This section deals with the first of the two processes involved in biological nitrogen removal: i.e. nitrification, where ammonium is converted into nitrate by autotrophic bacteria. The theoretical basis of this biological process will be presented. Furthermore model equations are developed which allow the steady state model for the activated sludge process, to be extended with the nitrification process. More specific, the following issues will be discussed:
- Nitrification kinetics
- Operational factors influencing the nitrification process
- Determination of nitrification kinetics
- Nitrification in activated sludge systems with unaerated zones
- Nitrification capacity and -potential
(1) Nitrification kinetics
Nitrification can be modelled with simple Monod kinetics, i.e. with the specific growth- and decay rates and the half saturation constant for ammonium. For the purpose of advanced modelling (for instance for simulation or process control), supplementary control functions may be included: e.g. for alkalinity and oxygen. However, when the objective is to design a nitrogen removal system, this will not be required. The effluent ammonium concentration of a completely mixed process is predicted from the value of the kinetic parameters and the applied (aerobic) sludge age. The minimum aerobic sludge age required for nitrification is calculated. Based on an extensive review of literature, values and temperature dependencies for the kinetic parameters are suggested. Click here to download this section.
(2) Operational factors influencing the nitrification process
Several factors influence the nitrification process and more specific the value of the specfic nitrifier growth rate: the mixed liquor temperature, -dissolved oxygen concentration and - pH value. Click here to download this section.
(3) Determination of nitrification kinetics
A method is presented to determine the values of the main kinetic parameters of the nitrification process: the specific growth rate, the decay rate, the half saturation constant of ammonium and the half saturation constant of oxygen. The method relies on the cultivation of a nitrifying sludge under steady state conditions and the subsequent application of respirometrics (oxygen uptake rate measurements). An extensive application example of the method is presented in Example A4.1. To download this section, click here.
(4) Nitrification with unaerated zones
In activated sludge systems designed for nitrogen removal, part of the reactor volume will not be aerated to allow for denitrification. The effect of this unaerated zone on the nitrification kinetics and the residual ammonium concentration is determined. A new parameter is defined: the maximum anoxic sludge mass fraction. This value is not to be exceeded if a certain nitrification efficiency is to be maintained. Other factors will set a maximum to this mass fraction other than that set by the requirements for nitrification. Click here to download this section
(5) Nitrification capacity and -potential
Two important parameters are introduced that relate to the nitrogen removal performance of an activated sludge system. The first one is the nitrification potential, defined as the Total Kjeldahl Nitrogen (TKN) concentration in the influent that is available for nitrification. The second one is the nitrification capacity, defined as the influent TKN concentration that is effectively nitrified in the activated sludge system. Click here to download this section.