RV black tank smell: causes and how to control it

A black tank smell inside an RV is common and almost always fixable. Most of the time, a part that is supposed to keep gas out of the living space has stopped working.

Find where the smell is coming from

Pay attention to when and where the odor is strongest. That narrows down the cause fast:

Too little water in the bowl

RV toilets seal the tank off with a flush valve, and a little water in the bowl completes the seal. Dometic’s manual says to keep about 1 inch of water in the bowl so holding tank odors stay out of the living space, and to use the “add water” function if needed. It also recommends adding water to the bowl before flushing solids and paper.

If the bowl won’t hold water, the seal is the usual suspect. Hard-water deposits on the flush ball or seal can cause a slow leak, and Dometic advises cleaning the ball and seal regularly, then replacing a worn seal. Thetford suggests replacing the toilet’s lip seal every 4 to 6 years, depending on use. Keep bleach and harsh household cleaners away from the toilet: both manufacturers warn they can dry out or damage the seals.

A plugged roof vent

The black tank has a vent pipe that runs up through the roof. It lets gas escape outside, above the RV. Dometic lists a plugged holding tank vent as a cause of odor when the toilet is flushed, and the fix is to clear whatever is blocking it.

Don’t climb onto the RV roof to do this yourself if you are not comfortable up there. An RV service technician can check that the vent and its cap are clear and sealed.

Dry sink and shower traps

The curved pipe under a sink or shower is called a trap. It holds a little water that blocks sewer gas from coming back up the drain. The South Davis Sewer District explains that this water evaporates when a drain isn’t used for a while, and faster in hot, dry weather. That is common in RVs that sit in storage or in drains you rarely use.

The fix is free: run water in every sink and shower for a moment. If the smell comes back after you refill the traps, a trap may be damaged or a vent may be blocked, which is a job for a technician.

A black valve left open

Leaving the black valve open at a full-hookup campsite is one of the most common causes of a stubborn smell. Thetford explains that the liquid drains away and leaves a pyramid of solids directly below the toilet, which is a source of odor and can plug the tank. Go RVing gives the same warning.

Close the valve, add water, and let the tank fill until it is at least half full before you dump. After you dump, rinse the black tank until the water runs clear, as Go RVing’s Sewer 101 guide recommends, using a hose that is never used for drinking water. Our guide on how often to dump your RV black tank has the steps.

Waste that sits too long

The longer waste sits, the more it smells. The University of Arizona Cooperative Extension recommends dumping holding tanks often so you need fewer deodorizers. If you do use a treatment, its advice is to follow the label and avoid formaldehyde and similar chemicals, which can harm the septic systems many campgrounds use. Our guide on how to treat an RV black tank lists what to keep out of the tank.

Stay safe

• Wear gloves when you handle the sewer hose and fittings, and wash your hands afterward.
• If a strong sewage smell fills the RV and anyone feels dizzy or sick, open the windows, get everyone outside and get help.
• Never put your head into an open tank or a dump station inlet to look or sniff.

If you have checked the bowl, the seals, the traps and the valve and the smell is still there, book an RV service appointment. A technician can test the vent, the seals and the tank fittings.