2009

Continuous-Flow System Produces Medical-Grade Water

Pressurized flowing water is heated by absorption of microwave power.

A continuous-flow system utilizes microwave heating to sterilize water and to thermally inactivate endotoxins produced in the sterilization process. The system is designed for use in converting potable water to medical-grade water. Systems like this one could be used for efficient, small-scale production of medical-grade water in laboratories, clinics, and hospitals. This system could be adapted to use in selective sterilization of connections in ultra-pure-water-producing equipment and other equipment into which intrusion by microorganisms cannot be tolerated. Lightweight, portable systems based on the design of this system could be rapidly deployed to remote locations (e.g., military field hospitals) or in response to emergencies in which the normal infrastructure for providing medical-grade water is disrupted. Larger systems based on the design of this system could be useful for industrial production of medical-grade water.

altThe basic microwave-heating principle of this system is the same as that of a microwave oven: An item to be heated, made of a lossy dielectric material (in this case, flowing water) is irradiated with microwaves in a multimode microwave cavity. The heating is rapid and efficient because it results from absorption of microwave power throughout the volume of the lossy dielectric material.

In this system, a copper tube having a length of 49.5 cm and a diameter of 2.25 cm serves as both the microwave cavity and the sterilization chamber. Microwave power is fed via a coaxial cable to an antenna mounted inside the tube at midlength (see figure). Efficient power transfer occurs due to the shift in wavelength associated with the high permittivity of water combined with the strong coupling of 2.45-GHz microwaves with rotational-vibrational transitions of the dipolar water molecule. The sterilization chamber is thermally insulated with closed-cell foam to reduce heat losses.

To enable a sufficiently high degree of sterilization and inactivation of endotoxins during the time of transit of water along the tube, it is necessary to heat the water to a temperature significantly above traditional autoclave temperatures. In order to do this, it is necessary to choose an appropriate combination of microwave power and flow rate, and to maintain the flowing water at a pressure high enough to prevent boiling at the desired maximum temperature. For example, typical steady-state operating conditions in experiments on this system included a flow rate of 13 mL/min, a pressure of 0.69 MPa, and a microwave power of 150 W, resulting in a sterilization temperature between 155 and 158 °C. Under these conditions, the system demonstrated effective sterilization and inactivation of endotoxins when challenged with influent water containing a mixed culture of Bacillus stearothermophilus, E. coli, and Pseudomonas aeruginosa.

Downstream of the sterilization chamber, the water is cooled at reduced pressure in a sterilizable connector section that contains a cooling coil situated between two pressure regulators. At the beginning of operation, for the purpose of sterilization, the sterilizable connector section is surrounded by removable insulation and maintained at full sterilization temperature and pressure using pressure regulator 2. For subsequent steady-state operation, insulation is removed from this section, causing the temperature to decline to the point where pressure regulator 2 can be opened to the atmosphere, requiring adjustment of pressure regulator 1 to maintain full sterilization pressure in the sterilization chamber.

This work was done by James R. Akse, Roger W. Dahl, and Richard R. Wheeler, Jr. of UMPQUA Research Co. for Glenn Research Center.

Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-18159-1.

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