The Canadian

Centre for Environmental Modelling and Chemistry

Sewage Treatment Plant (STP) Model

Version 1.50 - October 2001

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 STP designs and operating conditions. 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
  • molecular mass
  • biogradation data ie. half-lives in primary clarifier,
    aeration vessel, and final setting tank
  • Type 1 chemicals

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

    • - partition coefficients

    STP Design Properties:

  • primary clarifier area
  • primary clarifier depth
  • aeration tank volume
  • final clarifier area
  • final clarifier depth

    • Operating Conditions:

  • influent rate
  • influent VSS
  • fractional removal of VSS in primary sludge
  • VSS concentration in primary sludge
  • sludge recycle as fraction of influent
  • waste sludge as fraction of influent
  • MLSS concentration in aeration vessel
  • VSS concentration in effluent
  • air flow as multiple of aeration tank
  • chemical concentration in influent

    • 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

    • Minimum system requirements:
    Pentium with 8 Mb of RAM
    Monitor with SVGA (800 x 600) resolution
    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 August 12, 2011.