An effluent processing apparatus comprises a separator device arranged to receive an effluent mixture to separate oil and water from the effluent mixture and thereby to provide clean air. A member selectively allows either only oil or only water of the separated oil and water to pass from a first side of the member through openings in the member to a second side of the member. A sump is arranged to (i) receive and retain the oil on the first side if only water passes from the first side to the second side, and (ii) receive and retain the oil on the second side if only oil passes from the first side to the second side. Clean air from the separator device is exhausted through an air outlet to atmosphere, and water is drained through a water outlet to atmosphere.