Encyclopedia of Environmental Science and Engineering, Volume I and II

WATER AND WASTE MANAGEMENT SYSTEMS IN SPACE 1251

made more effective. The enzymes are bound to support, usu-

ally, a silica-based material, which is in a form that makes it

easy to utilize in a Unibed. Specific enzymes are used depend-

ing upon the compound or class of compound, to be removed.

These Unibeds have to include iodine removers at the inlet,

and iodinating resin beds at the outlets to maintain bacterial

control in the water supply system of the spacecraft.^29

Fuel Cell Product Water

When hydrogen and oxygen are combined in a fuel cell to

produce electric power, pure water is also produced. This

system was used in both the Apollo and Orbiter spacecraft.^5

Urine and wastewater can be electrolyzed to produce hydro-

gen and oxygen, thus providing a closed loop system (see

Electrooxidation above).

COMPARATIVE PERFORMANCE OF TECHNOLOGIES

Many promising technologies for wastewater treatment and

water recovery aboard the space station have been described

briefly. Their performance levels vary depending upon the

characteristics of the wastewater feed. The performance levels

and the basic features of each system are compared in Table 3.

WATER QUALITY CONTROL

A major problem in these regenerative systems is bacterial

contamination and biofilms forming on the internal surfaces

of the water supply system. These biofilms are the result

of colonies of bacteria forming on the surface. They occur

even in nutrient-limited environments. Biofilms accelerate

corrosion and are difficult to remove, or to prevent from

forming.^31 Pathogenic bacteria have been shown to sur-

vive in sterile water and at elevated temperatures for many

hours.^32 This bacterial contamination poses both health and

long term mechanical problems in any recycling system.

Adequate continuous sterilization procedures and equip-

ment will be required in any recycling system. A high con-

centration of micro-organisms is found in shower water,

reinforcing the necessity of ensuring the disinfection of

the recycled potable water supply. A special soap is used

for all washing on the Orbiter and is proposed for use on

the space station, which reduces foaming and consequent

problems in the recycling process.^33 Great care is taken to

avoid the possibility of contamination due to cross con-

nection of wastewater and processed water systems.

Pre- and Post-Treatment of Recycled Water

Pre- and post-treatment of water is generally necessary in

order to obtain the desired water quality after the treatment

of recycled water. NASA concentrated much of its research

effort on distillation processes. The combination of distil-

lation and ion exchange is quite successful in removing

inorganic contaminants but is less successful in remov-

ing volatile organic contaminants. These volatiles must be

removed to avoid their accumulation to toxic levels. Organic

TO AIR REVITALIZATION SUBSYSTEM

N 2 , CO 2 & H 20

OXYGEN

URINE &

FLUSH

HYGIENE &

WASH WATER

SOLID WASTE SLURRY

(IF DESIRED)

Heavy Liquids Processor

RESIDUAL INORGANIC SOLIDS

SCWO PRODUCT WATER

OXYGEN

POTABLE

WATER

VENT GASES

VPCAR BRINE

VAPOR PHASE CATALYTIC

AMMONIA REMOVAL

UNIT (VPCAR)

New

VPCAR

Processor

SUPERCRITICAL WATER

OXIDATION UNIT (SCWO)

SCWO

FIGURE 6 Schematic flow diagram of integrated water reclamation system utilizing a combination of VPCAR unit and

SCWO unit. Source: Refs. 10 and 11.

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