The amount of dissolved oxygen is influenced by water temperature, atmospheric pressure, salinity and other factors, and reduced in the highly polluted water because of much oxygen consumption. And then, if aeration stops, they become a mass of aggregate, namely, a flock, rapidly settle, and supernatant liquor get clear. Such coagulative sludge formed is called activated sludge that is a mass of aerobic organisms that mainly consist of bacteria called Zooglea. An activated sludge method is one by which wastewater is purified by making use of such activated sludge. Since an activated sludge process makes sludge gradually increase and an amount or more of sludge makes its function deteriorated,an amount of sludge must be pull off.
Excess sludge means such increasing and unnecessary sludge.
This layer is called scum. This means the amount of suspended solids in a condition under which sludge and wastewater are mixed in an activated sludge aeration tank. Since it is difficult to measure the amount of organisms useful for wastewater decomposition in sludge, a MLLS value measured replaces its amount. The greater value of settled sludge means lower subsidence property. In an activated sludge process, the desirable value is usually deemed to be around 100.