For several decades, lobe and multistage blowers were the tried-and-true blower technologies for wastewater treatment plants. Over the past 15...
Vacuum sewer technology helps city solve contamination in area with a high water table Think of Albuquerque, N.M., and images of spectacular western panoramas and Native American culture come to mind. Located in the Rio Grande Valley, the city is known for its Southwestern art and the annual International Balloon Fiesta, when hundreds of hot air balloons gracefully drift across the New Mexico sky.
The natural beauty of Albuquerque was in jeopardy in the late 1980s. Population in the unincorporated areas of Bernalillo County had been growing steadily for years. The farms and ranches that used to dot the countryside were replaced by several small communities. Most of the new homes were beyond the reach of the city’s services, so 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 become serious.
The issue reached the state level in the early 1990s, and in 1993 the state legislature allocated $12 million to address the problem. There was, however, an important stipulation: the money had to be encumbered within 12 months, an incredibly tight timeline for a major public works project.
“Typically it takes up to a year to get an engineer hired,” said Bill Zimmerman, P.E., of HDR Engineering. At the time of the project, Zimmerman worked for Wilson & Co., the firm that was hired to manage the design and installation process. “Greg Olson, a leader in the development of the vacuum sewer system in the region, accelerated the advertising for the project and really got things moving.”
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, but it solved a number of other problems that would have stalled the project and ballooned the budget.
The 1993 job was the first in a series of vacuum sewer projects that has helped Albuquerque solve most of its groundwater contamination problems. Even 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 come to appreciate vacuum sewer technology as a time and money saver that is a perfect fit given the area’s unique geographic features.
“I’m very happy with it,” said Zimmerman. “It was a good decision at the time and I still believe it is a good decision.”
It is tempting to think that the 1993 funding bonanza Albuquerque experienced was a little like the old TV show “Supermarket Sweep,” where shoppers run through the store throwing items into their shopping baskets before time runs out, but that was not the case. Local public works officials were compelled to seek the best solutions to the problem at hand, given some very specific restrictions.
“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. “I rented a hotel meeting room and we met for three days to brainstorm alternatives. We talked about gravity sewers, pressure sewers, vacuum sewers and combination sewers. I gave a presentation about the geography of the area and then we discussed the various options.”
The committee came to one quick and obvious conclusion: gravity sewers would be a costly, time-consuming option. Albuquerque is located in the Rio Grande Valley, so the groundwater table is usually only 4 to 6 ft below the surface. An intricate 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% slope on the 8-in. collection line means the sewer is dropping 21 ft every mile just to meet the minimum slope requirements. It is obviously very difficult to install sewers 21 ft 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.”
According to Zimmerman, the committee created a matrix of sewer options and assigned numbers to the alternatives. When all the issues were considered, vacuum sewer technology came out on top.
“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, Ind., to tour the AIRVAC test facility there. Then we went to Nashville [Tenn.] to look at an actual installation. Once we came back from that trip we were convinced vacuum sewers were the way to go.”
A New Old Idea
The concept of vacuum sewers has been around since the late 1800s and used in Europe for years, but the technology has only been widely used in the U.S. in the last 30 years. Only a handful of engineers and public works officials knew much about vacuum sewers in the early 1990s, including those designing the sewers for Albuquerque. When they began searching for sewer options, they eventually turned to AIRVAC , a pioneer in vacuum technology.
Vacuum sewer systems first 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. When wastewater in the sump reaches a predetermined level, usually about 10 gal, the interface valve is activated and the sewage enters the collection line. One feature of the AIRVAC differential valve is that it works pneumatically, and no electricity is required, making installation simpler. The valve itself is separated from the collection sump in a sealed fiberglass enclosure, so routine valve maintenance does not involve contact with raw sewage.
Differential pressure of about 16 to 20 in. Hg within the collection line propels the wastewater slug at relatively high velocity, about 15 to 18 ft 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 buildup common to gravity sewers. The wastewater collected at the vacuum station then is transferred through a force main to the nearest treatment plant.
The differential air pressure associated with vacuums provides additional energy compared to natural gravity flow, thus level or even uphill transport is possible. The net result is that collection lines can be buried much shallower, typically about 3 to 5 ft deep. This was a significant design feature for the Albuquerque design team.
“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. There is also the fact that we can serve about 1,000 homes with a single vacuum station, as opposed to about 15 lift stations to serve the same number of homes with a gravity system. The station maintenance alone is a big savings.”
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 gradually is 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.”