Role of Solids Retention Time on Complete Nitrification: Mechanistic Understanding and Modeling

Effects of solids retention time (SRT) on both steps of the nitrification process (e.g., ammonia oxidation and nitrite oxidation) and the nitrifier community in a complete-mix activated sludge process were studied. At a SRT less than or equal to 20 days, the effluent ammonia concentration was lower than the effluent nitrite concentration. At the 40-day SRT, however, the effluent nitrite concentration became equal to or less than the effluent ammonia concentration. Quantitative Polymerase Chain Reaction (QPCR) assays indicated that increasing SRT significantly increased nitrite-oxidizing bacteria/ammonia-oxidizing bacteria (NOB/AOB) ratio. Further investigation indicated that Nitrosomonas europaea/eutropha were the dominant AOB for ammonia oxidation under all SRTs. However, for nitrite oxidation, Nitrobacter-like NOB played the key role at low SRTs, while Nitrospira-like NOB played the key role at high SRTs. Modeling results indicated that, for AOB and NOB, the values of maximum specific growth rate (μ) were 0.24 and 0.18/day, respectively, while the values of decay coefficient (Kd) were 0.066 and 0.045/day, respectively. The half-velocity constants (KS) for both AOB and NOB were approximately 0.02 mg/L, and the maximum specific substrate utilization rate (k) for AOB and NOB were 1.3 and 3.0 g-N/g-VSS-day, respectively. These values suggest that AOB had an advantage over NOB at a lower SRT, mainly because AOB grew faster than NOB did. On the contrary, the increased NOB/AOB ratio at a longer SRT was likely due to NOB having a smaller kd. The very low KS values for both AOB and NOB imply that a plug-flow reactor may not have much kinetic advantage over a complete-mixed reactor for a complete nitrification process. The mass concentrations of AOB and NOB in the activated sludge can also be calculated based on the measured nitrification capacity and their k value.