DBP THM Removal
In-tank aeration is the process of exposing finished drinking water to air and has been proven to be an effective way to remove Trihalomethanes (THMs). There are several different systems to choose from, but the capital and energy costs of technologies vary greatly.
In a bubble aeration system, air is injected at the bottom of a tank to create bubbles. As the bubbles rise, THMs diffuse from the water into the bubble, until the bubble becomes saturated with THMs. Eventually, the bubble reaches the surface and pops - delivering THMs into headspace.
In a surface aeration system, a motorized device churns the water and air together, allowing THMs to transfer from water into the air.
Spray aeration is the process of pumping water from the floor of the tank to the ceiling, and spraying it down through a set of spray nozzles. As each droplet flies out of spray nozzle, THMs evaporate out of the droplet and into the air.
Energy Costs Over the lifecycle of an in-tank aeration system, energy costs dominate. Different aeration technologies can vary in their effectiveness and energy usage, so it is important to look at the energy consumption versus MGD of water treated. Read the comparison between In-Tank Aeration Systems.
Science of Aeration In-tank aeration is the process of exposing finishing drinking water to air any volatile chemicals that are in the water (such as disinfection byproducts) will evaporate into the air.
Effect on Trihalomethanes (THMs) Trihalomethanes are formed from the reaction between natural organic matter in raw water and chlorine (Cl) disinfectant. They continue to form as water travels through distribution system and ages, especially in tanks, and are the most common regulated DBP. In-tank aeration deals with THMs where they are the highest and is very effective at removing THMs.
Effect on Residual Chlorine and chloramine are more stable in water than THMs, and we have seen little to no loss of residual in our in-tank aeration installations. In side-by-side measurements, we measured roughly 10% reduction in residual chlorine in a system that removed 50% of TTHMs. At pH levels lower than 7, we expect a greater fraction of residual chlorine to exist in the form of hypochlorous acid, which is slightly volatile. Therefore, we generally expect more residual chlorine to be lost due to aeration in low-pH systems.
Effects on Water Chemistry There are other benefits of in-tank aeration besides THM reduction. Mixing is an essential component of in-tank aeration - it contributes to THM reduction and helps lower any other volatile compounds in the water, such as hydrogen sulfide (H2S). Aeration can also lower carbon dioxide (CO2) levels. Some municipalities have found that pH levels rose naturally in their water due to the removal of CO2 by the aeration process and no longer needed to feed caustic soda into their system, saving roughly $5,000 a month.