Friday, February 3, 2023


Avoid black tank “pyramid” problems

By Chris Dougherty
Chris Dougherty is a certified RV technician. Here is a letter he received from a reader while he was serving as’s technical editor.

Dear Chris,pyramidman-copy
We’ve been stationary for almost a month, so far. I’ve noticed what I think is a pyramid formed at the pipe base in the black tank. This has occurred twice in two weeks. I use one of those magic wands to clear it. We use probiotics in all tanks and Scott toilet paper. I use the flush king after every emptying. Any thoughts or suggestions? —Alan

Dear Alan,
This is a common problem, and you’re fortunate that you can actually see into your tank, as many RVers can’t. The problem is a result of too little water in the black tank. This can be an RVer’s error, or it can be a malfunction.

When it comes to sewage, water makes the world go ’round … and the sewage go down. Some people use no extra water in the holding tank because they’re afraid they’ll fill it too quickly, especially when dry camping. Others, especially if they’re on a seasonal site, leave the black tank valve open all the time, thinking everything will just drain. It doesn’t, which causes a buildup of sludge. Some folks won’t use chemicals, which can also do the same thing.

From the malfunction standpoint, the black tank system could have a leak. It could be as simple as a poorly sealing termination valve, or it could be more serious like a cracked tank or a failed pipe connection at the tank. If the tank has failed there will be signs of leakage and odor. If your unit has an enclosed underbelly, there should be staining where the liquid has drained, and sometimes pooling which will weigh the underbelly enclosure down causing it to bow.

Finally, if the leak is in the valve, and this is the most common malfunction, it will be obvious when the cap is removed from the termination to dump the tank after use. If you are leaving your hose connected all the time, then there will be no sign of this, of course.

This is often caused by one of a couple of things: There is some stuff jammed in the o-ring part of the valve, the o-ring has failed, or the valve is in need of lubrication to make it close properly.

The best way to fix this is to service the valve, especially if it has been in use for a long time. To do this, disassemble the valve, clean the parts, replace the seals (a new kit is only about $9 on Amazon), reassemble and lubricate using Dow Corning 111 valve lubricant.

I hope this helps!

##RVT838 ##RVDT1434


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Wayne A.
2 years ago

“Unique” brand “RV Digest-It”. My last encounter with this “pyramid problem” was 5 years ago.

2 years ago

In 30 years of trailering this has never happened to us. I keep the black tank valve closed until the tank is FULL then flush. The water is the carry that makes the tank empty 99 percent of the contents. The grey water side I leave the valve open the whole time we’re there as there’s not a build up of stuff that’s any concern. When we go home I close the black valve and make sure there’s a half a tank of contents or water for the trip home. The drive home will liquefy any contents the tank then I flush it at home. A little disclaimer I installed a cleanout at our drive way to flush the trailer out so I have that convenience. I am a plumbing contractor.

2 years ago

I regularly use Rid-X in both tanks. When we get back from a trip I flush both tanks out a couple of times and then I put a healthy dose of it in the tanks and let it sit for several days before dumping. It’s amazing the amount of junk that comes out from what appeared to be clear water after just rinsing. Sometimes I repeat the Rid-X process the second time. After this process my tank indicators show empty.

Tom G
4 years ago

I’ve noticed that even with a fair amount of water in the toilet at times a major dump will – shall we say – pile up in the bottom of the tank. I would up using a 3-foot long stick to knock the pile into smaller chunks in order to get the toilet to drain, This is even with about ten gallons of water int he tank. The soft stool just piles up. The black water flush will usually break the pile apart. Just one of the idiosynchronies of RVing.

4 years ago

I recently purchased a 2005 40′ Beaver MH. I believe inherited a significantly large solid Pyramid in my Blk water tank. I understand previous owner dry camped a lot. Blk water tank gauge reads 40% full when supposedly empty. When I add regular tap water and the gauge reaches 100% full, water goes up the stack and comes out onto the RV roof. I’ve even roto rooted back through the supposedly empty dump tank hose (real tricky) without success. I’ve tried various chemicals for extended periods. My limited research suggests that if left to dry out over a number of long storage periods it will solidify into almost concrete. There are lots of Blk tank chemicals out there and I’ve tried a few without success. Chemicals appears to be the only solution. Do you recommend any in particular. Any suggestions would be appreciated.

Norm W
2 years ago
Reply to  Ron KREDBA

I use 10# of store bought ice cubes to dump down the toilet. Drive around for 30 minutes making turns to the left and right….then go empty your tank. The ice cubes will scrub your tank guts clean…and they go down the sewer hose just as water.

4 years ago

I have that leaky black water tank. It’ll take about 3 days before the level get high enough to breach the small crack, located where the 4″ pipe comes out of the tank. Very very hard location to get to and apply Plasti-Mend. I had a huge patched area, just for this tiny crack. Did good……for about a month. Then the entire patch came off. The liquid is just to “corrosive”. Further swabbing with the product was temporary. Then I tried the Flex-seal tape. Nope. Any suggestions our there? So every 3 days I look for a drip on the rag I have situated under and before the t-handle. I’m probably never going to get the Flex-seal tape off that pipe either. Help me!

4 years ago
Reply to  Ken

The tank is probably made out of polyethelene which is only repairable with hot air welding. If you scratch with your finger nail and it feels waxy it is polyethelene. If it is very hard and your finger nail slides across the surface tap and it almost rings it is ABS which is cementable with the proper cement.

4 years ago

Chris, can I suggest that if someone is going to take off the black or Gray water valves it’s easier to fit new ones and only cost a couple of bucks more.. Those two valves are on my “every second year” list, so every second year I fit new ones. We have a 40 gallon black tank and after rinsing I refill it to 1/4 full.


2 years ago
Reply to  Richard

I try to add a lot of water to my tank but am never sure how much I am adding. How do you know when you have added 1/4 tank?

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