Twin-vault composting latrine

Break the faecal–oral route and return the nutrients — the highest lives-saved-per-effort structure a settlement can build.

Build Tier 1 · Neolithic Water Easy to build, requires discipline to run 8–12 person-days Crew 3

Home: Sanitation, Level 0 · Triage, level 1 of 2 in its track. Serves 3 steps across 3 subjects.

Every build on this site has hazards that injure or kill — quicklime, carbon monoxide, molten metal, collapsing ground. Read the safety notice before starting, and read this build's own failure modes below.

**Inside a sawdust latrine — seat, chute, and the bin of cover material that makes the system work.** Composting toilet · 2015-08-07 16:26:40 · Rene Cortin · CC BY-SA 4.0 · Commons ↗

What it is and why this shape

Diarrhoeal disease is the largest killer in every collapse, every refugee camp and every siege on record. It is also the most completely solved problem on this site: separate human waste from human water and hands, and the problem largely disappears.

A simple pit latrine does that, and for a moving group it is the right answer. But a settled group has two additional problems a pit does not solve. Pits fill and must be re-dug, progressively contaminating more ground and eventually the water table. And every pit is a permanent export of soil fertility — nitrogen and phosphorus that came out of your fields, buried where nothing can reach it. Before synthetic fertiliser, a settlement that buries its nutrients is slowly mining its own soil.

The twin-vault design solves both. Two sealed, above-ground chambers used alternately: one in use, one resting. By the time the first is full, the second has been sealed for a year — long enough that time, dryness and temperature have destroyed the pathogens. You empty a chamber of soil-like material rather than of sewage.

The rest period is the entire safety mechanism. It cannot be shortened because the vault is full, and this is the rule most likely to be broken.

Section and plan Fig 1

Earth / subsoilMasonry / brickTimberMetalLoose fill / grainGravel / aggregateVoid / air

The vaults are above ground with a sealed floor — nothing leaches into the soil or the water table, which is what makes this safe to site near a settlement. The vent must be black, in the sun, and taller than the roof, or the draught reverses and the room smells.

Tippy-tap handwashing station Fig 2

Earth / subsoilTimberLoose fill / grainWaterVoid / air

Handwashing is the other half of the intervention and it is worth as much as the latrine itself. The point of a tippy tap is that nothing is touched with dirty hands — the vessel is tipped by a foot cord. A shared bowl of standing water is not handwashing; it is a way of sharing an infection.

Keyed parts

1	Vent pipe, painted black, with fly screen	Does two jobs: solar heating drives an updraught that removes odour, and the screen traps flies that go up toward the light.
2	Squat slab or seat, per vault	Smooth, washable, sloping to the drop hole.
3	Drop hole with a lid	Kept covered. An open hole lets flies in and lets the vent draught short-circuit.
4	Vault A — in use	Sealed, above ground, watertight floor. Twelve months' capacity for the household.
5	Vault B — sealed and resting	The one doing the actual work. Twelve months minimum, undisturbed.
6	Ash / dry-material store and scoop	A handful after every use. Controls moisture, odour and flies at once.
7	Removal hatch	Sealed with mortar or a gasketed door while the vault rests. Opened once a year.
8	Urine diversion (optional)	Keeps the vault dry, which speeds pathogen die-off. Diverted urine is a good diluted fertiliser.
9	Handwashing station	At the exit, with water and soap. Not optional — the building without this does half a job.

Specifications

Vault size	1.2 × 1.0 × 0.9 m each	≈ 1.1 m³ — about 12 months for a household of 5–6 with ash added
Number of vaults	2, used alternately	Two is the minimum that works. One vault is a pit latrine with extra steps
Rest period	≥ 12 months	The entire safety mechanism. Longer in cold climates
Vault floor	Sealed, impermeable	Above ground level. Nothing leaches to soil or groundwater
Vent pipe	≥ 150 mm dia, black	Extending above the roof ridge, screened at the top
Distance from water	≥ 30 m and downhill	Less critical than a pit because the vaults are sealed, but keep the margin
Cover material	1–2 handfuls per use	Wood ash, dry soil, sawdust or lime. Ash also raises pH, which speeds die-off
Moisture	Keep it dry	Urine diversion or extra ash. Wet vaults smell, breed flies and process slowly
End use of compost	Non-root crops only	Orchards, cereals, fibre and fodder — not salads or root vegetables

Bill of materials

Material	Quantity	Notes and substitutes
Masonry or rammed earth	≈ 3 m³	Brick, block, stone or stabilised earth for the vault walls.
Impermeable floor	≈ 2.5 m²	Lime concrete, rendered masonry, or well-laid slabs bedded in clay.
Squat slabs or seats	2	Cast or carved, smooth and washable, sloping toward the hole.
Drop hole lids	2	Close-fitting. Flies and odour both depend on these being used.
Vent pipe	1 × 3 m	Fired clay, timber box or metal. Painted black — solar gain drives the draught.
Fly screen	1	Fine mesh or woven cloth at the vent top. Traps flies heading toward the light.
Hatch covers	2	Sealed with mortar or a gasketed door while the vault rests.
Superstructure	1	Walls and roof for privacy and weather. Keep the interior dim — flies go toward the brightest opening, which should be the vent.
Tippy-tap set	1	Vessel, cord, posts, soap or ash dish, and a gravel soakaway.

Tools: Trowel and level · Shovel · Saw and auger · Mould for the slabs

Construction

Site it: downhill, downwind, and close enough to be used

At least 30 m from any water source and downhill of it. Downwind of the dwelling and the kitchen. But close enough and pleasant enough that people actually use it at night and in the rain — a well-sited latrine that is unpleasant to reach gets bypassed, and then none of this matters.

