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Gravity vs Pump vs Pressure-Dose Conversion for a Failing Field

Reference Process Updated Jun 19, 2026 ยท Septic Services

Why this matters

When a drainfield fails, the replacement is rarely a like-for-like swap. Site conditions, soil, slope, depth to water table or bedrock, and available area dictate whether the new field can run on gravity, needs a pump to lift effluent to it, or needs pressure-dosing to distribute effluent evenly and let the soil rest between doses. Choosing wrong means either an oversimplified design that fails again or an over-engineered one that costs the customer for capability the site does not need. The conversion choice is also heavily code-governed; the local health authority and the soil evaluation usually constrain the options before preference enters the picture. This matrix frames the three distribution methods, what each demands of the site, and how the field decision flows from soil and topography rather than from habit.

The options

Gravity distribution. Effluent flows from the tank to a distribution box and out to the trenches by gravity alone, no electricity, no moving parts. Simple, durable, and cheap to operate, but it requires adequate fall from tank to field, suitable soil, and enough vertical separation to the water table or bedrock. It distributes unevenly (the nearest trenches load first), which contributed to many original-field failures.

Pump (demand-dose) distribution. A pump tank lifts effluent to a field that sits higher than the tank or too far for gravity, dosing it in batches when the pump tank fills. It overcomes a lack of fall and gives some dose-and-rest benefit, but adds a pump, floats, a control panel, and an alarm, plus the maintenance and failure modes those carry.

Pressure-dose (low-pressure pipe / pressurized) distribution. A pump pushes effluent through a network of small-diameter, perforated pressure laterals that distribute it uniformly across the entire field, followed by a rest period that lets the soil recover and the biomat process the load. It maximizes the use of a constrained or marginal soil, extends field life, and is often required on difficult sites (shallow soils, high water table, tight clays, or where even distribution is mandated). It is the most complex and maintenance-intensive of the three.

When gravity wins

Choose or retain gravity distribution when:

  • The site has adequate fall from the tank to a field that sits below the tank.
  • Soil percolation and depth to limiting layer (water table, bedrock) meet code for a conventional gravity trench.
  • There is sufficient area for a properly sized conventional field.
  • The customer and code both favor the lowest-maintenance, no-power option.

Gravity is the default where the site allows it; do not add a pump the topography does not require.

When pump distribution wins

Choose pump (demand-dose) when:

  • The replacement field must sit higher than the tank, or beyond gravity reach, so effluent must be lifted.
  • The site has adequate soil for a conventional field but lacks the fall for gravity.
  • Batch dosing is beneficial but full pressure distribution is not mandated by the soil or code.

The pump solves an elevation problem; it does not, by itself, fix a marginal-soil problem the way pressure-dosing does.

When pressure-dose wins

Choose pressure-dose when:

  • Soils are marginal (shallow, tight clay, or rapidly permeable sands) where even distribution and dose-and-rest are needed to make the site work or to meet code.
  • Depth to the water table or bedrock is limited, requiring a shallow, uniformly loaded field.
  • The local code or the soil evaluation mandates uniform distribution.
  • The available field area is constrained and must be used to its full hydraulic capacity.

Pressure-dosing extracts the most treatment from a difficult site and is frequently the only compliant option on the soils that caused the original field to fail.

Field decision flow

  1. Start with the soil and site evaluation, not a preferred method. Percolation or soil-morphology results, depth to water table and bedrock, slope, and available area are the governing inputs. The local health authority and code constrain the menu first.
  2. Check fall from tank to field. Adequate fall to a lower field with suitable soil = gravity candidate. Field must sit higher or beyond gravity reach = pump or pressure-dose.
  3. Assess the soil's tolerance for uneven loading. Good, deep, well-drained soil tolerates gravity or simple pump dosing. Marginal, shallow, or tight soil needs the uniform distribution and rest of pressure-dosing.
  4. Apply code mandates. Many jurisdictions require pressure distribution on specified soil or separation conditions; the code choice overrides preference.
  5. Size for the design daily flow and confirm the chosen method meets the loading rate the soil can accept.
  6. Document the soil data, the governing code provisions, and the method selected so the conversion is defensible to the health authority and the customer.

The conversion rule: the site picks the method. Gravity where fall and soil allow, pump where elevation demands a lift, pressure-dose where marginal soil or code demands uniform loading. Method selection follows the soil evaluation and the local code, never the crew's default.

Drainfield conversions are permitted, code-governed work. A failing field replacement almost always requires a soil evaluation, a design by a qualified designer, a permit, and an inspection by the local health authority before backfill. Installing a field, or changing its distribution method, without the required permit and inspection exposes the customer and the contractor to enforcement, and an undersized or non-compliant field will fail again and surface raw sewage, a public-health hazard.

References

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