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This article was published in the October, 2005 issue of Public Works Magazine.
You can click the image below to download the PDF.

To view a video and additional photos of the installation of vacuum sewers
in Albuquerque, New Mexico go to our Municipal Project Gallery.

Vacuum sewers save time, money

Albuquerque, N.M. evokes images of spectacular western panoramas, desert sunsets, and Native American culture. This natural beauty was put in jeopardy in the late 1980s. Population in the unincorporated areas of Bernalillo County had been steadily growing for years. The farms and ranches that used to dot the countryside were replaced by small communities. Most of these homes were beyond the reach of the city’s services, so the families there relied on septic tanks and well water. Over time, a number of these septic tank systems began to fail, increasing the amount of sewer discharge into the valley’s groundwater. By 1990 the situation had gotten serious.

In 1993, the state legislature allocated $12 million to address the problem. However, there was an important stipulation– a commitment to spend the allocated funds within 12 months had to be made, which is a tight timeline for a major public works project. In 1994, the legislature followed through with an additional $15 million in funding.

“Typically it takes up to a year to get an engineer hired,” said Bill Zimmerman, P.E., of HDR Engineering, Albuquerque, a firm brought in by the city’s public works department to handle the wastewater project. At the time of the project Zimmerman worked for Wilson & Company, the firm hired to manage the design and installation of the sewer system. “Greg Olson, P.E., city engineer and a leader in the development of the vacuum sewer system in the region, accelerated the advertising for the project and really got things moving, he said.”

“Realizing that time was of the essence, one of the first things we did was to schedule a marathon meeting with some city public works employees and engineers,” said Zimmerman. In a three-day meeting the committee discussed gravity sewers, pressure sewers, vacuum sewers and combination sewers. They created a matrix of options, assigned numbers to the alternatives, and reached a quick conclusion¬– gravity sewers would be a costly, time-consuming option. When all the issues were considered, vacuum sewer technology came out on top.

Albuquerque’s groundwater table is usually only 4 to 6 feet below the surface. A series of deep irrigation ditches cuts through the valley at various locations. These ditches are vital to the area’s agricultural needs and could not be disturbed, posing yet another obstacle. And, because the area is very flat, the collection lines would require deep trenches with numerous lift stations.

“Urbanization was occurring in a rural area that was dead flat,” said Bob Paulette, P.E., an engineer with Wilson & Co., who has worked on Albuquerque’s vacuum sewer project for 10 years. “A 0.4 percent slope on the 8-inch collection line means the sewer is dropping 21 feet every mile just to meet the minimum slope requirements. It is obviously very difficult to install sewers 21 feet deep in high groundwater conditions. Factor in the difficulty in crossing the irrigation ditches and the cost would be staggering. It also would take years to complete.”

“None of us had had any experience with vacuum sewers, we had only read about them,” said Zimmerman. “We made arrangements to travel to Rochester, Indiana, to tour the AIRVAC test facility there. Then we went to Nashville to look at an actual installation. Once we came back from that trip we were convinced vacuum sewers were the way to go.”

Within a few months work began on the installation of a new vacuum sewer system. Vacuum sewer technology not only made for a speedier installation, which was vital for this project, it also solved a number of related problems that would have stalled the project and ballooned the budget.

A NEW OLD IDEA
The concept of vacuum sewers dates back to the late 1800s. While vacuum swers have been used in Europe for years, the technology has only been used in the last 30 in the United States. Many engineers and public officials, including those designing the sewers for Albuquerque, were not 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 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 slug 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 associated with vacuum 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 design 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 installed in this project, which is 95percent complete. The projects 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 1,000 homes, as opposed to about 15 lift station 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, according to Jerry Morse, the utility’s maintenance superintendent. “We have nine vacuum stations, each serving up to 1,000 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 after the initial rush job of 1993, Albuquerque is gradually eliminating all sources of sewage pollution of its groundwater. Vacuum sewer technology is a proven commodity with local public works officials and engineers who continue to expand the vacuum system into additional 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.”

This article appeared in the October, 2005 issue of Public Works Magazine. If you would like to read the entire story as it appeared in the magazine, you can download a pdf, or you may request hard copies. Additional information and photos of the installation of vacuum sewers in Albuquerque, New Mexico can be found in our Municipal Project Gallery.

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