Desludging is the first process in the cleaning of the tank and this is where most of the oily sludge from the tank is removed. The desluding process may begin when the oxygen level has fallen below 8 per cent. The nozzles are operated one at a time, alternating between all of the nozzles supplied with the system. Since the sludge may contain solids, water and trapped, liquefied oil it may be necessary to add cutter stock to re-liquefy the sludge.