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Supercritical Carbon Dioxide-Based Cleaning and Sterilization of E. Coli and S. Aureus Biofilms from Stainless Steel Substrates

Pamela L. Garner1, Gifty Prempeh1, Leah N. Williamson1, Kenneth L. Roberts1, and Geoffrey D. Bothun2. (1) Department of Chemical Engineering, North Carolina A&T State University, 1601 East Market Street, Greensboro, NC 27411, (2) Department of Chemical Engineering, University of Rhode Island, 205 Crawford Hall, Kingston, RI 02881

Supercritical carbon dioxide / water mixtures were used in removing biofilms of average weight 0.008g from stainless steel of dimensions 1/8in x 1/2in x 3/4in at three different temperatures (37oC, 47oC and 57oC) and four different pressures (1200psi, 1350psi, 1500psi and 2000psi) for each temperature. The biofilms were produced by Escherichia coli bacteria and were grown in aqueous media. Six and twelve hours experimental runs were performed for different condition. It was observed that the minimum removal of 2.13 x 10-5g/cm3 (15.98%) was at 37oC and about 1360psi for 6 hours run time whilst the maximum of 5.75 x 10,sup>-5g/cm3 (43.13%) was at 57oC and about 2079psi for 12 hours run. The results mostly indicated that increases in temperature and run time increases removal.

Supercritical CO2 / water mixtures were also used to remove biofilms from stainless steel coupons with the dimensions of ⅛in x ½in x ¾in. The biofilms were produced by Staphylococcus aureus bacteria and were grown in two different liquid media; high salt concentration (4.278 M) and low salt concentration (0.0856 M). Biofilms grown in high salt conditions were investigated at a pressure range from 1272 to 2744 psi. The maximum amount of biomass occurred at 2744 psi (0.01605 g). Biofilms grown in the low salt conditions were investigated in a lower pressure region (1262–2040 psi) and a higher pressure region (2619-3261 psi). The highest amount of biomass removed occurred at 2040 psi (0.0080 g) for the lower pressure region and 2869 psi (0.0069 g) for the higher pressure region. The highest amount of biomass removal occurred at 2263 psi (0.0089 g). All experimental runs were carried out for 6 hours at a temperature of 57oC.

This work will examine the strengths and challenges of the using supercritical CO2 / water mixures for biofilm cleaning and sterilization. Results have revealed a strong resistance by the bacteria to supercritical CO2–based sterilization. The effects of pressure, temperature, mixture composition, and mass transfer will be presented. The possible effects of SC-CO2 on cell chemistry and structure will be discussed. Also the effects of cell sterilization on cleaning will be discussed.