Sketch of the septic system drop box or D-Box - US EPA 1980 Septic System D-Box Inspection Procedures
Inspect & Test the Septic System Distribution Box

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Septic system D box inspection & problem diagnosis: procedures for inspecting or diagnosing problems at the the septic drainfield distribution box, or the "D-box" or "drop box". If the D-box is leaking, smells, or is tipped, clogged, or otherwise not working this article describes how to diagnose & fix the trouble.

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Septic System the D-Box Inspection & Troubleshooting

Septic system D-box with algae growth or "slime" (C) InspectApediaInspection of the Septic System Drop Box - the Septic System Distribution Box

The distribution box (more than one may be in use) connects a single effluent line from the septic tank to a network of absorption system components such as drainfield leach lines or to a network of seepage pits or galleys.

The photo (left) shows the adjustable weir outlets that permit balancing flow among drainfield lines. (Source EPA who used photo from Ayres Associates.) More sketches of D-box layouts and configurations are shown in this EPA drawing.

Inspection Points at the Septic Drop Box

Regulating effluent distribution: In good system design the outlet openings from the distribution box to each drainfield line can be adjusted to regulate the flow among the various absorption lines.

Elegantly simple, a plug with an eccentric hole is inserted into the end of each leach line fed from the D-box. By turning the plug in the end of the leach line pipe one can place the eccentric hole higher or lower with respect to the bottom of the distribution box, thus compensating for a slightly tipped box, differences in leach line length, or differences in leach line condition.

Uneven effluent distribution: If a distribution box becomes tipped (or clogged) effluent may be routed to only a portion of the absorption system, thus overloading it and leading to a "breakout" of effluent at the surface or to clogging and system backup.

An examination of the septic system distribution box interior may show flood lines in the box if the drain field has been clogged or saturated in the past even if at the time of inspection the box is not flooded.

If the septic drainfields have been flooded you should be pessimistic about the remaining life of the absorption system.

If the septic drainfield distribution box is tipped and/or septic system effluent arriving from the septic tank has not been uniformly distributed among the drainfield lines (assuming they are of equal length and in equally good soils), only a simple adjustment of the outflow may be needed.

Round plugs with eccentric openings may be present or can be inserted in the D-box outlet openings to regulate flow among the individual absorption lines. (C)Trap Daniel Friedman Copyright Protected text.


Tipped or flooded distribution boxes, resulting in uneven loading of soil absorption system lines. This condition can flood one or two lines leading to early field failure.


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