a third zone called the sedimentation zone 16 located in another upright containment vessel 112 the flocs and aggregates that were created inside the two preceding zones 10 and 15 are attracted by gravity toward the funnel shaped flooring 22 of downstream vessel 112 heavier particles are therefore more likely to sink to the flooring 22 of the sedimentation zone 16 and do so more quickly than lighter ones which is the interest of coagulation and ballast flocculation in order to improve the efficiency of the water treatment system a scraper 20 which may carry a device such as an inverted cone 21 at its center is given a rotational movement along the plane of the sedimentation zone 16 through a motor 18 driving a rotatable upright shaft 17 the purpose of shaft 17 is to rake the flocs of contaminants which have deposited on the radially inwardly downwardly sloped walls of flooring 22 of a sludge recovering cavity 19 located beneath inverted cone 21 in the center of the sedimentation zone 16 the flocs of contaminants thus gather inside the sludge recovering cavity 19 the mass of which consequently forming sludge the inverted cone 21 may be replaced by other suitable structures for example by a horizontal perforated plate spacedly supported over pit flooring 22 the perforations of such a perforated plate would enable free passage of the sludge at such a flow rate that the sludge would not be returned to a state of suspension this sludge which contains a relatively large volume of water shall hereinafter be called diluted sludge to optimize the operation of this water treatment this diluted sludge needs to be treated in order.