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As the durations of manned space missions increase, so will the need for compact and reliable water recycling systems. Optimization of such water-recycling systems involves computer simulation of process elements and subsystems. The operations of water recycling systems are simulated at the Ames Research Center using commercial software called ASPEN-PLUS. Ion exchange is a part of the multifiltration subsystem, used for final polishing of recycled water and in some cases as a complete water treatment. Ion-exchange resins remove hazardous ions from solution by exchanging them with innocuous ions according to selection parameters. The ion-exchange operation is not provided in the ASPEN-PLUS multiprocess simulator package, but FORTRAN-callable modules may be added. Therefore we have adapted a FORTRAN program simulating multicomponent adsorption by ion-exchange resins, for use both as an ASPEN-callable module and as a free-standing simulator of the ion-exchange bed.

Membrane performance parameters have been obtained for high water recovery operation in the pressure range up to 1.4 MPascals (200 psig) for system modeling. Simple equations can be fitted to these measurements, from which RO system performance can be predicted or simulated as part of a model of system performance. A single-pump configuration with feedback has been found useful in reaching high brine osmotic pressures in continuous-flow operation. Enhanced brine concentration as well as an enhanced fraction of product/feed water recovery can be obtained by recycling a second-stage permeate of intermediate quality. For spaceflight recycling of hygiene water this permits lower pressure-vessel ratings, or higher water recovery, or both.

Water needs, water sources, and means for recycling water are examined in terms appropriate to the water-quality requirements of a small crew and spacecraft intended for Mars exploration. Inorganic, organic, and biological hazards are estimated for wastewater sources. Sensitivities to these hazards for human uses are estimated. The water recycling processes considered are humidity condensation, carbon dioxide reduction, waste oxidation, distillation, reverse osmosis, pervaporation, electrodialysis, ion exchange, carbon sorption, and electrochemical oxidation. Limitations and applications of these processes are evaluated in terms of water-quality objectives. Computer simulation of chemical processes is examined. Recommendations are made for development of new water recycling technology and improvement of existing technology for near-term application to life support systems for humans in space.

Electrodialysis is used to remove salts from waste or other water streams, to yield a concentrated brine and a substantially deionized product water. During the electrodialysis process, the boundary layer adjacent to the ion selective membrane can become depleted of ions, resulting in severe pH changes sometimes accompanied by precipitation, and power losses, by a process known as “water-splitting.” In order to optimize the applied electric current density, to achieve maximum deionization without exceeding the limiting current at any point along the path, a simulation program has been created to plot ion concentrations and fluxes, and cell current densities and voltages, along the electrodialysis path. A means for tapering the current density along the path is recommended.

Inclusion of a recycling loop for partially-desalted water from second-stage reverse-osmosis permeate has been shown useful for achieving high-recovery at moderate applied pressures. This approach has now been applied to simulated wash waters, to obtain data on retention by the membranes of solutes in a mixture comparable to anticipated spacecraft hygiene wastewaters, and to generate an estimate of the maximum concentration that can be expected without causing membrane fouling. A first experiment set provides selectivity information from a single membrane and an Igepon detergent, as a function of final concentration. A reject concentration of 3.1% Total Organic Carbon has been reached, at a pressure of 1.4 MegaPascals, without membrane fouling. Further experiments have generated selectivity values for the recycle configuration from two washwater simulations, as a function of applied pump pressure.

Several quite different waste-water processing subsystems have been tested for space mission applications. Mission choice, and choice of priority for further development, involves processor requirements for (electrical) power, launch and resupply weights, and space occupied, and developmental factors of reliability and reparability. For each subsystem one can identify theoretical power, etc., requirements, and current realistic requirements, and potential realistic subsystem requirements. Compared are reverse osmosis, vapor-compression-distillation, and TIMES processes, in terms of power, weight, and space requirements. Discussions of electrodialysis and CELSS are included as components of water treatment systems.