Abstract: A sewer basin has a tube mounted longitudinally in the basin to isolate the level sensing elements from particulate and non-soluble materials present in the basin which could contaminate the elements, which are used for selectively activating a pump in the basin. The liquid level in the tube is kept in equilibrium with the liquid outside of the tube by an aperture below the normal low liquid level. A filter is provided in the tube between the aperture and the level sensing elements. The filter is formed by a stack of filter elements mounted on an axial post extending into the tube from a bottom end cap. The elements are limited in their axial mobility on the axial post.