A filter underdrain (100, 300) for supporting a filter media (105, 305) between a plurality of dividers forming at least one cell (102, 302), the filter underdrain comprising: at least one porous plate (104, 830, 1030) for supporting the filter media for filtering a fluid, the porous plate spanning across at least one of the dividers; the plurality of dividers comprising: a plurality of one-piece cell dividers (500, 810, 1010), each one-piece cell divider comprising: an upper portion which is configured to act as a partition between two cells; a first flange configured to support the porous plate; and a lower portion comprising a bottom surface configured to attach the one-piece cell divider to a base-plate, wherein the first flange is located between the upper portion and the lower portion, and wherein the first flange is substantially perpendicular to the upper portion and the lower portion; and 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, the divider portion and the support member being separate components; wherein the porous plate is positioned between the one-piece cell dividers and supported by the one or more multiple-piece dividers, and wherein the filter underdrain is assembled without the use of caulk. at least one porous plate (104, 830, 1030) for supporting the filter media for filtering a fluid, the porous plate spanning across at least one of the dividers; the plurality of dividers comprising: a plurality of one-piece cell dividers (500, 810, 1010), each one-piece cell divider comprising: an upper portion which is configured to act as a partition between two cells; a first flange configured to support the porous plate; and a lower portion comprising a bottom surface configured to attach the one-piece cell divider to a base-plate, wherein the first flange is located between the upper portion and the lower portion, and wherein the first flange is substantially perpendicular to the upper portion and the lower portion; and 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, the divider portion and the support member being separate components; wherein the porous plate is positioned between the one-piece cell dividers and supported by the one or more multiple-piece dividers, and wherein the filter underdrain is assembled without the use of caulk. an upper portion which is configured to act as a partition between two cells; a first flange configured to support the porous plate; and a lower portion comprising a bottom surface configured to attach the one-piece cell divider to a base-plate, wherein the first flange is located between the upper portion and the lower portion, and wherein the first flange is substantially perpendicular to the upper portion and the lower portion; and 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, the divider portion and the support member being separate components; wherein the porous plate is positioned between the one-piece cell dividers and supported by the one or more multiple-piece dividers, and wherein the filter underdrain is assembled without the use of caulk.