AM-13.02 — Closed loops: why 90% recycling does not mean autonomy

1 — What is a “closed loop”?

A closed loop recovers material after use, treats it, and sends it back into useful service instead of discarding it. The easiest mental picture is water: crew use water; part of it becomes urine, humidity condensate, wash water or water trapped in waste; equipment recovers as much as practical, purifies it and routes it to approved uses.

NASA already operates water recovery and air revitalization systems aboard the International Space Station. Mars raises the stakes because resupply becomes slow and mission-dependent.

2 — Five words before any calculation

Stock is the amount available now. Flow is an amount moving per unit time, such as 80 kg/day. Recovery rate is the fraction returned in reusable form. Loss is what does not return to the useful stock. Make-up supply is fresh material added to compensate for that loss.

A 90% recovery rate means that out of 100 kg entering the educational example, 90 kg return and 10 kg must eventually be replaced if the same useful inventory is to be maintained.

3 — Why 90% can still require a lot of resupply

If the same 10 kg loss occurs every day for 30 days, the make-up requirement is 10 × 30 = 300 kg. This is why high recovery numbers must always be translated into kilograms, days and mission logistics.

4 — 90%, 95% and 99% are not small differences

For a 100 kg/day educational flow, losses are 10, 5 and 1 kg/day respectively. Over 365 days that becomes 3,650 kg, 1,825 kg and 365 kg of make-up material. Improving from 90% to 99% divides the loss by ten in this example.

But the last percentage point may demand more equipment, energy, consumables and maintenance. Engineering optimizes the complete system rather than one headline percentage.

5 — A water loop is a treatment plant

A real system uses pumps, separators, filters, adsorbent beds, thermal or membrane processes, sensors, tanks, valves, bypass lines and software. NASA’s ISS Water Recovery System is a useful example: wastewater is treated and water quality is checked before it is returned to use.

6 — Different materials require different loops

Water

Chemical and microbiological quality dominate. A settlement may use different grades so that every liter is not treated to drinking-water quality when a technical use needs less.

Oxygen and carbon dioxide

Crew consume oxygen and exhale carbon dioxide. NASA’s SCOR work illustrates why long-duration missions seek greater oxygen recovery from metabolic CO₂ rather than assuming endless resupply.

Metals

Metals can be reused, remachined or remelted, but alloy chemistry and contamination still matter.

Polymers

Thermal history and oxidation can degrade recycled polymers. Recycled feedstock suitable for a protective cover may be unsuitable for a critical structural part.

Biological waste

It contains water and nutrients but also microbiological hazards and contaminants. Safety comes before the “zero waste” slogan.

7 — Purity matters as much as quantity

Recovering 99 kg out of 100 is not a success if the product contains a contaminant that makes it unusable. Every loop therefore needs four answers: how much is recovered, in what condition, with what variability, and for which uses is it actually acceptable?

8 — Hidden losses: power, filters and catalysts

Pumps consume electricity; furnaces consume heat; filters clog; membranes age; catalysts can be poisoned; sterilization needs energy. A Mars settlement therefore tracks both a material balance and a support-equipment balance. A loop that needs a unique Earth-made filter every six months is not fully autonomous.

9 — Close the maintenance loop too

Autonomy becomes stronger when the settlement can clean, regenerate, repair or manufacture parts of its own recycling equipment: seals, bearings, sensors, filters, adsorbents and test instruments. The deeper question is not only “what percentage do we recycle?” but “how long can we keep recycling after failures and wear?”

10 — Worked example: reserve stock

A base uses 60 kg of water per day and recovers 96%.

So 288 kg covers the calculated make-up loss for 120 days before safety margin. A real design would also account for failures, variable demand and accidental losses.

11 — Levels of autonomy

1. Everything imported and discarded.
2. Part of the material flow recovered.
3. Losses covered by Earth stocks.
4. Some make-up material produced locally.
5. Most operating consumables produced or regenerated locally.
6. The settlement can maintain and progressively reproduce the machines that close the loops.

This is why “100% recycled” and “100% autonomous” are not equivalent statements.

12 — Three reasoning traps

13 — Exercises with answers