In this section the focus will be on denitrification, the second of the two processes involved in the biological removal of nitrogen, in which nitrate is converted into nitrogen gas. The theoretic principles of denitrification are discussed and model equations are developed to extend the steady state model for the activated sludge system. More specific, the following issues will be discussed:
- Prerequisites for the denitrification process
- Configurations for nitrogen removal
- Denitrification kinetics
- Denitrification capacity
(1) Denitrification prerequisites
The necessary conditions for the denitrification process to develop in an activated sludge process can be summarised as:
- Presence of a facultative bacterial mass, capable of using oxygen and nitrate or nitrite
- Presence of nitrate and absence of dissolved oxygen in the mixed liquor (i.e. an anoxic environment)
- Suitable environmental conditions for bacterial growth
- Presence of an electron donor (nitrate reductor): i.e. organic material
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(2) Configurations for nitrogen removal
Denitrification requires organic substrate for the reduction of nitrate to nitrogen gas. The first distinction is whether the source of the organic substrate is internal (i.e. present in the wastewater) or external (i.e added to the wastewater). The second distinction is between pre-denitrification (pre-D) systems, where the denitrification reactor precedes the nitrification reactor, and post-denitrification (post-D) systems, where it is the other way around. Combinations of pre- and post denitrification are also quite common, as in the popular Bardenpho configuration. In this section the design principles, advantages and disadvantages of the main configurations for nitrogen removal will be discussed. Click here to download this section.
(3) Denitrification kinetics
The kinetics of the denitrification process are determined for both pre-D and post-D anoxic reactors. The minimum anoxic sludge mass fraction is determined, which maximizes nitrate removal with the easily biodegradable material present in the influent. Click here to download section.
(4) Denitrification capacity
Similar to the nitrification capacity, the denitrification capacity indicates the amount of nitrate that can be removed per litre of influent. The value of this parameter is different for pre-D and post-D anoxic reactors and is a function of the kinetic parameters of the denitrification process, the influent COD composition and the applied sludge age. Click here to download this section. Several examples illustrate the application of the concepts of nitrification- and denitrification capacity: Example 4.5. and Example 4.6