NOVELTY - The photobioreactor comprises: a treatment chamber (12) for receiving a culture of algae; a water inlet for supplying waste water to the chamber; a light source provided within the chamber for providing light to the culture; and a water outlet for removal of treated water. The water outlet is further arranged in use to selectively remove a proportion of the biomass produced within the chamber when the biomass reaches a predetermined maximum level so as to maintain a continuous culture of algae within the chamber. The light source comprises a high frequency flashing light.    USE - The photobioreactor is useful for treating waste water (claimed).    ADVANTAGE - The photobioreactor can be used on an industrial scale to economically treat the wastewater with improved efficiency and reduced energy consumption.    DETAILED DESCRIPTION - The photobioreactor comprises: a treatment chamber (12) for receiving a culture of algae; a water inlet for supplying waste water to the chamber; a light source provided within the chamber for providing light to the culture; and a water outlet for removal of treated water. The water outlet is further arranged in use to selectively remove a proportion of the biomass produced within the chamber when the biomass reaches a predetermined maximum level so as to maintain a continuous culture of algae within the chamber. The light source comprises a high frequency flashing light arranged to emit light at an intensity above a pre-determined threshold intensity for damaging the culture of algae. The light source is in the form of an elongate column. The elongate column is arranged in use to extend substantially parallel to the longitudinal axis of the chamber. The light source comprises LED. The flashing light is arranged in use to flash so that the ratio of the period of time in which light is emitted to the period of time in which light is not emitted, is 1:2. The light intensity provided by the light source is variable. The light source is arranged in use to provide an increasing light intensity over a predetermined period of time. The bioreactor further comprises an insulating layer, a membrane located adjacent to or at the water inlet and arranged in use to maintain the sterility of the culture, and an inlet for supplying metals to the chamber. Wavelengths of light emitted from the light source are calibrated to enhance photosynthesis. The light source is calibrated to provide light having predominantly a wavelength in the region of 620-645 nm. The outlet comprises a first outlet for removal of treated water from the chamber and a second outlet arranged in use to selectively remove a proportion of the biomass. The membrane is arranged in use to retain the culture within the chamber while allowing treated water to be removed by the outlet and to selectively allow a proportion of the biomass through the outlet when the biomass within the chamber reaches a predetermined maximum level. The outlet comprises a first outlet (20) for removal of treated water from the chamber and a second outlet (24) arranged in use to selectively remove a proportion of the biomass. The membrane is arranged within the chamber so as to be located between the first and second outlets. INDEPENDENT CLAIMS are included for:    (1) a waste water system;    (2) a kit for waste water treatment; and    (3) a method for treating waste water.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a photobioreactor.    Treatment chamber (12)    First outlet (20)    Second outlet (24)    Gas diffuser (26)    Nanodiffusers. (34)