A pumpbox apparatus includes a reservoir volume having a first inlet for receiving a feedstock stream and a second inlet for receiving a water stream, the reservoir volume being in communication with a discharge outlet disposed to discharge accumulated liquid from the reservoir volume. The reservoir volume is operable to accumulate the feedstock stream and the water stream in the reservoir volume while withdrawing a discharge stream through the discharge outlet to cause a flow of liquid through the pumpbox. The first inlet defines a first flow velocity region between the first inlet and the second inlet and a second flow velocity region between the second inlet and the discharge outlet. The first flow velocity is lower than the second flow velocity to facilitate flotation of a low specific gravity portion of the feedstock through the first region toward an upper surface of the liquid accumulated in the reservoir volume.