Design of a WWTP with nitrogen removal

Modelling and control of a WWTP with nitrogen removal
Case study: the Sharon process for coupling with Anammox


Partial nitrification techniques are often denoted as very promising for improving sustainability of wastewater treatment. Their realization of nitrification/denitrification with nitrite as intermediate is beneficial both in terms of oxygen consumption, translated into aeration energy costs, and COD-source addition costs.

Up till now, the Sharon (Single reactor High activity Ammonia removal Over Nitrite) process is the only one in which partial nitrification of ammonium to nitrite is successfully established over a long time period. Its set-up consists basically of a continuous, completely mixed reactor without sludge retention, operating at high temperature (35C) and an average pH-value of about 7. Under these conditions, ammonium oxidizers grow faster than nitrite oxidizers, which can be washed out by sufficiently lowering the hydraulic retention time. Denitrification is performed for pH-control, by only cyclically aerating the reactor and adding a COD-source such as methanol in the unaerated phases. As the effluent ammonia concentration is independent of the inlet ammonia concentration, the Sharon process is better suited to achieve substantial ammonia conversion at high reaction rates for relatively concentrated flows, rather than to meet strict effluent standards. It is very well applicable for the treatment of reject water from sludge digestion, where high temperature and pH are taken advantage of.

The Sharon process was developed more than five years ago at the Delft University of Technology. Microbial kinetic and stoichiometric parameters for ammonia oxidation were determined under the actual process conditions by lab-scale (1.5 l) tests. Making use of both these experimental data and literature, the development of a dynamical process model in Matlab 4.2 - Simulink 3.0 for further process design and scale-up was started. This model combines microbial kinetics with chemical dissociation equilibria and gas-liquid mass transfer. The insights gained by the simulations allowed the immediate design and construction of a full scale process without the need for a costly and time consuming intermediate pilot test. The full-scale process is already operational since January 1999 at the Rotterdam Sluisjesdijk sludge treatment plant. More information on this full-scale implementation, as well as on the main principles of the Sharon process, is to be found on http://www.bt.tudelft.nl/bpkf.htm

In the last few years, the idea to couple the Sharon process with a so-called Anammox (anaerobic ammonia oxidation) process, in which equal amounts of ammonium and nitrite react to form nitrogen gas under anaerobic conditions, finds more and more support. The required ammonium/nitrite ratio for Anammox can be produced by operating the Sharon process without pH control. Benefits of this combined Sharon-Anammox process compared to the Sharon process with denitrification are the reduction of aeration costs, the omission of the need for additional COD source, and the possibility to obtain low nitrogen effluent concentrations.

To gain insight into the coupled Sharon-Anammox process, the former simulation model has been upgraded for use in Matlab 6.0 - Simulink 4.0. Temperature and pH dependency of the model parameters have been thoroughly studied. A feasibility study for adjusting the ammonium/nitrite ratio by pH control has been performed. The stoichiometric matrix has been rewritten in terms of yield and biomass composition to allow easy incorporation of the latest experimental results. A procedure for sensitivity analysis of the different model parameters is being developed, to provide a means of estimating the influence of inaccurate or uncertain parameter values. In future, the model will be used to design and tune controllers.

Project duration: 2000-02-01 - 2006-02-01


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Contact

Eveline Volcke
Department of Applied Mathematics, Biometrics and Process Control
Coupure Links 653
9000 Gent
Belgium
Tel: +32(0)9 264.59.37
Fax: +32(0)9 264.62.20
Email: eveline.volcke@biomath.rug.ac.be


Last update: 01 december 2008, webmaster@biomath.ugent.be


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