Bacteria and High Purity Water Systems

George Younan
ProcessHQ, Inc.

In high purity water systems, one of the greatest maintenance concerns is the development of colonies in the form of biofilm. Biofilm may be described as structure of bacterial flora composed of micro colonies of cells. These cells are attached to the walls and components of the water system. The bacteria in biofilms characteristically secrete a substance; primarily a polysaccharide, which is filmy and sticky. This film encourages the attachment of other organisms, acts as containment for metabolic nutrients and protects the organisms from the disinfectants. The flora that make up biofilms may be 500 times more resistant to anti-bacterial agents1.

The presence of biofilm in a water system may be very difficult to detect. A low bacterial count may not indicate a system that is free from bacterial contamination. In fact, low counts often yield a false sense of security due to the fact that sample is so dramatically diluted. On the other hand, samples may occasionally indicate a very high contamination level when a large piece of plaque contaminates the sample2, 3. The most indicative sampling technique seems to be the swabbing of specific areas in the system which are more susceptible to biofilm growth i.e. heat exchangers, dead legs, and mechanical connection points.

Biofilm is not only difficult to detect, but also, difficult to remove. The shearing forces generated by the velocity in most water systems (1.5 m/s to 2.5 m/s) is not sufficient to remove the biofilm. In fact, the forces generated by these velocities may not be enough to prevent the adsorption of biofilm forming organisms to the walls of the system4. Traditional biocides are also not very effective against the removal of biofilm due to the lack penetration into the polysaccharide coating and must be rotated to prevent the development of a resistance factor. Oxidative biocides such as blends of hydrogen peroxide and peracetic acid have been shown to be more effective during the same exposure times5.

Biofilms in high purity water system have adapted a unique ability to survive in invasive nutrient poor environment. It is apparently prudent to minimize the proliferation of these biofilms instead of trying to eradicate them after they are established. The best treatment seems to be consistent and regular disinfections in a system that has been designed with the unique adaptability of these organisms in mind.

Bibliography

1. Costeron, Annual Review of Microbiology, Vol. 49 pp 711-745.
2. Fleming, Werkstoffe and Korrosion, Annual Review of Microbiology, Vol. 45 pp 40-53.
3. FDA, Guide to the Inspection of High Purity Water Systems, July 1993.
4. Powell, Biotechnology and Bioengineering, Vol. 24 pp 2527-2537.
5. Maltais and Stern, Minntech Corp., Minneapolis MN.