A New Old Idea
The concept of vacuum sewers dates back to the late 1800s. While vacuum systems have been used in Europe for years, the technology has only been used in the past 30 years in the United States. Many engineers and public officials, including those designing the sewers for Albuquerque, were not that familiar with vacuum systems in the early 1990s. When they began searching for sewer options, they eventually turned to AIRVAC, Rochester, Ind., which had been working on similar projects for more than 30 years.
Vacuum sewer systems rely on gravity to move wastewater from each house to a nearby valve pit. The valve pit houses a collection sump and a vacuum/gravity interface valve. There is typically one valve pit for every two houses attached to a vacuum system, and no electricity is required for the valve to operate. When wastewater in the sump reaches a predetermined level, usually about 10 gallons, the interface valve pneumatically opens and this “slug” of sewage enters the collection line.
Differential pressure of about 16 to 20 inches of mercury within the collection line propels the wastewater slug at a relatively high velocity, about 15 to 18 feet per second, to the vacuum station where it collects in a tank. The velocity of the sewage through the line provides a scouring effect that prevents grease build-up common to gravity sewers. The wastewater collected at the vacuum station is then transferred through a force main to the nearest treatment plant.
The differential air pressure in a vacuum system provides additional energy compared to natural gravity flow so level—or even uphill—transport is possible. As a result, collection lines can be buried much shallower, typically about 3 to 5 feet under ground. This was a significant feature for the Albuquerque design team.Many Benefits
The 1993 job was the first in a series of vacuum sewer projects that have helped Albuquerque solve most of its groundwater contamination problems. Today work continues on new sections of vacuum sewers, as well as new potable water lines in other areas of the city. The Albuquerque Bernalillo County Water Utility Authority has found vacuum sewer technology to be a time and money saver that is well-suited to the area's unique geographic features.
Approximately 200 miles of vacuum sewers have been or are being installed in this project, which is 95% complete. The project's purpose is to provide sanitary sewerage to all areas of the valley for public health reasons. The vacuum systems are being installed in flat, rural flood plains of the Rio Grande River. “The cost of installing the AIRVAC system is much lower in this type of flat topography,” said Paulette. “You avoid all the issues and costs associated with deep trench excavation. Also, a single vacuum station can serve about 1000 homes, as opposed to about 15 lift stations being required to serve the same number of homes with a gravity system.”
Vacuum technology also provided an important ancillary benefit to the engineers. Vacuum collection lines do not leak. If there is a crack in a vacuum line, the vacuum pressure prevents wastewater from entering the environment.
“The possibility of contamination is zero,” said Paulette. “This allowed us to obtain permission from the New Mexico environmental agencies to install the new sewer lines close to existing potable water lines, in some cases within a foot. This meant we could put the sewer line in narrow trenches without relocating the water line. It would have been impossible to do that with gravity sewers.”
Maintaining the system is relatively easy, said Jerry Morse, the utility's maintenance superintendent. “We have nine vacuum stations, each serving up to 1000 connections. System wide, we receive about four or five service calls a month. Most of them are solved in about 15 minutes without ever coming in contact with raw sewage,” said Morse. “The vacuum pumps at the stations give us very little trouble. As long as you change the oil regularly, everything is fine.”
Now, more than 10 years into the project, Albuquerque is gradually eliminating all sources of sewage pollution of its groundwater. Vacuum sewer technology has proven itself with local public works officials and engineers who continue to expand the vacuum system into new neighborhoods.
“The installation of vacuum sewers has helped tremendously in eliminating the groundwater pollution we were getting from septic tanks,” said Paulette. “It fit perfectly into the conditions we had and we were able to install it with very few problems.”
— Steve Gibbs is a freelance writer in Germantown, Tenn.