Installation for screening an effluent which is charged with materials in suspension, having a succession of rotating screening groups (Gi) with external supply which are in contact with the effluent to be filtered and which allow the effluent to pass through with the materials in suspension being retained on the external surface of the screen in order to remove them by means of scraping, each screening group (Gi) comprising - a filtering drum which is formed by a cylinder having a substantially horizontal axis and which is rotatably driven and whose peripheral surface is constituted by a screen having a defined cutoff threshold, - a tank which receives the effluent to be screened, having a spillway which is applied externally against the screen in order to form a contact zone between the effluent bath of the tank and the outer surface of the screen, - a scraper which is pressed against the outer surface of the screen, downstream of the spillway in the rotation direction of the drum in order to scrape the materials accumulated on the screen, - a trough which is installed below the drum in order to receive the filtered effluent (Ex) which has passed through the screen, this trough being provided at the lower portion with an outlet, the cutoff thresholds of the screens of the screening groups (Gi) decreasing from the group at the highest position to the group (Gn) at the lowest position, the effluents passing successively by gravitational force from the outlet of one group to the inlet of the group located below it, which installation is characterised in that - the outlet of the trough of the group (Gi) is connected via a tube to the inlet of the lower portion of the base of the tank of the group (Gi+1) downstream, - the tank has a base in the form of a horizontal cylinder portion having a circular cross-section which accommodates an agitator having blades which are carried by an axle and which rotate about the horizontal axis of the cylinder which forms the base. - a filtering drum which is formed by a cylinder having a substantially horizontal axis and which is rotatably driven and whose peripheral surface is constituted by a screen having a defined cutoff threshold, - a tank which receives the effluent to be screened, having a spillway which is applied externally against the screen in order to form a contact zone between the effluent bath of the tank and the outer surface of the screen, - a scraper which is pressed against the outer surface of the screen, downstream of the spillway in the rotation direction of the drum in order to scrape the materials accumulated on the screen, - a trough which is installed below the drum in order to receive the filtered effluent (Ex) which has passed through the screen, this trough being provided at the lower portion with an outlet, - the outlet of the trough of the group (Gi) is connected via a tube to the inlet of the lower portion of the base of the tank of the group (Gi+1) downstream, - the tank has a base in the form of a horizontal cylinder portion having a circular cross-section which accommodates an agitator having blades which are carried by an axle and which rotate about the horizontal axis of the cylinder which forms the base.