The specification discloses an improvement in rotary screening apparatus for the filtration of liquids and especially water. Such apparatus comprises (a) a stationary structure including a delivery channel for the feed water and a screened water holding tank wherein a lower elevation of screened water is maintained and (b) a screen drum disposed to receive feed water from the channel and to rotate within the tank on a horizontal axis with the uppermost section of its cylindrical surface above the feed for cleaning purposes. The improvement resides in a seal between the holding tank and the drum which provides improved protection against leakage and utilizes a flow of screened water which drives the feed water away from the seal if and wherever there may be a leak.