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In-situ liquid storage capacity measurement of subsurface wastewater absorption system products.


In the United States, onsite wastewater treatment systems serve 25 percent of existing homes and 37 percent of new residential construction. Conventional systems consist of a septic tank that discharges to a subsurface wastewater infiltration system (SWIS) constructed with gravel-filled trenches. Over the past 30 years, gravelless products have been replacing gravel in the trenches. These alternative products include molded plastic chambers, precast concrete galleys, multiple bundled pipes, plastic and geo-textile composites, and pipe and expanded polystyrene (EPS) bundles. All these manufactured products provide temporary storage for septic-tank effluent and a permeable conduit to soil infiltration surfaces, thus eliminating the need for gravel and pipe installation within an excavated trench.


The proper functioning of either a conventional gravel-filled trench SWIS or an alternative gravelless SWIS requires a certain quantity of liquid storage capacity to accommodate peak flows. When septic-tank effluent discharges into the SWIS at a rate that exceeds the rate of infiltration into the soil, effluent is temporarily stored within the SWIS, providing time for infiltration to occur. Storage is essential because household water usage may periodically be elevated or the soil within and surrounding the trench may get saturated (Kropf, Laak, & Healey, 1977). In both of these situations, the SWIS provides a factor of safety against system failure and is integral to the proper functioning of the system. When the SWIS fills to capacity with wastewater and the wastewater cannot infiltrate into the soil, the water may either pond at the ground surface or back up into the plumbing system.

A multitude of factors determine the long-term successful operation of a SWIS. The type of soil in which the SWIS is installed is of primary importance. Other factors include the rate at which septic-tank effluent is discharged, the nature and quantity of the septic-tank effluent (as a result of homeowner use or abuse), local topography, and vegetation. With the advent of gravelless SWIS technologies came reductions in the total length of the trench. Sizing reductions are typically determined as some fraction of the trench length or basal area of a conventional gravel system. The reduction in liquid storage capacity must also be considered with SWIS sizing reductions. If, compared with the length of a gravel trench SWIS, the length of a gravelless SWIS is reduced by 25 percent, the percentage reduction in liquid storage capacity can be, and often is, much greater. Reduction in liquid storage volume depends, almost exclusively on the in-situ porosity of the product.

Some states have pursued regulation of liquid storage capacity for SWISs that employ gravelless products. For example, in Virginia, "a gravelless system that uses plastic or other types of media must allow for the storage capacity that is substantially equivalent to that available in a gravel system" (Virginia Department of Health, 1999, page 4). In Washington, the state guidance document says, "The total measured volume of any installed gravelless system must be equivalent to or greater than the void volume provided by a conventional gravel-filled trench system" (Washington State Department of Health, 1999, p. 8). Other states that have addressed liquid storage capacity as part of their onsite wastewater system regulations include Arkansas, Georgia, Louisiana, Mississippi, Missouri, Oklahoma, South Carolina, and Tennessee. The U.S. Environmental Protection Agency (U.S. EPA) Onsite Wastewater Treatment System Manual (U.S. EPA, 2002) also addresses the importance of liquid storage within a SWIS. Unfortunately, among all of these regulations, no guidelines or protocols exist for quantifying liquid storage capacity.

The intent of this work is not to define a necessary liquid storage capacity for a particular product type. The objective is to present a method for ascertaining the in-situ liquid storage capacity of a SWIS, use the method to measure liquid storage capacity, evaluate the differences in storage volume among products, and demonstrate that company-reported storage volumes may be inaccurate if they were not measured under fieldlike conditions. Establishment of a reliable testing methodology will allow the onsite-waste-water-system community to make comparisons of liquid storage capacity values among products developed by different manufacturers, and it will allow state and county officials to obtain reliable data to ensure that state regulations are being met.

Materials and Methods


In addition to gravel, three product types were evaluated for in-situ storage capacity: plastic chambers, perforated multipipe bundles, and expanded polystyrene bundles. The first two have a high degree of rigidity under the weight of soil, the latter less so. The chambers were manufactured by Infiltrator Systems, Inc. (ISI); the multipipe bundles were made by Plastic Tubing Industries (PTI); and the EPS bundles were produced by Ring Industrial Group (EZflow). The authors believe that each of these three products is representative of the generic group of which it is a member. Products manufactured by ISI and Ring were obtained from local distributors, and the PTI products were ordered from a supplier located in Florida. Table 1 lists the products evaluated and their dimensions and configurations.

