How to empty grey and black water

What grey water and black water are, where to empty them, how to stay clean and what must never go on the ground.

Updated: 2026-07-25

Two systems beginners mix up most often. Let's sort out what is what, and empty them in the right order.

Grey and black are different

Grey water comes from the sink and shower. It collects in a tank under the floor, drains through a large valve and smells only mildly.

Black water comes from the toilet. In most motorhomes this is a cassette — a pull-out tank of about 20 litres that you take out through a hatch outside and carry to the disposal point by its handle. Some larger motorhomes have a fixed tank, but the principle is the same.

They must not be mixed: they have different disposal points.

Where to empty

Both only at a service point or campsite, into the dedicated points. Grey water goes into a drive-over grate in the tarmac. Black water goes into a separate hatch or a disposal toilet, often with a rinse tap for the cassette.

Never empty grey water onto the ground, into bushes or a storm drain: it carries grease and food residue. Many countries fine you for it, and a campsite will ask you to leave. Black water — even more so.

Step by step

  1. Empty grey first: park over the grate, open the valve, let it run. A couple of minutes.
  2. Then black: take out the cassette, carry it to the disposal point, open the spout cap, and pour while holding the vent button — that stops the glugging and splashing.
  3. Rinse the cassette: fill with clean water, shake, empty. Repeat until the water runs clear.
  4. Add fresh fluid and a little water so the bottom is covered.
  5. Wash your hands — service points usually have a separate tap.

Toilet fluids

Green and blue fluids differ: blue breaks waste down more aggressively, but some campsites refuse it because of formaldehyde. Look for a biodegradable label — with it you are welcome everywhere. Tablets are an option too: easy to carry and they cannot spill.

How often

For two people a cassette fills in 2–3 days, grey water in 3–4 days of showering. Do not wait until full: a full cassette is heavy (about 20 kg), and the level sensor warns you late.

Before a long stay in nature, empty both — that is how you last longer off-grid.