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'Life support' covers the theory and practice of sustaining life in environments or situations in which the human body is incapable of sustaining its own natural functions. There are essentially only three practical, non-exclusive ways to ensure the biological autonomy of man when isolated from his original biosphere: provide all required consumables at the start of the mission or resupply them, regenerate life-support materials during the mission, or utilise in-situ resources (in the case of manned missions on planets).
Historically, air, water and food were taken on board and the waste stored and returned to Earth. This was a completely open-loop life-support system used successfully for short-duration space missions. As space missions get longer, however, supply loads get heavier and soon prohibitive, effectively limiting the duration of such missions, however exciting and potentially important they may be. It becomes crucial then to close some vital loops to permit longer missions.
When we consider the three vital loops of a life-support system, i.e. air, water and food/solid waste, the most demanding in terms of mass constraints is the water loop. Indeed, water represents approximately 92% by mass of the total life-support consumables. Closing the water loop by recovering potable water from waste water will therefore already provide for 92% of human needs, i.e. 92% autonomy of man in space.
Waste water can be roughly classified according to its degree of contamination. It is now generally accepted that highly contaminated water, such as urine, must be subjected to a process involving phase change before it will be regarded as suitable for re-use. Such phase-change systems have been studied for several years, notably in Russia and the USA, and include techniques such as AES (Air Evaporation System), TIMES (Thermo-electric Integrated Membrane Evaporation System) and VCD (Vapour Compression Distillation).
Moderately or slightly contaminated water, such as hygiene (washing, showering) water, condensate recovered from the air-conditioning system, product water from the air-revitalisation (oxygen recovery) system and possibly also the product water from the urine processing system, can be treated in other ways which promise to be less complex, consume less power and provide a higher percentage recovery rate.
It is known that, in manned space missions, over 90% of the expected waste water can be classified as 'moderately contaminated'. If, in addition, the product water from the processing of the highly contaminated waste stream is regarded as 'moderately contaminated', the need, as a first priority, for an effective, reliable and efficient "core water recycling system" for processing moderately contaminated water becomes evident.
Based on several studies financed by ESA, a core water recycling system was designed, aimed at recovering potable water from hygiene water, typified by shower water. The system uses a combination of filtration and reverse-osmosis units in successive stages to eliminate solids, organic and inorganic molecules, including micro-organisms, from the product stream. The aim is to produce water meeting the ESA quality standards for potable water defined in ESA PSS-03-402.
To validate the technology, a development model has been designed, built and tested. This development model water-recovery unit is contained in a rack approximately 2 m wide, 2.1 m high and 0.6 m deep, and consists of four successive membrane units: one ultra-filtration (UF) unit based on a mineral membrane, and three successive reverse-osmosis (RO) units. It is sized to produce approximately 2 litres of drinking water per hour.
The role of the first (ultra-filtration) unit is to reduce the turbidity of water, i.e. to exclude particulate materials and high-molecular-weight macromolecules. Elimination of low-molecular-weight organic molecules as well as ionic compounds (salts) is the task of the three successive reverse-osmosis units. The test bed operates nearly automatically, controlled by software specifically designed for that purpose, the main exception being the periodic purges needed to maintain membrane performance, which are done manually.
Major aspects of previous studies
A first phase was related to the selection of:
After validation on separate membrane equipment, a development model has been designed, composed by four membrane filtration steps, as shown on the flow diagram, and tested, proving, during three 100-hour runs, its capability for producing potable water and its efficiency against bacteria and viruses.
- reference water and soap for shower
- microbial stabilisation procedures
- membrane materials, configuration and techniques
- a well suited combination of membrane filtration steps, to reach ESA's standards in terms of water quality
- regulated pH values of feed tanks
Major conclusions of these tests are the following:
During the testing, particular emphasis was placed on the following aspects:
The capability of the Water Recovery System to eliminate microbial contamination totally was tested four times:
- elimination of any microbial contamination
- quality of recovered water
- performance of membranes
- performance in terms of the percentage of water recovered
Microbial contamination was induced by the addition of micro-organisms to the waste water.
- preliminary test of microbial retention by the UF unit alone
- monitoring of microbial elimination during the first 100-hour duration test
- simulation of a microbial accident (important microbial contamination) during the second 100-hour duration test
- simulation of 2 microbial accidents (important microbial contamination coupled with a failure in bactericide delivery) during the third 100-hour duration test
The recovered water complied with the ESA standards for drinking water with one exception, namely the Total Organic Carbon (TOC) concentration. This was due to the selected microbial stabilisation procedure.
Membrane performance was according to specifications and constant during the tests and the water recovery always had a yield over 95%.
Further validation of the integrated system during a longer duration (5 to 6 months) has been performed after breadboard improvements: the automation level of the breadboard has been increased and different membrane filters were tested and finally implemented.
Conclusions and perspectives
Throughout this long duration test, the recovery yield was, in the aggregate, greater than 93%. The water produced has never been microbiologically contaminated, even if microbiological operating conditions were not optimized (breadboard configuration, open tank, shower water stabilisation at the beginning of the test). Membranes used for this 6-month test have never been replaced.
The microbial monitoring of the recovered was carried out with a particular condition: the first shower water supplies were highly contaminated leading to an immediate contamination of the first membrane loop of the breadboard. The results of the monitoring show two important points:
A particular emphasize has to be dedicated to the innovative aspect of this process:
- the first filtration loop stops the contamination inside its own loop and protects the following parts of the breadboard during all the trial. The produced water has never been contaminated.
- biofilms were quickly established in the NF1 loop and all the chemical disinfection trials of the loop failed with the particularity to select resistant fungi as permanent contaminants.
Production of potable water from grey water has never been investigated so deeply and no other production equipment for this particular application exists.
If one considers that the choice of the soap is definitive, other points should be further investigated, particularly on the following aspects:
- stabilisation of shower waters
- protection of the breadboard against potential contamination
- definition of decontamination protocol in case of failure of the two above mentioned procedures, either by use of chemical bactericide agents or by implementation of physical techniques
Last update: 18 November 2007