The Canadian

Centre for Environmental Modelling and Chemistry

Sewage Treatment Plant (STP) Model

Version 2.11 - March 2006

New in version 2.11!

Software Description

The STP model estimates the fate of a chemical present in the influent to a conventional activated sludge plant as it becomes subject to evaporation, biodegradation, sorption to sludges and to loss in the final effluent. The most critical and uncertain variable is the biodegradation rate constant and its dependence on biomass concentration.

Intended Uses
This model is useful for establishing the general features of a new or existing chemical's behaviour, in a sewage treatment plant. It is not intended as an accurate simulation of chemical fate in a specific plant, rather it is a screening level model whcih seeks to estimate the likely proportions of the chemical which leave the system by biodegradation, in sludge, by evaporation, and in the water outflow. These proportions are dictated by both the physical chemical properties of the substance and the characterisitics of the treatment plant. The effect of varying chemical properties such as half-lives and operating conditions can be readily explored.

Features of the STP model software:
Provides a database of chemicals and chemical properties.
Permits temporary additions/changes of chemicals and their properties to a simulation.
Permits permanent additions, changes and deletions of chemicals and their properties to the database.
Provides a database for treatment plants. A single configuration is supplied however, permanent additions, changes and deletions can be made by the user.
Permits temporary additions/changes of STP designs and operating conditions to a simulation.
Provides context-sensitive Help.
Displays and prints the STP model calculations, as performed by the software.
Allows the printing of simulation tables and the summary diagram.
Allows the program results to be saved as a comma separated value (*.csv) file readable by standard spreadsheet software.

This program was based on the following publication:

Environ. Sci. Technol. Vol. 29, p. 1488, 1995 by B. Clark, J.G. Henry, and D. Mackay.

The required input data are:
Chemical Properties:

  • chemical name
  • temperature
  • molar mass
  • biodegradation data as half-lives in a primary clarifier, aeration vessel, final setting tank, aerated and facultative lagoon or as the Ready test result
  • Type 1 chemicals

  • - water solubility
    - vapour pressure
    - log Kow
  • Type 2 and 3 chemicals

  • - partition coefficients

    Treatment Plant Properties:

  • number of tanks
  • influent flow and VSS concentration
  • gas and liquid phase mass transfer coefficients for each tank or lagoon

  • - area and depth
    - biomass fraction of VSS
    - sludge VSS concentration
    - outflow (ML)VSS concentration
    - aeration rate, if applicable
    - recycled fraction, if applicable
    - removed fraction, if applicable

    Model output includes:

  • Z and D values
  • steady state phase concentrations
  • process stream fluxes
  • relative amounts of chemical that are likely to be stripped or volatilized, sorbed to sludge, biodegraded, and discharged in the effluent
  • summary diagrams and charts
  • Minimum system requirements are an IBM-compatible PC running Windows 98 or XP. This model will not run under Windows NT or 2000.

    STP Version 2.10, released August 2004 and Version 1.50, released October 2001 continue to be available.
    For non-Windows users the original, DOS-based version is also available.

    This software is only available in compiled form. A "readme.txt" file with more detailed technical information is included.

    Please read the STP SOFTWARE LICENSE before downloading the software. Use of the software constitutes your agreement to abide by the terms and conditions set out in the license agreement.

    Bug Report


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    Last updated June 12, 2013.