Full-time blackwater odor question

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Well-known member
Feb 6, 2022
Harrisburg, PA
Has anyone experienced this: When parked in one place for a few weeks (or months), the odor from the blackwater tank (from the small amount of air that escapes upward when the toilet is momentarily flushed) is much stronger than when we are on the move, punctuated by short, few day long stops. I'm thinking that when we're moving, the contents of the tank are being sloshed and mixed around, letting the blackwater chemicals work much more effectively that when we're standing still, thereby killing the odor more effectively.

Anyone else experiencing something similar?
Do you have the overhead vent open? The fan on? Even just the vent open can cause some smells to escape from the toilet when flushed under the right wind conditions. Another cause can be if you have the black tank drain valve open while parked on a full hookup site. The recommended method is to keep the black tank valve closed until the tank is at least 1/2 to 3/4 full to avoid solids buildup.
Do you have a black tank flush system installed? You might try once a week or so, using it to almost completely fill your black tank and then dumping it all at once. Doing it a second time will help get the solids out of the tank. If you don't have a flush system installed, you can do the following two things:
  1. When I am parked for several days, and after I have dumped the black tank so it is empty, I will fill the toilet up almost to the brim by using the foot pedal and then flush it to move things around in the tank. Do this two or three times. Then I will open the dump valve and see if more gunk comes out.
  2. And if nothing else works, you can try filling your tank half-full and going for a short drive, stopping and starting suddenly a couple of times to slosh things around.
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The "secret" is to keep all solids completely covered with water at all times.
And as NY-Dutch said, do not open the roof vent until after the commode has been flushed. It's a matter of air moving the path of least resistance, which should be the waste water vent pipe, but it's only 2" diameter, the roof vent is 10".
There is likely a couple things at work here. First, the black tank contents don't have to be stinky. Using plenty of water is the secret to that. The contents should be a slurry, not mud. That little bit of air that escapes may not be noticeable. Second, why is the air escaping? The black tank is vented to the roof, so there should not be any pressure build-up unless the vent is clogged. Or maybe if there is a breeze blowing DOWN the vent. The other possibility is suction inside the RV pulling the fumes from the tank. You say nothing like that, but consider other air vents besides the bathroom ceiling fan.
One other thing I do is to use a dishpan. I wash dishes in it and also hold them over the dishpan when rinsing them. (Very little water goes down the grey water drain.) When I am done, I have a full dishpan of soapy water, so I dump it into the toilet. I figure the toilet always needs extra water, but the grey tank does not.

Basically, we all try to save water at one time or another, and this means we and our family tend to not flush with enough water to dissolve all the TP and keep stuff covered, so the dishpan water helps.

Also, if you dump your tanks on your way home or just before you leave a campground and are going to leave your RV parked for a while, there is a good possibility the "leftovers" in the tank will dry out and create a solid block. So, add a couple gallons of water before you store your rig, more if you live in a very dry area. That will keep the stuff at the bottom dissolved so it will dump out more easily next time.

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