A 2011 report by the American Society of Civil Engineers – "Failure To Act; the economic impact of current investment trends in water and wastewater treatment infrastructure" – cited studies indicating the country's sewer collection systems are in disrepair and treatment capacity is inadequate. As a result, these systems discharge an estimated 900 billion gallons of untreated sewage each year into the environment. This troubling news comes at a time when federal and municipal agencies are cash-strapped and funding for major infrastructure projects is limited. In 2010, the U.S. Environmental Protection Agency (EPA) estimated a cost of about $91 billion to upgrade America's potable and wastewater treatment systems. However, only $36 billion of this need was funded, leaving a $55 billion funding gap.
Clearly, when it comes to fixing America's water infrastructure, money is tight and difficult to obtain. Therefore, municipal leaders must be prudent when they have to spend public money on new sewers. For civil engineers faced with replacing or installing a new collection system, the options generally are limited to gravity sewers, low-pressure sewers, or vacuum sewers. In recent years, more engineers are turning to vacuum sewer technology because, in many situations, vacuum sewers may be less expensive to install and easier to maintain than other options, including gravity sewers.
Depending on topography and soil type, excavation may be very expensive. This is particularly true in coastal areas or other places where there are high water tables and sandy soil. Installing collection lines for gravity sewers may require deep trenching, perhaps 15 feet deep or more, and you incur all of the related problems and costs associated with digging that deep, including expensive dewatering. Fripp Island, S.C., faced this issue a couple of years ago when utility officials began considering sewer options.
"We quickly discovered that gravity sewers were going to be expensive," said Ernie Wilson, director of the Fripp Island Pubic Service District. "They also would create a lot of disruption to our roads and utilities because of all the deep trenches that would be needed."
Deep trenches require larger excavation equipment, trench reinforcing, and in situations where there is a high water table, dewatering. The installation process takes longer and is far more disruptive than with vacuum technology, which usually requires a trench only 4 to 6 feet deep. These trenches can be dug relatively quickly and with smaller equipment, and the collection lines (typically SDR 21 or Schedule 40 PVC pipes) can often be placed by hand. Also, vacuum systems such as AIRVAC do not have manholes, further reducing installation time. If an unexpected obstacle is encountered underground, rerouting the line around or beneath the obstacle is simple and rarely requires a change order. All this means less work, less disruption, and less risk for workers.
Installation of collection lines and construction of lift stations can create enormous disruption within a community. Streets and sidewalks must be torn up, utilities and traffic rerouted, and commerce is interrupted. There are also significant liability issues. In Alloway, N.J., engineers calculated that new collection lines would need to be 22 to 24 feet deep. Such deep excavation would have disrupted the community for months. They avoided the issues associated with extremely deep trenches by installing an AIRVAC system. Community life was relatively uninterrupted by the new sewer project and the sidewalks and streets that were affected by the work were repaired quickly and easily.
With vacuum sewer technology, every house and business must be connected to a valve pit buried near the street (see Figure 1). Often, a single valve pit will serve two or three houses. The valve operates pneumatically; no electricity is required. Installation is simple and several installations can be accomplished in a single day.
Another significant cost issue is the number of lift stations necessary to convey sewage. Cities built on flat terrain may require many lift stations, each one a significant capital investment in land, construction, and equipment. In Oak Island, N.C. (pop. 7,800), engineers estimated they would need 70 to 80 lift stations. The city decided to install an AIRVAC sewer that needed only nine vacuum stations to serve the entire community. The cost for vacuum technology was approximately 25 percent less than the cost of a comparable gravity system. Considering the cost to purchase a parcel for a lift station, the cost savings may be even greater for areas with high property values.
Operational cost savings
Faster, easier installation is a great benefit, but operational costs have a greater long-term impact on a city's budget. Therefore, it is important to consider the day-to-day costs of operating and maintaining any sewer system.
Daily maintenance of a vacuum sewer is simple and predictable. Workers visit vacuum stations on a daily basis for a few minutes to check gauges and fluid levels. Among other things, the gauges reveal pressure levels within the lines and help detect if a rare leak has occurred. If so, the leak can be isolated quickly and, because of the shallow depth of collection lines, excavated and repaired, usually within a few hours.
"With our AIRVAC system, you always know if you have any infiltration or exfiltration because we have system monitoring at the vacuum station," said Chuck Martin, maintenance coordinator, JEA, in Jacksonville, Fla. "In a gravity system, you may have to run a camera in the sewer main for days to find a leak. Gravity sewers tend to require a lot of expensive maintenance equipment and a lot of man-hours. It can be a big operation. That's not the case with vacuum systems."