✓ Check: Walkable from every dwelling in the dark, in under a minute.

Build both vaults at once

Two chambers with a shared central wall, above ground, on a sealed floor. Build the removal hatches into the rear or side of each. Do not build one vault and plan to add the second later — the alternation is the design, and the second vault is always needed sooner than expected.

✓ Check: Both vaults watertight: fill each with water and check for seepage before roofing.

Cast the slabs and drop holes

One per vault, smooth and washable, sloping gently to the hole so it can be swilled clean. Make close-fitting lids. If you are diverting urine, form the diversion into the slab now — retrofitting it is far harder.

✓ Check: Water poured on the slab runs to the hole and leaves no standing puddle.

Fit the vent

A vent from the vault in use, up past the roof ridge, screened at the top. Paint it black. The sun heats the pipe, air rises, and the draught pulls odour out of the vault and down through the drop hole rather than up into the room. Flies in the vault fly toward the light at the vent top and are caught on the screen.

✓ Check: Hold a wisp of smoke at the drop hole — it should be drawn down into the vault, not blown back.

Keep the interior dim

This is a genuinely counter-intuitive detail. Flies emerging in the vault head for the brightest opening. If that is the drop hole or a bright window, they come into the room and then into the settlement. If the vent top is the brightest thing, they go there and die on the screen.

✓ Check: Standing inside with the door shut, the vent is the brightest opening visible from the vault.

Build the handwashing station at the exit

A tippy tap on the path out, with soap or ash beside it and a gravel soakaway beneath. Position it so leaving without passing it is awkward. Handwashing is worth as much as the latrine, and it only happens if it is easier to do than to skip.

✓ Check: Refilled daily and visibly wet at the end of every day.

Commission the first vault

Seal the second vault's slab hole. Put a bucket of ash and a scoop beside the seat. Establish the rule from day one: a scoop of ash after every use, and the lid closed. Habits set in the first month persist; habits introduced later mostly do not.

✓ Check: Ash bucket needs refilling regularly — proof it is being used.

Switch vaults when the first is full

When vault A is about 300 mm from full, seal its slab hole and its hatch, and open vault B. Mark the sealing date in writing on the vault and in the settlement log. Move the vent connection to the vault now in use.

✓ Check: Sealing date recorded in two places, one of them on the vault itself.

Empty only after twelve full months

When vault B fills, vault A has been sealed for a year and can be opened. The material should be dark, crumbly, earth-smelling — not recognisable as waste. Empty it with a shovel, wear gloves, wash thoroughly afterwards, and put the material through a second composting heap with vegetable waste for extra margin before it goes anywhere near a field.

✓ Check: Material is soil-like and odourless. If it is not, reseal it and wait another six months.

Use it on the right crops

Apply to orchards, cereals, fibre crops, fodder and tree crops. Not to salad crops or root vegetables that are eaten raw or that grow in contact with the soil. This closes the nutrient loop that a pit latrine breaks — the reason for the whole build.

✓ Check: A written rule of which fields receive it, and everyone growing food knows the rule.

Tolerances that matter

Dimension	Target	Why it matters
Rest period	≥ 12 months	The only pathogen barrier in the system. Shortening it defeats the entire design.
Vault floor	impermeable	A leaking vault is a pit latrine with worse siting.
Vent height	above the roof ridge	Below the ridge, wind eddies reverse the draught and the room smells.
Moisture	as dry as possible	Wet contents go anaerobic, smell, breed flies and process far more slowly.
Interior brightness	vent is the brightest opening	It determines whether emerging flies leave via the screen or via the room.

How this fails

Emptying a vault early because the other is full. The single dangerous failure. It puts fresh pathogens onto fields and hands. If you are running out of capacity, build a third vault — never shorten the rest.

One vault instead of two. There is then nowhere for waste to rest, and the design collapses back to a pit.

A leaking vault floor. Contaminates soil and possibly groundwater, invisibly, and undoes the reason for building above ground.

No handwashing station, or one that runs dry. You have controlled the waste and left the transmission route wide open. Hands are how it actually reaches food.

A bright window opposite the drop hole. Flies then leave through the room instead of the vent screen, and flies are the vector that turns one case into ten.

No cover material. Odour, flies, and a wet anaerobic mass that will still be recognisable as waste a year later.

Applying the compost to salad or root crops. Even well-composted material should not go on crops eaten raw.

No written sealing date. Somebody will open a vault on the basis of a guess, and it will be the wrong guess.

How you know it worked

VerificationVault contents after twelve months are dark, crumbly and earth-smelling with nothing recognisable; there is no fly nuisance and no odour in the superstructure; the handwashing station is refilled daily; and no case of diarrhoeal illness in the settlement has been traced to it over a full year.

Scaling and variants

Immediate and minimal: a simple trench or pit latrine with a lid, ash after each use, and a handwashing station gets most of the health benefit within a day. If you have just arrived somewhere, dig that now and build this later. Do not let the better design delay the basic one.

Larger: size vault volume at roughly 0.06 m³ per person per year including cover material, and add vaults rather than enlarging them — a bank of four small vaults on a rotation is far more robust than two large ones.

Urine diversion: separating urine keeps the vault dry, which speeds die-off and cuts odour sharply. Urine is close to sterile as it leaves the body and is a good nitrogen fertiliser diluted about 1:10 with water. This single addition improves the system more than any other refinement.

Cold climates: die-off slows considerably near freezing. Extend the rest period to 18–24 months, insulate the vaults, and rely on the second composting heap for margin.

The same thing, built

**A raised composting latrine, vented and reachable from outside.** Compost Toilet 03 · 2022-01-01 14:56:09 · Jugraj Singh Sehri · CC BY-SA 4.0 · Commons ↗