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Ask Dave: Black tank smells but not until a few days after dumping. Why so late?

Dear Dave,
We have been full-time in this rig for five years now and lately are having a lot of issues with black tank smells. Since we live in SW FL, the heat requires frequent dumping. In cooler months we can easily go four weeks without issues. We dump and thoroughly clean once a week and have been using Camco TST Max treatment in-between. We dump, attach the water hose and let that fill up, and empty it twice. Another thought was that maybe we weren’t using enough water to flush after use, so we increased that. Also, we checked the vents, which appear to be fine. We use an RV-safe water softener cleaner when cleaning, Unique Tank Cleaner.

We have re-leveled the rig thinking maybe that would help (and it has a few times). We’ve emptied and filled the tank multiple times. We haven’t found any leaks anywhere. We have stuck a hose down the toilet to spray from that direction thinking maybe something got missed. There seems to be a thick scaling right below the flush valve and I’m not sure why since we use plenty of water when flushing. Some of the scaling has come off when we clean. But if it’s not that, I have no idea what else to try. That scaling doesn’t want to budge and because it’s so high, we can’t really fill the tank that much… or can we? It’s literally 12 inches from the point of entry.

Despite all this, after a couple of days, we will flush the toilet and the odor is back. There is nothing left we can figure out to try. As much as we would love to have someone fix her, we can’t afford it and my husband is very handy. We are at a complete loss. Any ideas? —Jenna, 2018 Heartland Mallard M27

Dear Jenna,
The first thing I always ask when I get a question like this is: “Has it always been like this or just recently?” You indicated it just started lately, so something has happened in your tanks perhaps with venting, or a new product you’ve used. Second, is there a specific area the odor is noticeable, like the bathroom or the kitchen? You stated that after a couple of days you flush the toilet and the odor is back. So, I assume it’s in the bathroom and that you don’t have the smell the first couple of days.

You also stated that you inspected all the vent pipes, which appear to be fine. I would suggest looking a little further and even replacing the roof vent with Lippert’s 360 Siphon roof vent cap.

Lippert's 360 siphon roof vent cap

Check those roof vents again

Since your rig probably has the traditional vent pipe coming up to the roof with a cap, I’m not sure what you “checked” for the vent and verified it was good. Just looking at the cap on the roof doesn’t verify the pipe at the tank is secure or even hooked up. If you have a “cheater vent,” which is a vent pipe coming off the tank and coming up under a countertop that can not go to the roof, they use a siphon cap which is designed to allow air intake to eliminate a vacuum when dumping but not let odors out. These almost always fail!

Clean the black tank to get rid of smells

Next, I would clean the tank with Thetford’s Tank Blaster product, since you also indicated there was buildup in the tank. This is a great product to thoroughly clean the tank and all sludge and “gross stuff” on the sidewalls of the tank.

And finally, I would switch to Thetford’s enzyme-based holding tank treatment, as it uses what we call the “good bugs” to decompose waste. If you do not use the proper treatment product, the “bad bugs” will create a sulfurous or rotten egg smell and not properly treat the waste. I am not stating the product you are using is not correct. I just do not have experience with it and do know our tests have shown the enzyme-based products work extremely well.


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Ask Dave: Black water tank is clean but there are still smelly odors. Why?

Dear Dave,
After flushing the black water tank, there are still smelly odors like a sewage smell in the bathroom that seems to come from under the sink in the cabinet. I flush the waste tank until the water comes out clean and there is still a strong smell. Is there a one-way valve or something like that that may be stuck open? Thanks. —Luis, 2020 Winnebago Micro Minnie 1706FB

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Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”

Read more from Dave here

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