In a water treatment system and a water treatment process, a scale inhibitor is supplied to water to be treated containing Ca ions, SO.sub.4 ions, carbonate ions, and silica, and the water to be treated is adjusted to a pH at which silica is soluble. The pH-adjusted water containing the calcium scale inhibitor is separated in a first demineralizing section into concentrated and treated water. In a first crystallizing section, seed crystals of gypsum are supplied to the first concentrated water, whereby gypsum is crystallized and removed from the first concentrated water. Silica in the water to be treated is removed from the first concentrated water on the downstream side of the first crystallizing section. Calcium carbonate in the water to be treated is removed from the first concentrated water on the upstream side of the first demineralizing section or the downstream side of the first crystallizing section.

In a water treatment system and a water treatment process, a scale inhibitor is supplied to water to be treated containing Ca ions, SO.sub.4 ions, carbonate ions, and silica, and the water to be treated is adjusted to a pH at which silica is soluble. The pH-adjusted water containing the calcium scale inhibitor is separated in a first demineralizing section into concentrated and treated water. In a first crystallizing section, seed crystals of gypsum are supplied to the first concentrated water, whereby gypsum is crystallized and removed from the first concentrated water. Silica in the water to be treated is removed from the first concentrated water on the downstream side of the first crystallizing section. Calcium carbonate in the water to be treated is removed from the first concentrated water on the upstream side of the first demineralizing section or the downstream side of the first crystallizing section.

A filtration device (1) has a housing (4) and a filter module (2) with hollow fibers (32) surrounded by an outer shell (5). The hollow fiber bundle (6) is sealed to the outer shell (5) at each end by an adhesive (11) transverse to the longitudinal direction (10). Each end of the hollow fibers (32) is unclosed. An unfiltered product chamber (20) is between the outer shell (5) and a wall (19) of the filter housing (4) and a filtered product chamber (13) is inside the filter module (2). The bottom end of the filter module (2) connects with an intermediate section (3) that connects with a receptacle (17) of a bottom portion (15) of the filter housing (4). The intermediate section (3) forms a connection chamber (26) facing the hollow fiber ends (29) and radial feed channels (28) connect the connection chamber (26) with the unfiltered product chamber (20).