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Grey Water Tank

The holding tank that collects drain water from sinks and the shower in an RV. Drained at a dump station or sewer hookup.

Also called: grey tank, gray tank, grey water, graywater, sink water tank

The grey water tank (also spelled “gray tank”) collects drain water from the kitchen sink, bathroom sink, and shower. It’s distinct from the black water tank, which collects toilet waste.

Despite the name, grey water is fairly clean compared to black water — it’s mostly soap, food particles, and water. It still needs to be contained, transported, and disposed of at a dump station or sewer connection, but it’s not regulated the way black water is.

Why the grey tank fills faster than the black tank

Two reasons:

  1. You use more water at the sink and shower than you use to flush a toilet. A 5-minute shower in an RV is about 10 gallons. A typical toilet flush is under 1 gallon.
  2. Most RVs have larger black tanks than grey tanks. A common configuration is 30-gallon black and 40-gallon grey, but the grey fills faster because of usage volume.

For most renters, the grey tank is the constraint on how long you can boondock. You’ll fill grey 1–2 days before you fill black.

How to extend grey tank capacity

  • Take shorter showers. Even a 3-minute shower with the water off-and-on (Navy shower) cuts usage in half.
  • Use the campground bathhouse when one is available. Saves both fresh water and grey tank space.
  • Don’t pour dishwater into the sink. Use a small wash tub, then discard outside (where legal — at most public-land dispersed sites this is fine; in campgrounds it’s not).
  • Open the grey valve at full-hookup sites. Unlike black tanks, grey tanks can be left open at hookup sites without consequence — there’s nothing solid that needs to sit and dissolve.

At the dump station

The grey tank dumps after the black tank in the standard sequence. The grey water flushes the sewer hose clean of any black residue. See the black water tank page for the full sequence.

Common rental mistakes

  1. Putting food scraps down the sink. They clog the grey tank’s sensors and reduce capacity. Use a strainer or wipe plates into the trash first.
  2. Pouring cooking grease down the sink. It congeals inside the tank. Catch grease in a can and discard in the trash.
  3. Forgetting to close the grey valve before driving. At hookup sites with the valve left open, water sloshes when you drive. Close it, drive, reopen at the next hookup site.