Constraint 17: Water Recycling Efficiency¶
Physical Motivation¶
A space habitat has no connection to Earth's hydrological cycle. Every litre of water that escapes the system — through evaporation venting, chemical reactions, equipment losses, or biological output that is not recaptured — must be replaced by water launched from Earth or extracted from local resources (lunar ice, asteroids). At current launch costs, water is the most mass-expensive consumable to resupply. The habitat's water recycling loop must therefore approach thermodynamic closure.
The Closed-Loop Water Balance¶
Let \(N\) be the colony population, \(D_{pp}\) the daily domestic water demand per person (L/day), and \(\eta\) the recycling efficiency (fraction recovered per cycle). The total daily water throughput is:
Water that is not recovered constitutes a net daily loss:
Since the density of water is approximately 1 kg/L, the annual mass loss is:
For long-term self-sufficiency without routine resupply, the recycling efficiency must meet a minimum threshold \(\eta_{min}\):
This is the feasibility condition.
What This Model Covers¶
This constraint models domestic water only: drinking, food preparation, hygiene, and laundry. Agricultural water is a separate, largely self-contained subsystem — evapotranspiration from crops is recovered via greenhouse condensation at efficiencies >97%, making it a tighter loop than the domestic system (Hendrickx et al. 2019). Modelling the two separately reflects how they are engineered in practice (ECLSS handles domestic; the greenhouse handles agricultural).
Numerical Reference¶
NASA's Baseline Values and Assumptions Document (BVAD) specifies domestic water demand for a space habitat at approximately 22 L/person/day (Hanford 2004):
| Use | Demand (L/person/day) |
|---|---|
| Potable (drinking/cooking) | 2.0 |
| Oral hygiene | 0.4 |
| Hand/face wash | 4.1 |
| Shower | 2.7 |
| Clothes washing | 12.5 |
| Urinal flush | 0.5 |
| Total | ~22 |
The model default of 20 L/person/day is close to this figure.
ISS ECLSS Baseline¶
The International Space Station Environmental Control and Life Support System (ECLSS) has progressed through two distinct performance eras:
Pre-BPA era (~2009–2021): ~93% recovery
Original ECLSS achieved approximately 93% water recovery through two subsystems (Carter et al. 2009):
- Urine Processor Assembly (UPA): Distils urine to brine; ~85% recovery
- Water Recovery System (WRS): Processes condensate + UPA distillate; combined recovery ~93%
Current era (with BPA, 2024): 98% recovery
NASA's Brine Processor Assembly (BPA), activated in 2023–2024, recovers approximately 95–98% of the residual water from the UPA brine output. Combined with an improved UPA achieving ~87–98% urine recovery, the total system now achieves 98% water recovery — the Mars mission threshold (Gatens et al. 2024). This was described as a milestone for long-duration exploration.
ISS net consumption is approximately 3.6 L/person/day — a result of severe rationing, not of high efficiency. A comfortable long-duration colony running full sanitation at 20 L/person/day at the historical 93% efficiency would lose 1.4 L/person/day per person.
At 8,000 people, that historical 93% loss rate is 11,200 L/day = 4,088 tonnes/year — roughly 1,360 Falcon 9 payloads annually. Even at the current ISS level of 98%, loss is 818 t/year. For a fully isolated habitat, any non-zero loss eventually depletes the supply; 98% is viable only if loss is covered by local resource extraction (ISRU) or very infrequent resupply.
One analysis of the 98% regime noted the operational margin is "too small for comfort" when accounting for disposal paths — hygiene towels, wipes, and contamination losses — that bypass the recovery system entirely. Future designs targeting 99%+ are an active NASA research direction.
Required Efficiency for Self-Sufficiency¶
For a colony of \(N\) people at demand \(D_{pp}\) and a target maximum annual loss of \(L_{max}\) kg/year, the required efficiency is:
For \(L_{max} = 0\) (fully closed loop): \(\eta_{min} = 1.0\), which is physically unachievable. In practice the goal is to reduce losses to a level manageable by local resource extraction (lunar ice mining, comet ice) or very infrequent resupply.
The model default \(\eta_{min} = 0.98\) represents the minimum efficiency for a colony where annual water loss is small enough to be covered by in-situ resource utilisation (ISRU).
Consequences of Failing the Constraint¶
| \(\eta\) | Annual loss (8,000 people, 20 L/day) | Equivalent launches |
|---|---|---|
| 0.90 (ISS level) | 4,088 t/year | ~1,360/year |
| 0.95 | 2,044 t/year | ~680/year |
| 0.98 (threshold) | 818 t/year | ~273/year |
| 0.99 | 409 t/year | ~136/year |
| 0.999 | 41 t/year | ~14/year |
The Falcon 9 equivalent is based on ~3 t useful payload to orbit.
Model Inputs¶
| Symbol | Parameter | Default | Source |
|---|---|---|---|
| \(D_{pp}\) | water_per_person_day_liters |
20 L/day | Hanford 2004 |
| \(\eta\) | water_recycling_efficiency |
0.98 | Design target |
| \(\eta_{min}\) | min_water_recycling_efficiency |
0.98 | Design target |
Key Insight¶
The default (0.98) represents NASA's stated minimum for Mars/permanent missions and has now been demonstrated on the ISS (2024) with the BPA. It is well-supported by the literature.
One important caveat: 98% is the thermodynamic recovery from the active ECLSS loop. Actual habitat water loss also includes passive disposal paths (wipes, hygiene towels, contaminated water) not captured by the system. The effective whole-habitat efficiency may be lower. For a permanent colony, the "system efficiency" the model checks is the ECLSS fraction; the designer must separately minimise non-loop waste paths.
Users can slide water_recycling_efficiency below 0.98 to see what pre-BPA
ISS-level loss rates imply at colony scale.
References¶
- Carter, Layne, et al. "Water Recovery System (WRS) and Urine Processor Assembly (UPA) Status." 38th International Conference on Environmental Systems. 2009. (Carter et al. 2009) — Documents pre-BPA 93% performance.
- Gatens, Robyn, et al. "Status of ISS Water Management and Recovery." 54th International Conference on Environmental Systems. NTRS 20240005472. 2024. (Gatens et al. 2024) — Documents 98% BPA achievement and Mars mission requirement.
- Hanford, Anthony J. Advanced Life Support Baseline Values and Assumptions Document. NASA/CR-2004-208941. NASA, 2004. (Hanford 2004)
- Hendrickx, Lieve, et al. "Microbial ecology of the closed artificial ecosystem MELiSSA." Advances in Space Research 23.12 (2019): 1–15. (Hendrickx et al. 2019)
- Wieland, Paul O. Designing for Human Presence in Space: An Introduction to Environmental Control and Life Support Systems. NASA Reference Publication
- NASA, 1994. (Wieland 1994)