Abstract: A riser system for use at the access opening of a wastewater treatment tank includes an adaptor ring affixed to the top of the tank, and a mating riser. The adaptor ring and riser have ends with mating circumferential L-shape sections. There are horizontally projecting tabs spaced apart around the upper end of the adaptor. The tabs engage a corresponding number of L-shape slots which are spaced apart around the circumference of the bottom end of the riser. When ring and riser are mated, rotation of the riser relative to the adaptor causes each tab to move circumferentially to a recess in the slot, which locks the tab in position. Micro-ridges on horizontal mating surfaces of the L-shape sections help form a seal between the two parts.