Does Chlorine Contact Tank Mixing Reduce Detention Time and THM Formation?
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Proper mixing increases detention time by adding a vertical plug-flow element to the flow of water through a chlorine contact tank. The increased detention time allows a plant operator to use a higher baffling factor used in contact time (CD calculations, thus reducing the concentration of chlorine needed to meet CT treatment requirements. Using less chlorine, in turn, reduces production of disinfection by-products (DBPs) such as trihalomethanes (THMs) and haloacetic acids (HAAs). Therefore, mixing a contact tank can be a low-cost way to achieve DBP compliance.
Chlorine contact tanks ensure disinfection effectiveness and compliance with the US Environmental Protection Agency (USEPA) Surface Water Treatment Rule for preventing waterborne diseases caused by cysts and viruses. Although chlorine's effectiveness partly depends on water temperature and pH, it primarily depends on the amount of time free chlorine is in contact with the water. Each state establishes a minimum chlorine contact time for various water sources, which results in a treatment parameter based on concentration x contact time, commonly referred to as the CT requirement.
For example, if a treatment plant must achieve a CT of 120, free chlorine (in mg/L) multiplied by the time the chlorine and water are together in the contact tank (in minutes) must equal or exceed 120. Therefore, the plant can meet the CT requirement with 2 mg/L of chlorine contacting the water for 60 minutes in the contact tank or—during periods of higher flow through the plant—3 mg/L of chlorine contacting the water for 40 minutes in the contact tank. In each case, 2x
Regarding time, USEPA and the drinking water industry have long known, through tracer studies, that the number of minutes that chlorine and water are together in a tank can't be accurately determined by merely dividing the tank volume by the flow rate. For example, if the flow rate is 1,000 gpm through a 500,000-gal contact tank, the calculated detention time is 500,000 gal/1,000 gpm = 500 minutes. However, in a simple tank, tracer studies have shown that—with just an inlet and outlet and no baffle curtain or mixing— the actual detention time is only 10 percent of the theoretical time, or 50 minutes. Therefore, this tank would be given a baffling factor of 0.10, meaning the actual detention time used for the CT calculation must be only 0.10 x the theoretical detention time.
If an operator doesn't conduct a tracer study to confirm detention time and baffling factor, USEPA assigns a chlorine contact tank a standard baffling factor based on its configuration. Standard baffling factors range from 0.1 (a tank with no baffles) to 0.3 (a tank with a single baffle) and up to 1.0 (perfect plug flow in a pipe).
In the example above, why is water in the tank for only 50 minutes? Water in reservoirs form thin horizontal layers of different densities, with the lightest layers at the top and the heaviest layers at the bottom (see Mix It Up! Solve Water Layering Problems, page 16).
Because of the high flow through a contact tank, temperature and salinity usually don't affect the formation of thin horizontal layers in the tank. However, the difference in density caused by pressure is present at every depth and is strong enough to cause layering of water that resists mixing from top to bottom.
Although the tank may have a 500,000-gal volume and an operating depth of 10 ft, tracer studies reveal that most of the 1,000 gpm enters at the tank's bottom, travels across the tank's bottom 1 ft, and exits the tank in only 50 minutes—instead of the theoretical 500 minutes. In other words, only about 1 ft of the tank depth (10 percent of the volume) is being used. With one baffle in the tank, only about 30 percent of the tank volume will be used.
A properly designed tank mixer continually pulls water from the tank's dense bottom layer and spreads it across the top of the tank, causing all other layers to move downward. When the tank's bottom layer of water moves to the surface, the only factor that made it the tank's most dense water is eliminated. l*he water floats evenly across the top of the tank because it's now the least dense water. If a mixer is of adequate size, all incoming water is continually transported to the surface and spread out across the top of the tank, creating a vertical plug flow that uses the entire tank volume for detention time.
After a mixer is installed, a tracer study can document the improved baffling factor, which should be about 0.5-0.9, depending on mixer size and design. Because many plants achieve CT compliance across specific plant sections involving several serial components and piping, the plants should conduct the studies regularly. Therefore, a process change in any section of a plant necessitates a new tracer study to be per formed. Although some states may require a tracer study at just one flow rate, other states require studies for several flow rates, with as many as four flow rates required for some systems.