Method and apparatus for decreasing the content of impurities in raw water

A method for decreasing the contents of at least one impurity selected from the group consisting of metals, metalloids, nitrate and nitrite in raw water supplied to a hermetically sealed basin through in the basin horizontally arranged feeding pipes so as to flow through said basin containing naturally occurring filter material, whereby in case of decreasing the contents of metals and/or metalloids water containing oxygen or oxygen-releasing substances and/or metabolic reactions of naturally occurring micro-organisms and in the case of decreasing the contents of nitrate and/or nitrite water containing denitrification organisms and/or substrates therefore, termed conditioned water, is intermittently introduced to the basin through at least two infiltration means, said at least two infiltration means is also used for periodically withdrawing said conditioned water so as to regenerate said conditioned water, whereby water having decreased content of said at least one impurity is extracted by at least one vertically arranged extraction well and the extraction of said water being such that a constant negative pressure is created around said at least one extraction well,

• comprising the steps of arranging said at least two infiltration means by placing at least one horizontally above and at least one horizontally below said horizontally arranged feeding pipes, and alternating withdrawal and infiltration of said conditioned water between said at least two infiltration means so that essentially a vertical flow of water is created between these means, thereby enhancing the mixing between conditioned water and raw water to make the reaction between these waters more effective and distributing precipitation of impurities over a greater portion of the basin.