end pipe 18 when the level of liquid in the vessel 2 reaches the low level 37 the electronic circuit 42 instigates the stopping of the pump 24 and the closing of the electrically operated valve 23 with a temporal offset of a duration such that the level of liquid in the vessel 2 reaches a bottom low level 43 near the lower wall 7 or is completely discharged the electronic circuit 42 then instigates the opening of the electrically operated valve 27 so as to bring about a secondary influx of dilution liquid into the vessel 2 when the level of the dilution liquid reaches the intermediate low level 38 the electronic circuit 42 instigates the closing of the electrically operated valve 27 when effluent is carried into the vessel 2 the level of liquid contained in this vessel rises and the cycle that has just been described recommences starting from the level 38 when the liquid discharged from the vessel 2 passes through the exposure channels 13 of the treatment members 10 a and 10 b it undergoes a decontamination treatment through being exposed to the ultraviolet light radiation from the lamps 14 in an alternative form the lamps 14 may be permanently illuminated in another more economical alternative form upon detection of the intermediate level 39 the electronic circuit 42 instigates the illuminating of the lamps 14 and when the low level 43 is reached the electronic circuit 42 instigates the switching off of the lamps 14 of course the time spent conveying the dilution fluid that raises the level from the intermediate level 39 to the high level 40 must be long enough for the lamps 14 to warm up to ensure that the length of time for which the dilution fluid is supplied may.