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This editorial was published in WE&T Magazine's May 2011 issue.

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Understanding Vacuum Sewer Technology

An alternative collection system

 

Public works directors take a long view when they make infrastructure decisions. The true cost of a project or technology is determined by many factors over a period of years, often decades. Installation costs, manufacturer support, operations, maintenance and service are all part of the value equation.

For centuries, gravity has been the usual choice for wastewater conveyance. It is a known system that’s relatively easy to understand. Newer technologies, like vacuum sewers, are something of an unknown.

Public works officials who have first-hand knowledge of vacuum sewer technology will tell you that installing and maintaining a vacuum sewer system is easy and the service life is measured in decades. The first vacuum sewer was installed in the United States in 1972, and 30-year-old vacuum systems are still functioning well with minimal maintenance.

Vacuum sewers have proven to be an affordable, efficient, reliable technology. Knowing how they function and are maintained will shed light on why many municipalities are turning to vacuum sewers as their preferred conveyance system.

An AIRVAC field service manager assists contractors during installation of a valve pitHow it Works

Some may think that vacuum sewer systems are complex and fragile. In fact, the opposite is true. Vacuum systems operate on simple principals of physics and have proven to be extremely reliable over many years of service.

Homeowners usually don’t notice the difference between vacuum sewers and other systems because gravity is used to transport wastewater from homes and businesses to the first collection point, the vacuum valve pit. The valve pit is usually buried near the street and consists of a small collection sump and a pneumatic vacuum valve mechanism located in a chamber above the sump. Two homes are typically connected to a single valve pit.

When the wastewater in the valve pit sump reaches a predetermined level — usually corresponding to about 38 L (10 gal) — it triggers the pneumatic valve that releases the wastewater into the vacuum main. In the vacuum main, negative pressure propels the wastewater at speeds up to 5.5 m/s (18 ft/s) toward the vacuum station. The speed of the wastewater within the vacuum main helps scour the line and break up solids.

The PVC vacuum main is laid in a sawtooth profile to ensure adequate vacuum levels at every point along the line. Burial depths typically range from 1.2 to 1.8 m (4 to 6 ft), shallower than most gravity sewers mains. When the wastewater reaches the vacuum station, it is collected into a tank and then pumped into a sewer main that transports the wastewater to the treatment facility.

Vacuum stations often are designed to look like other structures in the neighborhood. Each station contains two or more vacuum pumps, two discharge pumps and a collection tank. A single vacuum pumping station can collect wastewater from houses located as far as 3 km (2 mi) away. Because vacuum sewers are closed systems, there is no odor at the station or anywhere along the vacuum main.



BEST APPLICATIONS
The technical characteristics of vacuum sewers have made them a popular option for areas with flat terrain and high groundwater, such as coastal communities. Many inland areas also have installed vacuum sewers for a variety of reasons, including environmental concerns. Because vacuum sewers are a closed system under pressure, groundwater can’t enter into the system and wastewater can’t leak into the environment.  If a leak occurs within the collection system, monitors at the vacuum station alert system operators.

Vacuum sewers are a good option for established neighborhoods, too. Vacuum mains require relatively shallow trenches, so there is typically less traffic disruption, less excavation, and less dewatering. Vacuum lines also can be rerouted easily around underground obstacles, usually without change orders. Communities transitioning from septic tanks to a central sewer system have found vacuum technology appealing because vacuum system installation creates less neighborhood disruption. Municipalities with gravity sewers can add vacuum sewers to their collection system with little or no problem.


Lightweight polyvinyl chloride pipe is used for vacuum collection lines.PRE-INSTALLATION
Depending on geographic conditions, vacuum sewer design and installation can be different than gravity sewer installation. Vacuum pressure in the collection lines moves wastewater without the benefit of gravity. Therefore, slope is a less significant factor in vacuum sewer design. The shallow depth of the vacuum main helps reduce installation cost and disruption during installation. It’s also easy to make last-minute changes during pipe installation. If an unexpected underground obstacle is encountered, the vacuum main is simply re-routed around it; change orders are usually unnecessary.

