A grit removal unit for a wastewater system, comprising: a round grit removal chamber defined by a bottom surface and a substantially annular vertical wall extending up from said bottom surface; an input opening through said annular vertical wall, said input opening having a bottom surface (40,48) above the bottom surface of the grit removal chamber, with a sloped bottom surface extending between said input opening bottom surface and said chamber bottom surface; an output opening through said annular vertical wall, said output opening having a bottom above the bottom surface of the grit removal chamber; an influent channel for directing wastewater with grit into said chamber through said annular vertical wall input opening, said influent channel having two side walls (42,44); an effluent channel for wastewater output from said chamber through said output opening; and an output guide member having a guide wall extending upwardly from a substantially horizontal bottom along an edge of said output guide member bottom, wherein said guide wall has a forward end spaced from said chamber annular vertical wall at said output opening to define an output flow opening width "C" between said chamber annular, vertical wall and said guide member guide wall forward end, and a rear end extending through said output opening and into said effluent channel, and said output guide member horizontal bottom extends from said guide wall within said grit removal chamber into said effluent channel across substantially the entire width of the output opening whereby effluent passing through said output flow opening width flows above said output guide member horizontal bottom to said output opening through said annular vertical wall output opening to said effluent channel; characterized in that: said output guide member is adjustably mounted relative to said effluent channel whereby said output flow opening width may be selectively adjusted, whereby the guide wall forward end is located at a selected spacing from said annular vertical wall with said output guide member horizontal bottom extending from said chamber into said effluent channel across the width of the output opening. a round grit removal chamber defined by a bottom surface and a substantially annular vertical wall extending up from said bottom surface; an input opening through said annular vertical wall, said input opening having a bottom surface (40,48) above the bottom surface of the grit removal chamber, with a sloped bottom surface extending between said input opening bottom surface and said chamber bottom surface; an output opening through said annular vertical wall, said output opening having a bottom above the bottom surface of the grit removal chamber; an influent channel for directing wastewater with grit into said chamber through said annular vertical wall input opening, said influent channel having two side walls (42,44); an effluent channel for wastewater output from said chamber through said output opening; and an output guide member having a guide wall extending upwardly from a substantially horizontal bottom along an edge of said output guide member bottom, wherein said guide wall has a forward end spaced from said chamber annular vertical wall at said output opening to define an output flow opening width "C" between said chamber annular, vertical wall and said guide member guide wall forward end, and a rear end extending through said output opening and into said effluent channel, and said output guide member horizontal bottom extends from said guide wall within said grit removal chamber into said effluent channel across substantially the entire width of the output opening whereby effluent passing through said output flow opening width flows above said output guide member horizontal bottom to said output opening through said annular vertical wall output opening to said effluent channel; characterized in that: said output guide member is adjustably mounted relative to said effluent channel whereby said output flow opening width may be selectively adjusted, whereby the guide wall forward end is located at a selected spacing from said annular vertical wall with said output guide member horizontal bottom extending from said chamber into said effluent channel across the width of the output opening. a guide wall extending upwardly from a substantially horizontal bottom along an edge of said output guide member bottom, wherein said guide wall has a forward end spaced from said chamber annular vertical wall at said output opening to define an output flow opening width "C" between said chamber annular, vertical wall and said guide member guide wall forward end, and a rear end extending through said output opening and into said effluent channel, and said output guide member horizontal bottom extends from said guide wall within said grit removal chamber into said effluent channel across substantially the entire width of the output opening whereby effluent passing through said output flow opening width flows above said output guide member horizontal bottom to said output opening through said annular vertical wall output opening to said effluent channel; characterized in that: said output guide member is adjustably mounted relative to said effluent channel whereby said output flow opening width may be selectively adjusted, whereby the guide wall forward end is located at a selected spacing from said annular vertical wall with said output guide member horizontal bottom extending from said chamber into said effluent channel across the width of the output opening.