Project Wastewater collection, Monroe County, Fla.

Civil engineer Wade Trim

Product application Vacuum sewer system provides a cost-effective, unified approach to wastewater collection in the Florida Keys.

Economical vacuum sewer systems help protect fragile South Florida ecosystems.

Wastewater treatment has been a catch-22 for Monroe County, Fla. Protecting the fragile ecosystems of the Everglades and the Florida Keys from water pollution is a priority, but flat topography and coral-based lime rock present in the bedrock eliminate many cost-effective options. The result is a collection of ineffective, poorly-organized systems that fail to meet updated nutrient-removal requirements.

To streamline the entire system and reduce the amount of harmful nitrogen, ammonia, and phosphorous present in processed wastewater, the Florida legislature created a mandate for the county, requiring, among other things, that an alternative collection system be chosen and installed as the primary means of sewage collection by July 2010. Monroe County settled on a vacuum sewer system, which has already been successfully implemented in various Keys jurisdictions, as well as 300 other sites in the United States.

A technology that remained under the radar for years because of a lack of involvement in engineering curricula, vacuum sewer systems were introduced to the United States in the 1960s. The lack of practical knowledge concerning this system in today’s engineering field is the chief reason it hasn’t been utilized elsewhere. In many cases, it is more economical than more traditional methods.

A series of suction lifts, or physical lifts in the pipeline, are created by 45-degree fittings necessary to adjust for grade differentials, reducing the depth that the sewer lines in these systems are buried. Source: AIRVAC

Vacuum sewers work on the principal of a pipe network under negative pressure, using the force of 16-inches to 20 inches of mercury to draw sewage to a receiving tank at a vacuum station. Waste is then transferred to a treatment plant by pumping through a force main using suction from the vacuum receiving tank. A vacuum pit—established in the public right-of-way for access by maintenance personnel—is installed at the point of service and connected to the wastewater source by a gravity service lateral. This pit is a self-contained, at-grade unit with a sump at atmospheric pressure and a sealed compartment above that containing the vacuum valve.

When sewage reaches a pre-set level in the sump, a pressure transducer sends a signal to open the vacuum valve and expose the contents to the negative pressure in the system, forcing an evacuation and drawing air into the system before the valve closes.

In addition, to reduce the depth that the sewer lines in these systems are buried, a series of suction lifts, or physical lifts in the pipeline, are created by 45-degree fittings necessary to adjust for grade differentials. Burying the sewer lines at a more shallow level can reduce excavation costs.

Burying vacuum sewer lines at a more shallow level can reduce excavation costs.

Given the alternative structure of this system, design requirements for vacuum sewers are unconventional. To operate properly, the air-to-liquid ratio in the pipe must be at 3-to-1. A lower air-to-liquid ratio can result in the system becoming "waterlogged," reducing conveyance efficiency. In addition, each vacuum station must be designed for the requirements of the area it serves—the air evacuation pump capacity must maintain the necessary level of vacuum in the network. A typical vacuum pit can serve three to six single-family homes (or Equivalent Dwelling Units), but serving more than three units can put undue strain on the entire sewer system, which can include hundreds of vacuum pits.

The Florida Keys is a unique environment for the United States and the world, and implementing the vacuum sewer system is an excellent way to unify local communities in an effort to preserve it. By following this mandate and creating a sewer system that is unified and more efficient, the state of Florida, Monroe County, and local residents are taking steps toward preserving the health and well-being of the coral reefs, supporting the sustainability of the Keys’ ecosystem for years to come.


Jeff Gilbert, P.E., and John Pantelis, P.E., work for Wade Trim, a nationwide provider of engineering, planning, land development, landscape architecture, and transportation services. Gilbert and Pantelis are currently managing design and construction of sewer projects and wastewater treatment system improvements for the Key Largo Wastewater Treatment District. Gilbert can be reached in Wade Trim’s Key Largo office at jgilbert@wadetrim.com; Pantelis works in Tampa and can be reached at jpantelis@wadetrim.com.