“AIRVAC developed most of the specifications used in our vacuum system design, so it was good to have their expertise at the job site. They have worked with us to create a system that works very well and requires minimal maintenance,” said Don Eckler, P.E., president of Eckler Engineering, whose firm helped design a vacuum sewer system in the Florida Keys. Airvac is a vacuum sewer manufacturer based in Rochester, Ind.

Field service technicians serve as a liaison between the contractor, engineer, and owner. Their experience and knowledge of vacuum sewer technology ensures maximum system efficiency, secure connections and more importantly, they provide experienced advice if installation problems arise. This type of service helps reduce installation time and cost, and above all, ensures the system is installed properly.


TRAINING
Vacuum sewer technology is easy to understand once you see it in operation.

Ernie Wilson, general manager for the Fripp Island, South Carolina, Public Service District, was skeptical of vacuum sewers until he saw the technology first hand and received instruction on the operation and maintenance of the system.

“We went to the manufacturer’s headquarters in Indiana and we visited some cities with vacuum sewer installations. “The training we received was outstanding,” he said. “They taught our staff how to install valves and all other aspects of the system.”

Once the system is installed, system owners can choose their level of service from the manufacturer. A field service technician is available for as long as necessary, even permanently, to help train public works staff on how to maintain the system or to provide everyday system maintenance.

JEA, the electric, water and sewer utility for Jacksonville, Fla., chose to have their staff trained on-site by a field service representative. According to Chuck Martin, JEA’s maintenance coordinator, it was the right decision for his utility.

“The training we got from the manufacturer was excellent,” said Martin. “They were very thorough. They taught us how to perform routine maintenance on the vacuum pumps about once a week. I can’t recall a single instance where we’ve had to repair any part of the vacuum sewer infrastructure.”


Vacuum sewer stations, such as this one next to a playground, often are designed to blend in with neighborhood architecture. MAINTENANCE
Other municipalities and sewer districts have chosen to contract with system manufacturers to provide temporary and/or ongoing system maintenance. The Tri-Lakes Regional Sewer District in Columbia City, Indiana, has approximately 1800 customers, making it one of the 10 largest vacuum sewer systems in the United States.

A contract maintenance agreement was a perfect solution for Tri-Lakes, said Nel Mann, Tri-Lake’s district administrator

“The field service technician did a great job of managing our system for an extended period, making some necessary system adjustments, and training our on-site personnel,” she said. “We are extremely comfortable knowing that they are always just a phone call away.”

After more than a decade of contracting the operation and maintenance services to a local maintenance company, Tri-Lakes decided to operate and maintain their system themselves. They signed a 6-month contract with AIRVAC to take over daily operations and maintenance responsibilities and to train their new employees so they could eventually provide operations and maintenance services themselves.

“A well-qualified AIRVAC service technician was assigned to us for a 6-month period,” explained Nel Mann, Tri-Lake’s district administrator. “During this time, he not only responded to service calls, he also trained our new employees on the technology and fine-tuned our system. At the end of his 6 months, our vacuum sewer system was running better than ever and our employees were well trained to keep it that way.”


Even communities that prefer servicing their own vacuum sewers have found the process to be easy, efficient and safe. Because vacuum sewers are closed systems, workmen rarely come in contact with wastewater. Vacuum stations are clean and odor free; and daily system checks take only a few minutes. If a vacuum line is accidently cut or damaged, the leak is easy to locate and can be repaired within a few hours with minimal digging or disruption.

“One thing that struck me from the beginning was the simplicity of the vacuum system,” said Robert Holland, utilities superintendent for the City of Groveland, Florida. “I expected a lot of technical issues, but we were trained on the system in a matter of a few days. It is really a very simple system, which was a pleasant surprise.”

Holland said it takes a maintenance staffer 15 minutes a day to check the gauges at the vacuum station. If a vacuum valve ever becomes stuck in the open position, it can be fixed in less than five minutes. He noted that the vacuum valve pits, which operate pneumatically and require no electricity, are easily accessible and simple to understand.

PARTS AND SERVICE
As part of a routine maintenance plan, utilities choose to have their vacuum system vendors provide an annual or bi-annual system evaluation. Technicians visit to monitor and analyze your system’s performance and make necessary recommendations. This can help prolong the life of vacuum components and ensure uninterrupted service to customers.



This article appeared in the May 2011 issue of WE&T. 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.



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