Sludge transfer is normally the second most significant contributor to MBR energy demand after aeration.
Costs deriving from sludge pumping or transfer from the membrane tank to the process tank, and between different regions within the process (for nutrient removal), are primarily a function of the pumping rate.
Other key parameters affecting the rheological properties of the sludge (solids concentration and temperature) do not vary significantly across large-scale municipal applications; the sludge solids concentration is normally around 8-12 g/L in the process tank and 10-15 g/L in the membrane tank for an immersed system. For a sidestream system subjected to high loads the solids concentration may be higher.
Generally speaking, the rate of transfer between the membrane and process tank is between 3 and 5 times the rate of the flow of water through the process. Flows between different process tanks to achieve nutrient removal are of a similar order.