Space Toilet Water Recovery: Vapor-Compression Distillation

Distillation Drum Controls

Heater power 1.2 kW

Drum rotation 1800 rpm

Cold-plate temp -5 °C

Running — nominal

Live Readouts

Water recovery

0.0%

Distillate purity

0 ppm

Brine concentration (solids)

4.8%

Waste tank

10.0 L

Distillate tank

0.00 L

How it works

Real spacecraft toilets (ISS-style) feed urine into a spinning Vapor Compression Distillation (VCD) drum. Rotation replaces gravity: centrifugal force flings the liquid into a thin film against the drum wall so it can be heated and evaporated even in microgravity, instead of floating as loose droplets.

Evaporation:  ṁ_evap = η(ω) · P_heat / L_vap
Brine conc.:  c = m_solids / (m_solids + m_water) · 100%
Recovery:     R = V_distillate / V_total_feed · 100%

η(ω) is the centrifugal-separation efficiency: too slow and the film is too thick and unstable to evaporate cleanly, too fast and flow becomes unstable — efficiency peaks near a design RPM. P_heat is heater power (kW) and L_vap ≈ 2260 kJ/kg is water's latent heat of vaporization.

Heater power — drives the evaporation rate directly (more power boils off water faster).
Drum rotation — sets η(ω); far from the ~1800 rpm design point, more contaminated droplets get entrained in the vapor instead of staying in the brine.
Cold-plate temperature — colder condenses more of the vapor into liquid distillate instead of losing it as vented vapor.
Non-volatile solids (urea, salts) never evaporate, so brine concentration rises as water is pulled out — real systems stop and swap the brine bag around ~32% solids, the crystallization limit modeled here.

This is the same mass/energy balance behind the ISS Water Recovery System, which recovers roughly 90%+ of wastewater as drinkable water.

Waste tank (brine) Distillate tank Vapor / distillate droplets

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