| The objective of this study was to measure the maximum specific growth rate of nitrifying bacteria active in municipal wastewater treatment plants. The operation of the plants was simulated using bench scale sequencing batch reactors (SBRs) operated at solids retention times ranging from eight to 20 days at 20°C. A total of four reactors were operated over the course of two years using settled wastewater from two plants located in New York City. Once steady nitrification of ammonia was achieved in the SBRs, nitrification rates were measured and used to compute the maximum specific nitrifier growth rate, muAMAX. The value of muA MAX was also determined using a second method that is commonly known as the "High F/M Method".; The measured nitrifier decay rates, bA ranged from 0.09 to 0.11 d-1, more than twice the value of 0.04 d -1 typically used in engineering calculations. When calculated using the experimentally derived nitrifier decay rates, muA MAX averaged 43% and 49% greater than the values calculated using the decay rate of 0.04 d-1. The experiments demonstrated that the value of bA was critical in calculating the associated mu AMAX. Thus, the maximum specific growth rate and decay rate must be treated as a pair, and always reported as such. The maximum specific nitrifier growth rates determined from the exponential growth batch experiments were always higher than the values derived from methods using the sequencing batch reactors. For example, the rates determined from exponential growth tests were 5% higher than SBR derived values for one facility, whereas for a second facility the exponential growth tests were 37% higher. Though higher values of muAMAX may require a higher safety factor for design purposes, the high F/M method is less labor intensive, of shorter duration, and is less costly, making it more appealing to municipalities.; For the two wastewaters tested the corresponding values of mu AMAX and decay rate were 0.55 d-1, 0.11 d-1, and 0.83 d-1, 0.09 d -1, respectively. The findings suggest that varied between facilities, and must be measured prior to utilizing the parameter for determination of the nitrification capacity of the facility. Furthermore, the nitrifier decay rate should be measured in tandem with muAMAX in order to assure proper design of the biological nitrification process at a cost effective basis. |
