A process of treating sewage for safe discharge allows the relatively lightweight particles of untreated sewage to be returned to the digestion basin. PICT:1011264/C1/1 PICT:1011264/C1/2 A sewage tank 10 is divided by baffles 54 and 68 into a digestion tank 64, settling basin 74 and clarified liquid basin 76. Aeration is by aerator 96 and 26 and 28 are inlet for sewage and outlet for liquid respectively. Baffles 40 and 42 and vanes 106, 108, 118 and 120 give a circulatory movement to the sewage and cause a region of reduced pressure which attracts a flow of liquid and buoyant particles through the troughs 100 and 116. The flow through opening 114 into channel 116 can be controlled.
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