Liquid is caused to circulate in a shallow tank which has an operational depth less than half the effective maximum distance across the tank. One wall 17 defines with another wall a corner 18 and a bubble curtain 19 is provided along the wall 17 adjacent the corner 18 which imparts to the liquid in the tank an upward vertical component of movement which is asymmetrical along the wall 17, causing greater vertical movement of liquid at said corner than at the other end of the wall. Liquid rising up the wall 17 moving away therefrom by reaction therewith will circulate with a horizontal component due to the asymmetrical arrangement of the bubble curtain. The invention can be applied to the separation of materials of different densities e.g. the separation of suspended solids or emulsified oils from waste water.