Septic distribution box inspection - US EPA Septic System D-Box Covers
Function of the Cover on a Septic System Drop Box; Cover Leaks, Cover Safety

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Septic system drop box cover installation, repair, requirements: why we need a safe, water-tight cover over the septic system D-box.

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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Distribution Box Cover Troubleshooting & Repair

Septic D box in trouble (C) Daniel Friedman N DayQuestion: How to repair a bad D-box lid or cover

My D box is level, it flows nicely and is made of concrete. Unfortunately the lid has failed and the sides are starting to crumble. Do I have to replace the whole box or could I simply form and pour concrete around the old one? Would this be an acceptable repair method? - Scott 5/1/12


Scott, if you can repair the dbox to be water tight in place that's fine, but I worry its a wasted effort ad if it's crumbling the original concrete mix may have been bad - if so the box will continue to crumble - replacement may be in order and not too costly. Certainly you don't want to pour a new "lid" in place that seals the D-box making it impossible to open, inspect, adjust, in the future.

Question: standing water over the [septic tank?] cleanout covers


our cleanout access covers get about an inch of water on it when we run the washer and shower in the house. The water comes up quickly on just the cover and then two minutes later it's gone. We have the tank cleaned once to twice a year; we call the company that cleaned it and the are telling us they need to dig up the D box. I have checked the yard and there is standing water anywhere. Does digging up the D box sound correct. Or should they just check for a clog? - John 6/26/12


John if the septic tank or dbox is flooding as you describe it sounds as if the fields are clogged or flooded. Your system is in trouble, needs diagnosis and repair for sure. Because it's easy and quick it's a great quick-check to look into the D-box to see what's going on.

Open the D-box covers, flush a few toilets in the home, or run the washer, and watch in the D-box. If you see effluent entering and flooding the box then we figure the outlet lines and drainfield are either blocked or saturated.

Start a more throughout inspection at the septic tank, include piping to the D-box, the D-box, and the fields. For example if the tank inspection shows us that the tank baffles are gone, we know we've been pushing solids into and ruining the fields.

Question: how to seal the concrete lid on a septic D-Box?

2017/04/26 Ed said:

What do you use to seal the concrete lid on a d box?

Reply: which Septic D-Boxes & D-box covers need a seal?


The D-Box should be protected from surface runoff & flooding without a special sealing cover

Pre-cast concrete Distribution Boxes are sold usually by local septic tank and system suppliers and typically include gasketed openings for the effluent distribution pipe connections and a flat concrete lid that simply mates with the flat edges of the D-box without a gasket and without use of a sealer. Below I warn that using an effective sealant between the D-box body and D-box cover risks being unable to remove the cover later when needed.

If the septic system distribution -box is protected from surface runoff and local flooding, as it and the rest of the septic system should be if it is to be functional, and if the septic soakbed or drainfield is itself working, then neither groundwater nor septic effluent will make trouble flowing into nor out of the D-box without extravagant gaskets or sealants.

Sealing a Deep Septic D-Box Cover

Really? Well not always. A most-effective septic drainfield is not buried deep into the ground because there's not enough oxygen down at depths more than a couple of feet. But in cold climates such as Northern Minnesota, the top of a septic tank itself may be four feet below ground and septic soakaway or drainfield beds may be still deeper. It can be more difficult to keep seasonal ground water out of such systems.

If you have an unusual situation that requires actually sealing the septic D-box lid to the box body, and as usual the concrete D-box design doesn't include gaskets and grooves to mate them, you can use butyl sealant on a clean dry surface - it will bond very well - perhaps too well as the D-box won't leak but it can be hard to get the lid off again if you use too much sealant.

For that reason some septic installers often provide a flexible rubber gasket (or a makeshift one using a neoprene gasket material) that will seal the lid of the D-box while making its removal easy when inspection or adjustment are needed.

Sources of Septic D-Boxes with Seal-able Covers

Tuf-Tite makes a plastic D-box that includes built-in seals around the lid as well as around the connecting pipes. [Tufftite]

Polylok makes a plastic D-box with a sealing cover for round Distribution boxes. [Polylock]

Advanced Drainage Systems produces a polyethylene "drain tube distribution box" sold at WalMart stores. This box includes a friction-fit plastic access cover.

Corex provides a polyethylene plastic D-box taht to me looks identical to the Advanced Drainage Systems product. These products are ribbed for structural strength and may be more suitable for surface water drainage.

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