science, research, technology 257 - 124 = 133 mg/L deficiency In this example, alkalinity is insufficient to completely nitrify influ-ent ammonia, and supplementation through denitrification or chemical addition is required. Remember that this is a minimum; you still need some for acid buffering in downstream processes, such as disinfection. Bioavailable Alkalinity Most experts recommend an alkalinity residual (effluent residu-al) of 75 to 150 mg/L. As previously identified, total alkalinity is measured to a pH endpoint of 4.5. For typical wastewater treat-ment applications, operational pH never dips that low. When measuring total alkalinity, the endpoint reflects how much alka-linity would be available at a pH of 4.5. At higher pH values of 7.0 to 7.4, where wastewater operations are typically conducted, not all alkalinity measured to a pH of 4.5 is available for use. This is a critical distinction for the bioavailability of alkalinity. Therefore, in addition to the alkalinity required for nitrification, additional alkalinity must be available to maintain the 7.0 to 7.4 pH. Typically, the amount of residual alkalinity required to maintain pH near neutral is between 70 and 80 mg/L as CaCO3. Proper Alkalinity Levels for Treatment Alkalinity is a major chemical requirement for nitrification and can be a useful and beneficial tool for use in process control. 32 | RUMBLES SEPTEMBER 2015 Figure 1. pH versus nitrification rates at 68ºF (maximum nitrification rate occurs at 8.0–8.5 pH). Source: EPA-625/4-73-004a, Revised Nitrification and Denitrification Facilities Wastewater Treatment, U.S. Environmental Protection Agency Technology Transfer Seminar. Several things to keep in mind: • Alkalinity provides an optimal environment for microscopic organisms whose primary function is to reduce waste.
RUMBLES - September 2015
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