Inspection chamber with filter

A water reservoir (1) for use in an underground water discharge system comprises: a first compartment (14) including a first opening (2) for connection to a supply side of the water discharge system so as to receive water to be treated, or to a discharge side of the water discharge system so as to discharge treated water from the reservoir (1), and a second opening (3) for connection to the other one of the supply side and the discharge side of the water discharge system so as to receive water to be treated or discharge treated water from the reservoir (1), respectively; a second compartment (16); and a water treatment means (10) disposed between the first opening (2) and the second opening (3) for treating water flowing from one (2; 3) of the openings to the other (3; 2). The water treatment means (10) comprises: an inner shaft (11), the inside of which communicates the first compartment (14) with the second compartment (16); and a helical sedimentation channel (12) surrounding the inner shaft (11) and communicating the second compartment (16) with the first compartment (14). The first and second compartments (14, 16) are separated from each other by a partition plate (15), comprising a first through hole (17) for accessing the inner shaft (11) and a second through hole (19) for accessing the sedimentation channel (12).