Location and Trench Preparation

All experiments were conducted within a pasture field on the Swine Farm of Clemson University in Clemson, South Carolina. The landscape was typical of that found in the Piedmont. An area of Cecil sandy loam soil (fine, kaolinitic, thermic Typic Kanhapludults) was selected for the field work. Cecil soil is the most common soil series in the Piedmont of South Carolina. At the site, the profile consisted of 8 to 10 inches of a sandy loam Ap overlying a clayey Bt horizon that extended below 42 inches.

Trenches were excavated by backhoe. Trench depth averaged about 36 inches, with lengths of 10 to 12 feet for the ISI products and approximately 13 feet for the EZflow and PTI products. The gravel-filled trenches were excavated to 10 feet in length. After the backhoe excavation work was complete, the trench bottoms were leveled. Leveling was accomplished by fine grading of the soil with a shovel, tamping by foot, and repeated checking of elevations with a level. The leveling continued until variations in the trench bottom measured no more than 0.02 feet. Figure 1 and Figure 2 illustrate the trench and product design and dimensions.



After the soil leveling was completed, the drainline product was placed in the bottom of the trench, and elevation measurements were collected on the upper surface of the product. If variations greater than 0.02 feet were measured, additional leveling was conducted. The product was removed from the trench, and the bottom of the trench was checked again and releveled if necessary. The product was then reinstalled. This procedure was repeated as necessary to ensure that the drainfield product rested horizontally at the trench bottom.

Pre-installation System Preparation

Metal rods (3 feet long and 1/8 inch in diameter) were used to establish the invert height and the full-capacity height of an EZflow bundle. One metal rod was placed on the bottom of the 4-inch-diameter pipe (invert height) contained within the interior of the bundle. The other rod was affixed to the top of the bundle. Plastic tubes, one on each bundle, were inserted into the bundles to minimize the entrapment of displaced air during liquid filling. The plastic tubes extended above ground to facilitate the escape of displaced air.

End plates were securely attached to the ISI chambers. Holes were drilled in one end plate to allow water entry into the chamber. Nails were taped in position on the end plate at the water-entry end to mark the height of the invert and the height at which the chamber was full of liquid. To minimize air entrapment during the filling procedure, holes were drilled into the chamber dome at two locations and plastic tubing was inserted in a manner that would allow displaced air to escape. The plastic tubes were taped to the top of the chambers and extended above ground. As with the EZ-flow products, metal rods were used to fix the invert height and the full-capacity height of the PTI products.


Once fitted with markers to indicate liquid fill heights and allow the release of displaced air, each product was placed within an impermeable plastic membrane liner and placed in the trench. Care was taken not to disturb the height markers and tubing. Measurements were made and repeated, if necessary, to ensure that the systems were level. At the water inflow end of each drainline product, a 4-inch-diameter plastic pipe was placed vertically within the plastic membrane liner. These pipes were placed securely against the bundle or chamber and extended above grade. A 2-inch-wide, 14-inch-long notch was cut out of the side of the pipe facing each system. The notch created a visual observation port and allowed direct observation of water heights within the system once liquid filling commenced.


When the manufactured products were in the trenches and the observation ports secured, soil was placed in the trench in a manner that minimized disruption of the systems. Backfilling was initially performed by hand shoveling. When the systems were completely covered with soil, the backhoe was used to backfill the remainder of the trench to achieve a 2-foot backfill height. Care was taken to ensure that the impermeable plastic membrane liners were not punctured during the backfilling process.

For the gravel trenches, the excavations were lined with plastic, and a fabric cushion layer was placed on the trench bottom. Six inches of gravel were placed in the trench in a manner that protected the impermeable plastic membrane liner and ensured that the gravel surface was leveled. Two sections of perforated pipe were placed horizontally across the gravel bed. After a vertically oriented observation pipe was placed in the tee joining the two sections of perforated pipe, the trench was filled with gravel to a depth of 14 inches above the trench bottom. (South Carolina requires 14 inches of gravel; other states have different requirements.) A 2-inch square had been cut out of the bottom of the tee that joined the two sections of perforated pipe, allowing water to enter the pipe during filling.


Water-Filling Process

Water was supplied through a hose that had been placed in the observation port. Care was taken to prevent the hose from contacting the nails or rods used to define the invert- and total-capacity heights. Water volume was measured with a calibrated Model 25 Recordall water meter manufactured by Badger Meter. To establish the accuracy of the meter, the authors weighed a known volume of water on a certified scale and compared the corresponding volume to the volume indicated on the water meter.

