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science, research, technology Together we can do a world of good. Denver • Boulder • Lakewood • Highlands Ranch Contact Jack Bryck 303 316 6535 RUMBLES SEPTEMBER 2015 | 31 normally used to maintain reasonable nitrification rates, and for locations with low-alkalinity waters, alkalinity is added at the water resource recovery facil-ity to maintain acceptable pH values. The amount of alkalinity added depends on the initial alkalinity concentration and amount of NH4-N to be oxidized. After complete nitrification, a residual alkalinity of 70 to 80 mg/L as CaCO3 in the aeration tank is desirable. If this alkalinity is not present, then alkalinity should be added to the aeration tank. Why Is Alkalinity or Buffering Important? Aerobic wastewater operations are net-acid producing. Processes influencing acid formation include, but are not limited to: • biological nitrification in aeration tanks, trickling filters, and rotating biological contactors; • the acid formation stage in anaerobic digestion; • biological nitrification in aerobic digest-ers; • gas chlorination for effluent disinfec-tion; and • chemical addition of aluminum or iron salts. In wastewater treatment, it is critical to maintain pH in a range that is favorable for biological activity. These optimum conditions include a near-neutral pH value between 7.0 and 7.4. Effective and efficient operation of a biological process depends on steady-state conditions. The best operations require conditions without sudden changes in any of the operating variables. If kept in a steady state, good flocculating types of microorganisms will be more numerous. Alkalinity is the key to steady-state operations. The more stable the environment for the microorganisms, the more effectively they will be able to work. In other words, a sufficient amount of alkalinity can provide for improved performance and expanded treatment capacity. How Much Alkalinity Is Needed? To nitrify, alkalinity levels should be at least eight times the concentration of ammonia in wastewater. This value may be higher for untreated waste-water with higher-than-usual influent ammonia concentrations. The theoreti-cal reaction shows that approximately 7.14 mg of alkalinity (as CaCO3) is con-sumed for every milligram of ammonia oxidized. A rule of thumb is an 8-to-1 ratio of alkalinity to ammonia. Inad-equate alkalinity could result in incom-plete nitrification and depressed pH values in the facility. Plants with the ability to denitrify can add back valu-able alkalinity to the process, and those values should be taken into consider-ation when doing mass balancing. (For the Operations Challenge event, the decision has been made to not incor-porate the denitrification step in pro-cess profiling.) To determine alkalinity requirements for plant operations, it is critical to know the following param-eters: • influent ammonia, in mg/L; • influent total alkalinity, in mg/L; and • effluent total alkalinity, in mg/L. For every mg/L of converted ammonia, alkalinity decreases by 7.14 mg/L. There-fore, to calculate theoretical ammonia removal, multiply the influent (raw) ammonia by 7.14 to determine the mini-mum amount of alkalinity needed for ammonia removal through nitrification. For example: Influent ammonia = 36 mg/L 36 mg/L ammonia × 7.14 mg/L alka-linity to nitrify = 257 mg/L alkalinity requirements 257 mg/L is the minimum amount of alkalinity needed to nitrify 36 mg/L of influent ammonia. Once you have calculated the minimum amount of alkalinity needed to nitrify ammonia in wastewater, compare this value against your measured available influent alkalinity to determine if enough is present for complete ammonia removal, and how much (if any) additional alkalin-ity is needed to complete nitrification. For example: Influent ammonia alkalinity needs for nitrification = 257 mg/L Actual measured influent alkalinity = 124 mg/L Passion. Commitment. Success. Denver Boulder Lakewood www.arcadis-us.com Imagine the result


RUMBLES - September 2015
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