Waste water purification system

A biological water-treatment unit for biological treatment of a waste water stream, comprising:

• an inlet for waste water, a reactor, an outlet for purified water, and ventilation means configured to provide ventilation to the reactor, said inlet, reactor and outlet being in fluid communication;
• wherein the reactor is divided into two or more successive compartments by one or more mesh screens disposed along a length of the reactor, wherein each compartment is in communication with the ventilation means, and comprises a biofilm on one or more surfaces, wherein the reactor further comprises a powder or granular substrate of biofilm growth that is heavier than water, said substrate being substantially contained in each compartment within the reactor by the one or more mesh screens;
• and
• wherein the reactor is configured to expose at least a portion of the waste water to the biofilm in the presence of air supplied to above the waste water stream by the ventilation means, thereby biologically treating the waste water stream into purified water;
• wherein the reactor further comprises a rotating drum which comprises the two or more compartments, said rotating drum being configured to rotate so that a film of the waste water is formed on at least a portion of the reactor and the mesh screen as they rotate through and out of the waste water stream, thereby exposing the film to air above the waste water stream supplied by the ventilation means;
• and
• wherein at least a portion of an inner surface of the rotating drum is adapted to increase the friction between the substrate and the rotating drum such that, in use, rotation of the rotating drum causes at least a portion of the substrate to be raised out of the waste water and into the air supplied by the ventilation means.