Water flow into the ISI chambers and PTI bundles was approximately 4 gallons per minute (gpm). Water flow into the EZflow products was about 2 gpm. Initial testing demonstrated that equilibrium between water heights in the observation port and within the EZflow products required a lower filling rate.

When water in the observation port reached the level of the nail or rod and remained constant for 5 minutes, a water volume reading was measured and recorded. When the total-capacity reading had been collected, the test procedures for the product were complete.

Test Replication

Between three and nine replications were performed for each product type; the minimum number of unique tests to be performed on each product was pre-established at three. Additional tests, beyond the minimum of three, were performed until researchers considered that the data set accurately reflected the performance characteristics of the product. The number of replications was determined through consideration of two primary factors: 1) variability in results between unique tests conducted on a specific product and 2) variability between field-measured and previously reported performance data. Tests were performed as necessary to establish a more robust testing data set that would serve either to reduce uncertainty where test-to-test data variability was identified during the study or to verify that variability between study data values and previously reported values was valid. All replications were distinct tests performed with a dedicated trench and new, unused materials.




The mean field-measured invert and full-capacity liquid storage capacities of the drain-line products are shown in Figure 3 and Figure 4, respectively. The error bars indicate the standard deviation for each product measured. Variability among measurements was small, indicating a high degree of precision in the determination of both invert and full-capacity volumes.

Stone and Pipe

The total storage volume of the gravel and pipe trench was 10.2 gal/ft, with a standard deviation of 0.3 gal/ft (Figure 4). Because a SWIS of this type is not a manufactured product, no manufacturer-reported data exist on liquid storage capacity Certain states have established a method for determining the storage capacity of a gravel trench. The Georgia Department of Human Resources establishes the linear storage capacity using the following relationship (On-site Sewage Management Systems, 2001, 2005):

Storage capacity = width x height x 1 ft x 0.35 (gravel porosity) x 7.48 gal/[ft.sup.3]

For the 3-foot-wide, 14-inch-deep trenches constructed as part of the study, the reference liquid storage capacity was 9.2 gal/ft in accordance with Georgia regulations. It would have been 9.6 gal/ft for the 4-inch pipe. Using the numerically calculated liquid storage capacity as a reference, the mean field-measured capacity for three replicates was 11.3 percent greater than the reference value. The authors had determined that the porosity of the gravel being used was 0.40, which results in a calculated storage volume of 10.2 gal/ft, the exact mean value that they found. With gravel systems, differences of 10 percent between calculated and measured storage volumes can be expected.

Manufactured Products

All four of the chambers had invert capacities greater than the capacity of the gravel-pipe system (Figure 3). The differences between the chambers and gravel-pipe system result from the large void spaces within the chambers and the heights at which effluent enters the system. Three of the four chambers also had total storage capacities greater than that of the gravel-pipe system; one chamber had a capacity of about 50 percent (Figure 4).

Both of the multipipe bundles exceeded the gravel-pipe invert volume. As with the chambers, this is a consequence, in part, of the elevation at which effluent enters the system. The multipipe bundles had 80 and 92 percent, respectively, of the full-volume capacity of the gravel-pipe system (Figure 4).

None of the ESP bundles had capacities equal to the invert and full-capacity volumes of the gravel-pipe system (Figure 3, Figure 4). The small invert volumes are due, in part, to the low elevation of pipe within an ESP bundle. The range of full-capacity volumes of the four ESP products ranged for 72 percent to 28 percent of the capacity of the gravel-pipe system.


The authors believe the data presented in Figure 3 and Figure 4 accurately express in-situ liquid storage capacities for the products tested. Measurements were conducted with the bottom of the trench level and with backfill hand-placed around and above the installed products. Plastic tubes placed in the systems allowed air to escape through the plastic membrane liner, preventing air entrapment that would result in a reduction in measured capacity. By comparison, when a contractor installs a system with a backhoe, the trench bottom may not be as level as the trench bottoms constructed in this evaluation. Also, loose soil may be present in contractor-installed systems, while it was removed during the installation work for this evaluation.

