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Open-bottom polyethylene chambers have come to the fore as
a popular solution for onsite water dispersal—and in
the process have made the workday much easier for thousands
of septic system installers.
Plastic chambers first appeared in the late 1980s as a progression
from pre-cast concrete models that do the same thing. “Our
innovation,” recalls Jim Bransfield of Infiltrator Systems
Inc., “was to take the concrete version and manufacture
it out of molded plastic.”
Within a few years, plastic galleys were surpassing concrete,
and several more plastic-molding competitors had emerged.
As of 2002, more than 700,000 systems had been installed domestically,
according to the EPA. Presently, one in four septic installations
nationwide is a plastic-chambered system (nearly all of the
others being the long-standing gravel trench system). Infiltrator
Systems (Old Saybrook, CT) dominates the niche, owning about
three-quarters of it, says Bransfield, who is the product-line
marketing manager. Infiltrator also exports to 24 countries.
More Dispersal
in Less Space
Why the explosive success? “Two things,” says Shawn
Luton, national product manager of Hancor Inc., a chamber-maker
based in Findlay, OH. First, the lightweight, pre-molded systems
are “easier and faster to install,” he notes, “because
there’s no pipe and gravel to haul, and you can dispense
with textile fabrics.” Second, due to their arched spaciousness,
they can hold more water than a comparably sized gravel system.
This confers several advantages, beginning with the fact that
the open-bottom design—which eliminates rocks—allows
chambers to deliver greater dispersion performance, foot-for-foot,
than the old way.
Better still, this boast (and several others) are now scientifically
confirmed by repeated field and lab studies. Research at Clemson
University in South Carolina last year confirmed, for example,
that one representative chamber brand provided as much as
twice the storage capacity of stone trenches of equal width
and depth. An earlier analysis by EPA researchers also calculated
the relative space-saving advantage at 25% to 50%, all things
being equal. (Differences in performance tending to decrease
with lower effluent volumes.)
What this means, for one thing, is that septic fields can
be downsized without loss of performance, as sizing is usually
specified by local regulatory codes using projected effluent
volume and soil-absorptive assumptions. Among state and local
regulatory bodies, plastic chambers are now universally accepted,
Bransfield and Luton point out, either as “approved alternatives”
or, increasingly, as delivering superior capacity. This means
some states (most recently, North Carolina and Indiana) permit
a smaller plastic-chambered square-footage in lieu of a larger
gravel drainfield. Apart from accomplishing this regulatory
milestone, plastic chambers may sometimes encounter minor
issues in the permitting stage, such as the adequacy of the
manufacturer’s warranty term, and whether installers
are adequately certified.
Reduction in footprint means less disruption to the landscape
and, above all (at least to the laborers who get to install
them), elimination of the need for hauling in tons of gravel
and transport or dispersal equipment. In turn, the absence
of stone or gravel for the bedding reduces undesirable soil
compaction (which, when it occurs, impairs absorption), not
to mention the disruption to nearby shrubs, trees, or flower
beds, Luton notes.
Moreover, the chambers’ greater storage capacity compared
with gravel-filled trenches offers, he adds, “a nice
safety factor … to enable a site to hold peak effluent
surges” and avoid some risk of failure. During rainy
seasons the chamber acts as a kind of buffer to minimizing
the risk of ponding as the groundwater gets soaked. Chamber-equipped
sites can also more easily accommodate heavy surges in water
usage, such as on big laundry days or when houseguests are
adding extra baths.
Still another advantage of the plastic chamber is its more
even dispersal. In conventional stone systems, effluent tends
to discharge most heavily through the first holes in the perforated
pipes; this may lead to biomat buildup and concentrated saturation
at the inflow point. By contrast, in a typical chamber system,
there are no perforated pipes; the inflow gushes freely to
disperse itself throughout the chamber. No “masking”
occurs over the gravel, and the biomat gets plenty of oxygen.
Plastic Building-Block Components
The chambers themselves are typically arched or domed at the
top, measuring 15 inches to 36 inches wide at the bottom,
with sidewalls 12 inches to 18 inches high. (These and other
measurements obviously vary according to the specific manufacturers
and products concerned). Lengthwise, the concave sectional
lengths range from a few feet to 8 feet; they’re modular,
so multiple sections can be attached end-to-end rather easily
to extend the dispersion range up to 150 feet or more—making
systems remarkably scalable.
