Then, at most treatment plants, the water slowly moves through a presedimentation basin where much of the sand and silt settle to the bottom.
(the water; slowly moves; through a presedimentation basin; T:Then; L:at most treatment plants) (much of the sand; silt settle; to the bottom)
the water much of the sand
slowly moves silt settle
through a presedimentation basin T:Then L:at most treatment plants
to the bottom