A number of biological sewage treatment plant types are in use at sea but nearly all work on what is called the extended aeration process. Basically this consists of oxygenating the liquor either by bubbling air through it or by agitating the surface. By so doing a family of bacteria is propagated which thrives on the oxygen content and digests the sewage to produce an innocuous sludge. The imprison that bubbling air through the sewage serves to oxidize it thus reducing BOD is not strict case. It is the bacteria that reduce the BOD by converting the organic content of the sewage to a chemically and organically inert sludge. In order to exist, the bacterial need air and nutrient. The nutrient is in the form of body and galley wastes. If the nutrient source is cut off, i.e. the plant shut down or by-passed for say an extended ocean passage, the bacterial die and the plant cannot function correctly until a new bacteria colony is generated. This process can take from 7 to 14 days. Bacterial which live in the presence of oxygen are said to be aerobic. When oxygen is not present, the aerobic bacterial cannot live but a different family of bacterial is generated. These bacterial are said to be anaerobic. While they equally capable of producing an inert sludge, in so doing hydrogen sulfide, carbon dioxide and methane are formed. Some sewage processes are designed to work anaerobically but they are not usually adopted for shipboard use.