Abstract
Legionella is a kind of bacteria that has been detected by many researchers in diffrent Water Heating Electrical (EWH) or solar (SWH) systems installed at various places. In the capital city Tripoli - Libya, most of water heater systems used are of electric type. Simultaneously, over 100 solar water heating (SWH) systems provided with electric heaters were installed under the supervision of the Centre for Solar Energy Research and Studies (CSERS) in the residential buildings in Tripoli in the period between (2000 and (2020). Ten hot water samples were collected from the outlet points in SWH systems and 43 water samples from diffrent water draining points at Corinthia hotel and Palm City Residences water systems, for the purpose of legionella detection and enumeration in the systems. A questionnaire was designed and distributed to be filed by the 10 SWH system users whom the ten water samples were taken from. Th purpose was to collect information about the building, the hot water system, and the user’s satisfaction. As the water source is untreated ground water from private wells, analysis of Escherichia coli (E.coli), total Coliform bacteria and total bacterial count were conducted to see whether any other probable bacteria kind is available. According to the results obtained, both 43 samples and 10 SWH samples investigated are free of legionella bacteria, while the total bacteria count for 4 out of 10 SWH systems was less than 60 CFU/g. Ths was due to water temperature not lowered to the bacteria colonisation temperature, and the daily water temperature gained from solar energy was high enough to be more than 55°C for at least an hour daily and maximum daily temperature exceeds 60°C for 80 days in addition to summer time, which is high enough to destroy legionella bacteria

Key words: Solar water heating, Legionella Bacteria, Maximum daily temperature