As shown in Table 2, the field measurements were within 10 percent of those reported by manufacturers for the chambers and multi-pipe bundles. The authors found considerable differences between the field-measured and manufacturer-reported liquid capacity values for the EPS bundles (Business Profit Improvement, PLLC, 2001). Field inspection of these products prior to installation showed that the configuration of the products varied. The 14-inch-diameter products were tightly packed, with the 4-inch plastic pipe in the top of the bundle in all cases. The 12-inch-diameter bundles were not packed as tightly, and the position of the interior pipe varied from end to end of the bundle. Inspections after measurements were made showed significant compression in the smaller-diameter bundles, with minimal compression for the larger, more tightly packed bundles. All products were backfilled with 2 feet of native sandy loam and clay soil, a practice that is representative of backfill conditions in a typical contractor-installed system. The fill depth and soil texture provided a representative state of geostatic stress load on the products tested. The authors' data agreed quite well with manufacturer-reported measurements for the two rigid-type products (MACTEC Engineering and Consulting, 2003; Plastic Tubing Industries, 2004), but lack of rigidity of some of the EPS bundles resulted in their compression under field conditions.

Laboratory work was also performed to verify the mean 50-plus percent differences between field-measured and manufacturer-reported liquid storage capacity values for the triple 12-inch-diameter EPS bundle. Measurement of the ex-situ porosity of the EPS aggregate showed a value of 42 percent. The in-situ cross-sectional area of the product was measured at 284 square inches. Using this area and the laboratory-measured 42 percent porosity value, the authors calculated the total linear capacity of the bundle to be 6.2 gal/ft. This value is within 5 percent of the field-measured capacity of 6.1 gal/ft, validating the field capacity measurements.

The low standard deviations found in these experiments suggest that the data are accurate and representative of the liquid storage capacity of the products. If data from the study were in error, one would expect measured values to be slightly higher than actual capacities, but the authors expected measured values to be with 10 percent of true values. The most likely source of bias would be the plastic membrane liner used to hold liquid, which could have left pockets outside the product where soil did not completely conform to its contoured surface, thus allowing excess water to be stored. Excavations made following the liquid storage capacity measurement trials indicated close contact between the product and the undisturbed native soil and backfill; thus, capacities created by voids outside the products were small. The authors also expected field-measured liquid storage capacity values to be within 10 percent of the manufacturers' reported capacities. This degree of variation was expected because of the nature of the product installation process and inherent variation that comes with installing products in the subsurface environment.

Table 3 shows the relationship between the sizing of a typical SWIS and liquid storage capacity. To demonstrate this relationship, the authors designed a three-bedroom home with 30-minute/inch soil according to Georgia regulations (Onsite Sewage management Systems, 2001, 2005). As indicated in the table, two of the three manufactured products provided less liquid storage capacity than required under Georgia regulations. In contrast, liquid storage capacities derived from manufacturer's reported values show that all three manufactured products would provide the state minimum standard liquid storage capacity.

The considerable differences between field-measured and manufacturer-reported liquid storage values identified in the study reported here serve to underscore the need for a uniform, standardized method of measurement for liquid storage capacity. The method presented here is one possibility. A standardized method will allow the onsite wastewater community to make comparisons of liquid storage capacity values among products developed by different manufacturers, as well as among manufactured products and conventional gravel and pipe trenches. With regulations and policy in place or being developed in Arkansas, Georgia, Louisiana, Oklahoma, South Carolina, Virginia, and Washington, the need for a common means of parameter testing and measurement exists.


The objectives in performing this work were to present a method for ascertaining the in-situ liquid storage capacity of a SWIS, utilize the method to measure liquid storage capacity, and demonstrate that differences exist between field-measured liquid storage capacity data and data reported by manufacturers. For three manufactured products tested, multiple pipe bundles and chambers provided liquid storage capacities that were within approximately 10 percent of the manufacturer's reported values. This degree of variation is acceptable considering the way these products are manufactured and installed. By contrast, EPS bundles provided liquid storage capacities that varied by more than 50 percent compared with the manufacturer's reported values. This considerable difference has the potential to affect SWIS performance in a field installation, as a system designer may base the design on the manufacturer's reported values, which may overestimate actual liquid storage capacity.

Given the magnitude of and considerable range in differences between field-measured liquid storage capacities and manufacturers' reported data, the authors advocate the establishment of a uniform, widely accepted method for measuring liquid storage capacity of SWIS manufactured products and conventional gravel-and-pipe trenches. A uniform testing methodology with standardized procedures would allow valid comparisons between liquid storage capacity data from different product manufacturers. The ability to make these comparisons would be of great value to regulators, engineers, designers, installers, and ultimately, homeowners.

Corresponding Author: Dennis F. Hallahan, Technical Director, Infiltrator Systems, Inc., 6 Business Park Rd., P.O. Box 768, Old Say-brook, CT 06475. E-mail:


Business Profit Improvement, pllc. (2001). Determination of soil interface areas and void volumes for EZflow brand product configurations under various load conditions. Germantown, TN: Author.

