Sucking in the Savings
A veteran of both gravity and vacuum sewer systems crunches the numbers.
Sewers require at least some attention every day. They represent a significant part of the public works budget and require a substantial amount of labor. Perhaps most important, they’re vital to a city’s daily operation.
Cedar Grove, Fla., looked at all of these factors when considering sewer technology options about nine years ago. When its research was complete, the city chose to install vacuum sewers, a decision that has reaped benefits almost every day since the system became operational.
That’s great news for me, because I became the city’s public works director in 2002.
Cedar Grove has both gravity sewers and vacuum sewers, so we’re familiar with the operation and maintenance of both. When it’s time to submit an annual budget, it’s clear why my staff and I are so pleased with the vacuum system’s performance. It’s easy to operate and maintain, requires less of our time, is more reliable, and presents fewer safety concerns.
FEWER SAFETY AND LIABILITY CONCERNS
Any public works director who has to send employees into a gravity sewer worries about the dangers: 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, and contact with raw sewage.
Because of the potential for injury, we always assign at least two employees for maintenance or repair work.
Vacuum sewers present much less risk. There are no manholes, no confined spaces, and access to the collection lines is easy because they’re usually buried 3 to 5 feet deep. A vacuum valve’s moving parts are completely self-contained and there’s never any contact with raw sewage at the valve pit. We have about 500 vacuum valve pits that are easily accessed at curbside. No special tools are required; and because the pits operate pneumatically, no electricity is involved.
As the person responsible for employees’ safety, I have no problem sending a single employee out to perform vacuum sewer maintenance and repairs.
SIMPLE DESIGN CUTS REPAIR TIME
Although both our gravity and vacuum sewer stations require daily servicing, we spend about 2 to 3 hours on the vacuum stations compared to 49 hours on the lift stations in any seven-day period (for details, see chart).
The gravity sewer collector lines require regular cleaning with a jet truck. The vacuum lines are self-scouring, so no jet cleaning is necessary. Also, vacuum sewers are inherently tight which prevents storm water and groundwater from entering the system, thus saving costs at the treatment plant. Manholes and lateral connections are notorious for leaks, which vacuum sewers eliminate.
The gravity lift stations put crews in contact with sewage: They have to open the lift station hatch, clean the floats, add degreaser, exercise the pumps, and hose out the wet well. It’s a tedious and often dirty job. Because all of the vacuum system’s equipment, including the collection tank, is completely sealed, sewage is contained and the vacuum station remains clean and odor-free.
Last year we had two after-hours call-outs for the vacuum sewer. Both involved the vacuum valve controller, which caused pipeline vacuum levels to decrease slightly. But the inherent vacuum reserve in the pipeline, coupled with additional run time of the vacuum pumps, was sufficient to maintain system vacuum so that no customers lost service while the problem was being fixed. These types of problems typically are corrected by one employee in less than an hour, including travel time, time to isolate the faulty valve, and repair time.
Cedar Grove is located just a couple of miles inland of the Gulf of Mexico. As you would expect, we get several major storms every year and the occasional hurricane.
Every storm presents the possibility of an electrical power outage, which can be disastrous. The loss of power shuts down the lift and the vacuum stations. Gravity sewer lines quickly back up into homes and businesses creating a public health—and public relations—nightmare. We have to shuttle portable generators between the lift stations to keep the gravity system functioning.
Not so in our vacuum system, where a permanent generator located at each vacuum station keeps them running. Furthermore, vacuum sewers are closed systems so rainwater can’t infiltrate the lines.
When electrical power is lost, nearly all of our staff is dedicated to keeping the gravity system running. With the vacuum system, the only thing we have to do is keep gasoline in the generator.
MAKING THE CHANGE
Public works directors who have no experience with vacuum technology may think that vacuum sewers are complex systems that require years of specialized training to maintain.
Not so. We recently assigned a new hire to service our vacuum sewer system. Within a week he was handling daily maintenance schedules by himself; and after a week of specialized hands-on training, he was handling nearly all aspects of repair and service.
As with all systems, parts occasionally wear out and sometimes mains are damaged. Replacing components is easy, largely because the system is amazingly simple and offers easy access to the valve pits, vacuum station, and vacuum mains. Repairs typically take minutes and the component parts are relatively inexpensive.
Vacuum sewer technology is safe, efficient, reliable and simple to operate and maintain. We hope to expand our vacuum system soon (that’s easy, as well), so our department will become even more cost effective. For topographic conditions like we have along the coast, vacuum sewers are by far the best option for sewage collection. PW
Two-thirds of Cedar Grove’s homes—3,000—are on a gravity sewer system and the
rest are on a vacuum sewer system, giving public works ample opportunity to compare
the performance of the two.
This article appeared in the October, 2008 issue of Public
Works. 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 AIRVAC vacuum sewer installation in Ceadr grove, Florida can be found
in our Municipal
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