The Urine-Diverting Dry Toilet

The device is a separating seat over two chambers, and the separation is the only clever part. THE PEDESTAL. The critical component is a bowl divided into two zones: a small forward chamber, roughly 200 millimetres wide, draining to a urine outlet, and a large rear opening dropping straight into the vault. The divider between them must be a smooth, forward-sloping ridge — a sharp step catches material and a flat divider lets urine wash backwards. At bench scale, mould one from fibreglass over a plaster former, or cut and bond it from 3 millimetre HDPE. Commercial units are single mouldings and cost little, and there is no shame in buying one and building everything else. GET THE GEOMETRY RIGHT, because this is where the design succeeds or fails. The urine collection zone must be far enough forward that it catches urine from a seated adult and far enough back that it does not catch faeces. The divider crest typically sits 180 to 220 millimetres from the front of the seat. Too far forward and urine misses it, which floods the vault and ruins everything. Too far back and faeces land in the urine channel, which blocks it and smells. THE URINE LINE. From the front outlet, a 32 millimetre pipe falling continuously at 2 percent or more to a sealed jerrycan. Two rules: no dips, and no long horizontal runs. Urine deposits struvite — magnesium ammonium phosphate — as a hard scale on any surface it sits on, and a low spot becomes a solid plug of it within months. Design so the pipe can be rodded, and expect to do it. Do NOT fit a water trap on the urine line. A trap will scale up solid, and there is nothing to trap anyway: fresh urine is almost odourless. The smell of urine is ammonia from urea breaking down, which happens over hours to days, so a sealed container and a short pipe give you nothing to seal against. THE TWO VAULTS. Two chambers below the pedestal, each about 400 litres, with a movable chute or a repositionable pedestal so one is in use while the other rests. This alternating pair is the core of the safety case: material in the resting vault sits undisturbed for six to twelve months while the other fills, and time plus dryness is what kills the pathogens. Each vault needs a door for emptying and a small passive vent — 50 millimetre pipe to outside. This vent is NOT doing the VIP's job. In a VIP the vent is the odour control. Here it just removes water vapour, which is a much smaller duty, and the vault is not relying on it. Raise the vaults above ground or in a shallow plinth. Not needing an excavation is one of the design's real advantages and burying it discards that. COVER MATERIAL. A bucket of dry, carbon-rich, absorbent material beside the seat with a scoop: wood ash, sawdust, dry soil, rice husk, coir. Ash is best where it exists — it is dry, absorbent, and strongly alkaline, and the high pH is itself a pathogen kill. A scoop after every use, no exceptions.

The entire claim is that separated faeces dry instead of rotting. Test it with a surrogate rather than the real thing — this is a bench experiment, and you do not need pathogens to measure moisture. THE SURROGATE. A mixture that matches fresh faeces closely enough for the physics: roughly 75 percent water by mass, with a solid fraction of bran, sawdust and a little vegetable matter. Make up 200 gram portions. SET UP FOUR IDENTICAL VESSELS, each with a small vent, kept in the same place: VESSEL A, the UDDT as designed. One portion of surrogate, one scoop of ash on top. VESSEL B, no cover material. Surrogate alone, vented, dry. VESSEL C, the diversion defeated. One portion plus 1.2 litres of water, which is a realistic daily urine volume for one person, plus a scoop of ash. VESSEL D, a pit latrine analogue. Surrogate plus urine volume, sealed except for a small vent, no cover. WEIGH ALL FOUR DAILY for three weeks and plot mass against time. Also record the smell on a simple scale of nothing, faint, obvious, bad — subjective, but consistent if one person does it. WHAT YOU SHOULD FIND. Vessel A loses mass steadily and is dry and friable within two to three weeks. It stops smelling within a day or two, because the cover material both absorbs surface moisture and physically blocks the release of what is produced. Vessel B also dries, more slowly, and smells for longer at the start. Cover material accelerates drying and suppresses odour immediately; it is not doing the whole job but it makes the difference between acceptable and not on day one. Vessel C does not dry at all. The added water swamps the evaporative capacity, the mixture stays saturated, it goes anaerobic within days and smells strongly. One scoop of ash does nothing against 1.2 litres of liquid. Vessel D is the same as C, worse, and is what a simple pit latrine is: wet, anaerobic, odorous. THE CONCLUSION IS SHARPER THAN PEOPLE EXPECT. The difference between a UDDT that works and a pit latrine is not the ash, and it is not the vault, and it is not the vent. It is the urine. Vessel C has every feature of the design except the diversion, and it behaves exactly like the thing the design was meant to replace. That is why the pedestal geometry matters so much, why a UDDT used by someone who will not sit properly fails, and why every failed UDDT project reported in the literature reports the same failure: the diversion stopped working, or was never used correctly, and the vault turned into a pit. Do the arithmetic on your own numbers. Compute the drying rate from vessel A in grams of water per day and compare it against the 1.2 litres per person per day that a defeated diversion adds. The evaporative capacity of a passively vented vault is on the order of a few hundred grams a day. It cannot possibly keep up, and no amount of ventilation will make it.