Variably shaped chambers that are narrower or taller can
be ordered to accomplish greater sidewall absorption, or to
nestle into reduced ground space.
As for the spacious capacity previously noted, a typical
6-foot section might hold about 80 gallons and yet weigh so
little during the installation phase that several lengths
can be easily carried by one or two workers.
Chamber designs include molded in lots of sidewall corrugation.
This increases the support strength, while also extending
the effective surface area, thus enhancing infiltrative performance—and
lifting the volumetric capacity even higher. Strengthened
arches also mean that the backfill can be left looser and
uncompacted—increasing the soil’s absorptive capacity
too.
Bottomless Caverns, Louvered Sides
As noted, what perhaps distinguishes the chamber most of all
isn’t what it adds, so to speak, but what it removes.
The length-long open-bottom allows unimpeded wastewater infiltration
directly into soil, without a rock barrier. In fact, soil
scientist Robert Siegrist offers this assessment: “I
don’t consider it a ‘leaching chamber’; I consider
it just an infiltrative surface that’s maintained open
by having a chamber, as opposed to having a gravel-filled
or a synthetic material–filled trench.” Siegrist
and others have conducted or supervised “ a number of
studies” pertaining to the open systems’ performance;
most recently he authored a report in Small Flows Quarterly
(2004), which measured the comparative improvement in water
absorption of open systems versus stone. Siegrist (a professor
and director of environmental science and engineering at the
Colorado School of Mines in Golden), notes too that, “What
we’re finding is that, on a soil-infiltrative surface
with a wastewater effluent applied, the embedment, as well
as fines and other impact associated with solid objects, like
stones or synthetic materials, reduces the infiltration rate
or capacity of that infiltrative surface, compared to an open
system.”
In addition to having no barrier at the bottom, the chamber
walls are louvered. This allows lateral effluent dispersal
out, and oxygen transfer in, while also inhibiting soil intrusion.
A descriptive overview of chamber louvers by Texas A&M
notes that some of these designs also “lend themselves
well to root-level irrigation of shrubs, flowers, and trees.”
Another study in Small Flows Quarterly (2002) reported impressively
low relative failure rates of Infiltrator’s products
vis-à-vis conventional aggregate-based drainage, and
found “no significant difference” (failure rates
being less than 2% for both). Moreover, the article stated:
“Of the failures that were observed, none appeared related
to the reduction in the infiltrative basal area,” but,
rather, seemed to have resulted from “poor site maintenance.”
All in all, the consensus of research data indicates that—when
properly sited, installed, designed, and maintained—chambers
should work well for at least 20 years.
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Molding Even
Better Mousetraps
Because of the housing boom and resulting product demand,
chamber makers have multiplied the size-and-configuration
options, and have introduced assorted innovations, e.g., a
4-foot-long chamber, multiple-port end caps, quick-coupling,
and overlapping groove connection systems. For one example,
Advanced Drainage Systems (ADS), which manufactures the EnviroChamber,
offers a swivel connector that, notes ADS’ Gerry Snowden,
“allows a chamber to bend at up to 22 degrees left or
right” in order to fit terrain needs. Similar jointed
connectors, spacers, and devices to allow contouring at sloping
sites or to bypass obstructions, etc., have been introduced.
(Late note: ADS announced in August that it completed the
purchase of Hancor Inc., but Snowden—who is ADS’s
national sales manager for onsite markets—indicates the
two firms “will continue to operate independently”
for the immediate future.)
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PHOTO: CULTEC INC. |
| Installation of an open-bottom polyethylene chamber. |
A significant variation on the basic chamber is the pipe-augmented
design manufactured by Cultec Inc. Effluent is delivered to
the chamber by a standard 4-inch PVP pipe, but instead of
stopping at the endcap, it runs atop the chamber’s length.
Twin perforation rows in the pipe “allow the effluent
to feed into the chamber from the outside,” notes Cultec
product specialist Chris Ditullio. Water trickles along the
outer surface through a non-woven filter fabric, instead of
being piped directly in, she explains. This allows for greater
oxygenation and additional treatment, because the soil around
the top of the chamber is more fully utilized. Moreover, the
approved filter fabric covering “allows for the ‘wicking’
effect [improved dispersion],” she adds, while preventing
soil intrusion. The fabric absorbs effluent, which eventually
dries and flakes off, “So it’s like a constant treatment,”
Ditullio says.
