A grit removal unit including an influent channel directing wastewater with grit into a round grit removal chamber and an effluent channel receives wastewater output from the chamber. A divider wall extends upwardly between the sides of the influent channel, and gates at the upstream end of the divider wall are adjustable to selectively open channel portions on opposite sides of the divider wall. An output guide member is mounted to allow selective adjustment of output flow opening width “c” to the effluent channel. The output guide member has a horizontal bottom extending from the chamber into the effluent channel across the width of the output opening, and a guide wall extending upwardly from the guide member bottom. The guide wall has a forward end spaced from the chamber annular vertical wall to define the output flow opening width therebetween.