With AIRVAC technology, utility workers rarely, if ever, come in contact with sewage because vacuum sewers are a closed system. For that reason, vacuum stations are typically clean. And, because there are no manholes, workers never have to venture into cramped spaces to check sewer lines.
"Safety and liability are areas of concern for every public works director who has to send his personnel into a sewer," said Thomas Voght, former Public Works director for Cedar Grove, Fla. "For gravity sewers, there are numerous dangers involved in routine maintenance and repair work. Falling into manholes, working in confined areas with the threat of hydrogen sulfide gas, the moving parts and electricity associated with a lift station, traffic hazards, contact with raw sewage, and many others. By comparison, there are very few injury risks associated with vacuum sewers."
With vacuum sewer systems, workers typically check valve pits on a scheduled basis, but this is easy duty because the valve pits are easily accessible, relatively maintenance free and there is no danger of electrical shock. Valve pit problems are rare, but if they do occur, they are easily fixed using ordinary tools, usually in a matter of minutes.
Vacuum sewers also can reduce costs at the treatment plant. Chuck Adams, operations manager for New Bern, N.C., said infiltration was an enormous problem for his city. With their gravity systems, a major rainfall event would increase the burden at the treatment plant from its normal 3.5 million gallons a day (mgd) to 11 mgd. Since 1994, the city has installed several AIRVAC systems that allow no infiltration. Imagine the treatment cost savings over one, two or three decades.
It's important to note that AIRVAC systems have an excellent record for durability and longevity. The first municipal vacuum systems in the United States are now more than 40 years old and still going strong, with minimal, easy maintenance.
Saving money is wonderful; saving the environment is even better. Vacuum sewers can help with both. As an alternative to leaky sewers and septic tanks, vacuum technology is an environmentally wise choice that pays off in numerous ways.
The Florida Keys are a popular vacation destination and much of the area's economy is based on tourism. In the 1980s and 1990s, it was discovered that inadequate wastewater conveyance and treatment were contributing to a serious environmental problem – the magnificent coral reefs and marine life in the area were being threatened by high levels of nitrogen and phosphorous. The problem was so bad that swimming became unsafe in some areas. Public works officials prioritized their problems and went to work installing vacuum sewers in places like Key Largo and Marathon. Today, the water in and around the Keys is much cleaner. It is safe to swim, the aquatic life is thriving, and the local public utilities have won environmental awards for their work in creating sustainable water quality. They also protected their most valuable industry – tourism.
Additionally, places such as the Florida Keys and cities along the Eastern and Southern coastlines are susceptible to hurricanes. When electrical power is lost during a storm, sewer service also may be disrupted and lift stations can no longer convey sewage. The result can be a serious health risk to local residents.
"We have 39 lift stations and just two generators for our gravity sewer system. When there is a power outage, keeping them running is tiring and stressful for the entire staff. With the vacuum station, we have nothing to worry about. The back-up generator kicks in and the system continues to function normally," explained Robert Holland, Utilities Superintendent for the City of Groveland, Fla.
Selecting a new or replacement sewer is a cost equation with many variables – installation, maintenance, manpower, efficiency, longevity, and economic development value.
AIRVAC systems are equipped with backup generators, so if power is lost there is no disruption of sanitary sewer service, and a single vacuum station can serve hundreds or even thousands of homes. Also, hurricanes often cause flooding and an infiltration of stormwater and sand into gravity sewer lines and treatment facilities. Closed AIRVAC systems don't have this problem.
Modern vacuum sewer technology also can be a terrific selling feature for communities as they compete for business and residents. AIRVAC sewers enhance property values and can make your city more attractive to businesses that appreciate reliable, environmentally responsible public utilities.
Total cost effectiveness
Selecting a new or replacement sewer is a cost equation with many variables – installation, maintenance, manpower, efficiency, longevity, and economic development value. When you consider all of these factors, vacuum sewer technology becomes a cost-effective alternative. The proof is already in the ground, as hundreds of communities across the country have installed systems that offer evidence of the immediate and long-term value of vacuum sewers.
Steve Gibbs is a freelance writer based in Memphis, Tenn., with more than 25 years of experience covering public works and construction projects for regional and national publications. This article is a compilation of project case studies highlighting AIRVAC vacuum sewer technology.