Outer-surface dispersion with Cultec’s design increases
transevaporation and root irrigation. The fabric also helps
gather more wastewater within the corrugation grooves, enabling
even more copious dispersal. Finally, perforations along the
chamber top dome and sidewalls allow effluent to enter and
soak into the soil below, as usual—so the chamber’s
key benefits can be fully realized. Bottom line: Cultic claims
an increased absorption with the same soil.
Moreover, says Ditullio, adding the length-long pipe enables
either pumping of the effluent or gravity dispersal. “Most
often,” she says, customers prefer pumping, desiring
more even distribution “to make sure it’s getting
spread out to a larger area, and to make sure it’s getting
treated better and more quickly.”
Other chamber makers offer similar piping and pumping capabilities;
the use of controlled dosing may theoretically reduce the
required depth of soil underneath.
Easier, Cheaper Installation
It’s not quite Tinkertoys or LEGOs, but plug-and-play
modular parts make chamber-system assembly relatively quick
and painless. Chamber sections typically can be stacked for
easy transport in a small pickup, and are light enough for
two or even a single person to load and position.
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PHOTO: ADVANCED
DRAINAGE SYSTEMS |
| Installation using ADS Envirochamber's
swivel connectors to fit terrain needs. |
Using a small backhoe or trencher, trenches typically 2 feet
to 3 feet wide are dug to a prescribed level depth, leaving
below the usual separation necessary from local groundwater,
rock, or other horizon (usually at least 2 feet). Design requirements
are spelled out by manufacturers and/or by local health codes.
Layouts for a single-family septic dispersal usually comprise
two or more trenches; drainfields for a higher-volume commercial
or multi-residential system might require several more trench
lines, commonly spaced 1 or 2 feet apart, notes Ditullio.
Forget gravel handling; chambers are simply dropped into place,
section by section, then connected (often by snaps or other
built-in couplers). The 4-inch pipe from the septic tank is
attached to the end and the trench is loosely backfilled with
the native dirt. As finishing, the surface should be re-planted
with grass or other vegetation and left unobstructed; don’t
drive over it with the pickup or other vehicles.
Elapsed time? Reportedly, as little as half that needed for
a stone bed. Some septic installers who have worked with lightweight
chambers are disinclined to go back to gravel. The latter
demands hauling and handling truckloads of gravel, requiring
extra manpower and extensive site cleanup.
One installer who recently did his first bed configuration
(i.e., plugging together lots of elements for a large housing
development) comments tersely, “They work well.”
But, he confesses, he had difficulty attaching base plates,
and he found the connection device “awkward.… Sometimes
it wouldn’t slip in, it wouldn’t click, and we had
to cut a hole in it,” he says. “It was kind of a
pain in the butt. Besides that,” he adds, “it worked
fine. I could deal with it. No complaints.” He adds,
“I guess you could climb in one if you wanted to. I’m
not going to.”
Chambers can be installed in a variety of topographies, soil
environments, and configurations—not only basic trenches
and beds, but fill and mound systems; in serial distribution;
and at-grade, step-down, and drop-box systems.
How about maintenance? It should be about the same as for
gravel systems; pumping-out the tank is usually needed every
two or three years to gather the accumulated biosolids. Chambers
come with a molded-in section to accommodate above-ground
ports for internal monitoring, and, should the system need
to be inspected or accessed for troubleshooting or whatever,
the absence of gravel should make it easier.
As for cost, installed chamber systems for one residence
can range upwards from $3,000 to several times that figure,
depending on the soil type, flow volumes, and local regulatory
sizing codes. If gravel is unusually cheap locally, the relative
cost of plastic may look a bit steep, one sales executive
notes.
However, Luton points out that the key saving isn’t
so much the product itself but the reduction usually realized
on installation time, labor, and cleanup—all being much
easier. The other potential saving is real estate. As Luton
explains, “A lot of states and regulated communities
will now grant the chamber a reduction in total length of
leach field,” reflecting EPA’s reported 25% to 50%
benchmark. Accordingly, an excavation contractor might save
a dozen feet or more in trenching work, and so on. Thus, all
things considered, a chamber can often come out as the cheaper
choice.
