ISS IATCS Coolant Loop Biocide Implementation 2008-01-2159

The proliferation and growth of microorganisms in the Internal Active Thermal Control System (IATCS) aboard the International Space Station (ISS) has been of significant concern since 2001. Initial testing and assessments of biocides to determine bacterial disinfection capability, material compatibility, stability (rate of oxidative degradation and identification of degradation products), solubility, application methodology, impact on coolant toxicity hazard level, and impact on environmental control and life support systems identified a prioritized list of acceptable biocidal agents including glutaraldehyde, ortho-phthalaldehyde (OPA), and methyl isothiazolone. Glutaraldehyde at greater than 25 ppm was eliminated due to NASA concerns with safety and toxicity and methyl isothiazolone was eliminated from further consideration due to ineffectiveness against biofilms and toxicity at higher concentrations. Therefore, OPA, an aromatic dialdehyde compound with the formula C₆H₄(CHO)₂ was selected as the optimum antimicrobial for use in the ISS IATCS and was approved for use at concentrations <109 ppm (1).

A key aspect of the biocide use on the ISS was the method in which the biocide would be delivered to the coolant system. A non-intrusive implementation technique was required in order to add the OPA to the IATCS. The method developed utilized the current NiRA (Nickel Removal Assembly) packed bed hardware including a 2 liter canister, flex hoses and quick disconnects. An immobilization of the OPA to an inert resin substrate was selected as the process to package for delivery. Development of the immobilization procedure involved determining proper OPA loading density to the resin material as well as proper placement of the biocide loaded resin in the packed bed to meet requirements for biocide elution profile and final coolant concentration. Significant ground testing at both the 1/10^th and full scale levels demonstrated that adequate concentrations could be added to the IATCS in the required time envelope while maintaining compliance with system requirements and safety.

In addition to the development of the OPA delivery resin material, means to adequately determine on-orbit aqueous OPA concentrations were also investigated. Since the frequency of on-orbit sample return is usually greater than 3 to 6 months for laboratory verification of biocide concentration, on-orbit determination of biocide concentration was deemed imperative. A partnership between Hamilton Sundstrand and Branan Medical Corporation was undertaken to investigate the feasibility of manufacturing OPA colorimetric concentration test strips. Test strip development for OPA concentrations from 25 ppm to 200 ppm was achieved and successfully manufactured for on-orbit use.

The OPA delivery resin and test strips were delivered to the ISS on flight STS-120 and subsequently used during that mission. This paper reports on all development activities associated with the OPA delivery resin and test strips as well as the successful implementation of OPA to the ISS IATCS.