The nitrification and denitrification unit processes can be divided into two broad classes, the attached growth systems, and the suspended growth systems.
In the attached-growth (biofilm) process (Chapter 5), the bulk of the blomass is retained on a medium and it does, therefore, not require a solids separation step for returning the solids to the nitrification reactor. The media that carry the nitrifying biofilm can be anything from plastic media to Nitrogen ion-selective zeolites. Trickling towers, Rotating Biological Contactors (RBC), Upflow Fixed Bed Reactors (UFBR) are the most widely used for biofilm systems.
Suspended-growth (activated sludge) processes (Chapter 6) take place on suspension of the biological solids in a mixed liquid. The result is the activated sludge processes, based on only nitrifying bacteria, or on a combination of oxidative and nitrifying bacteria, depending on the influent waste water. A subsequent clarification stage is required to return the microorganisms to the nitrification stage.
The activated sludge and the biofilm systems can be further subdivided into systems which use different variations of combined oxidation-nitrification processes and separate stages of nitrification or denitrification processes. Table 2.1 gives an overview of the different applications.
Further details of the different biological unit operations for the removal of nitrogen are outlined in Chapter 5 for the attached-growth systems and in Chapter 6 for the activated sludge processes.
The demarcation between the biofilm and the activated sludge processes is not always very clear. For example in the fluidized bed, the medium consists of solid particles covered with a biofilm, and moves in the reactor. This principle is, therefore, similar in some ways to the activated sludge process.
Table 2.1 Classification of different combined nitrification/denitrification and separate stage nitrification or denitriflcatlon units._
Combined carbon oxidation and nitrification processes (secondary treatment).
Suspended growth processes (activated sludge processes).
Activated sludge system. Single stage Two-stage
Attached growth processes (Biofilter processes). Trickling filters, with different filling material. Rotating Biological Contractors.
Upflow Fixed Bed Reactor (UFBR), applying different media.
Combination of Biofilter and activated sludge process in two stages. Combined nitrification and denitriflcatlon process.
Suspended growth processes.
Activated sludge systems with alternated oxic conditions.
Attached growth processes (biofilter processes).
Simultaneous nitrification and denitriflcatlon applying N-ionselective media in an upflow Fixed Bed Reactor (UFBR).
Separate stage nitrification processes (tertiary treatment).
Suspended growth processes. Activated sludge processes.
Attached growth processes (Biofilter processes). Trickling filter.
Rotating Biological Contactor (RBC) Upflow Fixed Bed Reactor (UFBR). Fluidized Bed Reactor.
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