11/11/2008
Eckart V: Physico-Chemical
Life Support Subsystems
V.2 Water Management
Objectives
Functional Requirements: Supply potable water. Potentially also supply hygiene water, collect atmospheric condensate (respiration and perspiration) and wastewater (hygiene, urine and ultimately fecal) as well as monitor quality throughout processing stages.
Requirements depend largely on water quality to be processed, output water quality desired, and degree of system closure.
- Hygiene vs. Potable water standards
- Recovery water streams incorporated vs. processed as waste
- Water may also be derived from or sent to other subsystems (e.g. fuel cell, CO2 removal, sublimator heat exchanger, etc.)
Couple of related articles…
http://science.nasa.gov/headlines/y2000/ast02nov_1.htm
http://science.nasa.gov/headlines/y2006/30oct_eclss.htm?list134004
http://www.cnn.com/2008/TECH/space/11/10/space.station.additions.ap/index.html
Vapor Compression Distillation (VCD)
Thermoelectric Integrated Membrane Evaporation (TIMES)
Vapor Phase Catalytic Ammonia Removal (VAPCAR)
Air Evaporation and other processes
Urine Recovery
Vapor is compressed to raise saturation temperature and condensed on an evaporator
Thermally (net) passive system from balanced heat flux between evaporation and condensation, so active heat control not required
Waste heat ~ambient
Energy required for compression and thermal/mechanical losses
Can recover >96% water from urine
Rotation allows separation in weightlessness - mechanical complexity? vibration source?
Unevaporated water is circulated until solid concentration reaches specified limit
Solids removed and stored as brine
Quality depends on ammonia and VOC’s that condense in the water
Acid pretreatment can reduce NH3 formation from the urea
Evaporator must be periodically evacuated to remove noncondensible gas buildup (CO2, N2, VOC’s, etc.) to restore operational efficiency
(similar system resources, but lower power requirement than TIMES)
Thermoelectric Integrated Membrane Evaporation Systems
(TIMES) (Fig V.25, pp 225)
Utilizes a thermoelectric heat pump to transfer heat from a water condenser to an evaporator
Urine is pretreated with ozone (or ideally UV) and sulfuric acid to fix free NH3, inhibit microbial growth, control odor and reduce foaming
Waste is heated then pumped through hollow fiber membranes exposed to reduced pressure to evaporate
Vapor condensed on cold side of thermoelectric modules
Latent heat of condensation is reused in evaporative process
Gas/liquid separation achieved via centrifugal pump
95% recovery achievable
Generates net heat
Acid pretreatment can reduce NH3 formation from the urea
Brine must be stored as waste
Reliability of membranes is low
Vapor Phase Catalytic Ammonia Removal (VAPCAR) (Fig. V.24, p. 223)
PC process that combines vaporization with high temperature catalytic oxidation of the volatile impurities that vaporize with the water (NH3, VOC’s)
Designed to eliminate need for expendable chemicals
Evaporator consists of hollow fiber tubes
Waste is fed through the fibers and vaporizes
2 catalyst beds
- First used to oxidize NH3 into nitrous oxide (N2O) and N2 and volatile hydrocarbons into CO2 and H2O.
- Second N2O is catalytically decomposed to N2 and O2, which can be used in cabin air resupply
Urine recycle and vapor loops maintained above pasteurization temperature (steam heat) for microbial control
Recovered water only requires pH adjustment to meet potable standards
Reliability of membranes is low and operating temp is high
Air Evaporation Systems (AES)
Pretreated urine is pumped through a particulate filter to a wick package using a pulse feed technique
Circulating heated air used to evaporate urine water from wick, leaving solids
Dry down process results in ~100% water recovery
Aqueous Phase Catalytic Oxidation Post-Treatment System
(APCOS)
High pressure oxygen injected into heated gaseous feed water then two phase mixture passes through catalyst to oxidize organic compounds
Super Critical Water - or Wet - Oxidation (SCWO)
Makes use of water in supercritical state (>647K and 2.21x207 Pa) during oxidation process to destroy organic compounds
Does not require catalyst
Normally insoluble organic compounds and oxygen become soluble at supercritical state and permits oxidation to occur in single phase
Creates potable water from all input sources
Drawbacks include high temp and pressure, material corrosion, and need for post treatment to remove toxic product gases
Former
Urine processed by evaporation recovery and steam condensation, then purified by sorption
Used aboard MIR for generating oxygen by electrolysis
Regenerable, 80% urine reclamation produced all oxygen requirements.
Reverse Osmosis (RO)
Multifiltration (MF)
Electrodialysis
Osmosis refers to
transfer of solvent (transfer of solute is called dialysis)
Solvent - A substance in which another substance is dissolved, forming a solution
Solute - A substance dissolved in another
substance, usually the component of a solution present in the lesser amount
Hygiene Water Recovery and Potable Processing
0.9 L / hr recovery capacity for hygiene use
Reverse Osmosis (RO) and Ultrafiltration (UF, lower pressure than RO) (Figs. V.26 & V.27, pp. 230-231)
Osmosis driven transport (less concentrated to more)
RO – opposite, facilitated by applying pressure to exceed osmotic pressure and force water transport across a semi-permeable membrane, leaving ions and organics
Filters most suspended particles, but small organic compounds pass through
Small volume of permeate must be further processed
Staged process: UF (to remove larger contaminants) à RO
High temp used to pasteurize
Projected power consumption 10 Wh/kg water
Concerns again with hollow fiber clogging
Multifiltration (MF)
MF of condensate from THC subsystem and CO2 reduction processes
Flow through filters and packed columns in series
1 – particulates removed by filtration
2 – suspended organics removed by activated charcoal
3 – inorganics removed by cation/anion exchange resin beds
4 – system heated to pasteurize or chemically treated (iodinated MCV)
Delta P only needed to overcome filter backpressure
Simple design, but high need for consumables
Utilizes ion exchange resins and membranes to deionize feed water
Electrical potential directs ion transport through membrane into adjacent compartment
Ion exchange resin is continuously electrically regenerated, no chemicals required
Outputs – brine and purified DI water (‘polished’ water)
Highly complex system
Water Recovery from Condensate
Former USSR System used on Salyut and MIR
Treated to potable standards with mineral salts and preserved with ionic silver
Used for food prep and shower
4 kWh / m3 water processed
100% recovery
System mass – 0.2 kg / kg recovered water (assumed from consumables?)
Water Quality Monitoring
Need for rapid enumeration of pathogens and other ‘undesirables’
Frequent - PH, NH3, TOC, Conductivity, Microbial
Periodic - Color, odor, turbidity, foaming, heavy metal concentrations
Taste
Continuous (or frequent) measurements
Silver purification
Iodine addition AND removal before consumption
MCV = Microbial Check Valve
ACTEX = Activated Carbon/Ion Exchange assembly used on shuttle