It is also important that there is some balance of flux in the system. Because of the high recoveries used in some cases, the flux rates of tail elements in the last stage can be very low. To compensate for this, lead elements must operate at higher fluxes. This can be compensated by using booster pumps or turbo boost pumps. More recently, new membranes have come on the market with have thicker spacers and are more fouling resistant. The use of these membranes reduces the pressure losses, helping to improve flux balance in the system by providing more net driving pressure in the last stage.