Abstract: The present invention is directed to a one-piece cell divider for use in a filter underdrain system that supports a parous plate that supports a filter media for filtering a fluid. The one-piece cell divider includes: a divider portion; a porous plate support portion, including a flange that supports the porous plate; a support member portion, including a flange that attaches the divider to a base. A filter underdrain using a one-piece cell divider may include; at least one porous plate for supporting the filter media for filtering a fluid, the porous plate spanning across at least one of the dividers; the at least two dividers including: one or more one-piece cell dividers; one or more multiple-piece dividers, each multiple-piece cell divider comprising a divider portion and a support member, the support member being directly under the divider portion.