Unsicherheitsanalyse und regionale Sensitivitätsanalyse eines Modells zur Nitrifikation im Biofilm
Zusammenfassung der Projektergebnisse
Mathematical modeling of biological processes is the basis for studying, designing, and improving biological wastewater treatment processes. Uncertainties in model parameters and model structure result in uncertainty in the model predictions. In this project, uncertainty and global sensitivity analysis methods were applied exemplarily to a mathematical model describing two-step nitrification in a biofilm. A qualitative and a quantitative method for sensitivity analysis were compared with respect to parameters identified as unimportant. Both methods identified the same set of unimportant parameters. The qualitative and computational less demanding sensitivity analysis method can, therefore, be used to identify unimportant parameters before applying a qualitative and computational high demanding sensitivity analysis for a more thorough analysis of the reduced parameter set. Furthermore, the results from both sensitivity analysis methods showed that parameters describing reactor operation had a larger impact on model results than kinetic parameters. Based on this result, the impact of a broad range of operation conditions on the coexistence of both groups of nitrifying bacteria was evaluated. For some biological nitrogen removal processes for wastewaters with high ammonium concentrations but low organic carbon content, it is of interest to stop nitrification at the level of nitrite and to displace nitrite oxidizers from the biofilm. Parameters describing the operation conditions that were evaluated included dissolved oxygen concentration in the bulk phase, temperature, ammonium influent concentration, ammonium surface loading rate, and extent of mass transfer resistance into the biofilm. The influence of operation conditions on outcompeting nitrite oxidizers from the biofilm was evaluated for three different scenarios: reactor start-up from scratch, maintaining partial nitrification in long-term, and washout of nitrite oxidizers from a fully nitrifying biofilm. The driving mechanism for maintaining partial nitrification to nitrite in long-term was competition for oxygen. Ammonium oxidizers have typically a higher affinity to oxygen than nitrite oxidizers and win the competition for oxygen inside the biofilm. A higher affinity to oxygen for ammonium oxidizers than for nitrite oxidizers was crucial to maintain partial nitrification to nitrite in longterm. If ammonium and nitrite oxidizers have similar oxygen affinity constants, nitrite oxidizers can compete with ammonium oxidizers for oxygen, and are able to establish in the biofilm. For similar oxygen affinity constants, outcompetition of nitrite oxidizers was only possible at low nitrite effluent concentrations. Low nitrite effluent concentrations are, however, not aimed for when applying partial nitrification to nitrite in wastewater treatment. For outcompeting nitrite oxidizers from an already nitrifying biofilm, elevated temperatures were crucial that ensured higher growth rates for ammonium oxidizers than for nitrite oxidizers. Inhibition of nitrite oxidizers by free ammonia was not required to outcompete nitrite oxidizers or to prevent re-growth of nitrite oxidizers in long-term. Inhibition by free ammonia can, however, speed up outcompetition of nitrite oxidizers.
Projektbezogene Publikationen (Auswahl)
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(2007). Comparing global sensitivity analysis for a biofilm model for two-step nitrification using the qualitative screening method of morris or the quantitative variance-based fourier amplitude sensitivity test (FAST). In: 7th International IWA Symposium on Systems Analysis and Integrated Assessment in Water Management, May 7-9, 2007, Washington DC, USA
Brockmann, D. and Morgenroth, E.
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(2007). Comparing global sensitivity analysis for a biofilm model for two-step nitrification using the qualitative screening method of morris or the quantitative variance-based fourier amplitude sensitivity test (FAST). Water Science and Technology, 56(8): 85–93
Brockmann, D. and Morgenroth, E.
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(2008). Monte Carlo filtering to evaluate operation conditions for outcompeting nitrite oxidizers and maintaining partial nitrification in biofilm systems. In: IWA Conference on Biofilm Technologies, January 8-10, 2008, Singapore
Brockmann, D. and Morgenroth, E.
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(2008). Partial nitrification in biofilms: inhibition versus competition. In: IWA World Water Congress, September 7-12, 2008, Vienna, Austria
Brockmann, D. and Morgenroth, E.
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(2009). Evaluating operation conditions for outcompeting nitrite oxidizers and maintaining partial nitrification in biofilm systems using biofilm modeling and Monte Carlo filtering. Water Research
Brockmann, D. and Morgenroth, E.
