Sarasota County Water/Wastewater Hydraulic Models

To help develop a Wastewater Management Plan (WWMP), Sarasota County requested Jones Edmunds develop hydraulic models of its conventional lift station and force main systems using SewerCAD™ and use the models to evaluate proposed wastewater system improvements necessary to maintain the existing system and support future growth.

The County currently owns and operates complex and extensive wastewater collection/transmission systems comprised of around 83,700 sewer connections, over 700 conventional lift stations (LSs) with 300 miles of force mains, over 1,000 miles of gravity sewer mains, several alternative wastewater collection systems, and 4 WWTFs. At the time the models were initially developed, the County owned 10 WWTFs. Later, some of these WWTFs were decommissioned.

As part of the model development effort, Jones Edmunds developed 10 dynamic hydraulic wastewater models from the County’s July 2007 wastewater system geodatabase (GDB). The models include approximately 550 County-owned lift stations and associated force mains. Gravity sewer mains, which connect cascading conventional lift stations and force main systems; and gravity sewer interceptors, which connect conventional lift station and force main systems to WWTFs are included in the models.

The model network layout is primarily based on the wastewater GDB developed as part of the County’s Geographic Information System (GIS) Mapping project. Portions of the County’s wastewater GDB were updated by Jones Edmunds’ GIS Department before it was used as a basis for the model GDB. In addition, the County provided additional LS information including LS type (triplex, duplex, etc.), wet well dimensions, force main and gravity main diameters and routing, and elevations. Jones Edmunds spatially rectified numerous record drawing documents including scanned as-builts and digitized facility data to achieve a final wastewater GDB suitable for developing a wastewater model.

While developing the models, Jones Edmunds worked with the County operations staff to confirm the size and configuration of system components as much as possible. The lift station systems for which supervisory control and data acquisition data (SCADA) were available were calibrated to field conditions to the extent practical. Model calibration included adjusting model parameters (e.g., lift station inflow hydrographs, level controls, force main Hazen-Williams coefficients) to match model lift station run times and WWTP influent flow to actual values. Subsequently, we identified system deficiencies and proposed system improvements to overcome significant deficiencies, allow for growth, and to optimize use of existing pumping facilities. The models were set up to assist the County, with planning and identifying potential deficiencies within the system.

With a well calibrated dynamic model completed, we next developed an intuitive method to review deficiencies and benefits of improvement alternatives. Due to the extent of the manifolded lift station/force-main systems within critical portions of the County’s wastewater systems, several LSs periodically experience reduced capacities or deadhead conditions during high-flow periods. For large systems such as this, upgrading the system to eliminate all potential deadhead conditions, as determined through steady-state modeling, is rarely an economically viable solution. By developing a detailed model that reasonably represents the physical components and performance of the actual system and that contains all LS pumps and controls, it is possible through extended period simulation (EPS) to force the model to sequence pumps on/off as done in an actual system. By developing these detailed models and loading 24-hour diurnal flow patterns for given flow days, it is possible utilize a more realistic approach to assess system deficiencies that are expected to occur.

System deficiencies were assessed using EPS of various flow conditions. Using innovate techniques we were able to provide the County a map that indicates which lift stations experienced a deadhead condition, the number of deadhead events experienced, and the duration of each deadhead condition. In addition we showed the maximum depth reached in a given wetwell after its high-water alarm was activated. The presentation of this information in an intuitive color-coded map facilitates decision-making. Upgrades can be quickly inserted in the model, simulations re-run, and new maps compared to existing condition maps to validate decisions and compare impacts of improvements on reducing deficiencies.

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