Abstract: An ascending flux lamellar decantation apparatus for treating a liquid having a plurality of parallel lamellar modules which define a plurality of ducts disposed within an enclosure. The apparatus includes a channel aligned parallel to the lamellar modules for introducing liquids into the apparatus. The apparatus further includes a repartition device for maintaining hydraulic flux of liquids, which repartition device is positioned above at least one of the lamellar modules. The repartition device is made of a plurality of hydraulically independent sections of substantially equal size such that the treated liquid flow in each section is equal. The repartition device further includes a treated liquid recovery means for collecting treated liquid.