Water Environment Federation (WEF)

The Dangers of Uncalibrated Activated Sludge Simulation Packages

Activated sludge simulation packages are useful tools, but it can be dangerous to rely on them if
they are used incorrectly. How dangerous? To determine potential pitfalls, we compared the
activated sludge tank volumes predicted by three simulation packages used directly out-of-thebox
without calibration. We sought to identify the underlying reason for differences in predicted
required volumes.

We analyzed a simple activated sludge plant to determine the relative volumes required to achieve effluent total ammonia of 1 milligram per liter (mg/L) identify abbreviations. The programs show substantial variation in projected plant performance. Plant designs based on the simulation programs would thus differ substantially. This paper shows how plant designs would vary by analyzing solids retention time (SRT), effluent ammonia, apparent yield, and relative volumes.

We completed an extensive evaluation of the kinetic and stoichiometric parameters in each of the models. However, once these parameters were set to the same values, we continued to find variation in the projected plant performance.

Simulation packages for activated sludge modeling are used for a range of purposes, including option screening, process sizing, detailed design, plant optimization, and process control. Because of limited data, time, and budget, these simulation programs are often used without adequate influent wastewater characterization (i.e., definition of influent chemical oxygen demand and nitrogen fractions) and/or model calibration (i.e., adjusting kinetic and stoichiometric parameters within the model to achieve the same effluent quality of the actual plant).

We examined the differences in process sizes predicted by three uncalibrated simulation programs. We hypothesized that the differences in results from uncalibrated simulations are due to one or more of the following:

  1. Differences in the default values for distribution of biochemical oxygen demand (BOD) into chemical oxygen demand (COD) and nitrogenous fractions in wastewater influent,
  2. Differences in the default values for the kinetic and stoichiometric parameters used in biological growth and chemical equilibrium equations, and
  3. Differences in the inherent model structure, that is, the equations used to describe biological growth and chemical equilibrium.

While the first two items can be changed by the user, the third item is inherent to the activated sludge model selected for the simulation and—once a given model in a given simulation package is selected—cannot be altered.

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