Continuous Stirred Tank Reactor with Activated Sludge Recycle

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This Demonstration considers a model for the dynamics of a continuous stirred aeration tank processing activated sludge.


The activated sludge process involves the aeration and agitation of an effluent in the presence of a flocculated suspension of microorganisms that are supported on particulate organic matter. After a predetermined residence time, the effluent is passed to a sedimentation tank where the flocculated solids are separated from the treated liquid.


Contributed by: R. Ricardo Sánchez (August 2022)
Open content licensed under CC BY-NC-SA



The volumetric flow rate entering and leaving the reactor is the sum of the influent and recycle flow rates:


The recycle ratio is defined by:




The reactor residence time is defined as the volume of the reactor divided by the volumetric influent flow rate:


The concentration factor is defined as the ratio of the recycle biomass to effluent biomass concentrations:



: biomass concentration in reactor (),

: substrate concentration (),

: sludge concentration factor (dimensionless),

: recycle ratio (dimensionless).

The model is described in detail in [1, Exercise 3.3.2].

Suggestions for further exploration:

1. Vary the recycle ratio to see how the performance of the process is influenced.

2. Vary the values of the flow rate to see how the process is influenced.


[1] J. B. Snape, I. J. Dunn, J. Ingham and J. E. Prenosil, Dynamics of Environmental Bioprocesses: Modelling and Simulation, New York: VCH, 1995.

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