Nomad Water 11 min read

Grey water and toilets: the van's outlet circuit

By Azimut AI · September 21, 2026 · Updated October 14, 2026 · Our editorial policy

A conversion is always sized from the inlet: how many litres of fresh water to carry. Then, on day three, the shower backs up and the washing-up waits. It is not the fresh tank running dry, it is the grey tank overflowing — and that one you cannot top up at the nearest tap: you have to find somewhere to empty it.

1 kg/L

water weighs the same going out as coming in

≈ 1.3 L

urine per person per day

17–21 L

chemical cassette: 3 to 5 days for two

€1,500

ceiling of the fine (contravention) for emptying outside a designated point (art. R635-8)

The outlet sets the limit, not the inlet

In the vast majority of conversions, the grey tank is equal to or smaller than the fresh tank. Yet it receives everything the fresh tank delivered, minus what was drunk. In other words, the grey tank fills almost as fast as the fresh one empties — and since it is often smaller, it is the one that ends the stay.

Two people in frugal use get through 20 to 40 L a day, depending on whether they shower on board: an 80 L grey tank therefore fills in two to four days. Second effect, rarely anticipated: water weighs 1 kg per litre in both directions. A full 100 L grey tank is 100 kg carried at the worst possible moment — driving in search of a dump point, usually at the end of a stay, usually on a small road.

Grey tank, cassette or fresh water: which stops you first? The off-grid calculator works it out for your vehicle.

Toilets: what each option costs in days of autonomy

• Urine-solids separation — around 1.3 L of urine per person per day, plus a few hundred grams of solids. The smell is not a fatality: it comes from the mixing, urea breaking down into ammonia and the solids then fermenting without oxygen. Kept apart, they barely smell — and the chemicals become unnecessary.
• Chemical cassette — 17 to 21 L depending on the model, so a realistic 3 to 5 days for two. It goes to a dedicated emptying point, never into a café toilet bowl or a public WC: neither the additives nor the cassette are made for that.
• What van dry toilets do not do — they do not compost. They dry and store. Real composting takes months, off the vehicle, with its own rules. Claiming otherwise promises a treatment chain that does not exist inside a van.
• The real trade-off — separation removes the chemical and stretches autonomy, but means two containers to manage and a handling step that does not suit everyone. The cassette is simpler to live with and more limited in duration.

Emptying legally in France, and cutting the volume beforehand

In France, discharging grey or black water carried in a vehicle anywhere other than the places designated for it, without the consent of whoever has the use of the place, is a fifth-class offence: a fine of up to €1,500, and the vehicle can be confiscated (art. R635-8 of the Code pénal). The cassette is no exception. If the discharge reaches a watercourse or the sea and harms wildlife, plants or health, it becomes a criminal offence: up to two years in prison and a €75,000 fine (art. L216-6 of the Code de l'environnement). The least intuitive rule concerns storm drains: on a separate sewer system, rainwater goes to the river without passing through the treatment plant; on a combined system, storm overflows release untreated water as soon as it rains hard.

Never

On the ground, into a ditch, into a storm drain, into a café toilet bowl or a public WC. Including with a so-called biodegradable soap.

Conditionally

Campsites, some fuel stations, private aires: the service exists, but it is asked for and paid for where it is charged. Check that it takes black water, not just grey.

Always

A dedicated dump point: aires de service, many campsites, a few fuel stations. Spot the next one before the grey tank reaches three quarters.

Cutting at source shifts the constraint more effectively than enlarging the tank. Washing up under a running tap commonly uses 8 to 15 L; the same washing-up in a basin fits in 2 to 3 L, and the rinse water starts the next round. Biodegradable describes what happens in a treatment plant, not in a ditch: it is not a licence to empty on the ground. Same logic for waste — the bins on a free aire are there for the users of that aire, not for a week of accumulated rubbish. That is exactly how a free aire gets closed.

Where you may only sleep at campsites, every legal stop is also the dump point: emptying tanks without service areas: the Croatian case.

Key takeaways

The grey tank is equal to or smaller than the fresh one yet receives almost all of it: it is what ends the stay.
Water weighs 1 kg per litre in both directions — a full 100 L grey tank is 100 kg at the worst moment.
Toilet smell comes from urine and solids mixing; separate them and the chemicals become unnecessary.
Van dry toilets do not compost: they dry and store.
In France, emptying waste water carried in a vehicle outside the designated places costs up to €1,500 and possibly the vehicle; on a separate sewer system, a storm drain leads to the river, not the treatment plant.

FAQ

Dry toilet or chemical cassette?

Separation wins as soon as the stay runs beyond a few days without a dump point: it removes the chemical, and urine is emptied more often and more easily. It loses when there is no room for two containers, when nobody on board is comfortable handling solids, or when the vehicle is mostly used for weekends near equipped aires — there, the cassette is simpler and perfectly sufficient.

Does biodegradable soap allow grey water to be emptied on the ground?

No. Biodegradable describes breakdown in a treatment environment, with micro-organisms, time and dilution. In a ditch or a storm drain, the product reaches the natural environment almost intact, along with the fats and food residue from the washing-up. And the rule does not distinguish by soap: what is penalised is discharging outside the designated places.

What grey tank size should you aim for?

At minimum the equivalent of the fresh tank, otherwise the outlet limits the inlet and part of the water carried will never be used. Beyond that, the gain is paid in mass and volume: above 100 L you are mostly transporting dirty water. A grey tank slightly larger than the fresh one, plus serious reduction at source, holds up better than a big tank alone.

Sources

Facts checked on October 14, 2026