METHODS FOR MEASURING NITRIFIER MAXIMUM SPECIFIC GROWTH RATE: CRUCIAL FOR PLANT CAPACITY RATING

Nitrification kinetics is one of the most important parameters in activated sludge process design and determines the necessary solids retention time (SRT) and aeration tank volume to meet an effluent ammonia-nitrogen (NH4-N) concentration goal. The necessary SRT is directly related to the maximum specific growth rate of the nitrifying bacteria (μmax), a key term in the Monod growth kinetics equation. Reported values for μmax vary by a factor of five. In view of the uncertainty about nitrification kinetics, methods are needed to provide site-specific assessments of nitrification kinetics for use in plant capacity evaluation and optimization. The purpose of the work reported here was to evaluate three different bench-scale methods for μmax determinations.The three test methods are the Low F/M SBR, High F/M, and Washout test methods. The Low F/M method requires operating a sequencing batch reactor (SBR) for a time period equal to at least three SRTs, followed by a series of batch nitrification rate tests using the SBR mixed liquor. In the High F/M test, a small seed is placed in a reactor with a wastewater sample spiked with NH4-N and alkalinity, and exponential increase in nitrate and nitrite (NOx-N) concentrations is followed over about a 5-7 day period. For the Washout test, a nitrifying mixed liquor is placed in a reactor fed with wastewater spiked with NH4-N and alkalinity and the reactor NH4-N and NOx-N concentrations are monitored with time over about a 5-7 day period. The feed rate to the reactor is selected so that a low enough SRT exist and the nitrifying bacteria are washout out of the reactor over time. Tests were done with all these methods using wastewater from the Renton, WA. and Puyallup, WA. wastewater treatment plants. The model fitting methods to obtain max from the data obtained for each method are presented, and the results of experiments to assess some key operating variables for the Low F/M and High F/M methods are presented.All three methods provide reliable μmax values from test data, and advantages and disadvantages of using a particular method is presented. To obtain μmax values a value for the endogenous decay coefficient for nitrifying bacteria is critical. Tests at temperatures ranging from 12 to 22°C found a 20°C specific endogenous decay coefficient of 0.17 g/g-d, with a temperature correction factor of 1.03. A temperature correction factor of 1.072 was obtained for μmax. Reports on nitrification kinetics must include both μmax and decay coefficient values.