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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.


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.)

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.

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.

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.

OW - November/December 2005


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