Passive electromagnetic resonator for water treatment for improving the biological quality of water having an inlet (1) for introducing the water to be treated and an outlet (4), and comprising a single serial through-flow type system formed from copper tubes in between said inlet (1) and said outlet (4). The copper tubes form the edges of at least a first pyramid, in the edges which border the horizontal base of said first pyramid water flows round in the same direction, and water flows in the opposite direction in either of two neighbouring edges bordering the inclined planes of said first pyramid.