include a vertical portion 23a which is connected through a right angle bend 23b to a horizontal portion 23c in the arrangements shown in fig2 the gas lift draught tubes 23 also include a lower horizontal portion 23d which terminates in a broader member 23e which has an aperture not shown in its lower surface in operation the gas distribution member 9 of fig2 delivers gas into the gas lift tubes 23 which being immersed in the lower digester portion of the tank contain liquids and solids material being subjected to anaerobic digestion the emission of the gas bubbles at the foot of the vertical portions 23a of the gas lift tubes 23 causes liquid within these vertical portions to be raised vertically with a simultaneous drawing in of further liquid or sludge through the apertures in the lower surfaces of the broader members 23e and discharge of liquid from the upper horizontal portion 23c there is thus a continuous movement of liquid through the gas lift draught tubes 23 from the horizontal portion 23d into the vertical portion 23a and thence into the horizontal portion 23c from which the liquid is expelled the changes of direction in the movement of the liquid at the right angle bends at the opposite ends of the vertical portions 23a produce the reaction forces which cause the mixing assembly of fig2 to turn on the bearings 18 and 22 about the central member 5 and so move through the liquid and solids near the base of the lower digester portion to effect mixing a gas lift draught tube 23 positioned on the mixing assembly at the end of the gas distribution arm 9 remote from the central member 5 is.