ESEL Paper Review_20130922
By Hong Guo
Mail: hongguo@gist.ac.kr
Phone: (+82) (0)10 82276568
1, Title and Author
Title: Extension of ASM3 for two-step nitrification and denitrification and its calibration and validation with batch tests and pilot scale date
Journal: Water research
Authors:
David Kaelina, Reto Mansera, Leiv Riegerb, Jack Eugstera, Karin Rottermanna, Hansruedi Siegristaa,*
a Ewag,Swiss Federal Institute of Aquatic Science and Technology, Uberlandstrasse 133, P.O. Box 611, Dubendorf, Switzerland
b modelEAU, Universite Laval, Department de genie civil, Pavillon Adrien-Pouliot, 1065 av. De la Medecine, Quebec QC.
2. Summary of Paper
? The activated sludge model no.3 was therefore extended for two-step nitrification and two-step denitrification in order to better describe nitrite dynamics especially during the treatment of communal wastewater.
? Nitrite was included as a new model compound and as an intermediate product of biological processes, both of heterotrophic and autotrophic bacteria.
? Two new model compounds replace Xa, the original autotrophic biomass, Ammonium Oxidizing Bacteria, Xaob and Nitrite Oxidizing Baceria, Xnob.
? Growth and decay processes of nitrifiers were split into AOB and NOB processes (3 additional processes) and heterotrophic anoxic processes were also doubled in order o account for two-step denitrification (4 additional processes)
3. Results
Model extensions
? Starting from the original ASM3 the activity of autotrophic organisms is described with 6 processes rather than 3 previously. All processes are doubled, and the autotrophic population is separated into Xaob and Xnob. The anox respiration of nitrifiers is assumed to be negligible with regard to nitrite reduction therefore only anoxic respiration over nitrate is considered for the autotrophic microorganisms
? The activity of heterotrophic organisms is described with 12 processes instead of 8 previously. All anoxic processes are doubled, including a first step from nitrate to nitrite and second from nitrite to molecular nitrogen.
Model calibration
? Considering that AOB and NOB are exposed to the same mass transfer effects the oxygen Monod constants were set equal in the model
? In the model only the aerobic respiration rates with intermittent aeration are introduced. The anoxic rate is simplified as being a fraction of the aerobic rate therefore a reduction factor , the same for AOB and NOB is used.
? The net growth rate of autotrophic bacteria is significant for the calculation of the minimum sludge retention time with regard to the nitrification capacity. Series of dedicated batch tests were made to assess net growth rates and their temperature and to illustrate the differences between AOB and NOB
? In this study of the calibration was made according to ta process of manual trial and error . The generation and use of statistical indicators such as correlation coefficient, average error, root mean square error or modelling efficiency was of little practical help and expert knowledge and visual fitness of the modeled curved were rather sued for the ultimate parameter calibration
? The selection of sensitive parameters for the calibration was made according to indications from earlier studies and an experience based approach.
Model validation
? The aeration control strategy of period B had a similar and even stronger effect on nitrite production than in period A. Constant aeration in Bio3 induces deficient aeration in Bio2, which leads to incomplete nitrification and high nitrite values in the effluent.
? Furthermore, since the maximum net growth rate of AOB is slightly higher than that of NOB above 20 c the NOB get progressively washed out during period B and nitrite started to accumulate in Bio3
? Heterotrophic denitrification is a complex combination of parallel and sequential processes carried out by different groups of heterotrophic microorganisms. The suggested two-step denitrification model with only one heterotrophic microorganism is therefore a strong simplification, similar to the simplified ASM3 assumption having first storage of substrate and then growth on storage product instead of parallel processes including direct growth on substrate. This limits the application of the extended model to the simulation of nitrite accumulation in conventional municipal wastewater treatment and the tested pre-denitrification flow schemes mostly used in Europe.
? For other flow schemes, as intermittent aeration and SBR the validity of the model parameters has to be verified with corresponding data.
? The presented model cannot be applied to side stream processes with high ammonium inlet concentrations because the accumulation of ammonium oxidizers and heterotrophic nitrite reducers due to inhibition of nitrite oxidizers cannot be accurately modeled.
4. Contribution:
? The presented two-step nitrification/denitrification model with one heterotrophic microorganism is a strong simplification of reality and limits the application of the extended model to the simulation of nitrite accumulation in municipal wastewater treatment.
? The model was able to reasonably describe the dynamics of the investigated pre-denitrification full-scale plant with a standard set of kinetic and stoichiometric parameters. For other flow schemes, as intermittent aeration and SBR the validity of the model parameters has to be verified with corresponding data
This research studies the behavior of activated sludge and the obtained profiles are very appropriate date for future studies.
5. Contact (Mail address): leiv.rieger@gci.ulaval.ca