A total of 452 samples from hot water systems of randomly selected single family residences in the suburbs of two German cities were analysed for the occurrence of Legionella. Technical data were documented using a standardized questionnaire to evaluate possible factors promoting the growth of the bacterium in these small plumbing systems. All houses were supplied with treated groundwater from public water works. Drinking water quality was within the limits specified in the German regulations for drinking water and the water was not chlorinated. The results showed that plumbing systems in private houses that provided hot water from instantaneous water heaters were free of Legionella compared with a prevalence of 12% in houses with storage tanks and recirculating hot water where maximum counts of Legionella reached 100,000 CFU/100ml. The presence of L. pneumophila accounted for 93.9% of all Legionella positive specimens of which 71.8% belonged to serogroup 1. The volume of the storage tank, interrupting circulation for several hours daily and intermittently raising hot water temperatures to >60°C had no influence on Legionella counts. Plumbing systems with copper pipes were more frequently contaminated than those made of synthetic materials or galvanized steel. An inhibitory effect due to copper was not present. Newly constructed systems (<2 years) were not colonized. The type of hot water preparation had a marked influence. More than 50% of all houses using district heating systems were colonized by Legionella. Their significantly lower hot water temperature is thought to be the key factor leading to intensified growth of Legionella. Although hot water systems using solar energy to supplement conventional hot water supplies operate at temperatures 3°C lower than conventional systems, this technique does not seem to promote proliferation of the bacterium. Our data show convincingly that the temperature of the hot water is probably the most important or perhaps the only determinant factor for multiplication of Legionella. Water with a temperature below 46°C was most frequently colonized and contained the highest concentrations of legionellae. It is evident that the same factors affecting colonization by Legionella in large buildings also exist in small residential water systems. If temperatures are low there is no difference between large and small systems and Legionella counts are high in both. Since private residences are an important source of community-acquired legionellosis, these findings emphasize the need for preventive control measures in small residential buildings. In some situations it may be necessary to install filtration devices at the point-of-use.

This chapter evaluates the efficacy of copper-silver ionization that is reported to be a useful tool to control growth of Legionella. Copper-silver ionization was tested over a period of 1 year as the first step in a multiple-barrier system controlling Legionella. In Germany, additions of silver and copper to drinking water need special approval of the Ministry of Health and are permitted only with substantial restrictions. Measurable effects in Legionella reduction can be achieved only at silver concentrations significantly exceeding the German standard of 10 μg/liter. Copper values exceeded national standards, preventing a short-term high dosing of silver. Concentrations of both metals vary depending on water flow. Silver cannot be analyzed directly with in the system, causing a considerable feedback delay. Legionella counts stayed above targeted values (working standard, Legionella counts <1 CFU/ml) . Reduction is not as efficacious as in hot water systems maintaining about 60№C. Long-term effects (e.g., development of heavy-metal resistance) of adding silver and copper are yet to be evaluated.