, manual adjustments.
Imagine the same basic column of tablets suspended with only a portion of the bottom layer in contact with water. A reduced flow of water is introduced through the tank by means of a weir, which contacts the tablet bed when started and falls away from the tablets when turned off. Outside and attached to the erosion chamber is a transducer capable of generating ultrasonic waves. The transducer is capable of creating an acoustic wave that induces alternating compression and rarefaction fronts that produce cavitation.
The cavitation phenomenon works on microscopic inclusions on entrained air or vapor of the liquid. The release of energy during this process causes solids held together by binders to become separated and blend into solution at a consistent and predictable rate. These ultrasonic waves can be transmitted through the walls of the water chamber into the water, contacting the tablets without any measurable loss in intensity.
Using this process, it is possible to go from virtually no energy in the tank to a powerful energy capable of aggressively dissolving the hardest tablet with ease. An additional benefit is that the ultrasonic process keeps the interior of the erosion chamber perfectly clean with no buildup of calcium or other solids during the erosion process. Because the solution strength can be made stronger, less water volume is used in the process. Controlling ultrasonic energy is accomplished with a simple rheostat or digital input having zero dead-band between minimum and maximum power settings.
This concept makes good chemistry better simply because it makes the process easier and more precise to control. Having manufactured hundreds of tablet systems using all major technologies, I can attest that there are several good ways to approach this issue. With that being said, I believe that ultrasonic energy offers some exciting advantages over current technology. Instantly responding controls, less volume of water and much less maintenance are powerful arguments to give this technology a close look.