How Biological Process Modeling Serves as a Tool for Decision Making, Optimization, and Training


  • Brandt Miller PE, Joe Rohrbacher PE, Chamindra Dassanayake PhD, PE - Hazen and Sawyer
  • P.S. Arora PE, William (Rusty) Willard - City of Denton

The City of Denton (Denton) owns and operates the Pecan Creek Water Reclamation Plant (PCWRP) rated for an average annual flow of 21 million gallons per day (mgd) and peak 2-hour flow of 46-mgd. The PCWRP consists of preliminary treatment (screening and grit removal), primary clarification, conventional activated sludge with classifying selectors, secondary clarifiers, filtration and UV disinfection. The solids are thickened, anaerobically digested, dewatered and composted for a Class A biosolids product. The PCWRP consists of two plants, the North Plant and South Plant. The North Plant consists of five long narrow aeration basins each having a selector zone and aeration basin. The South Plant consists of three trains each having a selector zone followed by an aeration zone. Operating two plants within a single plant presents its unique challenges to operations staff. Besides routine operation, PCWRP recently received more stringent nutrient limits in their Texas Pollutant Discharge Elimination System (TPDES) permit, including <0.5 mg/L total phosphorus, summarized in the table below. The ever increasing need to be ahead of regulation and meet operations targets has resulted in Denton looking for a robust and credible basis to base the operational decisions.

In order to manage the more stringent discharge permit limits and challenges of two plants on one site, the City of Denton recognized an opportunity to improve operator confidence and process control by developing an operations and evaluation tool. Hazen and Sawyer (Hazen) assisted Denton in developing a calibrated process model using BioWinTM 4 (developed by EnviroSim Associates, Ltd.) software to serve as the operations and evaluation tool. The goal of the operations tool is to assist in process decisions such as:

  • Aeration strategies and optimization
  • Wasting strategies and impacts to sludge production
  • Seasonal operations targets (solids retention time, mixed liquor suspended solids, etc.)
  • Response to process upsets (loss of clarifier removal efficiency, solids processing upsets, etc.)
  • Maintaining operation during construction, taking basins off line
  • Wet weather flow management
  • Construction sequence impacts

In addition the Biowin model was also used to evaluate phosphorous removal process strategies to meet the 0.5 mg/l discharge permit.

For more information, please contact the author at

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