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

Inspect & Test the Septic System Distribution Box

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) InspectApedia

Inspection 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 above 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 our adaptation of a 1980 U.S. EPA drawing just below.

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.

Also see ALTERNATING BED SEPTIC SYSTEMS.

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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Citations & References

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In addition to any citations in the article above, a full list is available on request.

  • New York State Department of Health, APPENDIX 75-A WASTEWATER TREATMENT STANDARDS - INDIVIDUAL HOUSEHOLD SYSTEMS , [PDF] New York State Department of Health, 3 February 2010, retrieved 3/1/2010, original source: https://www.health.ny.gov/regulations/nycrr/title_10/part_75/appendix_75-a.htm
  • [1] Readers of this page should also see System Design Regulations 75-A.7 Distribution lines, distribution boxes, gravity flow, pressure distribution, dosing, siphons design specifications for septic systems.
  • [2] Thanks to reader Nicholas Day for discussing D-box troubles and repairs, September 2010
  • [3] Zoeller Pump Company, 3649 Cane Run Rd., Louisville, KY 40211, Phone: 1-800-928-7867, 502-778-2731 Fax: 502-774-3624. Technical support and/or quote related emails: zcotechnical@zoeller.com. Zoeller produces just about every kind of septic, sewage, effluent, grinder, and sump pump. Homeowners who need a sewage pump are asked to contact their local representative or retail sales outlet.
  • [4] "Drainfield Rehabilitation", Pipeline, Winter 2005, Vol. 16, No. 1, NESC, National Environmental Services Center, 800-624-8301 [copy on file as [/septic/D-boxes/NESC_2005-16_1.pdf
  • [5] Advanced Onsite Wastewater Systems Technologies, Anish R. Jantrania, Mark A. Gross. Anish Jantrania, Ph.D., P.E., M.B.A., is a Consulting Engineer, in Mechanicsville VA, 804-550-0389 (2006). Outstanding technical reference especially on alternative septic system design alternatives. Written for designers and engineers, this book is not at all easy going for homeowners but is a text I recommend for professionals--DF.
  • [6] Builder's Guide to Wells and Septic Systems, Woodson, R. Dodge: $ 24.95; MCGRAW HILL B; TP;
  • [7] Country Plumbing: Living with a Septic System, Hartigan, Gerry: $ 9.95; ALAN C HOOD & TP; Quoting an Amazon reviewer's comment, with which we agree--
  • [8] Onsite Wastewater Disposal, R. J. Perkins;
  • [9] Onsite Wastewater Treatment Systems, Bennette D. Burks, Mary Margaret Minnis, Hogarth House 1994
  • [10] Septic Tank/Soil-Absorption Systems: How to Operate & Maintain [ copy on file as /septic/Septic_Operation_USDA.pdf ] - , Equipment Tips, U.S. Department of Agriculture, 8271 1302, 7100 Engineering, 2300 Recreation, September 1982, web search 08/28/2010, original source: http://www.fs.fed.us/t-d/pubs/pdfimage/82711302.pdf
  • [11] Percolation Testing Manual, CNMI Division of Environmental Quality, PO Box 501304, Saipan, MP 96950
  • [12] Septic System Owner's Manual, Lloyd Kahn, Blair Allen, Julie Jones, Shelter Publications, 2000 $14.95 U.S
  • [13] Test Pit Preparation for Onsite Sewage Evaluations, State of Oregon Department of Environmental Quality, Portland OR, 800 452-4011. PDF document. We recommend this excellent document that offers detail about soil perc tests, deep hole tests, safety, and septic design.

    Readers should also see SEPTIC SOIL & PERC TESTS and for testing an existing septic system, also see Dye Tests
  • [15] US EPA ONSITE WASTEWATER TREATMENT SYSTEMS MANUAL Top Reference: US EPA's Design Manual for Onsite Wastewater Treatment and Disposal, 1980, available from the US EPA, the US GPO Superintendent of Documents (Pueblo CO), and from the National Small Flows Clearinghouse. Original source http://www.epa.gov/ORD/NRMRL/Pubs/625R00008/625R00008.htm
  • [16] Water Wells and Septic Systems Handbook, R. Dodge Woodson.
  • [17] Wells and Septic System, Alth, Max and Charlet, Rev. by S. Blackwell Duncan, $ 18.95; Tab Books 1992. We have found this text very useful for conventional well and septic systems design and maintenance --DF.
  • [18] The NSFC Products List has an excellent list of design manuals/modules National Small Flows Clearinghouse (NSFC) now (2019/12/13) hosted at http://www.nesc.wvu.edu/ Tel: 304-293-4191 e-mail info@mail.nesc.wvu.edu.

    he National Small Flows Clearinghouse (NSFC) was funded by the U.S. Environmental Protection Agency (EPA) to help America's small communities and individuals solve their wastewater problems through objective information about onsite wastewater collection and treatment systems.
  • In addition to citations & references found in this article, see the research citations given at the end of the related articles found at our suggested

    CONTINUE READING or RECOMMENDED ARTICLES.

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