Robert A. Governal

Oligotrophic bacteria in ultra-pure water systems: Media selection and process component evaluations

SummaryPresently, tryptic soy agar (TSA) medium is used in the semiconductor industry to determine the concentration of viable oligotrophic bacteria in ultra-pure water systems. Deionized water from an ultra-pure water pilot plant was evaluated for bacterial growth at specific locations, using a non-selective medium (R2A) designed to detect injured heterotrophic as well as oligotrophic bacteria. Results were compared to those obtained using Tryptic Soy Agar. Statistically greater numbers of bacteria were observed when R2A was used as the growth medium. Total viable bacterial numbers were compared both before and after each treatment step of the recirculating loop to determine their effectiveness in removing bacteria. The reduction in bacterial numbers for the reverse osmosis unit, the ion exchange bed, and the ultraviolet sterilizer were 97.4%, 31.3%, and 72.8%, respectively, using TSA medium, and 98.4%, 78.4%, and 35.8% using R2A medium. The number of viable bacteria increased by 60.7% based on TSA medium and 15.7% based on R2A medium after passage of the water through an in-line 0.2-μm pore size nylon filter, probably because of the growth of bacteria on the filter. Our results suggest that R2A medium may give a better representation of the microbial water quality in ultra-pure water systems and therefore a better idea of the effectiveness of the various treatment processes in the control of bacteria.

Persistence of MS-2 and PRD- 1 bacteriophages in an ultrapure water system

The persistence of bacteriophages MS-2 and PRD-1 was evaluated in tap water, in reverse osmosis (RO) permeate, and in three locations within an ultrapure water system; ultrapure samples included pre- and post-UV sterilization and post-mixed bed ion exchange tank. The inactivation rates for MS-2 were calculated as log10 reduction per hour and per day: k = − (log10Ct/Co)/t. PRD-1 was found to persist with no significant loss of infectivity in all water purity environments evaluated. Inactivation of MS-2 was dependent on water quality and pH. Short-term inactivation rates for chlorinated tap water, post-RO, pre-UV, post-UV and post-ion exchange sample locations were 0.028, 0.455, 0.231, 0.191 and 0.168 log10 h−1, respectively. Long-term inactivation rates for chlorinated tap water, post-RO, pre-UV, post-UV and post-ion exchange sample locations were 0.485, 0.911, 0.605, 0.632 and 0.684 log10 day−1, respectively. Since phages were found to remain intact as well as to lyse in the ultrapure water environment, the phages have the potential to contaminate the ultrapure water environments of the microelectronics, pharmaceutical and power generation industries in both colloidal and dissolved form. Further work is proceeding to generate standardized and cost-effective methods to detect viruses in water environments.

Removal of MS-2 and PRD-1 bacteriophages from an ultrapure water system

Viruses must be removed from the ultrapure water environment, as they have the potential to deposit on microelectronic devices and generate killer defects. Controlled and well-defined challenges by MS-2 and PRD-1 bacteriophages were treated in a pilot-scale ultrapure water system using ultraviolet radiation (UV), ozone, mixed bed ion exchange adsorption, and reverse osmosis filtration technologies typical of those used in industrial systems. Applying a first order kinetic model to the data generated rate constants for MS-2 removal by UV-185, 50 mg L−1 ozone, mixed bed ion exchange or reverse osmosis filtration of 15.5, 12.9, 3.9, and 10.4 min−1, respectively, and PRD-1 removal of 13.8, 15.5, 8.2, and 11.9 min−1, respectively. In all cases, removal of viruses by oxidative mechanisms such as ozone and UV were far superior to adsorption and filtration mechanisms. A theoretical viral population balance was generated to model the removal of the bacteriophages by these unit operations. This model relates the inlet time-dependent profile of viruses to the output, destruction, and accumulation profiles; it also relates these profiles to the unit operation’s treatment mechanisms including oxidation, adsorption, and filtration. This model is the first step in generating a site-independent theoretical model to project the persistence of viruses in ultrapure water systems.

Effect of system interactions on the removal of total oxidizable carbon from DI water polishing loops

Ultra-pure water systems in semiconductor plants consist of many components. Although the operating principles behind each component are well-known, the interactions between system components, including cancellation and synergism, are not generally understood and taken into account. In this study, two examples of these interactions are studied and analyzed: UV interactions with membrane filters and UV interactions with ion exchange units. The results indicate that the sequencing of UV and filter affect the total oxidizable carbon removal efficiency and it is preferable to have filter before UV. UV followed by ion exchange is an effective configuration for some impurities but can be undesirable for some contaminants and particles. >

Effect of component interactions on the removal of organic impurities in ultrapure water systems

The interaction among the components of the ultrapure water systems in semiconductor plants is explored. Two important examples of these interactions are investigated: interactions of ultraviolet (UV) radiation units with membrane filters and interactions of UV with ion-exchange units. Both experimental and theoretical techniques are developed to study these interactions. The results indicate that the sequencing of UV and filter units affect the efficiency of total oxidizable carbon (TOC) removal. In particular, the efficiency is higher when the filter is before UV. Interaction between UV and ion exchange is desirable for the removal of recalcitrant, low-molecular-weight organic compounds such as ethanol, but undesirable for the removal of high-molecular-weight charged compounds such as bacterial lipopolysaccharides, alginic acids, humic acids, methionine, and glycine. >