Hydraulic Modeling and System Optimization of the Marco Island Wastewater Collection System Expansion

This paper presents a hydraulic modeling method to optimize a cost-effective design to expand the Marco Island wastewater collection system. Currently, approximately 60 percent of the city is served by individual septic tanks (decentralized system) and the remaining 40 percent is served by the existing centralized wastewater system. Marco Island has started the septic tank replacement program, which includes 14 sewer districts to be completed in 7 phases from year 2006 to 2012. One of the biggest design challenges is how to calibrate the hydraulic model of the existing wastewater collection system. The accuracy of simulation results is so critical because it is one of the most important factors on which key design decisions are based. Pressure gauges are used to continuously record pressure data at several lift station discharge lines. The recorded pressure data are then analyzed and compared with the simulated data to calibrate the modeling parameters and improve the model's accuracy and reliability. This will help design engineers better understand the existing collection system, and improve the future system development. Comparing actual field pressure data with the all pumps running at build-out model scenario ("worst case") suggests that the force main sizing and pump selections should not always be based on the worst case, which may prove to be too conservative. This less conservative approach may be more appropriate when it is not practical to upgrade pumps or force mains. Another design challenge is how to cost-effectively expand the existing wastewater collection system in phases in coordination with the existing facilities. The goal is to maximize the utilization of the existing gravity lines, lift stations, and force mains. The construction sequence of these sewer districts is mainly determined based on the configuration of the existing transmission system and hydraulic modeling of proposed developments. Other factors are also considered including sewer districts' own features (size, location, population, etc.), and the schedule for other proposed construction projects. The hydraulic modeling proves to be a cost-effective method to optimize the Marco Island wastewater collection system. More importantly, this method can wisely manage budgets and allocate money to projects with higher priorities based on recommendations from hydraulic simulations.