In-tank aeration is a proven method for removing trihalomethanes (THMs) from finished drinking water storage tanks. However, aeration technologies can vary greatly in their effectiveness and energy usage and selecting in-tank systems can be complicated. Several factors must be taken into consideration when selecting a system and each system should be customized to the tank to maximize THM removal while minimizing energy cost. Below, we have answered four questions received during our last webinar on in-tank aeration to help explain how in-tank aeration systems are designed.
Q: How do you choose between spray and surface aeration when you design a system?
There are several factors we take into consideration when designing an in-tank aeration system, including tank type, amount of THM removal needed, energy efficiency and accessibility to the tank. Surface aerators are large pieces of equipment that weigh several hundred pounds. Lifting this large and heavy floating equipment up high into an elevated tank doesn't make sense - a crane may be required to install and service the equipment every time the system needs maintenance. For elevated tanks we recommend spray aeration with the pumps and controls located at the bottom of the tank and easily accessible. For ground storage tanks, surface aeration and spray aeration are both reasonable considerations - it often comes down to the type of tank, amount of THM removal needed and the energy consideration. When installing a spray aeration system in a concrete tank, the tank may need to be taken out of service for several weeks to anchor header pipes in the concrete and headspace. If taking the tank our of service is not an option, floating surface aerators are a better choice since they often can be installed without draining the tank. From an energy standpoint, spray aeration is more energy efficient than surface aeration.
Q. What is the required headspace and anticipated tank volume loss for in-tank aeration?
Surface aerators require at least four feet of headspace to operate effectively. For spray aeration to be economical, you need at least five feet of headspace. For a mixer and ventilation alone, you need only a small amount of headspace to let THMs out of the tank.
Q. Do you design systems that enable aeration to turn on or off based on water demand or water level changes?
We design control systems to allow in-tank aeration to be turned on or off not with respect to water level changes but rather to adjust to THM levels. For example, in San Jose, California an online THM analyzer continuously feeds data back to a controller which selectively turns components of the aeration system on or off based on THM levels and time of day. Many utilities will operate their aeration systems only in the spring, summer and fall when THMs are highest. They will turn the system off in the winter not just because of cold conditions and the potential for freezing, but also because their THM levels are lower during this time of year.
Q. Will aeration systems operate in winter?
Some aeration systems can operate year-round, but in general, exchanging cold air in and out of the tank can promote ice formation. Based on our experience with in-tank aeration in cold climates, we recommend you turn off the system in the winter but leave the mixer on year round. Mixers continuously blend water coming into the tank with the rest of the volume and are very good at preventing ice formation in water distribution tanks. We can also include a thermostat with the aeration system that will automatically turn the system off when temperatures drop below a predetermined temperature for a period of time.