Eljen GSF Low-Pressure Dosing Design and Module Layout

Why this matters

The Eljen GSF (Geotextile Sand Filter) module is the most-installed pre-treatment / dispersal product in the alternative-OSSF market, especially in the Northeast (NY, NH, VT, ME), the Mid-Atlantic, and the Southeast Coastal Plain where conventional drainfields fail in tight soils or high water tables. The system pairs a NSF/ANSI 40-listed septic tank with patented bio-matt geotextile cores; effluent is distributed via a low-pressure dosing (LPD) network rather than gravity. Designing an Eljen GSF LPD network wrong is the difference between a 30-year functional drainfield and a 3-year hydraulic failure. This is the design playbook a senior installer or designer pulls up before submitting plans to the local approver.

Module sizing and the GSF area calculation

Eljen GSF modules are sold in two sizes: A-42 (4 sq ft footprint, 42 inches long) and B-43 (4 sq ft footprint, 43 inches long, used for residential bedroom-loaded systems). Sizing is by design flow:

• Single-family residential: 75 gpd per bedroom for standard occupancy (varies by state - VT uses 70, MA uses 110, FL uses 100).
• Commercial: per the local code's design table (a small restaurant runs 40 gpd per seat in most states).

The module count is derived from the design flow and the soil's accepted Long Term Acceptance Rate (LTAR). For a percolation rate of 30 minutes per inch (loamy sand to sandy loam), Eljen's published LTAR is 1.5 gpd per sq ft of bottom area. For 60 min/in (clay loam), LTAR drops to 0.8 gpd/sq ft. For tighter than 60 min/in, Eljen requires sand-fill construction below the modules.

Example: 3-bedroom home, 225 gpd design flow, sandy loam soil, LTAR 1.5 gpd/sq ft. Required bottom area = 225 / 1.5 = 150 sq ft. Modules (4 sq ft each) needed = 150 / 4 = 38 modules. Confirm against Eljen's design tables in the GSF Technical Manual - the manufacturer's lookup overrides any rule-of-thumb calculation, and the local approver will check against the published table.

Bed layout - the row and module-per-row geometry

GSF modules are laid in parallel rows, typically 3-7 rows wide depending on lot geometry. Each row is a continuous string of modules with a single LPD lateral running its length.

Row width is set by:

• Lot footprint and setbacks (see local code).
• The lateral pipe sizing (1.25 inch Schedule 40 PVC for residential).
• The maximum row length Eljen permits for even distribution: 75 ft per row, 18 modules per row maximum for the A-42 in a single-line manifold.

A common 38-module layout: 4 rows of 10 modules (40 modules total, exceeds the 38 minimum) or 5 rows of 8 modules (40 modules, narrower lot).

Spacing between rows: minimum 4 ft on center (Eljen recommends 6 ft for ease of construction). Spacing between modules in a row: 0 - modules butt against one another with no gap.

LPD lateral design - the pressure-distribution math

LPD requires uniform discharge across all orifices in the network. The classic Otis pressure-distribution sizing applies:

1. Lateral diameter sized so the head loss along the lateral is less than 10% of the operating head at the orifice. For 1.25 inch Schedule 40 PVC, a 50-ft lateral with 1/8 inch orifices on 3-ft centers operates well at 3-5 ft of head.
2. Orifice size and spacing: 1/8 inch orifices on 30-36 inch centers along each lateral, oriented up (12 o'clock position) so orifices self-clean during dose-and-drain cycling.
3. Operating head at the distal orifice: 2 ft minimum, 5 ft maximum. Less than 2 ft loses uniformity; more than 5 ft creates lateral hammer.
4. Dose volume: 5 to 10 times the lateral void volume per dose. For a 4-lateral, 50-ft-per-lateral, 1.25 inch system, void volume is approximately 25 gallons; design dose = 125-250 gallons.

Pump selection

A residential GSF LPD network typically uses a 1/2 HP effluent pump (Liberty LE51A, Goulds PE51, Zoeller M267) sized for the total dynamic head (TDH):

• Static lift from pump chamber to highest orifice (typically 4-8 ft).
• Friction loss in the discharge line (typically 1-3 ft).
• Pressure head at the distal orifice (2-5 ft per design).
• Friction loss in the manifold and laterals (calculated, typically 1-3 ft).

Total TDH commonly lands at 12-18 ft for a residential GSF LPD. Confirm pump curve crosses the design flow at the design TDH; under-pumping is the most common installation error and produces uneven dosing.

Pump chamber sized for at least 1 day of design flow plus 25% reserve (per most state codes). Two floats: redundant off / alarm. Three floats: pump on, pump off, high-water alarm. NEMA 4X control panel with manual hand-off-auto switch, alarm light, audible silence button.

Module bedding and cover

Eljen modules require:

• 6 inches of ASTM C33 concrete sand below the modules across the entire bed (not just under the laterals).
• Modules placed directly on the sand, geotextile-wrapped side up.
• 1.25 inch Schedule 40 PVC lateral run along the centerline of each row, pipe resting on top of the module wings.
• 12 inches of cover soil over the modules (more if frost-line depth requires).

The sand fill is a non-negotiable Eljen design parameter. Substituting native soil or pea gravel voids the listing and the warranty.

Pump cycle programming

Most installs use a time-dosed controller (SJE-Rhombus IFC11W, Orenco DLDPC). Configure:

• Dose volume: 125-250 gallons (per design).
• Doses per day: design flow / dose volume. For a 225 gpd design at 150 gallons/dose, that is 1.5 doses per day - typically programmed as 2 doses per day with a flow-balance.
• Anti-flood limit: pump shuts off if it cycles more than 4 times its design count in 24 hours.

Demand-dosed (float-only) systems also work but lose the surging benefit on low-flow days. Time-dosed is preferred.

Inspection and startup

1. Hydrostatically test the pump chamber and manifold to 5 PSI for 30 minutes before backfill.
2. Walk every orifice with a clear water dose - confirm uniform spray pattern, distal orifice within 80-100% flow of the closest orifice.
3. Document orifice flow at proximal, mid, and distal positions of each lateral.
4. Photograph the assembled bed before sand-and-soil cover. Approver will require photo documentation.
5. Run system at design dose for 24 hours, confirm no surfacing.

Common design failures

References

• Eljen GSF Technical and Design Manual, Eljen Corporation document GSF-TDM Rev 11 (2024)
• NSF/ANSI 40-2023 - Residential Wastewater Treatment Systems
• US EPA Onsite Wastewater Treatment Systems Manual, EPA/625/R-00/008, Chapter 4 (Low-Pressure Dosing)
• Otis, R.J. (1985) "Design of Pressure Distribution Networks for Septic-Tank Effluent" University of Wisconsin Small Scale Waste Management Project
• ASTM C33-23 - Standard Specification for Concrete Aggregates
• New Hampshire DES Env-Wq 1000 - Subdivision and Individual Sewage Disposal System Design Rules