Encyclopedia of Environmental Science and Engineering, Volume I and II

WATER AND WASTE MANAGEMENT SYSTEMS IN SPACE 1247

Thermoelectric

Regenerator

Waste

water

Purified

Water

Membrane

Evaporator

Wastewater

Heat Exchanger

Thermoelectric

Elements

Condenser Latent

Heat

Hollow Fiber

Membranes

Water Vapor

Waste

water

FIGURE 2 Schematic diagram of the TIMES (Thermoelectric Integrated Membrane Subsystem). Source: Ref. 4.

Reverse Osmosis (RO)

Reverse osmosis is a process in which pressure is applied

to a concentrated solution which is on one side of a semi-

permeable membrane. The pressure forces the molecules of

solvent through the membrane to the pure solvent or more

dilute solution on the other side. Greater concentration dif-

ferential between the two solutions on either side of the

membrane require greater pressure. A test unit containing

a bundle of tubes, capable of operating at up to 4140 kPa

(600 psi), was evaluated. When wash water containing soap

was processed a soap gel film formed on the membrane sur-

faces.^12 Reverse osmosis is a candidate for producing high

quality water. A combination of several systems, including

RO, produced water that met National Committee for Clinic

Laboratory Standards. 13,14

Multifiltration (Unibed)

The multifiltration system comprises a heat sterilization unit

to kill microorganisms, followed by a series of progressively

finer particulate filters down to 0.5 microns to avoid par-

ticulates clogging the sorbent beds.^15 The water then enters

the Unibeds which remove the dissolved contaminants. The

Unibed is a single replaceable unit which utilizes a set of beds

of different sorptive materials arranged in a specified opti-

mum sequence (Figure 4). The beds are designed to remove

specified types and amounts of contaminants from a known

waste stream in such a way that all the beds are exhausted at

the same time. A sequence of Unibeds of identical design,

each comprising three five-tube replaceable subunits, may be

used. Unidentified compounds are generally removed by a

mixture of several types of activated charcoal and nonionic

sorbents near the outlet from the bed. Microbial control to

avoid bacterial fouling of the various beds is achieved by

iodinated resin beds at the inlet and outlet of each Unibed.

Detailed discussions of the system may be found in Refs. 11,

and 14–16.

Electrooxidation—Combined Electrolysis and

Electrodialysis

Electrodialysis alone removes ions more efficiently than does

reverse osmosis (RO); however, when combined with elec-

trolysis the organic compounds are oxidized in the electroly-

sis and the inorganic salts are removed by the electrodialysis.^7

A combined electrolytic and electrodialytic cell is illustrated

in Figure 5. No chemicals are used. Low voltages are used to

avoid unwanted side effects such as the production of chlorine,

or the formation of insoluble salts. Various types of electrode

materials have been tested. Efficiency was increased consider-

ably when the polarity across the electrodes was periodically

reversed. This procedure is termed Periodic Reverse Pulsed

Electrolysis (PRPE). The process has theoretical advantages

over RO, as increasing the brine concentration improves

the conductivity, and hence the efficiency of the process.

Electrooxidation effectively kills bacteria in the feed.^17

Supercritical Water Oxidation (SCWO)

Wastewater is heated to a temperature of 650
C under pressures

of 250 atmospheres. When water is above its critical point, its

properties as a solvent change. Organic compounds which are

insoluble at normal temperature and pressure become soluble.

The addition of sufficient oxygen then leads to the complete

oxidation of these compounds. Most atmospheric gases and

trace contaminant gases are also soluble in supercritical water,

and will also be oxidized. The process also has the potential

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