Expanding Onsite Horizons
When effluent steadily exceeds absorptive capacity, systems
fail, of course. A system may need to be replaced or expanded.
Gravel systems can be problematic in this situation due to
their sizing, the disruption they cause to existing landscaping,
and the considerable cleanup required. However, plastic chambers
change that, suggests Infiltrator’s Evelyn Laurenzi.
She points out that whenever a need for drainfield renovation
or repair arises, “chambers may be ideal” because
of their reduced size, ease of installation, and proven performance.
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PHOTO: CULTEC
INC. |
| Plastic chambers offer reduced
size and ease of installation. |
Another growth market seems to be larger-scale projects in
which leaching systems are serving scores or hundreds of homes
where, Bransfield suggests, “there’s a large community
drainfield or disposal system that drainfield chambers are
used for very commonly.” The housing boom has increasingly
turned to onsite cluster solutions as an affordable alternative
to sewer line extensions. Even commercial and light industrial
sites with higher waste strengths are using leaching chambers,
notes Hancor’s Luton, in conjunction with other pre-treatment
systems like grinder pumps, thereby postponing or even eliminating
the need to connect to city treatment plants.
Laurenzi, who is Infiltrator’s marketing communications
manager, sees the same trend occurring with that company’s
products. Although chambers were originally designed with
single residential sites in mind, she says, developers who
are “trying to fit more properties on smaller pieces
of land” are clustering systems in order to serve whole
subdivisions and home communities. “Aging municipal wastewater
systems are often unable to accommodate many more connections”
affordably anyway, she notes.
Another emerging application uses chambers to discharge effluent
away from environmentally sensitive waterways. For example,
lakeside homes will install individual adjacent septic tanks,
but their effluent is then aggregated for pumping to an outlying
drainage matrix perhaps hundreds of feet away. “The effluent,’
says Laurenzi, “is removed from environmentally sensitive
areas and is treated where soils are suitable and there’s
no risk of contaminating waterways.” Systems can even
incorporate intermediate plastic peak storage or overflow
tanks, which then feed into the leaching type.
Growing Acceptability
Where soil conditions are less than ideal, leaching chambers
can also be paired with sand-based filtration. Laurenzi says,
“A layer of the native material is removed and replaced
by sand with the right percolation capabilities.” The
sand filters and treats the effluent sufficiently so that,
“when it goes down to the next soil horizon, it will
pass through without causing clogging or failure.” Plastic
chambers abutting sand will thus increase the infiltrative
surface area better than other methods, with increased storage
capacity and better gas exchange as well. Chambers can be
paired with both intermittent and recirculating sand filters
and stone layer systems, as permitted by local codes, notes
Infiltrator’s technical director Dennis F. Hallahan,
P.E. Chambers can go either on top of the sand filter, to
allow slow percolation into the sand, or serve as the collection
system below the filter, Laurenzi explains. In the latter
scenario, after sand filtration occurs, the effluent enters
the leaching chamber and is slowly discharged into soil, which,
lacking the two-step preparation, would not be suitable as
a destination. Such flexibility enables chamber usage for
septic chores even in exotic, remote, or environmentally super-sensitive
settings, like mountaintop ski lodges, near pristine creeks
and rivers, on beachfronts and in deserts (see sidebar).
In sum, there’s an increasing awareness of the multiple
benefits of chambered dispersal, for which the plastic-molded
models are a welcome improvement. Market demand is spurred
by these advantages, as well as by a need to find alternatives
to the negatives associated with adding to “big pipe”
centralized sewage—drawbacks: financial, environmental,
and political. Laurenzi observes that onsite septic treatment
“was once looked upon skeptically and considered a ‘second-best’
alternative—but,” she says, “that’s starting
to change because we’re now recognizing that these systems
perform as well as and often better than these centralized
systems.” Within this new paradigm, she adds, “We’re
all being required to find better, more reliable ways to meet
the demands of the environment, of the regulatory community,
and in the way housing is being developed.… And this
is really the ‘next phase’ of what’s going
on in the onsite wastewater industry.”
Writer DAVID ENGLE is based
in La Mesa, CA, and specializes in construction-related topics.
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- November/December 2005
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