The underfloor tank connection (UTA) replaces the 10-liter urine canister of the BioTioo and directs the urine into an external tank, for example, 20 to 30 liters under the vehicle floor. This extends the emptying intervals without increasing the size of the toilet. BioTioo is the only brand with an underfloor tank/external tank connection (UTA). This removes a limitation that every mobile dry toilet otherwise has: the urine range depends on the size of the built-in tank.
Why we removed the tank from the housing
More urine volume usually means a bigger housing for mobile toilets. We did not want to accept this conflict of goals. The BioTioo measures 388 × 390 × 332 mm, so it fits into small swivel bathrooms; a larger canister would have sacrificed exactly that. So we opened the urine path outward instead of stretching the housing.
The calculation behind this is simple. The built-in canister offers 9.9 liters of usable volume, an adult produces on average 1.5 liters of urine per day. For two people, the canister is theoretically full after about 3 days. A 20-liter external tank lasts about 7 days for two people, a 30-liter tank about 10 days; alone, these values double. You are no longer dependent on the 3-day rhythm. The urine path in the overall system is shown by the function of the dry toilet.
A second loose canister seems like the simpler solution but only shifts the problem: you would have to store it additionally, carry its weight, and empty it in the end just like the first one, only twice. And it costs you exactly the flexibility that characterizes the 10-liter canister: emptyable at any toilet without a disposal station. The UTA therefore deliberately follows a different principle than "more of the same container." The canister remains the right choice if maximum flexibility on the road is more important to you than maximum range. The UTA is intended for those who prefer to avoid emptying altogether; for this, a tank of this size, whether 20, 30, or even 40 liters, requires a suitable disposal station instead of just the nearest toilet.
How the connection is designed
The underfloor tank connection sits in place of the canister and transfers the urine to a wastewater hose. The appropriate opening is prepared in every housing from the factory, so you do not cut a hole in the stainless steel. The urine is moved solely by gravity. We deliberately decided against any pump because an active component in the urine path means a potential failure point and power consumption. The module is designed and manufactured by us in Überlingen on Lake Constance.
Three planning rules determine the installation:
- Continuous slope. The line slopes continuously toward the tank. Urine remains standing in every dip, and standing urine releases odor and forms deposits.
- Short path. Every meter of line is an area for deposits. The tank should be placed as directly under the toilet as possible.
- Ventilation. Air must escape from the tank when filling and flow back in when emptying. Without ventilation, the flow stalls.
If the slope is correct, the line empties after every toilet use; no urine remains inside that could smell or freeze. For maintenance, you scale our acid routine to the larger volume: pour citric acid solution through the toilet, let it act, then drain. This cleans the line and tank in one go; the intervals are listed in Avoiding Urine Scale.
To be honest: every additional connection in the line is an additional possible leak point. Clamps and couplings must be properly installed and occasionally checked; this is normal maintenance of a vehicle sanitary line, not a special risk of our module, but also not something we downplay.
An installation project for fixed installations
The UTA is an installation project, not plug-in accessory. You drill a floor penetration, lay the line with slope, and securely mount the tank under the vehicle or building. That is exactly what we built it for: tiny houses, larger motorhomes, and facilities like forest kindergartens. In all these places, the toilet is fixed, and carrying canisters would become a permanent routine. Our installation checklist answers the most common installation questions.
We advise against it for weekend campers. For two trips a month, such a large tank is disproportionate. Added to that is the weight: 30 liters of urine weigh about 30 kg and are carried as a permanent load; what this means for payload in a 3.5-ton vehicle is calculated in Weight as a Design Goal.
The real comparison point for the UTA is not other mobile canister toilets but fixed black water systems, as commonly used in motorhomes, boats, and tiny houses for sanitary disposal. The UTA competes against these: an odor-tight, water- and chemical-free dry toilet with connection to a freely selectable tank instead of a closed black water tank with chemical additives. You usually empty the underfloor tank at a disposal station; in stationary installation, you clarify before the project where the contents may go. This question belongs at the start of planning, not at the end.
Frost on the external tank
Collected urine freezes in every toilet system. A full external tank becomes an ice block during prolonged frost, and ice expands; we prefer to say this before purchase rather than after. Three methods have proven effective: before frost periods, completely empty the tank. Or insulate and heat it, both common solutions from motorhome construction and explicitly not a BioTioo component that we promise you. Or switch back to the internal canister in the heated vehicle during deep winter, as all BioTioo components can be flexibly installed and removed.
The toilet itself is hardly affected by winter. It holds no water reserve, which is one less frost risk. The housing is made of stainless steel, bowl and lid of cold-impact-resistant PP-C; the material justification is provided in Stainless Steel and PP-C.
Availability
The UTA is available as a single module and fits any BioTioo because the hose opening is prepared in all housings. Details and scope of delivery are on the product page Underfloor Tank Connection, more modules in Dry Toilet Accessories. We offer a 3-year warranty on bowl and lid, plus a 60-day return policy.
