Septic distribution box inspection - US EPA Septic System D-Box Splitters for Drainfield Rest & Recovery
Use of Splitters at the Septic System Distribution Box

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Septic drop box splitters: use of special control valves in the septic system D-box or distribution box can give sections of the septic soak bed or leachfield or drainfield a rest, allowing for drainfield recovery. This article describes septic d-box splitters & their use and gives product sources - where to buy a D-box splitter for drainfield R&R.

Septic system D box installation, specifications, inspection, diagnosis, and repair: in this article series about septic system drop boxes we describe the best procedures for locating and inspecting, repairing or replacing the septic drainfield distribution box, or the "D-box" or "Splitter box".

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Septic System D-Box Splitters - Drainfield Resting

Reader Question: What's the difference between a septic system D-box and a Splitter?

What is the difference in splitter box and a D box? - T.C.

Reply: why we might need a Zoeller Tru-Flow Splitter in place of a conventional Septic D-box - alternating drainfields & effluent flow balancing

Basically there is no difference in function between a D-box and a Splitter box. All D-boxes have some capability of adjusting the flow of effluent among different D-box outlets - a technique that can extend drainfield life by giving some drainfield sections a rest for two years or so between services. In some literature the control that permits diverting effluent among drainfield segments, or shutting it off entirely, is called a splitter valve.[4] In other installations, a simple manual plug or cap is inserted or adjusted at each outlet opening inside the distribution box.

Some "splitter" boxes such as Zoeller's are more sophisticated than the simple concrete or plastic Distribution Boxes illustrated in the article above.[3]

Zoeller Pump Co who make septic system grinder pumps and other onsite wastewater piping and pump components, also produce and sell component that most of us call a "distribution box" and that accomplishes that function - routing effluent among drainfield lines: the Zoeller Splitter System. What's the difference?

In a conventional concrete D-box, it's just that: a plastic or concrete or fiberglass "box" into which one pipe delivers septic effluent from the septic tank, and from which two or more pipes carry effluent to drainfield trenches, galleys, or other disposal systems. The box routes effluent among the various lines and if properly installed we hope effluent is distributed evenly. Some D-boxes include a round cover with an eccentric hole that can be rotated up or down to balance the effluent load among different drainfield sections.

Higher velocity pumped septic effluent may require careful effluent distribution control in the D-box - the Splitter

Zoeller's Splitter uses a series of internal baffles to control the routing of effluent among drainfield lines. One raison we might need the baffles and more precise control of effluent distribution through the D-box (or Splitter) is that Zoeller, who manufacture sewage grinder pumps and septic effluent pumps, (we pose) may have found that pumped septic effluent arrives in the Splitter (or D-box) at a higher velocity than effluent that drains into the box from a septic tank by gravity.

At high velocity, effluent entering a conventional D-box may not flow uniformly among the multiple box outlets - instead it would charge across the box and flow mostly into the effluent line directly across from the inlet pipe. Zoeller's baffles appear intended to prevent that problem by diverting flow uniformly among the various Splitter box outlets.

Zoeller's Tru-Flow [D-box] Splitter system includes diverter adjustments that help you fine tune and balance the effluent flow among various drainfield sections. The system can handle varying septic flow rates depending on the number of openings in use in the "box" - that is, depending on the number of outlets and drainfield sections that are in use.

Septic Effluent Flow Rate Capacity - Zoeller's Tru-Flo Splitter [D-box]

Number of outlet openings (drainfield segments fed) Maximum effluent inflow rate handled by the Splitter
5 openings 30 gpm
4 openings 21 gpm
3 openings 17 gpm
2 openings 13 gpm

See our contact information for Zoeller just below and also at at the reviewers section at the end of this article for a link to the Zoeller D-Box-Splitter for septic effluent distribution system information.

Watch out: Zoeller warns that if you use this device, which does a more careful job of routing effluent among the different drainfield branches, it is clog-prone if you don't include a septic filter in the system at the effluent outlet end. Indeed, an effluent filter will protect and extend the life of any drainfield and its components.

  • Zoeller Pump Co.,3649 Cane Run Rd. Louisville, KY USA 40211, USA, Tel: 001-502-778-2731, Website:, Email for Technical Support:, International Accounts Phone: +1-502-778-2731 ext. 8208 Fax: +1-502-774-3624 Email: Zoeller also has offices also in Taiwan, Canada, and Mexico.
    "Zoeller On-Site Wastewater Products Tru-Flow Splitter System Installation Instructions and Preinstallation Checklist", retrieved 7/12/2014, original source:

Reader Question: one side of my fields never dries out, we installed speed levelers and a new D-box. Should we cap off the wet area lines?

I have 8 lines coming off my d box. my yard is slightly pitch to one side. that side also sees most of the rain water. The surface never seems to fully dry like my other lines. speed levelers were installed when the dbox was replaced. should those 2 lines be capped and shut off for a while to regain a better saturation point? i you, JB 5/10/12



I agree with your idea to cap off saturated drainfield lines. Drainfield trenches tend to wear out (clog the soil) beginning at the trench end most distant from the D-box, slowly working the clogging back towards the D-box. Giving the drainfield a rest for 5 years or more might help those sections recover provided that they are not being soaked from some other water source.

Keep in mind that effluent breakout at ground surface is not only a failure indicator, it is unsanitary. In sum, if the area is wet, the effluent distribution lines in that area are not treating effluent anyway, just contaminating the environment.

Finally, if the wet area were in say the last 5 feet of a 60-foot line, you might try excavating and capping off the wet ends to see if that gives you longer use of the rest of the leach line.


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