Pressure Distribution System Reference

Why this matters

Pressure distribution is the engineered approach to delivering septic effluent equally across a drainfield. Conventional gravity-fed drainfields rely on slope and the natural distribution behavior of effluent in trenches; in many sites, that produces uneven distribution and uneven biomat formation, which shortens drainfield life. Pressure distribution uses an effluent pump to push effluent under pressure to small orifices in pressurized laterals, ensuring every square foot of drainfield receives equal flow. The system is more expensive to install (a significant additional vs gravity) and requires the pump tank components (pump, controls, alarm), but the drainfield lasts 50 to 100 percent longer when properly maintained. A septic contractor who understands both gravity and pressure distribution can engineer the right system for the site and customer.

When pressure distribution is required

Pressure distribution is mandatory for:

• Mound systems (always pressure)
• Sand filter systems (always pressure)
• Drip distribution systems (always pressure)
• Drainfields with significant elevation difference (gravity uneven across the field)
• Drainfields with very small footprint (need maximum effective utilization)
• Time-dosed systems (intentional resting between doses)

State and county regulations may also mandate pressure distribution for:

• Specific soil types
• Specific high-water-table conditions
• Larger systems (over 1,500 gpd typical)
• Repair / replacement of existing systems in problem areas

Components of pressure distribution

1. Septic tank (conventional)
2. Effluent filter (in tank outlet)
3. Pump tank (separate chamber)
4. Effluent pump (1/2 to 1+ HP submersible)
5. Float switches (pump-on, pump-off, alarm)
6. Pump controller (timer-based dosing typical)
7. Force main (PVC pipe from pump tank to drainfield)
8. Distribution box or manifold (splits force main to laterals)
9. Pressurized laterals (PVC pipe with orifices)
10. Orifices (small drilled holes at intervals along laterals)
11. Cleanouts (at end of each lateral for flushing)
12. Alarm panel (visual + audible)

How it works

The pump operates on a timer (typically 1 to 10 minute pump-on, 2 to 6 hour pump-off). Each pump cycle:

1. Float switch or timer initiates pump
2. Pump pressurizes the force main
3. Effluent fills the manifold and all laterals simultaneously
4. Pressure pushes effluent through each orifice equally
5. Orifices deliver effluent uniformly across the drainfield
6. Pump stops at end of cycle
7. Pressure bleeds; system rests until next dose

The result: every square foot of drainfield receives approximately equal effluent flow per dose. Biomat develops uniformly; field capacity is maximized.

Lateral design

A typical residential pressure-distribution lateral:

The orifices are sized to deliver equal flow at the design pressure. Orifice spacing determines spatial distribution.

Pressure considerations

The pressure at each orifice should be:

• Minimum 2 ft of head (most installations design for 3 to 5 ft of head)
• Uniform across all orifices (within 10 percent of each other)

Pressure decreases along the lateral length due to friction. Manufacturers and designers use:

• Larger lateral diameter than minimum (reduces friction loss)
• Smaller orifices closer to the pump end (forces equal flow)
• Pressure regulators if needed (for steep grades)

Pump sizing

Pump must deliver:

• Total flow (gpm) = number of orifices x flow per orifice
• Head pressure = elevation change + friction loss + orifice pressure requirement

Typical residential pressure distribution pump:

• 1/2 to 1 HP submersible effluent pump
• Flow: 30 to 80 gpm at design head
• Brand: Liberty Pumps Pro280, Goulds GSP series, Zoeller 53/98, Champion 200

Inspection and service

Annual inspection includes

1. Pump tank inspection (same as gravity pump tank)
2. Verify pump operates correctly (manual cycle)
3. Verify pump dosing schedule
4. Verify float switches
5. Verify alarm panel
6. Inspect cleanouts at each lateral end
7. Open cleanouts; verify clean discharge

Squirt height test (lateral verification)

The squirt height test verifies orifice flow:

1. With pump running, observe the squirt of effluent from each orifice (visible at end-cap cleanout)
2. Squirt height should be 1 to 4 inches typical (varies by design pressure)
3. All orifices should have similar squirt heights
4. Significant variation indicates pressure issue or orifice blockage

Orifice cleaning

Orifices accumulate biofilm and small debris over years. Cleaning:

• Open the lateral cleanout at the far end
• Pump cycle while open (allows flushing)
• Probe individual orifices with a wire if specific ones are blocked
• Chemical flush (hydrogen peroxide or NaOH) for biofilm

Lateral inspection

Some systems allow lateral camera inspection (smaller diameter pipe is harder to camera; specific cameras needed). Look for:

• Root intrusion (rare in pressure systems due to active flow)
• Pipe damage
• Significant solids accumulation

Common pressure distribution issues

Pump runs but pressure not reaching laterals

• Pump impeller worn (low pressure)
• Discharge check valve stuck open
• Major lateral leak or break
• Manifold valve closed

Uneven distribution

• Some orifices blocked (clean per above)
• Pressure regulator (if equipped) failed
• Lateral pipe partially blocked
• One lateral with multiple blocked orifices forces others to over-flow

Pump cycles too often / too infrequently

• Float switch failure (pump-on or pump-off level wrong)
• Customer water use mismatched to dose schedule
• Adjust dose schedule to match actual flow

Lateral freezing (cold climate)

• In cold climates, the laterals at the surface may freeze between doses
• Insulation at the orifices (sock insulation)
• Pump dose schedule that maintains warmer flow

Force main freeze

• Force main below frost line typically; insulation if shallow

Customer expectations for pressure distribution

The customer should understand:

• Pump runs on a timer regardless of immediate water use
• Pump alarms must be addressed immediately
• Annual service is required (more than gravity systems)
• Drainfield is more delicate than gravity (no vehicles, no construction over it)
• The system delivers longer drainfield life as the trade-off

References

• EPA Onsite Wastewater Treatment Systems Manual.
• Wisconsin Mound Soil Absorption System Manual (industry reference).
• ASTM F1216 (Pressure-resistant pipe systems).
• State / county septic regulations.
• Manufacturer literature: Orenco AdvanTex, Bio-Microbics, Geoflow.
• Manuall internal: Septic Pump Tank Maintenance, Septic Mound System Service.