Water displacement from the expulsion of bubbles will cause a mixing action to occur, and the contact between the water and the bubble will result in an oxygen transfer. It is often difficult for this type of machine to aerate the entire water column. The low speed surface aerator is a device for biology laying sod with high efficiency. Thoses devices are often in steel protected by epoxy coating and generate high torque. The common power is going from 1 up to 250kw per unit with an efficiency around 2 kgO2/kw.