An activated sludge process is in effect a suspended growth system aimed at achieving solid/liquid separation. Various types of activated sludge processes exist, including the sequencing batch reactor (SBR), membrane bioreactor (MBR) and conventional activated sludge. Each of these processes uses a different configuration of activated sludge and equipment in achieving solid/liquid separation.

The process design engineer’s tasks include process unit and equipment sizing. In particular, the sizing of the biological reactor for the activated sludge process is critical to good process design because the downstream process units depend on its output.

Today, activated sludge processes can be modeled using commercially available software. For example, EnviroSim’s Biowin, a wastewater treatment simulator, ties together biological, chemical and physical process models. Simulators are used in design, upgrade and optimization of wastewater treatment plants. The simulator core is the proprietary biological model, supplemented with others that may include water chemistry models for calculation of pH and mass transfer models for oxygen modeling and other gas-liquid interactions.

Beyond design simulation, an activated sludge model is a useful tool for planning, operation and upgrading existing plants, as well as process optimization, research and training.

However, to date, it is standard practice for process designers to use in-house developed spreadsheets for sizing process units and equipment. The perception remains that modeling and simulation software cannot specify or validate a process based on actual equipment used and other plant specifics. For example, the headwords or hydraulics incorporated in a design might not be available as software models to complete an overall plant design. 

Next, this article will examine the implementation and application of a process simulation model like the one mentioned above. It is worth noting that increasing numbers of end users today demand system developers use modeling and simulation to confirm compliance with the desired effluent requirements.

Static process design

Figure 1. Biowin MBR overall plant model schematic diagram

Figure 1. Biowin MBR overall plant model schematic diagram

Static process design is widely used in the wastewater industry and is considered normal practice. Spreadsheets have long since been developed to size bioreactors and associated equipment, combined with design methods based on simple expressions developed from models for steady-state conditions, as well as kinetic and stoichiometric parameters.

In this regard, Metcalf & Eddy and the Manual of Practice design guidelines are in common use in North America. ATV, a German standard, is widely implemented in Europe. These static design guidelines are widely recognized as accurate. However, due to the use of more stringently treated effluent, especially biological nutrient removal (BNR), use of static design alone without running a simulation poses a risk.

Static design does not provide insight into how a process would behave with different or dynamic loading rates, and whether the calculated process unit sizes are sufficient to meet the quality of the effluent required for compliance.

Simulators can confirm that the process unit sizes obtained from the static design actually will meet the desired effluent requirements and whether adjustments have to be made.

It is best to first develop a static design and calculate process unit sizes and equipment in a more or less traditional way and then use those results as input for simulation and analysis.   

Model setup

A competent model will be based on prepackaged unit modules implemented by icons in a graphical user interface. A package used in wastewater management is applied to a biological process, but other icon-enabled capabilities include those for headworks, equalization, thickening, dewatering and digestion. The whole wastewater treatment plant flow sheet, except incineration, can be set up. In order to establish overall plant mass balance, returns from other process units that cannot be modeled are entered as an input to the head of the plant.

Generally, design input data, including flow rates and organic/ inorganic loads for the wastewater treatment plant, are either provided in the tender document by the client or established by the designer based on historical data.

The first step in generating a model is to input the design data. The model allows input of parameters that include flow rate, chemical oxygen demand (COD), biochemical oxygen demand (BOD), inorganic suspended solids (ISS), total Kjeldhal nitrogen (TKN) and total phosphorus (TP). Most of the time, BOD and COD would not match the available design data unless changes are made to the fractionation of the influent in the model.

In order to match the BOD and COD, the particulate unbiodegradable (Fups) can be adjusted up or down. Similarly, ISS can be adjusted so that total suspended solids in the model match what is given in the design data.

TP input can be an issue if it exceeds the TP associated with unbiodegradable COD, biomass and ortho (PO4) components in the model. In this event, warnings appear on the screen. The way forward is either to adjust the fraction of PO4 in the TP or increase the total phosphorus value in the influent, whichever is acceptable to the designer and the end user.

Results obtained from the static design are entered into the model. These include bioreactor sizes, primary clarifier and secondary clarifier underflows, sludge wasting rates from MBR or SBR, and sludge thickening and dewatering flow rates.

Steady state & dynamic

Figure 2. Diurnal influent flow variations

Figure 2. Diurnal influent flow variations

Once design input is complete, a steady-state simulation is run. The result of the steady state should be used to check the following:

  • Quality of effluent BOD, COD, TSS, total nitrogen and TP
  • Concentrations of the sludge thickening and dewatering
  • Mixed liquor concentration of the bioreactor
  • Hydraulic and solids loading rates of primary and secondary clarifiers

If the effluent quality is not meeting the desired quality, then volumes of the bioreactors or the mixed liquor concentration are adjusted. Denitrification is always a challenge, especially when a primary clarifier exists, which will remove some of the readily biodegradable COD required for denitrification and result in much larger anoxic tank.

Figure 3. Diurnal influent load variations

Figure 3. Diurnal influent load variations

Operation parameters of the process units can also be adjusted. Internal recirculation rate, dissolved oxygen level and return activated sludge are adjusted to meet the desired effluent quality. In the case of SBR, aeration schedule or the cycle time distribution can be fine-tuned. Air diffuser oxygen transfer efficiency is also adjusted, using model diffuser correlation parameters such as k1, k2 and Y.

Subsequent to the steady-state simulation, a dynamic simulation also must be run. This is to test the model under various flows and load conditions. During the design stage of the project, diurnal flow of the plant is not available. However, similar plant variation pattern can be used to generate flows and loads that can be used for dynamic simulation. Figures 2 and 3 show typical diurnal flow and load variations generated for a 500,000 cubic meters per day plant with 577 milligrams per liter COD. Simulation results must demonstrate that the model is robust and capable of handling input fluctuations. This is the main advantage of modeling as opposed to static design, which will not predict future plant performance.

Process design output

Figure 5. Typical dynamic simulated effluent for TP and ammonia

Figure 5. Typical dynamic simulated effluent for TP and ammonia

After making the process adjustments described, the final steady-state results are used to provide the following process design output:

  • Simulated performance of the effluent in steady state and dynamic state
  • Mass balance of the whole plant
  • Design flow rates of thickening and dewatering equipment
  • Design flow rates of return activated sludge and internal recirculation pumps
  • Biogas production from anaerobic digester
  • Air capacity for bioreactors and other aerated unit blowers

Application of models for process design is increasingly accomplished because of its advantages over static design, which uses simple steady-state models. Overall mass balance of the plant can be obtained from the model as well as the aeration requirements. The effluent quality can be predicted, and the plant process unit and equipment sizes can be optimized.

Mohamed Ahmed Salah, P.E. is a process engineering manager with Larsen & Toubro in Doha, Qatar. He may be reached at and For more information visit