Potential Pitfalls in Wastewater Process Model Calibration and Application


  • Joe Rohrbacher, Ron J. Latimer, Paul A. Pitt, Katya Bilyk - Hazen and Sawyer

Application of advanced activated sludge process simulators such as BioWin and GPS-X has increased significantly over the past decade. These software packages are powerful tools for evaluation, optimization, and design of wastewater treatment plants (WWTPs). Use of uncalibrated models presents risks of poor correlation with actual operations and performance; however, use of an incorrectly calibrated model that was calibrated to bad data or incorrect assumptions can be more probelmatic. Issues that can have major impacts on model calibration include sampling and analysis problems, non-ideal flow splitting, backmixing within reactors, flow meter calibration, and non-ideal DO control. Case studies will be used to demonstrate potential impacts to model calibration and application including the following:

Calibration with Poor Data:
Influent loading and sludge production are the two most fundamental modeling issues for calibration. However, many times the historical data related to these parameters has been found in error. Sampling location issues, not cleaning sample lines, interference from recycles and septage, etc. can have major impacts on the influent load estimates. Sludge production as recorded by flow meters and sludge sampling frequently present mass balance and yield calculation problems.

Not Accounting for the Whole Plant:
Calibration and application of the model using a whole plant approach is critical, especially from a nutrient removal perspective. Quantifying and understanding solids handling process impacts on the main plant is important, even if the plant does not have digestion processes. Case studies will be provided to demonstrate the whole plant issues.

Not Properly Accounting for Operational Issues:
Calibration of the process simulator without fully accounting for site specific operational issues can also lead to significant calibration errors. Extensive field work and calibration examples have demonstrated the importance of doing thorough profiles during sampling campaigns for calibration. Examples of the issues important to understand include the following:

  • DO and nutrient profiles
  • Backmixing and short circuiting
  • Number of CSTRs in series
  • Flow spitting/distribution
  • Clarifier blanket levels and reactions
  • Water treatment plant sludge impacts (P adsorption, enhanced primary removals)

Potential Pitfalls in Model Application:
Once the process model is calibrated, application of the model for optimization or design also has potential pitfalls such as:

  • Applying an improperly calibrated model
  • Not accounting for diurnal impacts including of diurnal load variations and solids processing sidestreams
  • Not using oxygen modeling to account for dissolved oxygen in mixed liquor recycles and entering post anoxic zones

For more information, please contact the author at jrohrbacher@hazenandsawyer.com.

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