[Legionella in membrane expansion vessels]

Membrane expansion vessels (MEV) are installed between cold drinking water supply and hot water system to compensate pressure changes and water losses. An elastic membrane inside the MEV divides a gas pressurized compartment from a water filled compartment, which--in case of flow-through type of MEV--is directly coupled to the water current. While heating the system the expanding water enters the last mentioned compartment. When cooling down, the water is flowing out. Thus MEVs may prevent water losses. Bacteriological investigations of two DEVs of the flow-through type showed the following results: 1. While 5 samples taken from the cold water supply did not yield Legionella, all 7 samples from the outlets of both DEVs tested positive for Legionella (average 21 CFU/ml). 2. In each of 4 samples taken from the outlet of a DEV, total colony counts exceeded the guide value of 100 colonies/ml demanded in the German Drinking Water Act (average 724 colonies/ml at 20 degrees C, 921 colonies/ml at 36 degrees C). 3. Swab and contact culture from one of the membrane indicated excessive surface colonization with Legionella. The findings show the necessity to include MEVs in investigations of hot water systems for Legionella.