Abstract: Waste water containing biodegradable matter is fed to a rotating biological contactor (RBC) (15) from an inlet tank (10) and is discharged from the RBC-containing chamber into a humus tank (21). The RBC is divided into separated upstream (15A) and downstream (15B) sections, the chamber (17) containing the downstream section being fed with water to be processed at a controlled rate from the chamber (16) containing the upstream section. A balancing chamber, for isolating the downstream section of the RBC from variations in flow of waste water to the inlet tank (10), includes the upstream section (15A, 16) of the RBC which can act sacrificially in respect of toxic incursions in the waste water.