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Development of a Dynamic Wastewater Treatment Plant Hydraulic Model Linked with Collection System

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Currently, the City of Atlanta serves approximately 1,600 miles of sewer mains (15% combined and 85% separated) in 132 square miles area. All sewage flow within the City is treated by four wastewater treatment facilities and four combined sewage overflow (CSO) control facilities to serve a population of 1.6 million with a total average daily flow of 182 MGD. In addition to the flow produced within the City, there are six adjoining municipalities contributing flow to the City of Atlanta sewer system that comprise 45% of the total flow.

In response to the First Amended Consent Decree issued from the US Environmental Protection Agency (EPA) and Georgia Environmental Protection Division (EPD), the City of Atlanta began to build a comprehensive, dynamic hydraulic model of its sewer system. The City sanitary system was divided into ten sewer basins based on the natural boundaries of the collection system and the wastewater service areas. Each of the sewer basin model has a free outfall at the last node to represent the wastewater treatment facility.

While evaluating the existing conveyance system and ensuring that it is sufficient to carry future dry weather flows and peak wet weather flows, the City is mindful not to allow the peaks at the free outfalls to exceed the design capacity of the corresponding treatment facility. The City currently has a static spreadsheet hydraulic model of each wastewater treatment facility that is not linked to the sewer hydraulic model. The City's static spreadsheet model has many limitations including single pass flow simulation, no calculation for backwater surcharging, limited “what if” scenario analysis capability for plant operation, maintenance and planning, and no real-time simulation mode option.

The objective of the study was to develop a dynamic hydraulic model for Utoy Creek Water Reclamation Center (WRC) linking with the existing sewer hydraulic model. InfoWorks CS was chosen as the software package to model the treatment facility as the City's collection system uses the same software. Utoy Creek WRC is the newest of the four treatment facilities in Atlanta and includes bar screens, drum screens, primary clarifiers, activated sludge with biological phosphorus removal (BPR), secondary clarifiers with return activated sludge (RAS) pumps, tertiary filters, UV disinfection and a staged effluent channel with a peak day plant design capacity of 74 MGD.

InfoWorks lends itself well to this application as it has some physical parameters that represent actual operations and processes in treatment facilities. Bar Screens and drum screens are link options in the software that were tailored to replicate the actual screen parameters. Primary and secondary clarifiers were set up as storage nodes and weirs, while the biological phosphorus removal (BPR) tanks and conveyance systems were setup as open rectangular channels and circular pipes. The process for preparing testing and calibrating this treatment facility model consisted of identifying the physical components and data from the treatment plant within Infoworks and obtaining actual flow data to compare to the simulation model results.

One of the challenges of modeling the treatment facility is the multiple real time controls (RTC)s required for adapting the system's variable frequency drive (VFD) pumps to adjust to fluctuations in the flow volume. Infoworks CS has the RTC control capability to handle various complex scenarios such as these.

After a preliminary model was setup based on the physical and hydraulic characteristics of the plant, flow verification and model calibration was required. This process was setup by installing an additional meter to monitor the effluent flow from the plant and survey benchmarks at critical locations to measure depth of flow in the miscellaneous processes. On a typical dry weather day (no rain for the three proceeding days), flow depths were measured and times noted at all benchmarks. The plant's supervisory control and data acquisition (SCADA) information was also collected to identify how many units within each process were in operation together with pump flows and wet well depths. Continuous level recorders were also installed at several locations to monitor the flow depth variations during daily flow fluctuations and rainfall storm events. A comparison of measured depth to modeled depth was used to adjust headlosses in the model to reflect the field data.

The development of this dynamic hydraulic model has numerous applications. The static model was originally used as a tool for permitting or for design of planned facility upgrades. The dynamic model can be used to plan maintenance procedures and to see the effects of taking parts of processes offline as well as show the effects of unplanned process failure due to power outages. The model also helps predict changes in plant flow due to discharge point river level fluctuations and backwater effect. Additionally, the dynamic model created in a widely used software package would have the advantage that it only needs to be created once and could be utilized and understood by the users at the City as well as by any consultant as necessary whereas the static model was recreated each time new upgrades were planned. In the long-term, the other three WRCs will also be modeled and merged with the conveyance system to produce a truly integrated flow model which will be very unique tool for modeling applications in wastewater industry.
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Keywords: Infoworks CS; RTC; Wastewater treatment plant; hydraulic modeling

Document Type: Research Article

Publication date: 2010-01-01

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