A module that dissolves oxygen in water at concentrations approaching saturation, without generating bubbles of oxygen gas, has been developed as a prototype of improved oxygenators for water-disinfection and water-purification systems that utilize photocatalyzed redox reactions. Depending on the specific nature of a water-treatment system, it is desirable to prevent the formation of bubbles for one or more reasons:

  1. Bubbles can remove some organic contaminants from the liquid phase to the gas phase, thereby introducing a gas treatment problem that complicates the overall water-treatment problem; and/or
  2. in some systems (e.g., those that must function in microgravity or in any orientation in normal Earth gravity), bubbles can interfere with the flow of the liquid phase.
Figure 1. Hollow, Porous Polypropylene Fibers contain flowing water. Oxygen diffuses through the fiber walls and is dissolved in the water.

The present oxygenation module (see Figure 1) is a modified version of a commercial module that contains >100 hollow polypropylene fibers with a nominal pore size of 0.05 µm and a total surface area of 0.5 m2. The module was originally designed for oxygenation in a bioreactor, with no water flowing around or inside the tubes. The modification, made to enable the use of the module to oxygenate flowing water, consisted mainly in the encapsulation of the fibers in a tube of Tygon polyvinyl chloride (≈25 mm). In operation, water is pumped along the insides of the hollow fibers and oxygen gas is supplied to the space outside the hollow tubes inside the PVC tube.

Figure 2. The Concentration of Dissolved Oxygen as a function of time was measured with water flowing through the module at a rate of 300 mL and with two different oxygen-flow rates.

In tests, the pressure drops of water and oxygen in the module were found to be close to zero at water-flow rates ranging up to 320 mL/min and oxygen-flow rates up to 27 mL/min. Under all test conditions, no bubbles were observed at the water outlet. In some tests, flow rates were chosen to obtain dissolved-oxygen concentrations between 25 and 31 parts per million (ppm) — approaching the saturation level of ≈35 ppm at a temperature of 20 °C and pressure of 1 atm (≈0.1 MPa).

As one would expect, it was observed that the time needed to bring a flow of water from an initial low dissolved-oxygen concentration (e.g., 5 ppm) to a steady high dissolved-oxygen concentration at or near the saturation level depends on the rates of flow of both oxygen and water, among other things. Figure 2 shows the results of an experiment in which a greater flow of oxygen was used during the first few tens of minutes to bring the concentration up to ≈25 ppm, then a lesser flow was used to maintain the concentration.

This work was done by Anuncia Gonzalez- Martin , Reyimjan Sidik, and Jinseong Kim of Lynntech, Inc., for Johnson Space Center. For further information, contact the Johnson Commercial Technology Office at (281) 483-3809. MSC-23138