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Technology
Coastal Communities Turning to Vacuum Sewers
(designbuild.construction.com - May/June 2005 issue)
By Tom Nicholson
... Airvac also will arrange long-term
operational support from a qualified technician if one is
located nearby.
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| Premanufactured
collection tank module is dropped into concrete block
vacuum station. (Photo by Ed Stephenson) |
For the design duties, Airvac
representatives recommended Eckler, which had done several
other Airvac jobs in Florida, including a project in Palm
Springs in 2001 involving 15,000 ft of pipe. Eckler’s
experience with vacuum system technology was highlighted by
its work with the U.S. Environmental Protection Agency, which
has asked the firm to assist in writing federal standards
for the burgeoning technology.
"When we talked with Eckler it
felt right, it felt like it would be a good team," Bates
says. Eckler was onboard not only to draw up design plans,
but to lend Calloway its vacuum sewer experience and guide
the team through the installation process.
The team broke ground in December 2003
and finished by May 2005. The time frame of just under two
years is much faster than a comparable gravity sewer system.
Time was saved by using integrated project delivery and the
much shallower pipe depths made possible with the vacuum sewer
system. "The deepest lines were about 8 ft and the average
depth was between 4 ft and 6 ft," says Bates. "With
gravity systems, you are at depths of 20 ft to 21 ft, which
is very disruptive and time consuming. That made this job
very easy, relatively speaking."
Design-build delivery helped the team
trim time by tapping the contractor’s opinion during
the design phase, enabling Eckler to establish the best, most
efficient, design plan. What resulted was a design that called
for the pipe to be buried beneath the roadway, as opposed
to alongside of it, which at first seemed the likely place
to lay the pipe, says Eckler President Don Eckler. Before
drawing up a design, "we asked, ‘What’s best
for the contractor in this?’" says Eckler. In response,
Callaway pointed out that the swale was so full of utilities,
including all the subdivision’s electric and gas lines,
that "it would have made it extremely difficult to dig
and lay the pipe there," Eckler says.
The plan to lay pipe under the roadway
was was made even easier by the fact that St. Johns County
already had removed much of the road surface in the subdivision
in preparation for a resurfacing project. "We saved a
tremendous amount of time doing it that way," Eckler
says. "We were able to see the design from the contractor’s
standpoint and we came up with the best design because of
that. That was a result of working in a design-build environment."
Although it was a design-build job,
the contract required Eckler to submit completed designs before
construction began. "It wasn’t a typical design-build
job in that aspect," says Eckler. "But since we
were teamed with the contractor we were able to work with
them throughout the design process." That provides "a
whole different mentality" than traditional project delivery,
says Bates. "It’s worked out well with Eckler. They
would advise us on the way things should be done and then
we would apply it in the field," he says.
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| Airvac
field services technician (in red hardhat) helps supervise
valve pit installation. (Photo by Ed Stephenson) |
Eckler’s vacuum system experience
came into play when the engineer suggested that the vacuum
be tested on a daily basis at the end of the workday, a frequency
that is above and beyond what’s required. "That
was not part of the contract but they asked us to do that
and that was based on their experience," says Bates.
He explains that as pipe is laid, the vacuum pressure must
periodically be tested to determine how much energy is lost
due to distance from the pump, or from diverting up and around
other utility lines. "We followed their advice and tested
daily," Bates says. "It was a good idea that helped
us foresee any conflicts down the road. If there was loss
of vacuum we would know where it was before we got too far
along."
Sweet Success
Vacuum sewer technology emerged about
30 years ago as an alternative to gravity systems where geography
or limited work zone space made deep digging impossible. With
technology advances, vacuum sewer systems are gaining viability
among designers and contractors, not only as a last resort
in special circumstances, but as an option based on its own
merit and strengths.
The typical system works like this:
A pump station containing three 25-hp pumps is installed centrally
to the scope of the system. From each home, a gravity service
line of small-diameter PVC pipe connects to a sump valve in
a fiberglass valve pit, typically buried just outside the
homeowner’s property line. The Ponte Vedra job used 359
such valves.
The pneumatically operated valve opens
when 10 gallons of wastewater accumulates in the sump. Air
from an intake pipe that emerges above ground at each home
then breaks the seal in the pit and allows the water to be
pulled by the vacuum through the sewer lines toward the pump
station.
The sewer lines are installed with a
slight slope toward the vacuum station. The lines are placed
in vertically zigzagging "saw tooth" patterns that
take advantage of gravity flow and vacuum assist. Lines use
45° elbows to go around obstacles such as utility lines.
Sewage is sucked through the mains at a rate of 15 to 18 fps
to the pump station and is collected in a tank until it is
forced out into a gravity main that carries it on its way
to a treatment plant.
On level terrain, a vacuum station can
serve up to 1,200 homes through mains up to 10,000-ft long.
There typically are three pumps housed within the station,
but the operation relies mainly on two, with the third serving
as a backup that kicks on during peak hours. The pumps run
on electric power.
Airvac has installed a total of 250
vacuum sewer systems in 27 states. Company spokesman Rich
Naret says its use is contingent on geography, noting that
the system needs level ground for operation and is not viable
for hilly terrain. "It’s being used mostly in coastal
areas," says Naret. "It works well in a place like
Ponte Vedra where it is flat. It is good for jobs where there
is a high water table like along the coast in Florida or other
coastal states. One of the hottest places right now for us
is North Carolina."
Vacuum sewers are attractive to owners
because "there are no manholes and no odors," says
Naret. "And since excavation is not as wide or deep as
on a traditional system there are no trench walls, which is
a safety issue. Trench cave-ins are never a good thing. Installation
is generally less intrusive than with a gravity system."
Time saved through shallower excavation
cuts construction costs, but Naret says that owners considering
vacuum sewers must weigh reduced installation costs against
the costs of installing necessary components, such as the
vacuum valve pits and a pump station. "There’s really
a trade off," Naret says. "It’s less expensive
but there is also a big pump station that has to be built."
Naret says vacuum sewer projects are
beginning to be performed more frequently as design-build
jobs after a slow start in general for sewer work. "Utility
sewer work has mostly gone the traditional route," Naret
says. "Design-build is popular for things where you can
see a structure. It’s a bit different with underground
work because you can’t see it and it has taken a while
for underground work to catch up with design-build."
Other design-build vacuum sewer projects
include a 259-valve Airvac installation that was completed
in 2003 in Provincetown, Mass., by the team of North Harwich-based
contractor Robert B. Our Inc. and Wakefield-based engineer
Metcalf & Eddy Inc. One massive project currently out
to bid is an estimated $180-million job in the Florida Keys
town of Marathon that is scheduled to be completed in 2010.
That project will involve 1,600 valves and seven vacuum pump
stations and will service 10,000 homes.
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