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science, research, technology How Alkalinity Affects Nitrification Use alkalinity profiling in wastewater operations to control biological activity and optimize process control The Water Environment Federation’s new Operations Challenge laboratory event will determine alkalinity needs to facilitate nitrification. Operators will evaluate alkalinity and ammonia by analyzing a series of samples similar to those observed in water resource recovery facilities. This event will give operators an under-standing of how alkalinity works in the wastewater treatment process to facili-tate nitrification, as well as the analytical expertise to perform the tests onsite. This provides the real-time data needed to perform calculations, since these analyses typically are performed in a laboratory that can present a delay in the data. What Is Alkalinity? The alkalinity of water is a measure of its capacity to neutralize acids. It also refers to the buffering capacity, or the capacity to resist a change in pH. For wastewater operations, alkalinity is measured and reported in terms of equivalent calcium carbonate (CaCO3). Alkalinity is com-monly 30 | RUMBLES SEPTEMBER 2015 measured to a certain pH. For wastewater, the measurement is total alkalinity, which is measured to a pH of 4.5 SU. Even though pH and alkalinity are related, there are distinct differences between these two parameters and how they can affect your facility operations. Alkalinity and pH Alkalinity is often used as an indicator of biological activity. In wastewater opera-tions, there are three forms of oxygen available to bacteria: dissolved oxygen (O), nitrate ions (NO-), and sulfate ions 23 (SO4 2-). Aerobic metabolisms use dis-solved oxygen to convert food to energy. Certain classes of aerobic bacteria, called nitrifiers, use ammonia (NH3) for food instead of carbon-based organic com-pounds. This type of aerobic metabolism, which uses dissolved oxygen to convert ammonia to nitrate, is referred to as “nitrification.” Nitrifiers are the dominant bacteria when organic food supplies have been consumed. Further processes include denitrifica-tion, or anoxic metabolism, which occurs when bacteria utilize nitrate as the source of oxygen. In an anoxic environment, the nitrate ion is converted to nitrogen gas while the bacteria converts the food to energy. Finally, anaerobic conditions will occur when dissolved oxygen and nitrate are no longer present and the bacteria will obtain oxygen from sulfate. The sulfate is converted to hydrogen sulfide and other sulfur-related compounds. Alkalinity is lost in an activated sludge process during nitrification. During nitri-fication, 7.14 mg of alkalinity as CaCO3 is destroyed for every milligram of ammo-nium ions oxidized. Lack of carbonate alkalinity will stop nitrification. In addi-tion, nitrification is pH-sensitive and rates of nitrification will decline significantly at pH values below 6.8. Therefore, it is important to maintain an adequate alka-linity in the aeration tank to provide pH stability and also to provide inorganic carbon for nitrifiers. At pH values near 5.8 to 6.0, the rates may be 10 to 20% of the rate at pH 7.0. A pH of 7.0 to 7.2 is By Mary Evans and Gary Sober


RUMBLES - September 2015
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