Abstract: A bioreactor for the biochemical treatment of liquids containing organic matter, comprises an elongated, upwardly extending tubular container having inlet means for receiving the liquid to be treated and outlet means for discharging the treated liquid. A plurality of spaced-apart first and second trays are alternatively arranged above one another inside the container, each tray being apertured to provide liquid flow communication between the inlet and outlet means and adapted to support a respective bed of microorganism cells capable of reacting with the organic matter. The apertures of the first and second trays are arranged relative to one another to cause the liquid to flow laterally across the respective cell beds of the trays as the liquid flows from one tray to another.