|
Storm Survival
Vacuum sewers withstand Florida's hurricane barrage.
Catastrophic weather is a public works director’s worse nightmare. A
massive storm can wreck infrastructure on an epic scale, and
the costs to the city and the local economy can be staggering.
Public works directors in Florida will forever remember 2004
as the “Year of the Hurricane.” Four major storms, all
Category 2 storms or worse, pummeled the peninsula during a
six-week period. The last time four hurricanes hit one
state in a single year was in Texas in 1886.
For officials responsible for sanitary sewer operations, hurricanes
are especially troubling. The loss of electrical power,
a common occurrence with hurricanes, means that pump stations
shut down. Raw sewage can back up into homes or spill
out into the environment. Public works personnel have
to scramble to get portable generators on line to keep sewage
flowing. Meanwhile, massive amounts of stormwater infiltrate
the system and overwhelm treatment plants, which then have
to bypass untreated combined sewer overflow into local streams
and rivers.
Public works directors all over Florida had to deal with these
and other sewer-related problems during the aftermath of hurricanes
Charley, Frances, Ivan and Jeanne. However, a few communities
using vacuum sewer technology survived the hurricanes with
few, if any, sewer issues.
 Hurricane Charley made landfall on Friday, August 13, along
the Southwest Florida coast. Packing nearly 150-mile per hour
winds, it was the most powerful storm to come ashore since
Hurricane Andrew in 1992. Stock Island, located just a few
miles northeast of Key West, took a glancing blow from Hurricane
Charley, knocking out electrical power for several hours.
In 2003 the community had installed an AIRVAC vacuum sewer
system to serve several hundred of its residents. The
vacuum station is housed in a building that is above the 100-year
flood level and strong enough to withstand a Category 5 hurricane. The
island also has force main gravity sewer system with 16 lift
stations. Both systems are serviced by Keys Environmental,
Inc. based in Key West Fla., under contract to Key West Resort
Utilities.
When power is lost at lift stations, crews often have to bring
in large portable generators to maintain sewer service. Christopher
Johnson, president of Keys Environmental, Inc., noted that
handling large three-phase generators in high winds and pounding
rain is not only difficult, it can be dangerous. Fortunately,
the vacuum sewer system required minimal attention both before
and after the hurricane hit. A standby generator kicked in
when electrical power was lost, and it maintained service until
power was restored. Johnson also noted that several mainland
communities with gravity systems were still without sewer service
weeks after the hurricane had come and gone.
With a vacuum system, one vacuum pump station often can replace
seven or eight lift stations, a significant benefit for public
works departments that must decide whether to invest in expensive
backup generators for each station and that have to prepare
each station for oncoming storms.
“When we assess our Hurricane Preparedness Plan, the advantages
of the vacuum system really stand out,” said Johnson. “The
preparation that is required to ready the vacuum system is
miniscule compared to the enormous amount of effort it takes
to prepare the force main/gravity system.”
For example, in Englewood, Fla., about 30 miles northwest
of Fort Myers, Fla., about two-thirds of the community’s
sewer system is vacuum and about one-third is conventional
gravity sewer. There are five vacuum pump stations, but
60 lift stations for the gravity sewers. “We have a lot of
work to do in preparation for a hurricane,” said Mike Ray,
operations manager for Englewood Water District. “We had all
kinds of problems with our gravity sewers but the vacuum sewers
never missed a beat,” Ray continued. “If all our sewers
were gravity sewers we would have 150-200 lift stations. You
can imagine the amount of work it would require to keep up
with that many lift stations before and after a hurricane. We
don’t have the budget or the manpower to handle that amount
of work. ”
As Florida was cleaning up the wreckage left by Charley, Hurricane
Frances made landfall on the state’s east coast on Sept. 5
as a Category 2 storm with winds up to 110 mph and sea surges
of 6 to 8 feet. Frances knocked out power throughout Palm Beach
County, and more than 40 gravity sewer lift stations were shut
down.
“One of our vacuum stations is housed in a building that can
withstand a Category 5 hurricane and is equipped with a back-up
generator,” explained Robert Campbell, vacuum system technician
for the Village of Palm Springs, Fla. “We didn’t have
to do anything [to the vacuum station]; it had been running
since the power went out.” The village’s other vacuum station
did not have backup power, but it was back on line quickly once
a portable generator was brought in.
 The third storm, Ivan, a Category 3 hurricane, came ashore
on Sept. 16 near Mobile, Ala., racking the Florida panhandle
with high winds and rain. National Weather Service warnings
provided some time to prepare, but most public works departments
lacked the manpower and resources to get everything secured
before the storm hit. Those with vacuum systems benefited
because they had backup electrical power to eliminate service
interruptions.
“Our vacuum stations required zero preparation before the
storm hit, so this allowed us to spend time on other things,
like filling sand bags, anchoring street signs and so forth,”
explained Thomas Voght, public works director in Cedar Grove,
Fla., which is near Panama City. “After the storm other
cities had problems with their lift stations. Cedar Grove
was the only town in Bay County that didn’t experience any
sewer problems.”
Sarasota County also took a glancing blow from Ivan, but Craig
Bliss, manager, Water Reclamation Facilities of Sarasota County
Utilities, said its vacuum sewers were unaffected. “We had
very good success with our vacuum system during the hurricane. The
vacuum station required very little preparation and we never
lost service in that area even though we lost electrical power.
We currently have about 500 gravity lift stations. That
many lift stations require a significant amount of preparation
before a storm hits and a lot of service afterward.”
The final storm to rumble into Florida was Hurricane Jeanne,
packing 130 mph winds and sea surges up to 12 feet. It
made landfall on Sept. 25, less than 10 days after Ivan. Cities
that had been hit by one or all of the preceding storms were
still dealing with the cleanup efforts.
Palm Springs’ Campbell and his staff were overwhelmed with
issues at their 40-plus gravity lift stations, but there were
minimal problems from the vacuum sewers. “We had zero occurrences
of sewage spilling on the ground from the vacuum systems,”
said Campbell. “Basically it was because of the holding
capacity of the valve pits and the sealed system. Other
municipalities were, understandably, allowing sewage to flow
onto the ground because they had no power.”
According to James Moore, assistant water superintendent of
Carrabelle, one of Florida’s panhandle cities, “Other than
checking the fuel and oil in the backup power units, the vacuum
sewers required none of our time during and after the storms.
The system had no downtime and continued to service our customers.”
Although vacuum stations are two to three times more costly
than a lift station, replacing seven or eight gravity lift
stations with one vacuum lift station reduces total system
costs by about 20 percent to 25 percent, according to AIRVAC.
From a design perspective, vacuum sewer systems are similar
to gravity systems, however, engineers must take into account
that vacuum systems have a hydraulic limit, which is a function
of elevation, flow, and pipe length (see Figure 1).
Gradually, several Florida communities are expanding the use of vacuum sewer
systems. For example, the Englewood Water District installed its first vacuum
system in 1996 and continues to expand the number of stations and customers served.
Sarasota County Utilities currently is installing vacuum sewer systems throughout
the county. “When the project is complete we will have more than 20 new vacuum
stations to service about 30,000 homes.” Said Bliss. “That will mean a
lot fewer headaches for us the next time a hurricane comes through.”
This article appeared in the April 2005 issue of
CE News. 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
Englewood, Florida can be found in our
Municipal Project Gallery.
Return
to Special Themed Articles page.
|
