Investigation of biofilms in copper tube corrosion and the survival of Legionella pneumophila on alternative plumbing materials

Walker, James Taggart (1994). Investigation of biofilms in copper tube corrosion and the survival of Legionella pneumophila on alternative plumbing materials. PhD thesis The Open University.



An unusual form of copper tube corrosion, occurring in two hospitals, was investigated during two site surveys and due to the presence of characteristic perforations became known as pepper-pot pitting corrosion. The corrosion was found to occur predominantly in soft water areas but mainly in hot water systems maintained below 50°C. When monitoring the hot water system at a particular site the water temperature was found to decrease overnight with a corresponding decrease in the dissolved oxygen concentration and assimilable organic carbon. Copious amounts of biofilrn were recovered from the pipe surfaces thus it was hypothesised that metabolically active and respiring biofilm bacteria contributed to the creation of aggressive corrosive conditions at the copper tube surface. At control sites where this type of corrosion was not reported, the water temperature was found to be maintained above 50°C with reduced biofouling.

A laboratory model, using filter-sterilised potable water as the sole carbon source, was developed to investigate the conditions under which corrosion and biofouling was occurring. Biofilm development was demonstrated up to 55°C. At 60°C biofouling was very much reduced; however, a decrease in the number of bacteria recovered from the planktonic phase was only observed at 65°C. Planktonic bacteria were found to be dominated by pseudomonads while the biofilm was dominated by other Gram negative bacteria. Control measures that would slow down or prevent corrosion were studied. Pasteurisation (60°C) was found to prevent biofouling as well as controlling re-established biofilms but was less effective against consortia that had been previously exposed to this temperature. For the removal and control of biofilm, sulphamic acid was more effective than citric acid which allowed re-growth to occur within 14 days.

This unusual copper tube corrosion has resulted in increased use of alternative plumbing tube materials and therefore colonisation of copper and competitive plastic materials were investigated in the model system. Plastic materials were shown to encourage growth of Legionella pneumophila at 40'C whereas copper suppressed the growth of this water-borne pathogen. Results obtained in this investigation suggest that plastic plumbing systems pose a potential health risk by providing a means for transmission of pathogens such as L. pneumophila.

The association of biofilms with pepper pot pitting led to new ideas about mechanisms of microbially induced corrosion of copper tubing. A number of techniques including SEM, SCLM and light microscopy were used to demonstrate the heterogeneity and metabolic activity of biofilms produced in the laboratory model and on pipe surfaces. Mosaic microcolonies, themselves are responsible for the initiation of differential concentration sites that are aggravated by exo-polysaccharides, metabolic activity and particulate matter in the aquatic environment. It is the localised distribution of initiated sites that could be responsible for the formation of multi-loci corrosion cells that are driven by an electrochemical potential forming the type of corrosion described as pepper-pot pitting.

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