NOVELTY - A water-well filter device comprises a microorganism filter element (27) having largest pore size of at most 100 microns contained within a housing (19). The microorganism filter element is connected to housing interior in such way that the housing interior is divided into two compartments and the only way air can travel inside the housing is through the microorganism filter element.    USE - For both residential and municipal wells (claimed).    ADVANTAGE - The inventive filter element prevents airborne microorganism from entering into the residential or municipal water well. The incorporation of pre-filter helps stop large particles in the air from reaching and clogging the microorganism filter element, thus prolonging the useful life of microorganism filter element. The provision of vacuum relief valve prevents any undesirable effects caused by negative pressure that is built up in the well when the microorganism filter element or pre-filter is clogged up.    DETAILED DESCRIPTION - A water-well filter device consists of a microorganism filter element, and a housing having a connection member and at least two openings. The largest pore of the filter element has a size of at most 100 microns. The microorganism filter element is connected to the housing interior in such way that the housing interior is divided into two compartments having respective openings, and the only way air can travel inside the housing from one compartment to the other is through the microorganism filter element. The connection member allows the filter device to be mounted on an opening of the water well either directly or indirectly in such way that one compartment of the housing is in gas communication with the open air and the other is in communication with the inside of the well.    DESCRIPTION OF DRAWING(S) - The figure is a side view of the inventive water-well filter device.    Connection pipe (7)    Housing (19)    Pre-filter element (21)    Microorganism filter element (27)    Open air (28)