Brand New Aero-Stream Septic Tank Riser Kit 23.5" Diameter by 26" The standard in septic tank riser safety Includes: safety barrier, riser, cover, mounting fasteners, sealant, and illustrated instructions. Base flange is 30.25" x 30.25" with a 23.5" riser ring inner diameter. Complete and ready for installation (assembly required) A septic tank riser makes installation of the Aero-Stream Remediator easier and even faster.