Hydraulic Modeling and Engineering Design of the Big Coppitt Wastewater Collection System

The Big Coppitt Wastewater Collection System, located in unincorporated communities in Monroe County, Florida, includes the Big Coppitt, Geiger, Rockland, and Shark Keys. The Equivalent Dwelling Unit (EDU) method is adopted to determine the overall wastewater design flow. According to the Monroe County Wastewater Master Plan, one EDU represents an equivalent of a single-family residence of 2.5 persons and contributes an equivalent of 165 gallons per day (gpd). Because of shallow ground water table and coral rocks, one of the biggest design challenges is to keep the depth of gravity pipes as shallow as possible. This can significantly reduce the construction costs, especially on trench excavations. With this shallow gravity pipe limitation, lift stations can only receive gravity sewer flow from a relatively smaller area (i.e., fewer EDUs). As a result, most of lift stations might serve smaller design flow less than 79 gpm, which cannot provide a minimum velocity of 2 ft/s through a 4-inch force main. This leads to the second design challenge to select grinder pumps instead of traditional non-clog pumps to match smaller force mains and design flows per FDEP requirements. The third design challenge is to minimize major lift stations on the transmission mains. The piggy-backing method is applied to group linked lift stations to collect and transport smaller flows within certain service areas into an adjacent collection area. There are at least two advantages. Firstly, it increases the design flow of the receiving lift stations, and provides the opportunities to select more cost-effective non-clog pumps instead of grinder pumps. Secondly, the overall collection system is better designed instead of over-sized lift stations and force mains. It shall be noted that without piggy-backing, most of lift stations on the transmission mains must have larger design flows excessively more than what is actually needed to meet minimum 2 ft/s velocity requirement. With piggy-backing, the largest force main to the wastewater treatment plant (WWTP) decreases from 12-inch to 10-inch, and the overall design flow to the WWTP is reduced by 15 percent. The hydraulic simulations of two scenarios (all pumps running and one pump running) are also performed to confirm that the selected pumps will operate well under both scenarios.