Encyclopedia of Environmental Science and Engineering, Volume I and II

1248 WATER AND WASTE MANAGEMENT SYSTEMS IN SPACE

for oxidizing much of the organic solid wastes produced on

the space station.^18 Inorganic salts are produced when human

metabolic wastes are subjected to supercritical water oxida-

tion. These salts have very low solubility in supercritical water

and precipitate out, and thus can be removed.^19 Metals are also

precipitated out, with the exception of mercury, which car-

ries over in the vapor phase, and has to be removed by ion

adsorption.^20

Electrodeionisation

Feed water moves through an ion exchange resin bed

which selectively removes certain ions from the water.

Simultaneously, the resin is regenerated by the action of

an electric field imposed upon the resin bed.^21 A schematic

process diagram may be found in Ref 21. Bacteria are not

completely removed. It appears that this process works best

to remove ionic contaminants. It is a candidate for the pro-

duction of reagent grade water. Both benchtop and industrial

capacity units are available.

Air Evaporation

A system with a heat pump and solar collectors for evapora-

tive heat was tested. The system is capable of 100% water

recovery from numerous types of contaminated sources. The

wastewater is pretreated with a chemical solution to pre-

vent decomposition and bacterial growth. It is then pumped

through a wick filter to remove particles, in a series of pulses.

The timing of the pulses is such that the liquid from one pulse

is distributed along the wick by capillary action before the

next pulse arrives. A heated air stream evaporates the water

from the wick, leaving the solids behind. When the wick is

full of solids, it is dried and replaced by a new wick. The air

and vapor stream passes through a condensing heat exchanger

and then through a water separator to extract the free water

from the stream and test it for quality. It is then transferred to

the post-treatment filter section and thence to the main water

storage and distribution system. The wick system can be ster-

ilized by heating it to 121
C while in a dry state.^22

Vapor Phase Catalytic Ammonia Removal (VPCAR)

Neither pre-treatment nor post-treatment of the feed and

product water are required. The high temperature employed

also destroys microbes to a very great extent.^23 The present

design utilizes the thin film evaporation technique of the

VCD process in a rotating disk evaporator, combined with

catalytic reactors for vapor phase chemical reactions. The

ammonia and volatile hydrocarbons which are evaporated

along with the water vapor are oxidized to innocuous gases

by catalytic chemical reactions carried out in the vapor phase.

The overall system schematic is shown in Figure 6. 10,11 The

vapor from the VCD boiler passes over two catalytic beds.

The first bed operates at a temperature of about 250
C, to

oxidize organic volatiles to CO 2 and water, and ammonia to

NO and water. The second bed operates at 450
C and reduces

the NO to N 2 and O 2. The O 2 produced is more than sufficient

for first bed usage. The vapor is recycled to maximize O 2

utilization. This high temperature vapor or steam supplies the

Outer Shell

Rotating Drum

Evaporator

Motor

Compressor

Condenser

2

FIGURE 3 Cross section of a VCD (Vapor Compression Distillation) still. Source: Ref. 4.

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