An interceptor unit for receiving mixtures of petrol, oil and water and for discharging relatively clean water comprises a vessel (2) divided into an inlet chamber (4), an interceptor (6) and an outlet chamber (8). An inlet (12) enters the inlet chamber (4), and an outlet (36) discharges from the outlet chamber (8). Flow entering the inlet chamber (4) is transferred to the interceptor (6) through a conduit (18), where separation of the petrol and oil on the one hand and the water on the other hand takes place. From the lower region of the interceptor (6), relatively clean water is transferred to the outlet chamber (8) through an upwardly extending transfer passage (28), to be discharged through the outlet (36). Under conditions of heavy flow, the conduit (18) is unable to accept all of the mixture entering through the inlet (12), and some flow takes place from the lower region of the inlet chamber (4) to the outlet chamber (8) through an overflow arrangement (38). IMAGE
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