Kropf, F.W., Laak, R., & Healey, K.A. (1977). Equilibrium operation of sub-surface absorption systems. Journal of the Water Pollution Control Federation, 49(9), 2007-2016.

MACTEC Engineering and Consulting. (2003). Volumetric drainline laboratory report. Charlotte, NC: Author.

On-site sewage management systems, (2001, 2005). In Rules of Department of Human Resources, Public Health (Chapter 290-5-26). Atlanta, GA: Georgia Department of Human Resources, Division of Public Health, Retrieved August 14, 2006, from

Plastic Tubing Industries, Inc. (2004). PTIMPS-9 multi-purpose rockless system [Brochure]. Orlando, FL: Author.

U.S. Environmental Protection Agency. (2002). Onsite wastewater treatment systems manual (EPA Publication No. 625/R-00/008). Cincinnati, OH: Author.

Virginia Department of Health. (1999, July). General memorandum on Policy #102. Richmond, VA: Author.

Washington State Department of Health. (1999, April). Gravelless drainfields recommended standards and guidance for performance, application, design and operation & maintenance. Olympia, WA: Author.

Virgil Quisenberry, Ph.D.

Philip Brown, M.S.

Bill Smith, Ph.D.
TABLE 1 Subsurface Wastewater Infiltration Products Evaluated for
Storage Capacity

Product Type
and Description Dimensions and Configurations

Plastic leaching
ISI Standard 6.25 feet long, 34 inches wide, 12 inches high
ISI Quick4 Standard 4.0 feet long, 34 inches wide, 12 inches high
ISI Equalizer 36 8.33 feet long, 22 inches wide, 13.5 inches high
ISI Equalizer 24 8.33 feet long, 15 inches wide, 11 inches high

Multipipe bundles
PTI 9-Pipe 10 feet long, 23.125 inches wide, 8.6 inches high
PTI 10-Pipe 10 feet long, 23.125 inches wide, 12.6 inches high

EPS bundles
EZflow 1201 10 feet long, 12 inches in diameter; 1 bundle with
 14-inch-diameter pipe
EZflow 1203H 10 feet long, 12 inches in diameter; 3 side-by-side
 bundles (total width 36 inches); 1 bundle with 1
 4-inch diameter pipe
EZflow 1401 10 feet long, 14 inches in diameter; 1 bundle with
 14-inch diameter pipe
EZflow 1402 10 feet long, 14 inches in diameter; 2 side-by-side
 bundles (total width 28 inches); each bundle with
 14-inch diameter pipe

TABLE 2 Comparison of Field-Measured and Manufacturer-Reported Storage

 In-situ Measured Reported Capacity
SWIS Capacity (gal/ft) (gal/ft) Difference(%)

Gravel pipe 10.2 -- --
ISI Standard 13.3 7.3
ISI Quick4 Standard 11.7 11.1 5.4
ISI Equalizer 36 10.2 10.5 -2.9
ISI Equalizer 24 5.4 6.0 -10.0
PTI 9-pipe bundle 8.2 9.0 -8.9
PTI 10-pipe bundle 9.4 8.6 9.3
EZflow 1201 2.9 4.7 -38.2
EZflow 1203H 6.1 12.6 -51.6
EZflow 1401 3.8 5.0 -24.0

TABLE 3 Storage Capacities for Four Drainline Products Based on Georgia

 Required Trench Required Storage
SWIS Length (ft) (a) Capacity (gal) (b)

3-foot-wide gravel pipe 250.0 1,963.5
ISI Quick4 Standard 187.5 1,963.5
EZflow 1203H 187.5 1,963.5
PTI 9-pipe 220.0 1,963.5

 Manufacturer- Measured % Difference
 Reported Capacity (Required Versus
SWIS Capacity (gal) (gal) (c) Measured)

3-foot-wide gravel pipe -- 2,185.7 +11
ISI Quick4 Standard 2,081.3 2,193.8 +12
EZflow 1203H 2,362.5 1,143.8 -42
PTI 9-pipe 1,980.0 1,804.0 -8

(a) Georgia requirement based on three-bedroom house with 30-minutes-
per-inch soil.
(b) Georgia storage volume requirement based on 1 foot of gravel in a 3-
foot-wide trench.
(c) Based on total storage capacity volumes measured in this study.
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Title Annotation:FEATURES
Author:Smith, Bill
Publication:Journal of Environmental Health
Geographic Code:1USA
Date:Nov 1, 2006
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